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55

CHAPTER 3

Eye Movements and Measures of Reading Time

Simon P. Liversedge University of Durham

Kevin B. Paterson University of Nottingham

and

Martin J. Pickering University of Glasgow

Abstract

In this chapter, we consider the use of reading time measures that sum fixation durations in order to gain an understanding of the difficulty experienced when reading texts. We draw a distinction between two approaches to summing the duration of fixations. One approach is to sum the duration of fixations that are spatially contiguous in the text, meaning that the fixations neighbour each other in a specified region of space. The other approach is to sum fixations that are temporally contiguous, meaning that they occur in a sequence over a specified period of time. It is argued that both types of reading time measure are needed if the experimenter is to understand the time course of the influence of a linguistic variable on readers' processing of text. We first discuss a number of hypothetical eye movement records in order to illustrate the differential sensitivities of qualitatively different eye movement measures. We then report an eye movement experiment investigating how people process reduced relative clause sentences with and without the focus operator only in order to examine the utility of different reading time measures. The results showed that measures summing temporally contiguous fixations can make an important contribution to the experimenters' understanding of the precise pattern of eye movements which occur when a problem is encountered in the text.

Eye Guidarice in Reading and Scene Perception/G. Underwood (Editor) �9 1998 Elsevier Science Ltd. All rights reserved

56 S.P. Liversedge, K.B. Paterson & M.J. Pickering

Introduction

Over the last century researchers have monitored people' s eye movements in order to investigate various aspects of vision. One area in which eye movement methodology has proved of particular benefit is that of reading research. Psychologists working in this area often conduct experiments in which they record the eye movements subjects make as they read text (usually from a computer screen). They then compute from the eye movement record how long people took to read different portions of the text and use this information to draw inferences about the underlying psychological processes. Importantly, there is an underlying assumption that, on the whole, there is a close correspondence between the pattern of eye movements made by a reader and the mental processes needed to understand the text that they are currently inspecting. Thus, the direction of gaze indicates (to a large extent at least) what part of the text is being currently processed, and the time taken to process the text is indicative of the ease with which processing occurred. These assumptions are warranted by the considerable evidence that the linguistic properties of text have a direct influence on the time it takes to read that text (see Rayner and Pollatsek, 1989, for a review). However, there is not a perfect yoking of eye and mind. Research suggests that readers are able to at least partially process text that they have not yet encountered (see Balota and Rayner, 1991, for a review; also, other chapters in this volume). Furthermore, if a portion of text causes the reader difficulty, then this difficulty can "spill over" and affect processing of subsequent text (e.g. Ehrlich and Rayner, 1983).

One of the major advantages of eye movement methodology over alternative reading time measures (e.g. self-paced reading) is that the experimenter can separate those fixations that were made when a region was first read from fixations that were made later in the eye movement record. This is important as it allows experimenters to determine when a characteristic of the text first influenced processing and therefore permits them to make inferences about the time course of processing during text comprehension. When text is read, the eye makes a sequence of fixations that are separated by saccadic movements. Studies have shown that information is extracted from the text during the fixation, but not during the saccades (again see Rayner and Pollatsek, 1989, for a review). Consequently, most (though not all) experimenters analysing eye movements treat the time spent fixating the text, and not the time spent making saccades, as a measure of reading time. Very often researchers sum fixations that readers make on portions of text according to several different algorithms. Thus, it is quite usual for researchers to consider a number of different reading time measures rather than considering only individual fixations. In this chapter we will focus on the different types of measures that have been widely used in psycholinguistic experiments. Our principle claim will be that reading time measures may retain the property of either spatial or temporal contiguity (or both),

Reading time measures 57

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235 [ . . . . . .

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Fig. 1. Hypothetical eye movement record one.

and when analysing a reader's eye movement record it is important to consider measures which sum temporally contiguous fixations and also measures which sum spatially contiguous fixations in order to avoid failing to detect effects (Liversedge and Picketing, 1995; Liversedge, Pickering and Traxler, 1996). We will explain what we mean by spatial and temporal contiguity below and use hypothetical data to illustrate our point. We will then present data from an experiment conducted by Paterson, Liversedge and Underwood (1998) to demonstrate that simply using measures which retain the property of spatial contiguity results in a failure to detect certain effects.

Figure 1 shows a hypothetical eye movement record containing a sequence of nine fixations. Each of these fixations had a duration of between 196 and 320 milliseconds, and the sum of the duration of these fixations represents the overall amount of time during spent reading the sentence.

The location of each fixation is represented by a small circle and the lines between circles indicate the trajectories of saccades made between each successive fixation. The sentence is the same as one used by Paterson et al., and has been divided into six regions, each spanning one or two words of the sentence. As would be expected, the reader starts by fixating the first noun phrase of the sentence. Paterson et al. were principally interested in the time subjects spent reading the verb invited in Region Four as this is the verb that syntactically disambiguates the sentence and is likely to be the first point at which disruption might be detected. Therefore we will often refer to Region Four as the critical region.

In most eye movement experiments investigating reading, researchers construct experimental sentences which are designed to cause a reader difficulty as they are read, and counterpart control sentences which are usually very similar but are constructed such that they do not cause the reader problems. The experimenter then compares reading times for regions of the experimental sentences with reading times for equivalent regions of control sentences. In this way, the experimenter is able to observe the degree of disruption to normal reading the experimental manipu-


lation induced. When measuring processing difficulty, researchers generally report three reading time measures: first fixation duration in a critical region along with first pass reading time and total reading time for the critical region, and other regions of interest (see Rayner et al., 1989, for a general discussion).

The first fixation duration in a particular region of text, as we might expect, is simply the time the reader spent initially fixating the region. This measure is generally taken to be the very earliest point at which we might expect to see an effect due to the experimental manipulation, as this is the first time the reader has directly fixated the region in which disruption to processing is anticipated. In Fig. 1, the first fixation duration in Region Four, the critical region, is 270 ms. It is a common finding that the duration of the first fixation is sensitive to processing difficulty experienced immediately on reading that word. For example, studies have shown that subjects have a longer first fixation for words that have a low frequency of occurrence in the language than words with a high frequency of occurrence, when word length is controlled (e.g. Inhoff and Rayner, 1986; Rayner and Duffy, 1986; Raney and Rayner, 1995). Similarly, readers have a longer first fixation for a word that disambiguates a sentence in favour of an initially dispreferred syntactic analysis, as compared to the same word in an unambiguous version of the sentence (e.g. Rayner, Carlson and Frazier, 1983; Murray and Liversedge, 1994). However, the first fixation duration measure does not always provide an indication of initial processing difficulty. Quite often the region of text that is predicted to cause difficulty contains either a long word, or several words, and is likely to be fixated more than once during the first sweep of the eyes through the sentence. In such a situation all of these fixations can contribute to initial processing, in which case a more sensitive measure may be first pass reading time.

First pass reading time (or gaze duration, if the region for which the measure is computed contains a single word) is defined as the sum of all the fixations made in a region until the point of fixation leaves the region either to the left or to the right. From Fig. 1 it can be seen that the gaze duration for Region Four is 513 ms (obtained by summing 270 ms and 243 ms). If effects are found on measures of either first fixation duration or gaze duration in one condition relative to another, the experimenter will usually conclude that difficulty was experienced immediately on processing that region of text.

Total reading time sums all fixations made within a region of text, including those fixations made when re-reading the region. The total reading time for Region Four in Fig. 1 is 748 ms, obtained by summing the first pass fixations in the region (i.e. 270 + 243 ms), and the duration of the fixation made on the region following a regressive saccade from Region Five (i.e. 235 ms). If an effect is observed for total reading time on a region, but not for earlier measures such as first fixation duration or first pass reading time, then this is generally taken as an indication of the manipulation having a relatively late effect on processing.


In addition to these reading time measures, researchers often provide a measure of the number of regressive saccades a reader makes from the critical region. This is usually done by computing the probability of a reader making a first pass regression (i.e. a regressive saccade that terminates the summation of fixations for first pass or gaze duration reading time). Such a measure is also usually taken to indicate that the reader is experiencing difficulty when processing the critical word. However, the probability of making a first pass regression does not provide an index of the time a reader spends on earlier portions of a sentence after making a regression. For instance, it is possible that a reader may regress to an earlier region of text an equal number of times under different conditions of an experiment but spend more time re-reading text under one condition than another.

To a large extent, the reading time measures (as defined above) have been widely adopted as "industry standards" within the psycholinguistic community. However, twenty years ago there was far less standardisation of measures than there exists today. In fact, early studies frequently failed to provide unambiguous definitions of the measures which were used. In recent years, there has also been some debate concerning the relative merits of different measures of disruption to processing during reading (e.g. Kennedy et al., 1989; see also Altmann, Garnham and Dennis, 1994; Altmann, 1994; but see Rayner and Sereno, 1994a,b). However, it is important to note that all these studies conducted comparative analyses of different measures in an attempt to resolve questions regarding the interpretation of data with respect to an underlying psycholinguistic theory. By contrast, in this article we make a comparison between reading time measures in order to address a theoretical question concerning the properties of the measures themselves. To our knowledge, there are few, if any, published articles to date that explicitly set out to do this.

Reporting a combination of measures, such as first fixation duration, first pass and total reading times minimises the possibility that an experimenter may fail to detect an effect. However, it does not rule out this possibility (see Konieczny, Hemforth and Scheepers, 1997). The effectiveness of this approach critically depends upon what eye movement behaviour occurs when a reader detects a problem. Consider the options that are available to a reader (in terms of eye movements) when they encounter a word in a sentence which causes them difficulty. First, the reader could remain fixating the problematic word until their difficulty is resolved. Sec- ondly, the reader could make a regressive saccade to permit them to re-read earlier parts of the text in an attempt to work out where they went wrong. Finally, the reader might make a rightwards saccade in order to read the next region of the sentence in the hope that this may help them resolve their difficulty.

Most discussion about the pattern of eye movements made on encountering processing difficulty during reading has been concerned with what happens when reading sentences that are difficult to parse. However, many linguistic phenomena other than garden path sentences induce processing difficulty, and since we wish to


'l'l~c leenagers allowed a party invited a juggler straightaway.

