revealing the basic properties of the visuospatial sketchpad: the use of complete spatial arrays

ELSEVIER Acta Psychologica 89 (1995) 197-216

acta psychologica

Revealing the basic properties of the visuospatial sketchpad: The use of complete spatial arrays

Alison Barton a,* Bill Matthews b, Eric Farmer c, Andrew Belyavin d

a King Alfred's College, Sparkford Road, Winchester, Hants S022 4NR, UK b Dept. of Psychology, University of Southampton, Highfield, Southampton S09 5NH, UK

c Human Factors Group, Flight Systems 06 Dept. Defence Research Agency, Farnborough GU14 6TD, UK d Centre for Human Sciences, Defence Research Agency, Farnborough GU14 6TD, UK

Received 21 October 1993; revised 3 March 1994; accepted 9 June 1994

Abstract

In working memory theory, there is currently increasing interest in the notion of a visuospatial slave system analogous to the articulatory loop. Evidence for such a system has been obtained using dual task methods from a variety of tasks, many of which would seem to confound central with specific resource demands. The experiments reported here attempted to avoid this confusion by exploring the effect of performing verbal and spatial secondary tasks during a short period of maintenance following the visual presentation of complete simple spatial arrays. The effects of varying stimulus duration, length of the maintenance period and pattern complexity were also