22o O

2M

I 195 I 198 0 257

Fig. 2. Hypothetical eye movement record two.

keep the discussion of reading time measures in this chapter at a more general level we will refrain from associating particular mental processes with specific patterns of eye movement behaviour. Instead, we will consider the differential sensitivities of each of the measures in relation to different patterns of eye movement behaviour that may occur as a result of experiencing processing difficulty. The simplest situation is where the reader makes a saccade to the right after detection of disruption. In such a situation, assuming that fixations in the critical region are not inflated due to detection of disruption, no measure of initial processing will detect an effect for that region as in such a situation there is no observable cost to processing. In such a situation any effects would presumably be detected on the subsequent region.

Figure 2 shows a situation in which the reader has remained fixating the region that caused them difficulty. In this case the reader makes three successive fixations on the critical region. In such a situation it seems likely that all three measures described above would detect the disruption the reader experienced. Summing the fixations on the region would inflate the reading times for that region relative to reading times for the same region of the control sentences where presumably the reader would spend less time. It is important to note that under these circumstances the increased reading time may be due to both detection of the problem and reanalysis processes that permit recovery from the processing difficulty.

It is the third scenario, in which a reader makes a leftward saccade upon encountering the problematic word, that we find most interesting. In such a situation, it is possible that upon reading the problematic word, the reader may make a long fixation, or even a series of fixations prior to making a regression. If this occurs, then as with the situation depicted in Fig. 2, the reading time measures described above will detect the effect. However, in the situation where the reader detects a problem and makes an immediate regression, such measures will fail to detect disruption because there may be no difference in the first pass reading time for the critical region of experimental and control sentences. Indeed, there may even


"the l e e n a g e r s a l l o w e d a party inv i ted a j u g g l e r s t ra ightaway . I 1% o . . . . . . . i . , .... .. I ...... 270

234

2 4 3 ~ 1 32tl 198 257

I Fig. 3. Hypothetical eye movement record three.

be a shorter first pass reading time for the critical region in the condition that introduced processing difficulty than in the control condition. Importantly, in such a situation, the reading time measures described above would not detect disruption to processing and researchers may fail to detect an effect.

After a reader has made a regression, there are a large number of possible patterns of eye movements they may make. Below we will consider several of these possibilities and point out important differences between them. This discussion should illustrate that if a researcher considers only first fixation, first pass and total reading time measures, then they may be unable to make claims about when an experimental manipulation influenced processing after detection of a misanalysis. That is to say, these measures alone provide only limited information regarding the time course of processing. Consider Fig. 3 which depicts one of the most simple patterns of eye movements a subject could make after a regression from a problematic region.

Having made a leftwards saccade from the critical region, the reader makes a single fixation on text in Region Two, prior to making a rightwards saccade in order to fixate as yet uninspected text. In such a situation first fixation duration, gaze duration and total reading time for the verb will be no different to the control sentences. The only way in which an experimenter may detect disruption to processing is by computing the total time for Region Two which will be slightly inflated due to the fixation of 243 ms. At this stage, two important points should be made. First, computing total reading times for each of the regions of a sentence should ensure that effects such as these are detected. They would only be missed if gaze duration and total reading times were computed for Region Four alone. Secondly, it might be argued that if a researcher computed the number of regressions a subject made from Region Four during the first pass, then they would also detect such an effect. However, our point is slightly more subtle than this. We do not dispute that the effects depicted in Fig. 3 could be detected without recourse to measures of reading time defined in novel ways. What we do claim, however, is that spatially


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243

Igt} 214

196 I

32O

-7_ 198

----o 257

Fig. 4. Hypothetical eye movement record four.

contiguous measures of reading time and regressions do not necessarily provide an indication of the first point at which a variable has an influence on reading time after a subject has made a regression. To expand upon this point, consider Fig. 4.

The eye movement record in Fig. 4 shows that upon encountering the disambiguating word, the reader makes a long regressive saccade to the first noun phrase of the sentence and made a series of fixations to re-read this portion of the sentence. After the fourth fixation during re-inspection (214 ms), the reader then makes a long saccade to Region Five and continues to read the sentence normally. This situation is interesting as, presumably, all the four fixations in Region One reflect the processes involved in recovery after detection of misprocessing. As with the situation shown in Fig. 3, disruption of this type would be detected if the total reading times for each of the regions was computed. However, note that the total time measure would include fixations made during the first pass of the eyes through the sentence along with the fixations made after the regressive saccade from Region Four. That is to say, the total reading time measure is not a measure of recovery time alone, but instead is a mixture of initial processing time and recovery time. This is an important point as will become clear if we consider the hypothetical data shown in Fig. 5.

Figure 5 shows a pattern of data where the reader makes a series of fixations, each in a different region after reading the problematic word. In this situation, the problem of failing to detect an effect is exacerbated. The problem here is twofold in that not only are the fixations made during reanalysis grouped with fixations made during the initial analysis of the sentence, but also, because fixations made during reanalysis were not made within the same region, they will never be grouped together to provide a measure of recovery from disruption. Thus, effects which might have occurred during reanalysis could be substantially weakened due to noise from first pass fixations included in the total time measure for a region and also due


The tcenagers allowed a party invited a juggler straightaway.

O [ - - 2.~4 ~ I 22~~ I _ 27o

.% I I I ~ 0 3211

J o 198 [ 257

i i I i i i i I I I I I I I I I _

Fig 5. Hypothetical eye movement record five.

to reinspection fixations being considered in isolation rather than being grouped together.

An important characteristic of the reading time measures discussed so far is that they sum spatially contiguous fixations. That is, the fixations which are summed for first pass reading time and total reading time all occur within the same region of a sentence. It is the property of spatial contiguity which can result in measures of this kind failing to provide an indication of the time course of a variable having an effect on processing during recovery, and possibly even failing to detect an effect, because in such a situation each subsequent fixation is not spatially contiguous with its predecessor. In order to circumvent this possibility, we require a measure which sums not spatially contiguous fixations, but temporally contiguous fixations. That is to say, we need a measure that groups fixations in terms of when they occur in relation to each other in time rather than in spatial location. Two measures that retain the property of temporal rather than spatial contiguity are regression path reading time (Konieczny, 1996) and re-readingtimet. We defineregression path reading time as the sum of all the fixations from the first fixation in a region up to but excluding the first fixation to the fight of this region. This measure provides an index of the time a subject spent detecting a problem and then re-reading the text prior to fixating novel linguistic material. The regression path reading time for Region Four in Fig. 5 is 1337 ms (comprising the fixations of 270, 243,214, 3 I0 and 300 ms). Re-reading time is defined as the regression path reading time for a region less the first pass reading time for a region. This measure simply provides an index of the time a subject spent re-reading the text after encountering a problem, but before they make an eye movement to fixate words to the fight of the problematic

1 A number of measures exist which are the same or similar tO regression path reading time. See for example, Brysbaert and Mitchell, 1996; Clifton, Kennison and Albrecht, 1997; Livers- edge, 1994; Konieczny et al., 1997.

64 s.P. Liversedge, K.B. Paterson & M.J. Pickering

region. Note that these measures will usually group together the fixations a reader makes after they have encountered difficulty. Additionally, re-reading time does not include fixations made as the sentence is initially processed. By using measures which sum temporally contiguous fixations in parallel with measures such as first pass and total reading time which sum spatially contiguous fixations, we should ensure that we do not fail to detect subtle effects during recovery. Also, if recovery from disruption is an ongoing process, then by summing temporally contiguous fixations the experimenter might obtain a clearer picture of the nature of this process over time. We stress that we are not advocating that measures summing temporally contiguous fixations will always be better at detecting effects which occur during recovery than measures which sum spatially cOntiguous fixations. Rather, we are suggesting that in some circumstances measures summing temporally contiguous fixations may be more sensitive to the time course of effects than measures summing spatially contiguous fixations. We therefore suggest that the two types of measure should be used together. This will maximise the possibility that an experimenter will detect any effects occurring as the reader reinspects text. It will also permit experimenters to examine the nature of eye movement behaviour that occurs after a reader experiences difficulty during sentence comprehension.

Consider Fig. 6: in this scenario, upon encountering the disambiguating region, the reader makes a regressive saccade and a brief fixation on the preceding word before making a rightwards saccade to continue reading the sentence its entirety. Only when they have inspected the whole of the sentence does the reader spend a substantial period of time re-reading portions of the sentence that preceded the disambiguating verb. This may be contrasted with the scenario depicted in Fig. 5, in which the reader spends a substantial amount of time re-reading portions of text preceding the disambiguating verb before inspecting the remainder of the sentence. The point is that the total reading times for Regions Two, Three and Four for the hypothetical eye movement records depicted in Figs. 5 and 6 will be the same, yet the patterns of eye movements made in the two scenarios are qualitatively different. It is our contention that such differences are likely reflect differences in the processes occurring during recovery. Such differences would not be detected if the experimenter relied solely upon reading time measures which sum spatially contiguous fixations.

In advocating the use of additional measures, we are aware that a preponderance of statistically dependent measures can result in inflated Type I error rates, and the possibility that different measures could produce conflicting results. We thank Alan Kennedy and Wayne Murray for bringing these points to our attention during the Chamonix workshop. This is certainly true of the existing measures. First fixation, first pass reading time and total reading time are all statistically dependent measures, as a summation of the same fixations contribute to all three measures of reading time. Also, Altmann, Garnham and Dennis (1992) report two studies in which


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244

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Fig. 6. Hypothetical eye movement record six.

measures of first pass reading time and the probability of a reader making a first pass regression favoured competing accounts of initial sentence processing. Of the two new measures discussed in this paper, only the regression path reading time includes a sum of fixations that also contribute to other measures of reading time for a region. The re-reading time measure is statistically independent of first fixation, first pass and total reading times for the region of interest. Furthermore, because regression path and re-reading times sum temporally contiguous aspects of the eye movement record, any discrepancy between these measures and measures of spatially contiguous fixations will be informative about the pattern of eye movements that occurred.

Our central claim is that reading time measures which sum spatially contiguous fixations (first fixations, first pass reading times and total reading times), do not necessarily provide data to permit the experimenter to make strong claims about the time course of disruption to processing and possible recovery. We therefore advocate the use of two further reading time measures in psycholinguistic experiments: regression path reading time and re-reading time. We now demonstrate the utility of this approach by considering a recent experiment by Paterson, Liversedge and Underwood (1998).

Experiment

In this experiment, Paterson et al. monitored eye movements as subjects read a series of reduced and unreduced relative clause sentences. Reduced relative clause sentences like (1) are temporarily ambiguous between two syntactic analyses: a reduced relative clause reading in which the phrase allowed a party modifies the subject noun-phrase (i.e. the teenagers), and a simple active reading in which the

66 S.P. Liversedge, K.B. Paterson & M.Z Pickering

noun-phrase a party is a direct object argument of the verb allowed. Such sentences are disambiguated in favour of the reduced relative clause reading on encountering the verb invited. Unreduced relative clause sentences (e.g. 2) are unambiguous due to the inclusion of a relative pronoun and auxiliary verb (i.e. who were).

(1) The teenagers allowed a party invited a juggler straightaway.

(2) The teenagers who were allowed a party invited a juggler straightaway.

It is a well established finding that readers often experience processing difficulty when reading reduced relative clause sentences in isolation (e.g. Bever, 1970; Frazier and Rayner, 1982). Most accounts assume that readers initially adopt a simple active reading of the reduced relative clause sentence, and on encountering the disambiguating verb must reanalyse the sentence in terms of the dispreferred relative clause reading. Readers are said to have been 'garden-pathed' when they are forced to reanalyse the sentence in favour of an initially dispreferred syntactic analysis.

Our experiment tested a recent claim by Ni, Crain and Shankweiler (1996) that the inclusion of the focus operator only will guide the initial processing of reduced relative clause sentences such as (3), and so enable the reader to avoid the garden path that is normally experienced when reading such sentences.

(3) Only teenagers allowed a party invited a juggler straightaway.

Ni et al.'s claims are derived from the Referential theory of sentence processing (Crain and Steedman, 1985; Altmann and Steedman, 1988), according to which the nature of the referential context can guide initial processing decisions regarding structural ambiguities. Ni et al propose that when readers process the subject noun-phrase only teenagers in sentences like (3), they construct a mental represent- ation in which a focused set of teenagers is contrasted with some other set. However, as no contrast set is explicitly mentioned in the preceding discourse (because the sentence is presented in isolation) the reader must infer one. Ni et al. argue that for reasons of parsimony, the reader will infer that there are two sets of teenagers, and must anticipate further modifying information, such as a relative clause, in order to specify the nature of the difference between these two sets. Furthermore, they claim that anticipating modifying information will influence the initial processing of sentences like (3), such that readers will preferentially adopt the reduced relative clause reading of the ambiguous phrase allowed a party, and will not be garden pathed when encountering the disambiguating verb.

While not disputing that the referential properties of only will influence the processing of the sentence, in contrast with Ni et al., we predicted that only would not guide initial processing decisions (see Paterson, Liversedge and Underwood, 1998, for a full discussion). We anticipated that the reader would be garden-pathed immediately on encountering the disambiguating verb of a sentence like (3).


However, we hypothesised that the referential p'roperties of only would facilitate reanalysis, so that readers would find it easier to recover from the garden path in (3) than in (1). We will report the duration of the first fixation and gaze duration as measures of initial processing that is spatially localised at the disambiguating verb.

Method

Subjects

Thirty-two students from the University of Nottingham participated in this experiment. All subjects had normal and uncorrected vision and were paid s

Materials and design

Subjects read a series of thirty-six sentences that were either temporarily ambiguous reduced relative clause sentences or unreduced relative clause sentences which were disambiguated by the inclusion of a relative pronoun and auxiliary verb (i.e. who were). The sentences began with one of two determiner types: either the definite article (the) or a focus operator (only). Examples of these sentences are given in sentences (4) to (7) below.

(4) The teenagers/allowed a/party/invited/a juggler/straightaway.

(5) Only teenagers/allowed a/party/invited/a juggler/straightaway.

(6) The teenagers who were/allowed a/p&"ty/invited/a juggler/straightaway.

(7) Only teenagers who were/allowed a/party/invited/a juggler/straightaway.

Prior to analysis the sentences were divided into six regions that spanned one or two words in the sentence. Region divisions are indicated by a slash in sentences (4) to (7). The critical region of interest was Region Four, which contained the disambiguating verb.

Apparatus and procedure

Eye movements were monitored using a SRI Dual-Purkinje Generation 5.5 eye- tracker produced by Fourward Technologies. The tracker monitored subjects' gaze location every millisecond and the software sampled the tracker's output to establish the sequence of eye fixations and their start and finish times.

Before the start of the experiment, subjects read an explanation of the eye-tracking procedure and a set of instructions. They were instructed to read at their normal rate and to read to comprehend the sentences as well as they could. Subjects were then seated at the eye-tracker and placed on a bite-bar and under forehead restraint to minimise head movements. Subjects then completed a calibration procedure.


Before each trial, a fixation cross appeared near the upper-left-corner of the screen. Immediately subjects fixated this cross, the computer displayed a target sentence, with the first character of this sentence replacing the fixation cross. This also served as an automatic calibration check, as the computer did not display the text until it detected a stable fixation on the cross. If subjects did not rapidly fixate the cross, the experimenter re-calibrated the eye-tracker. The experiment was conducted in two blocks, with a short intervening break while the experimenter set-up the equipment for the second block, and subjects were calibrated at the beginning of both blocks, with other re-calibrations performed every eight materials to maintain a high level of accuracy. This meant that the eyetracker was calibrated a minimum of 10 times during the experiment.

Once subjects had finished reading each sentence, they pressed a key, and the computer displayed a comprehension question. Comprehension questions followed all of the experimental and filler trials. Half of these questions had "yes" answers, and half had "no" answers. Subjects responded to the comprehension questions by fixating either the word "yes" or "no". These words were presented on the left and right hand sides of the screen below the comprehension question. Subjects' resp- onses were recorded by the experimenter without feedback.

The computer displayed each experimental list in a fixed Latin Square order, together with 32 fillers that were materials for an unrelated experiment, and an additional 11 items that appeared at the beginning of the two halves of the experiment, �9 and following each of the breaks for re-calibration.

Results and discussion

Prior to analysis, an automatic procedure pooled short contiguous fixations. Fixa- tions of less than 80 ms were incorporated into larger fixations found within one character, and fixations of less than 40 ms that were not within three characters of another fixation were deleted. We removed those trials where either subjects failed to read the passage properly, or where there had been track loss. More specifically, those trials were removed in which a zero first pass reading time was recorded for two consecutive regions of text. This accounted for 3.0% of the data. The data were analysed for all six regions of text. As the disambiguating verb provided the point at which we expected to detect garden path effects, the reading time results for Region Four are reported below. We also report the total reading times for Regions Two and Three.

The data were analysed using 2 (Determiner) x 2 (Sentence Structure) ANOVAs, treating subjects and items as random variables. The mean first fixation duration, gaze duration, total reading times, regression path reading times and re-reading times for the disambiguating verb, along with the total reading times for Region Two and Three are given in Table 1.


Table 1

First fixation, gaze duration, total reading times, regression path reading times, and re-reading times for regions of reduced and unreduced relative clause sentences beginning with either the or only

The Only

Measure Reduced Unreduced Reduced Unreduced

First fixation duration (ms) for 219 192 disambiguating verb

Gaze duration (ms) for disambiguating 272 238 verb

Total reading time (ms) for Region 2 549 337

Total reading time (ms) for Region 3 386 296

Total reading time (ms) for 446 310 disambiguating verb

Regression path reading time (ms) for 426 270 disambiguating verb

Re-reading time (ms) for 155 31 disambiguating verb

191 173

250 213

444 328

321 307

355 261

318 278

68 65

First fixation duration There was a significant main effect of Sentence Structure (F~(1,31) = 6.5, p < 0.05, MS~ = 2626;/72(1,35) = 4.4, p < 0.05, MSo = 3868), with a longer first fixation on the disambiguating verb when it was part of a reduced as compared to an unreduced relative clause sentence. There was also a significant main effect of Determiner (Ft(l ,31) = 14.0,p<O.OOl,MSe= 1292;/72(1,35)= 11.6,p<O.OI,MSe= 1550), with a longer first fixation at this region when sentences beginning with the as compared to only. Yet, crucially, there was no interaction of Sentences Structure and Deter- miner (F < 1). The results demonstrated that subjects encountered a garden path on the first fixation on the disambiguating verb of reduced relative clause sentences, and that this effect occurred regardless of whether the sentence began with the or only.

The garden path effect on the duration of the first fixation on the critical verb is in line with previous findings (Rayner, Carlson and Frazier, 1983; Murray and Liver- sedge, 1994). For present purposes, the main effect of Determiner is not important. However, the lack of an interaction demonstrates that the inclusion of only did not guide initial parsing of reduced relative clause sentences.


Gaze duration There was a significant main effect of Sentence Structure (F~(1,31 ) = I 1.0, p < 0.01, MS e = 3605; F2(1,35) = 6.3, p < 0.05, MS~ = 6217), with a longer gaze duration on the disambiguating verb of reduced than unreduced sentences. There was also a significant main effect of Determiner (F~(I,31) = 5.6, p < 0.01, MS~ = 3216;/72(1,35) = 7.6, p < 0.01, MS, = 2776), such that there was a longer gaze duration for sentences beginning with the compared with only. There was no interaction of Sentence Structure and Determiner (F < 1 ). The results were very similar to those of the first fixation duration, and showed that subjects were garden-pathed when reading reduced relative clause sentences despite the inclusion of the focus operator only.

The first fixation and gaze duration results are clear. Subjects experienced processing difficulty on reading the disambiguating verb of reduced relative clause sentences, regardless of the inclusion of the focus operator. However, it was possible that the inclusion of only may have facilitated reanalysis for the reduced relative clause sentences at a later point in the reading process. If this was the case then we should observe an interaction between Sentence Structure and Determiner on the total time measure. We examined the total readings times for Regions 2 and 3 along with the disambiguating region.

Total retuting time At the disambiguating verb, there was a significant main effect of Sentence Struct- ure (F~(1,31) = 38.2, p < 0.001, MS~ = 11022; F2(1,35) = 31.8, p < 0.001, MS~ = 14083), with a longer total reading time for the disambiguating verb of reduced as compared to unreduced sentences. There was also a significant main effect of Determiner (F~( 1,31 ) = 13.2, p < 0.01, MS~ = 11828; F2(1,35) = 27.8, p < 0.001, MS~ = 6471 ), such that there was a longer total reading time for sentences beginning with the as compared to only. However, there was no reliable interaction of Sentence Structure and Determiner (FI(I,31) = 1.8, p > 0.05, MS~ = 7824;/72(1,35) = 1.8, p > 0.05, MS~ = 8203).

The total reading times for the disambiguating verb have same pattern as the first fixation duration and gaze duration results. There was a longer total reading time for reduced than unreduced sentences, and a longer reading time for sentences beginning with the as compared to only. Yet, importantly, there was no interaction of Determiner and Sentence Structure that might be expected if only had facilitated recovery from the garden path effect experienced when reading reduced relative clause sentences.

At Region Two, there was a significant main effect of Sentence Structure (Fl( 1,31) = 69.4, p < 0.001, MS e = 12477;/72(1,35) = 81.7, p < 0.001, MS~ = 12075), with a longer total reading time for reduced than unreduced sentences. There was also a significant main effect of Determiner (F~(1,31 ) = 20.6, p < 0.001, MS~ = 4993; /72(1,35) = 12.5, p < 0.001, MS~ = 8541), with a longer total reading time for


sentences beginning with the than only. There was a significant interaction of Sentence Structure and Determiner (F~(1,31) = 9.0, p < 0.01, MS~ = 8175; F2( 1,35) = I 1.3, p < 0.01, MSr = 7771). An analysis of simple effects showed that there was a longer total reading time for reduced as compared to unreduced sentences beginning with the (F~(1,31) = 88.3, p < 0.001, MS~ = 8175;/72(1,35) = 106.9, p < 0.001, MS~ =7771), and for reduced as compared to unreduced sentences beginning with only (F,(1,31) = 26.6, p < 0.001, MS~ = 8175; F2(1,35) = 31.3, p < 0.001, MS~ = 7771).

A similar pattern of effects occurred for total time on Region Three. There was a significant main effect of Sentence Structure (F~(1,31) = 13.4, p < 0.001, MS~ = 6492; F2(1,35) = 12.8, p < 0.01, MS~ = 7531), with a longer total reading time for reduced than unreduced sentences. There was an effect of Determiner that was significant by subjects and marginal by items (F~( 1,31) = 5.0, p < 0.05, MS, = 4607; F2(1,35) = 3.8, p < 0.06, MS,= 6164), such that there was a longer total reading time for sentences beginning with the than only. Finally, there was a significant interaction of Sentence Structure and Determiner (F~(1,31) = 4.3, p < 0.05, MS, = 10462; F2(1,35) = 8.1, p < 0.01, MS~ = 7007). Simple effects showed that there was a longer total reading time for reduced as compared to unreduced sentences beginning with the (F~(1,31) = 12.2, p < 0.01, MS, = 10462;/72(1,35) = 21.5, p < 0.001, MS, = 7007), but no difference between reduced and unreduced sentences beginning with only (F < 1). The results for Region Two and Three produced the predicted interaction of Sentence Structure and Determiner. There was a longer total reading time for Regions Two and Three of reduced as compared to unreduced relative clause sentences beginning with the. There was also a longer total reading time for Region Three of reduced as compared to unreduced sentences beginning with only.

At this point we have evidence to suggest that while initial processing was not guided, reanalysis was facilitated by the inclusion of only in reduced relative clause sentences. More time was spent re-reading text contained in Regions Two and Three of reduced relative clause sentences beginning with the, and text contained in Region Three of the reduced relative clause sentences beginning with only, after encountering a problem at the disambiguating verb at Region Four. It seems likely that readers re-read these earlier portions of text in order to recover from the misanalysis experienced at the disambiguating verb, but these total reading times alone are difficult to interpret in two respects. It is not possible to determine if, upon encountering the disambiguating verb, subjects immediately made a regressive saccade and spent considerable time re-fixating earlier portions of the text prior to reading as yet uninspected text. Or alternatively, if subjects read beyond the disambiguating region before making a regressive saccade to re-read earlier portions of the text. This is exactly the point we highlighted in our discussion of the hypothetical eye movement records in Figs. 5 and 6.

To remind the reader, regression path reading times indicate the time subjects spend reading the disambiguating region, and re-reading text prior to the dis-


ambiguating region, before inspecting text to the fight. Also, re-reading times indicate the time subjects spent inspecting text immediately after making a regression from the disambiguating region, but before fixating text to the right of the disambiguating region. Using these measures we should be able to establish whether readers spent time re-reading the beginning of the sentence immediately upon encountering the disambiguating verb, or whether they returned to re-read earlier portions of the sentence after they had read the sentence in its entirety. If subjects did re-read immediately upon encountering the verb, then we should obtain a significant interaction between Sentence Structure and Determiner for both regression path and re-reading time measures. Alternatively, if subjects read the sentence in its entirety before re-reading the text, then there should be no interaction.

Regression path and re-reading time The regression path measure showed a significant main effect of Sentence Structure (El(I,31 ) = 27.1,p<O.OO1,MS~= 11418; F2(1,35)= 22.2,p<O.OO1,MS~ = 14262), with a longer regression path reading time for reduced as compared to unreduced sentences. There was also significant main effect of Determiner (F~(1,31) = 19.6, p < 0.001, MS~ = 4077; Fz(1,35) = 11.9, p < 0.01, MS~ = 7363), such that there was a longer regression path reading time for sentences beginning with the than only. Finally, there was a significant interaction of Sentence Structure and Determiner (Fl(l ,31) = 10.8, p < 0.01, MS e = 10098;/7'2(1,35) = 12.3, p < 0.01, MS e = 9781). There was a significantly longer regression path reading time for reduced as compared to unreduced sentences beginning with the (Fl(l ,31) = 38.9, p < 0.001, MS, = 10098; F2(1,35) = 42.3, p < 0.001, MS~ = 9781), but no difference in reading time for reduced and unreduced sentences beginning with only (F~(1,31) = 2.5, p > 0.05, MSe = 10098; F2(1,35) = 2.4, p > 0.05, MS~ = 9781).

The re-reading time showed a significant main effect of Sentence Structure (FI(1,31) = 20.4, p < 0.001, MSe = 6252; F2(1,35) = 13.8, p < 0.001, MS c = 9661), such that more time was spent re-reading earlier portions of reduced relative clause sentences. There was a main effect of Determiner (F~(I,31) = 9.7, p < 0.01, MS~ = 2247; F2(1,35 ) = 5.8, p < 0.05, MS~ = 3945), with more time spent re-reading to earlier portions of sentences beginning with the than only. There was also a significant interaction of Sentence Structure and Determiner (F~(1,31) = 8.3, p < 0.01, MS~ = 13965;/72(1,35) = 13.9, p < 0.001, MSe = 9415). Simple effects showed there was a longer re-reading time for reduced as compared to unreduced sentences beginning with the (F~(I,31) = 17.4, p < 0.001, MS~ = 13965;/72(1,35) = 28.0, p < 0.001, MS~ = 9415). However, there was no difference in the re-reading time for reduced and unreduced sentences beginning with only (F < 1).

The interaction between Sentence Structure and Determiner was significant for both these reading time measures, and we obtained the same pattern of results as for the total reading time for Region Three. This pattern of reading times suggests that


upon encountering the disambiguating region of reduced relative clause sentences containing the, subjects made a regressive saccade in order to spend more time re-reading preceding portions of the sentence than they did when they read reduced relative clause sentences containing only. The results show that recovery was initiated immediately upon detecting a problem at the disambiguating region, and furthermore, that during the recovery process readers spent time re-inspecting text they had already read prior to fixating as yet unread text. We suggest that for these garden path sentences subjects attempted structural reanalysis immediately upon encountering the syntactically disambiguating verb.

The analysis of the temporally contiguous fixations made an important contribution to our understanding of the pattern of eye movements which occurred immediately after subjects read the disambiguating verb. Only by examining reading times which summed temporally contiguous fixations were we able to determine when during the course of processing subjects spent time re-reading the sentence. In this case re-reading occurred immediately upon a reader encountering the problem, rather than occurring after the sentence had been read in full.

Conclusion

The results of our experiment confirmed that the focus operator only did not guide initial parsing decisions when reduced relative clause sentences were read. The inclusion of only did, however, facilitate reanalysis after detection of the initial misanalysis. Our conclusion is that the inclusion of a focus operator does not cause a reader to anticipate modifying information and therefore the focus operator does not guide parsing decisions for reduced relative clause sentences.

Turning to the reading times measures, we have provided an argument in favour of the use of reading time measures which sum temporally contiguous fixations. We advocate the use of these measures in conjunction with existing reading time measures which sum spatially contiguous fixations. By using measures which sum spatially contiguous fixations and measures which sum temporally contiguous fixations, the experimenter is better able to determine the time course of the influence of linguistic variables on processing, and distinguish between qualitatively different types of eye movement behaviour which may occur when processing difficulty is experienced. This approach enabled a fuller understanding of the data obtained in the present experiment. We suggest that the use of these measures will prove valuable in the interpretation of results obtained in other eye movement experiments.


A cknowledgements

Special thanks are due to Keith Edwards of the Psychology Department at the University of Glasgow who developed software to support the reading time measures described in this paper. We also thank Wayne Murray, Franqoise Vitu and an anonymous reviewer for providing comments on an earlier version of this paper. This research was completed when the first author was a Research Fellow at the University of Nottingham.

References

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77

CHAPTER 4

Determinants of Fixation Positions in Words During Reading

Ralph Radach Technical University of Aachen

George W. McConkie University of Illinois at Urbana-Champaign

Abstract

This chapter begins by reviewing previous findings concerning Where the eyes land (i.e., landing positions) in words following progressive saccades during reading. It then reports the results from an examination of landing positions of German readers in two previously unexamined situations" intraword saccades (refixations) and interword regressive saccades (regressions). No evidence was found to support the common distinction between intra-and inter-word progressive saccades; landing positions in refixations are continuous with those in interword progressive saccades. In contrast, interword regressive saccades do not show the normal linear relation between launch site and mean landing position in the word that is observed in other conditions. In all cases, eye movement control during reading appears to be word-based.

Eye Guidance in Reading and Scene Perception/G. Underwood (Editor) �9 1998 Elsevier Science Ltd. All fights reserved

78 R. Radach & G.W. McConkie

Introduction

Although the eye-movement pattern made while reading a passage is infinitely complex, at one level it can be represented simply as a series of individual decisions about when and where to move the eyes. The where decision, at least in reading languages having spaces between words, appears to be word-based: that is, each saccadic eye movement is intended to take the eyes to a specific word location (McConkie and Zola, 1984).' If this is so, then understanding where the eyes actually land in the text requires two theories: first, a selection theol , which indicates how one word rather than another is selected as the target of a saccade, and, second, a performance theory which indicates where the eyes land given the selection of a target word for a particular saccade. In this chapter we will deal primarily with the latter of these two issues: the landing positions of the eyes within words.

McConkie, Kerr, Reddix and Zola, (1988) conducted a quantitative analysis of the landing positions of the first saccades into a word (initial fixation positions), using a corpus of 40,000 fixations based on a sample of 66 readers. Included in the analysis were three factors of potential influence, word length, saccade launch distance and prior fixation duration. These authors concluded that early visual processes must parse the text into word-units, probably on the level of low spatial frequency objects that are separated by empty spaces. This provides candidates that could serve as the target of the next saccade. On some basis, which they did not investigate, a word-unit is selected and a saccade is launched to take the eyes to it. Where the eyes actually land (i.e., the distribution of saccade landing positions, or initial fixation positions) can be accounted for by a small set of basic visuomotor principles, which they attempted to explicate.

McConkie et al. (1988), like many others (see reviews in O'Regan 1990, 1992), assume that there is a functional target location near the word center to which the eyes are being sent. O'Regan (1990) suggests that this is a result of learning that the center of the word is an "optimal viewing position" for word identification. He suggests that the position in each word from which it can be most readily identified varies according to the information structure within the word, but that, on average, the word center is optimal. However, the actual distribution of landing positions in words of different lengths, often referred to as the "preferred viewing position" (Rayner, 1979), shows an inverted U shape, with a maximum somewhat to the left of

1 There are exceptions to this generalization. For example, Hofmeister (1997) has data indicating that landing positions following return sweeps (saccades to the next line of text) are distributed relative to the beginning of that line, without influence of word locations. Also, it has been proposed (Radach, 1996) that some saccades may be sent to word groups, particularly when a re!atively long word is preceded by a short function word. Further research is necessary to explore this possibility.

Fixation positions in reading 79

the word center. Thus, on most fixations the eyes are not at the optimal location. McConkie et al. proposed that this is the result of two sources of error in the

visuomotor system. A "saccadic range error" leads to a linear relation between saccade launch distance and the mean landing position, which we will refer to as the "landing position function". When saccades are launched from a distance of about seven letters relative to the word center (a figure close to the average saccade length in reading), the resulting normal distribution of landing positions has its maximum close to the center of the word. When saccades are launched from locations closer to the target word, landing sites are shifted to the right (overshoot); when they are launched from more distant locations, landing positions are shifted to the left (undershoot). The second source of error is a "random placement error" that causes the spread in landing positions and that leads to an increase in this spread or variance with distance of the center of the target word from the launch site) In summary, the "preferred viewing position" in a word is seen, not as a basic oculomotor phenomenon, but as the result of several combined factors of influence (McConkie et al., 1988; see also McConkie, Kerr, Grimes and Zola, 1990; McConkie, Kerr and Dyre, 1994).

This chapter begins with a summary of factors that determine the locations of initial fixation positions on words following forward intra-word saccades during reading, as obtained from an extensive analysis of a corpus of German reading data (Radach, 1996). It then presents analyses of the same corpus, examining initial fixations on words following saccades that lead to refixations of words, and regressive intra-word saccades, to see if they show the same properties. It ends with a discussion of theoretical issues regarding eye movement control during reading, arguing that the eyes are sent to a selected word (discrete decision) rather than a certain distance (continuous decision), and that where the eyes land in the word is determined by oculomotor error factors rather than by certain alternatives that have been recently proposed.

Methodology

Eye movement data were collected from four participants (German-speaking grad- uate students of physics) as they read a German translation of the first two parts of the book Gulliver's Travels (about 160 book pages). 3 The text was presented in

2 Saccade target undershoot, saccadic range effect and increase in variation for more distant targets are common phenomena in the literature on basic oculomotor and other motor control processes (Becker, 1989; Kapoula and Robinson, 1986; Poulton, 1981).

3 The data used in this study were collected by the first author while on a Fulbright scholarship at the University of lllinois in Champaign/Urbana. The authors gratefully acknowledge the support of Gary Wolverton, Paul Kerr and John Grimes.


screen pages of five to seven double-spaced lines of up to 72 ASCII characters each on a 15-inch VGA monitor in negative polarity. At a viewing distance of 80 cm each letter corresponded to approximately 0.25 ~ of visual angle. Participants were instructed to read the text at their normal pace in order to comprehend the main ideas and to be able to answer questions at the end of each of 32 text segments. Eye movements were recorded using a Generation 5 Dual-Purkinje eye tracker with a sampling rate of 1000 Hz. The algorithm used for saccade identification is described in McConkie, Wolverton and Zola (1984), and details on the calibration routine are reported in McConkie (1981 ).

After excluding blinks, cases of track loss and fixations outside the page, matrices of 47989, 47826, 59857 and 64226 valid saccade-fixations pairs were available for the four participants, respectively. Orthographic and lexical variables (e.g., letter frequency and word frequency measures) were either generated on the basis of the text itself (48334 words) or imported from the German CELEX corpus (Celex, 1995).

The method employed in this study is quasi-experimental, involving the analysis of a complete set of normal reading data. The key technique is the method of orthogonal sampling (Kliegl, Olson and Davidson, 1983) where a number of variables is controlled (held constant) within a subset of data while a target variable is systematically investigated by comparing cases with different values of that variable. This method has certain weaknesses. For example, effects may sometimes be influenced by variables not considered in the sampling scheme or mediated by hidden interactions. The method also has a number of strengths. In cases where aspects of behavior being studied cannot be experimentally manipulated, such as where the eyes go during free viewing, this method makes it possible to separate out the effects of different variables on behavior. Also, when the corpus includes very large data sets from different individuals, similarities and differences among participants can be investigated and problems commonly arising in group statistics can be avoided. This is true in particular for the present corpus, because it includes enough data to carry out a complete replication of the original study by McConkie et al. (n = 66) for each individual reader.

Where the eyes go when initially fixating a word

Owl the basis of the data corpus described above, we have replicated McConkie et al.' s analysis and extended it to include a broader range of variables (Radach, 1996; see also Radach and Kempe, 1993). In the following section we will concentrate on those aspects of our studies on initial fixation positions in words that are most relevant for ongoing discussions of theoretical issues and which also lay the ground for the analyses of refixations and regressions presented in the later part of the chapter.


As indicated above, we are convinced that initial saccades are being sent to specific words during reading. Furthermore, the control system seems to act as if a particular location in the word, near its center, were acting as the target for the saccade. At the same time, there is an "optimal viewing position" within a word from which the word is most easily identified (O'Regan, 1990). One way to identify this is to compute the probability of refixating the same word as a function of the initial landing position. The basic result is that the likelihood of making a second fixation on a word is minimal when the eyes first land close to the word center (O' Regan et al., 1984). The effect was first observed in normal reading by McConkie et al. (1989), who found that the U-shaped refixation curve can be well specified by a quadratic equation of the form:

Y = A + B ( X - C) 2

where X is a fixation location within a word, Y is the proportion of fixations at that location that are followed by a refixation of the word, A is the vertical offset of the curve, B indicates the slope and C is the minimum point. This minimum point is the horizontal offset of the curve, the location where refixation frequency is minimal.

The analysis Of the German corpus data found considerable interindividual variability with respect to the actual distributions of landing positions (the "preferred viewing position"). For three participants, the landing positions vary around locations about halfway between word beginning and word center, whereas for one participant they are much closer to the word center. However, there is no comp- arable variation in the refixation curves. For all four individual participants and at all word lengths studied (5 to 9), the minimum of each refixation curve is within one letter position of the word center. Therefore, we consider the word center to be an estimate of an interindividually stable optimal viewing position.

The actual frequency distribution of saccade landing positions for a given word length (the "preferred viewing position") is Gaussian in shape and can be fit well by a normal curve. The data in this global distribution can be partitioned into launch site contingent landing position distributions. As an example, one such distribution would indicate landing positions within a seven-letter word following saccades that come from a distance of ten letters left of the word center. These distributions are also normally distributed and, most importantly, there is a linear relation between launch distance and mean landing position, the "landing position function". In the 206 particular combinations of word length and launch site considered in Our analysis, the average shift of mean landing position with each increment in launch distance (the slope of the landing position function) is 0.35, 0.33, 0.55 and 0.38 letters for our four participants. In regression analyses, performed on the individual subject's mean landing positions as a function of launch distance for word length 4-15, more then 90% of the variance is accounted for by a simple linear model.


Fig. 1. Estimated mean landing positions of initial saccades as a function of launch site relative to the word center for 9-11 letter words. Each data point represents between 1054 and 133 observations. At about launch site -15 the peaks of the underlying landing site distributions start to move off the target word and the estimated means are more variable. For this data set the slope of the linear regression function is 0.53 (r 2 = 0.96). Launch distances of 5 or less include cases

where the saccade was launched from within the same word.

This key finding, which has been observed in several analyses of different sets of reading data in both English and German (see also Kerr, 1992; McConkie, Kerr and Dyre, 1994), is illustrated in Fig. 1. This figure was prepared in order to explore how far from the word center the linear landing position function is found. We have plotted the landing position function for words of length 9-11, based on pooled data from our four subjects. Data pooling was required in order to have a large enough sample size for the analysis. This word length range was selected for two reasons. First, words had to be relatively long to counter the tendency of landing site distributions to move off the left word boundary for very distant launch sites. Second, words had to be short enough to provide a sufficient number of observations. German text is well-suited to meet these constraints. As is apparent from Fig. 1, the mean landing positions become less stable at launch distance o f - I 5 due to reduced sample sizes but even at -21 (n = 133) the linear trend is still present. We conclude from this analysis that for inter-word progressive saccades the landing position function is linear over the entire range of launch distances that is found in normal reading.

Unlike McConkie et al. (1988), but in agreement with McConkie, Kerr and Dyre (1994), we also found a small effect of word length on mean landing positions for a given launch distance. The individual average increase (relative to the word beginning) in mean landing position was 0.16, 0. l 8, 0.14 and 0.11 letter position for each one-letter increase in word length. This is a much smaller effect than the launch


position effect reported above. In linear regression analyses, performed on the mean word length contingent landing positions for each launch site and participant, only a modest portion of the variance can be accounted for (r 2 > 0.34, see Radach, 1996 for further details). The effect can be interpreted as a "center of gravity" phenomenon (Findlay, 1982), modulating the launch position effect. Such an interpretation would follow O'Regan's (1990) suggestion that the landing position saccades can be influenced by the presence of further elements in the critical visual configuration, in the present case additional letters belonging to the same target word (see also below our discussion of the relative importance of range-effect vs. "center of gravity" effect).

A curious observation made during the analysis of the word length effect was that not only the length of the target word but also the length of the preceding word had an influence on initial landing positions. When further investigating this effect, we found that this was not caused by prior word length per se, but largely due to the fixation pattern on the preceding word (Radach and Kempe, 1993). With saccade launch position held constant, there is a substantial rightward shift in mean landing position in a word when the prior word received more than one fixation (effect size in the order of 0.5 letters) and a large leftward shift when the preceding word was skipped (effect size up to 1.5 letters). Radach (1996) suggests that the first effect may indicate that in the case of refixations it is more likely that a parafoveal word or letter cluster is being recognized and that the subsequent saccade is lengthened in a "skipping-like" fashion. The second effect, a leftward shift of initial fixation position after skipping a short word is more puzzling and has led Radach (1996) to consider the possibility that saccades may sometimes be aimed at units of two words in which case a small function word is not "skipped" but remains unfixated because it is part of the larger two-word target unit (see Chapter 2 for a discussion of the related issue of information acquisition from locations left of the actual fixation position).

A further variable of potential influence on saccade landing positions is the duration of the previous fixation, the 'latency' of the initial saccade. This variable is of particular interest, because it provides a link to temporal aspects of eye movement control that may well significantly modulate spatial saccade parameters. Several hypotheses on the issue are feasible. McConkie et al. (1988) and O'Regan (1990) have proposed that when a preceding fixation is long, the following initial fixation locations converge towards the optimal viewing position. An alternative hypothesis could be that, due to increased parafoveal preprocessing, there should generally be a rightward shift of fixation positions after longer fixations (PoUatsek, Rayner and Balota, 1986). 4

4 Although Pollatsek et al. do not explicitly state this hypothesis, it might be derived from their more general proposal that "... there are places in text where more complete processing of the material fixated (and that just to the right of fixation) takes more time but then leads to longer saccades".


We studied the role of the preceding fixation duration for a total of 40 conditions obtained by orthogonal sampling within the German data corpus: five launch sites from-10 to-1 (number of letter positions to the left of the center of the target word, in two-letter increments), two word length ranges (5-6 vs. 7-9 letter words) and four participants. For each of these conditions, mean initial landing positions were compared for the condition-specific quartiles of prior fixation duration. These comparisons used non-parametric Kruskal-Wallis tests (equivalent to ANOVA' s) and t-tests of the extreme quartiles in each condition. Interestingly, our analysis supported neither of these two hypotheses. Of the 12 conditions with significant effects of prior fixation duration on saccade landing positions, 11 showed an unexpected pattern. In these conditions landing positions were shifted to the right following shorter preceding fixations.

This result is likely to be related to the fact that refixations tend to be of shorter duration as compared to single fixations (e.g. Kliegl, Olson and Davidson, 1983; Underwood, Clews and Everatt, 1990; O'Regan et al., 1994; Rayner, Sereno and Raney, 1996), and that saccades following refixations tend to be longer (see the fixation pattern effect described above). With these considerations in mind, one could suspect that the observed rightward shift after shorter fixation durations is equivalent to a rightward shift of landing position following longer gaze durations which typically result from cases that include more refixations. Indeed this hypothesis was confirmed as part of recent explorations into interrelations between spatial and temporal aspects of eye movement control (Radach and Heller, in preparation).

An additional source of variation in initial fixation positions within words is the position of the target word within the current line of text. Mean landing positions in words at the beginning of the line are substantially shifted to the fight and those in words at the end of the line are shifted to the left. These are relatively large effects, producing up to one letter position difference in mean landing position, depending on word length and launch distance. This underscores the importance of line-level information (line length, line distance, layout) for spatial navigation through a page of text (Heller, 1982). A comprehensive model of eye guidance in reading will need to consider these aspects, especially the functional significance of fixations preceding and following return sweep saccades (Hofmeister, 1997).

The variables discussed so far (with the exception of prior viewing duration) were all "low-level" variables operating on the oculomotor and/or perceptual level. It is now generally agreed that these explain most of the variance in initial saccade landing position distributions (see, e.g., Chapter 11 for a converging perspective). However, in recent years there has been considerable controversy around the question of whether parafoveally available cognitive (semantic, lexical or sub- lexical) information can also influence initial landing positions. One popular variation of this idea is the "parafoveal guidance hypothesis" stating that saccades go


further into words that have a less informative (more redundant) word beginning (Underwood, Bloomfield and Clews, 1988; Underwood, Clews and Everatt, 1990; but see Chapter 9 for a more cautious view).

We have tested the original parafoveal guidance hypothesis by looking at the same 40 conditions (5 launch distances x 2 word length ranges x 4 participants) as described above. For each condition we compared mean landing position as a function the quartiles of initial trigram 'informativeness' (i.e., number of word forms that share the same initial trigram). Among the 40 conditions there were only four significant differences and only two of these were in the direction predicted by the hypothesis. Two similar analyses was carried out with token trigram frequency and word frequency as the dependent variables, again with negative results (for a similar corpus analysis, see Rayner, Sereno and Raney, 1996).

It could be argued that in our corpus analyses we may not have given cognitive variables a fair chance to show their influence on saccade landing positions. However, when we recomputed the range of trigram frequencies used by Radach (1996) and Radach and Kempe (1993), and compared it to values given by other researches (e.g. Liversegde and Underwood, Chapter 9) we found that the differences between our high and low trigram items are in the same range. Our results are in line with other failures to replicate word beginning informativeness and trigram frequency effects (Rayner and Morris, 1992; Radach et al., 1995). However, other studies exist that have found evidence for a small effect of orthographic manipulations on the mean landing position (e.g., Hytin~i, 1995).

In summary, we have found that in the eye-movement data of four German readers the landing positions on words following progressive inter-word saccades is primarily determined by the locations from which the saccades originated, or launch sites, with smaller effects due to word length and position of the word on the line of text. In these data, neither the duration of the prior eye fixation, nor the informativeness of the initial trigram of the word affected these landing positions.

The purpose of the study described below was to determine whether launch-site- contingent intraword and regressive saccades show landing position distributions having the same properties as those obtained from interword progressive saccades. This is of particular interest given the strong distinction often made between interword and intraword (refixation) saccades, and between progressive and regressive saccades. O'Regan's (1990, 1992) Strategy-Tactics theory assumes a general scanning routine containing two components: (1) the eyes are sent to successive words in the forward (rightward in English and German) direction, and (2) there is a within word tactic of repositioning the eyes (refixating) when their initial fixation position is at a non-optimal location. Averaged over words, the word center is considered to be the optimal location for their viewing. Given that interword and intraword saccades are assumed to be controlled on quite different bases, we might expect their landing position distributions to show different characteristics.


Similarly, Morrison's (1984) model of eye movement control during reading, as modified e.g. by Henderson and Ferreira (1990) and Pollatsek and Rayner (1990), makes a clear distinction between refixations and progressive interword saccades. The original version of this model included no mechanism for accounting for refixations; it has been necessary to suggest other mechanisms to account for these. Again, if refixations and interword progressive saccades are being generated by different mechanisms, it would be expected that they would show different characteristics.

Finally, neither O'Regan's model nor Morrison's model include a clearly defined mechanism for generating interword regressive saccades; typically, they are explained as resulting from processing difficulties at higher cognitive levels that require a reconsideration of earlier text, thus resulting from quite a different set of circumstances than the 'normal' progressive interword saccades that are the basis for existing theories.

If differences were found in the properties of the landing position distributions of refixations or regressions, as compared to progressive interword saccades, this would provide evidence for the psychological reality of the processing distinctions described above. On the other hand, if no differences are found, this would not eliminate the possibility of these distinctions, since it may be that the distinctions lie at some higher level, where selection of a saccade target is made, with the mechanism that produces the landing position distributions (the control system that gets the eyes to a selected target) being common to all of these processes. The present study examined our corpus of German reading eye movement data to determine whether the landing position distributions following interword progressive saccades, refixations, and interword regressive saccades do or do not show similar properties. It also examines the issue of whether eye movement control is discrete or graded, and considers two alternative hypotheses of how landing positions in selected words are determined.

Refixations vs. progressive interword saccades

In the literature on eye movement control in reading two alternative views on refixations have been proposed. According to the first view, refixations are initiated when cognitive processing is difficult in one or another respect. Such a processing difficulty can take several forms: it may be that lexical access is hampered or that due to cognitive overload at modules of syntactic or semantic analysis the oculomotor system is ordered to "slow down" and park the eyes on the current word for some additional time (for a review, see Rayner and Pollatsek 1989). To account for


refixations, Henderson and Ferreira (1990) supplemented Morrison's attention- based eye guidance theory with the notion of an oculOmotor deadline, in which a refixation is initiated if attention has not shifted to a new word. Contrary to this prediction, the first of two fixations on words has been shown to be shorter than single fixations (Kiegl, Olson and Davidson, 1983; Underwood, Clews and Everatt, 1990; O'Regan et al., 1994; Rayner, Sereno and Raney, 1996; Radach, Heller and Inhoff, 1997). Pollatsek and Rayner (1990) discuss a another possibility in relation to parallel interactive models of word recognition (e.g. Paap et al., 1982). They propose that the total "level of excitation" in the lexicon (including excitation from parafoveal preprocessing during prior fixations) may provide a cognitive base for refixation decisions (see also Chapter 11 for a new processing mechanism to account for refixations).

According to the second view, refixations are based on visuomotor factors" deviations of the initial fixation position from the generally optimal viewing position result in an increase of refixation frequency (see above). In his "strategy and tactics theory" O'Regan (1990) proposes that the eyes, driven by a global, preprogrammed scanning routine, attempt to go to the "generally optimal" viewing position located close to the center of the word. If the eyes do not land at the optimal position, "lexical processing will begin, but may not be able to terminate, since information about certain letters in the word is lacking" (O'Regan 1990, p. 427). Therefore, a refixation will be initiated that will bring the eyes to the opposite end of the current word. "When the eye is on one side of the word, it goes to the other. When it is near the middle, it goes to either one or other end. In other words, the eye is not attempting to get to the optimal viewing position. Rather, it is attempting to spread its fixations evenly over the word." (O'Regan 1990, p. 427).

Although O'Regan (1990, 1.992) does not make exact quantitative predictions with respect to landing positions of within-word saccades, it is evident that he proposes a qualitative difference between saccades across word boundaries (based on the "strategy") and within-word saccades (based on "within-word rescue tactics"). Such a difference should be evident when saccade landing positions are plotted as a function of launch site relative to the word (the landing position function), allowing a comparison between cases with launch sites lying to the left of the target word (initial fixations) and cases in which the eyes are launched from within the word (refixations). Specifically, there should be some type of discontinuity in mean landing position at the transition point between inter-word to intra-word launch sites.

The left panel in Fig. 2 shows the frequency with which progressive intraword saccades (refixations) are launched from different letter positions of 9- and 1 l-letter


Refixation Launch/Landing Distribution

Absolute Frequency 700

6 0 0 ~" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4 0 0 . . . . . . . - - - - - - -NN~,~- -~ -- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

~ ( ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

200 .. . . . . . . . . . .

100

0 -5 .4 -3 -2 -1 0 1 2 3 4

Center based launch/landing position

�9 * landing position + launch site

Mean Refixation Landing Positions

Letter in word 1 ~. 1

F 10

0 -5 .4 -3 -2 -1 0 1 2 3 4 5

Launch site relative to word beginning

+ wlen9 ~ wlen l0 Et wlenl !

Fig. 2. Left panel: Frequency distributions of progressive refixation launch sites and refixation landing positions for 9-11 letter words. Please note that a refixation launch site is equivalent to the landing position of the prior initial fixation. The scale on the abscissa indicates letter positions relative to the center of the word. Right panel: Mean landing positions of saccades as a function of launch site. The abscissa indicates letter positions relative to the space immediately to the left of the word. Negative numbers indicate launch sites of initial progressive saccades going into the target word, whereas positive numbers indicate launch sites from within the target word (for these refixations observations are identical to those in the left panel). Pooled data for four participants. Notice that the curves appear continuous from progressive interword saccades to intraword

(refixation) saccades.

words combined , relative to the center of the same word, and the f requency with

which the eyes land at these locations fol lowing the refixation saccades. 5 The great

majori ty of progress ive refixations are initiated f rom posi t ions at or to the left o f the

center o f the word. The right panel shows the landing posi t ion function on 8- to

5 There are two features of the right panel of Fig. 2 that deserve comment. First, the vertical displacement of the curves from one another is primarily due to the fact that they are plotted relative to the space before the word (letter position 0 here), which was done to preserve continuity with Fig. 3. If plotted relative to the centers of the words, the curves are largely superimposed on each other. Second, the curves appear to diverge somewhat in the refixation range. This is primarily due to the fact that in this figure means of the observed landing positions are given as opposed to fitting a normal curve to the data as in Fig. 1. This has the effect of underestimating the central tendency of the distribution when it migrates toward the far end of the word, because saccades landing off the word are not included in the calculation. At any given letter position this error in estimation is greater for shorter words, hence causing a separation between curves for different word lengths, as observed here.


12-letter target words following saccades launched from different locations (launch sites) relative to the space before the target words. This figure illustrates the linear landing position function that has been observed in previous studies. However, it also shows that this relation continues smoothly from interword to intraword progressive saccades. There is no discontinuity at the transition point between the two types of saccades, as one might anticipate from current theories distinguishing these categories. Thus, this analysis provides no evidence for a difference in the basis on which landing positions are determined for initial and refixation progressive saccades.

Regressive inter.word saccades

The frequencies of saccades made in a right to left direction (regressions) among our participants ranged from 16.7 to 36.1% (including only saccades that start and land on the same line of text), which is within the range typically observed among German readers. About 2/3 of these are inter-word saccades ranging from 10.6 to 29.6 percent of all saccades made by the four participants studied. Our goal in the following analysis was to determine whether the linear mean landing position function observed for data following progressive saccades is also observed in data for interword regressive saccades.

The frequency of regressions varies considerably as a function of factors like text difficulty and reading instruction (Heller, 1982). Regressive saccades have recently received much attention in the psycholinguistic literature, having been shown to be related to language processing difficulties. Several regression-related eye movement measures can be considered: the frequency with which the fixation of a target word results in the initiation of a regression, the frequency with which a word is the recipient of a regression and the duration of fixations after regressive saccades. Regression-contingent analyses of syntactic parsing mechanisms have been discussed (Altman, Garnham and Dennis, 1992; Rayner and Sereno, 1994) and complex eye movement measures involving regressions have been developed (e.g., Daneman, Reingold and Davidson, 1995; see also Chapter 3). However, there is currently only limited understanding of whether and how spatial information about word locations within a sentence is being acquired and used for the subsequent planning and execution of regressions (Kennedy, 1992).

The question addressed here concerns the landing position function following these regressions and, particularly, whether it shows the same linear functions of launch site as to progressive saccades. An analysis of launch sites of regressive saccades in our corpus of reading data shows that most regressions originate from positions relatively close to the target word. Of all regressive saccades made within one line of text to words of length 5 to 10, 26.0% come from within the same word


(regressive refixations), 49.4% come from the immediately following word and only 24.6% from more distant locations.

Figure 3 shows the landing position function for regressive saccades. The abscissa on this figure, which shows the launch site, indicates letter position relative to the space fo l lowing the fixated word. Thus, negative numbers indicate refixation cases, where-1 means that the eyes were launched from the final letter in the word (-1 with respect to the space following the word), while positive numbers indicate launch sites to the right of the word, or interword refixation cases. As can be seen in the figure, for every word length the refixation regressions (negative numbers on the abscissa) show the same linear relationship between launch site and the mean of the landing position distribution that progressive saccades show (see Figs. 1 and 2). The mean of the slopes for the different word lengths, for this part of the data, is 0.39. However, for interword regressions (positive numbers on the abscissa), the result is quite different. Here, for every word length, the curves go flat (mean slope of 0.06). For these interword regressions there is essentially no relationship between launch site and the mean of the landing position distribution; no matter from where the saccades are launched, the mean landing position in the word on which they land is the same. Thus, the mean landing position function shows a sharp bend in the region of the space before the word, which separates intraword from interword regressions.

Fig. 3. Mean landing positions of regressive saccades as a function of launch site. The abscissa is numbered relative to the space following the target word, with negative numbers indicating launch sites from within the word, and positive numbers indicating launch positions to the right of the word boundary. The ordinate, indicating mean landing position, is numbered with respect to the center of the word. Each graph represents between 3819 and 744 observations. Pooled data

from four participants.


These results indicate that the control of the eyes in making interword regressions is functionally different in some way than it is in the other cases studied. McConkie et al. (1988), following Kapoula and Robinson (1986), attributed the linear relation between launch site and mean landing position found with progressive saccades to a range effect that is frequently observed in other muscular systems (Poulton, 1981 ): a tendency to overshoot near targets and undershoot far targets. Thus, it is assumed that a word string within a line of text, perceptually delimited by preceding and following spaces, functions as a stimulus unit or 'blob', with the eyes being drawn to the center of that unit when it is selected as the target for a saccade. The center serves as a functional target location for directing the saccade. From this perspective, it appears that in making interword saccades, there is no range effect. Rather, the eyes go consistently, with some random error that produces the Gaussian distribution observed, to their target.

A collateral observation from examining Fig. 3 is that the mean landing position following interword regressions is very near the center of the word, regardless of the word length or the launch site. Thus, the two variables that appear to have the greatest effect on mean landing position in other cases, have little or no effect following these particular saccades. This has two implications. First, it provides further evidence that the center of the word is the functional target for saccades going to the word. Second, it indicates that the mechanism producing the range effect is not always operating, even in the reading task itself. It is curious that the range effect should occur in intraword regressions but not in interword regressions. It is not just the direction of the saccade that is critical, since intraword regressive saccades show its pattern. Neither is it the distance that the saccade target lies into the periphery; the range effect is observed in progressive saccades of similar length~ to the interword regressions that do not show its pattern. There is clearly something different about saccade control with interword regressive saccades. Research is now needed to better understand the mechanism that gives rise to the range effect, and the conditions under which it operates.

Finally, there is one more observation that indicates the distinctiveness of interword regressions. Radach, Heller and Hofmeister (1998) have data indicating that certain readers show a distinct peak of very short fixations (80-120 ms) in fixation duration frequency distributions that these investigators explain in terms of minimal latencies of saccades that have become unnecessary but could not be canceled soon enough (Becker, 1989; Mo~ison 1984). Interestingly, these very short fixations were never found before inter-word regressions, suggesting that progressive and refixation saccades are part of a mode of eye control that may automatically generate, but sometimes cancel, saccades. Apparently the decision to make a regression to a previous word is the result of a more deliberate mode of eye control.


Continuous vs. discrete control of saccades

In the existing research literature on reading, a number of variables have been found to have a local effect on the lengths of saccades. As one example, there is a marked effect of word frequency on the likelihood of refixating the same word (e.g., McConkie et al., 1989; O' Regan et al., 1994, Rayner, Sereno and Raney, 1996). The effect is basically a vertical displacement of the refixation function, as depicted in Fig. 4 (left panel), which results from an increase in the amplitudes of saccades launched from the initial fixation within higher-frequency words as compared to saccades launched from lower-frequency words. In most cases the sizes of these saccade length effects are less than one letter position (e.g., 0.82 letters for the observations in Fig. 4, right panel). This can give the appearance of an eye movement control system that adjusts the lengths of saccades in a graded fashion, slightly increasing or decreasing the lengths of saccades according to the local processing requirements during reading. The alternative to this position, and the one we espouse, is that the lengths of saccades are determined in a more discrete fashion, resulting primarily from which words are selected as targets of those saccades. In harmony with Morrison (1984) and Rayner, Sereno and Raney (1996) and others, we propose that eye movement control is primarily a discrete process, with most variables on the perceptual and cognitive level influencing which word is selected as the target of the next saccade. 6

Thus, the observed effect of a local variable on saccade length is interpreted as being probabilistic in nature, with that variable having an influence on some underlying attractiveness level of the words that serve as potential candidates as target for the next saccade. Since the eyes can be sent to only one of these candidates on a given saccade, a selection is made through some winner-take-all competition. For many saccades, one candidate is so strong that small changes in candidate attractiveness have no effect on the outcome; for other saccades where alternative words have attractiveness values that are sufficiently similar, such an influence can cause a different word to be selected than would otherwise occur. In such a case, the saccade length will change by several letter positions; in extreme cases, the direction may even change.

Within the framework just described, finding that a variable decreases the lengths of saccades by an average of 0.5 letter positions essentially indicates that the lengths of the great majority of saccades were not affected at all by the variable, even though an influence of the variable may have been present. For only a small proportion of the cases was the influence of the variable being studied great enough, and the

6 As noted earlier, some variables, such as those that affect the center of gravity in a word, may influence the landing position within the word, thus being a graded influence. We assume that variables of this type account for only a small part of the variance in saccade lengths in reading.


Refixation frequency as a function of initial landing position

5o Refixation frequency (percent)

4o

1 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0 | , ! ! ...... i i

-4 -3 - 2 . I 0 I 2 J

Word center based landing position 4'- infrequent words .4- frequent words

�9 Saccade landing position relative to next work beginning

20 Relative frequency (percent)_ . . . . . . .

18

16

14

12

10

8

6

4

2'

0 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7

Word boundary based landing position

infrequent words .4- frequent words

Fig. 4. Left panel: Frequency of refixating the same word as a function of the initial landing position within the word. Data for word length 8-12 from four participants were pooled. Negative numbers on the abscissa indicate initial landing positions left of the word center. The selection of 'infrequent' and 'frequent' words is based on a median-split of word form frequency (Celex, 1995). Right panel: Landing positions of progressive saccades following an initial fixation position of -3 relative to the word center (with observations identical to those for -3 in the left panel). Negative numbers on the abscissa indicate landing positions within the same word (refixations); positive numbers indicate landing positions of regressive inter-word saccades.

existing selection likelihood structure of alternative words similar enough, for the effect of the variable to be realized in behavior. In these cases, a large change in saccade length occurred, sending the eyes to one word rather than another. Theoret- ical and methodological problems related to this "frequency of effects" problem in reading have been outlined by McConkie, Zola and Wolverton, (1985).

An example of this type of influence can be seen in the case of the effect of the frequency of a fixated word on the length of the following saccade. The two local variables that appear to have the largest effect on the likelihood that a given word will be selected as the target of the next saccade are its location with respect to the currently fixated letter and its length (Kerr, 1992; McConkie, Kerr and Dyre, 1994; see also Chapter 6).

To see other influences it is necessary to control for these two variables, which can be done by selecting cases from a large corpus of reading data as we have described above. Figure 4 shows a frequency distribution of landing positions in all cases where the saccade's launch site was three letter positions left of the center of the currently-fixated word (word length 8-12). As can be seen, the frequency distribution is markedly bi-modal, suggesting two populations of saccades, one keeping the eyes on the currently-fixated word (refixations) and the other taking the


eyes to a following word. The two curves are formed by taking a median split of the data based on the cultural frequency of the currently fixated word, including all words of our corpus that are listed in the German CELEX Corpus. Figure 4 shows that the frequency of the word does not simply shift the distribution left or fight in a simple manner, as would occur if the effect occurred in a graded fashion. Rather, word frequency influences how many of the saccades remain on the current word vs. go to the next, suggesting a discrete choice between alternative possible saccade targets. The curves for high- and low-frequency words both appear to be the sum of the same two underlying distributions, having similar modes; word frequency affects the number of cases in the two component distributions, resulting from the frequency with which the two alternative candidate words are selected as the target of the saccade.

When launch site and the lengths of immediately surrounding words are controlled in the manner illustrated above, the landing position distributions actually change shape as word length configurations change. Bimodal distributions of the type shown in Fig. 4 are frequently observed, suggesting that choices are being made among alternative potential saccade targets. This is consistent with an eye movement control mechanism that is discrete and probabilistic in operation, rather than one that adjusts saccade lengths in a graded fashion on the basis of local variables.

Some theoretical issues

We have argued that eye movement control during reading is a discrete process, involving the selection of a target word, and the somewhat error-prone process of moving the eyes to that word. In this section we discuss several issues related to the landing position distributions on words.

Launch site contingent landing position distributions are very well behaved, being Gaussian in shape, with a mean that is a linear function of the launch site (with certain exceptions), and a standard deviation that increases with launch distance (McConkie et al., 1988; see also McConkie et al., 1990; McConkie, Kerr and Dyre 1994). We have attributed the linear landing position function to a saccadic range error. Vitu (1991 a, 1991 b) has argued for an alternative possibility. As we do, she assumes that a word is selected to be the target of the following saccade. However, she then proposes that the eyes go to the center of gravity of an attended region extending about seven letters from the beginning of the selected word. The blank space at the end of the attended word is thought to have sufficient influence on this center of gravity to account for changes in mean landing position; the farther the space lies to the fight, the further to the right lies the center of gravity within this attended region. This theory can successfully predict that landing positions within longer words that begin at the same distance to the right of the launch site tend to lie further from the words' beginnings (Vitu, 1991 a). However, it does provide a basis


for predicting the landing position function: changes in mean landing position within words of the same length lying at different distances from the launch site. In contrast, in our analyses if landing position is measured relative to the centers of words, then word length has very little effect on the landing position distribution, whereas distance of the launch site from the target word has a large effect. Thus, we do not see center of gravity within an attended area as an alternative to the concept of the range effect for accounting for the strong, linear landing position function that exists in reading data.

Rayner and Morris (1992) as well as Rayner, Sereno and Raney (1996) have suggested that the landing position within words is determined by which letters in the word have been previously identified, with the eyes tending to go beyond the identified letters, similar to the proposal made by McConkie (1979). This raises the question of how landing position distributions are affected by cognitive factors. In discussing this issue it is critical to distinguish between factors that influence the selection of the saccade target, and those that influence where the eyes land with respect to that word. Clearly, cognition does affect where the eyes go during reading; the question here is whether it influences where the eyes land with respect to a selected word, or only affects the selection process itself.

The preprocessing hypothesis could be the basis for an alternative explanation of the range effect: namely, the farther the launch site is from a target word, the fewer the letters that are likely to have been identified from it, thus resulting in the eyes landing closer to the front of the word. However, two aspects of our data are not in harmony with this explanation. First, the slope of the landing position function tends to be in the range of 0.35. Thus, for every letter position farther from a word a saccade to it is launched, the landing position in the word moves leftward by 0.35 letter positions. However, as distance of the eye from the target word is increased, the visibility of the word should fall off faster than this. Second, the landing position distribution continues to be linear for all launch site distances examined so far, whereas we would expect that a visibility-based effect would be negatively acceler- ated, especially at more distant launch sites where a word's visibility would be minimal (Rayner and Morris, 1981; Nazir, Heller and Sussmann, 1992).

There is another possible interpretation of the preprocessing hypothesis in the current context; namely, that number of letters identified from a target word before sending the eyes to it might affect the variability that we have called the random placement error. This is the source of error that produces the Gaussian shape of the landing position distribution and is indexed by the standard deviation of that distribution. We assume that this error ~ises from a combination of sources: natural lack of precision in saccadic programming, especially with limited response times, error in the accuracy of the eyetracking device, and variation in the center of gravity within the word which results from interword variation in the distribution of letters of different levels in density, reflected here as the number of pixels intensified in


creating each letter. However, it is also possible that there may be systematic variability in landing positions related to number of letters peripherally identified from the target word, a factor that itself may vary from saccade to saccade, thus contributing to the spread of the distribution. This raises the question of whether any of the variability in these distributions is due to momentary variation in aspects of the reader' s cognitive state. This must be a matter for further investigation, but we doubt that such influence will be found.

Conclusions

In accounting for eye behavior during reading, McConkie et al. (1988) argued for a word-based control system, with a sharp distinction between selecting a word to serve as the saccade target, and the process of getting the eyes to the target word. They developed the beginnings of a quantitative model to account for data concerning where the eyes land in words, or landing position distributions. This model has been further confirmed and extended by Radach and his co-workers (Radach and Kempe, 1993; Radach 1996). The primary goal of the current investigation was to determine whether this model, which was originally developed to account for progressive inter-word saccades, can be extended to two other conditions: refixations and interword regressive saccades. The original model appears to account for refixations, which raises questions about the sharp distinction that is often made in theories of reading between progressive and refixation saccades. However, interword regressive saccades clearly show different landing position characteristics than do the other cases. Neither word length nor distance from launch site had much effect on the landing positions of these saccades, and the saccadic range effect, typically found with progressive saccades, was absent. Interword regressions appear to be sent to the centers of words, with landing position distributions in which "optimal" and "preferred" viewing positions are very similar. Thus, the original model must be modified to account for regressive inter-word saccades, and further research is needed to understand the basis for the difference. Interestingly, intraword regressive saccades (refixations) did not show these unusual characteristics.

Several issues were explored regarding the basis for eye movement control during reading. We have argued that eye movement control is discrete, based on the selection of word targets, rather than being a graded type of control, and have explored some of the implications of this position. In particular, we pointed out that even when a variable produces a significant effect on saccade length, it is likely that the lengths of relatively few of the saccades may have actually been changed. We also examined and argued against alternative positions to our model" specifically, that the landing position function may be due to center of gravity effects, and that the landing positions in selected words may be due to parafoveal letter processing rather than visuo-oculomotor factors.


In sum, we have argued that this one aspect of eye behavior during reading, namely, where the eyes go with respect to selected saccade target words, is the result of low-level visuo-oculomotor control factors, almost completely unaffected by higher cognitive processes. However, this should not be taken to indicate that we think there is no role for cognition in eye guidance. After all, the purpose of reading is to understand text and eye movements are made to serve this purpose. Elsewhere in this book (Chapters 6 and 11) it is argued that cognition plays a significant role in Selecting words as saccade targets. We agree with much of what is said on the issue of "word skipping" in these chapters (see also McConkie, Kerr and Dyre, 1994). In this chapter we have presented evidence for a cognitive component, in the form of a word frequency effect, in the decision of whether to refixate or make an inter-word saccade. Finally, in the case of regressive inter-word saccades, the saccade parameters we have looked at suggest a control mode different from the low-level default routines. Although we have not investigated this in the present chapter, it is very likely that not only the large 're-inspection' regressions (Kennedy and Murray, 1987) but also many regressions coming from an adjacent word are based on cognitive grounds.

The determination of landing positions within selected saccade targets, is highly regular and can be quantitatively described using models that account for most of the variance in the data. We suggest that investigators who obtain effects of cognitive factors on saccade landing positions consider the possibility that these factors are operating on the target selection stage rather than directly modulating saccade amplitude computation. Separating out these two sources of variance can be a difficult task. The predominant low-level determination of saccade landing positions within selected target words can be good news for researchers studying psycholinguistics processes: while there must be controls for this aspect of eye behavior, it does not appear to be a source of useful dependent variables, thus reducing somewhat the complexity of data analysis.

Acknowledgements

Preparation of this chapter was supported in part by grant No BMH1-CT94-1441 from the European Union under the BIOMED Programme. The authors are indebted to Dieter Heller, Albrecht Inhoff, Wayne Murray and two anonymous reviewers for helpful discussions and comments on earlier drafts of the manuscript.

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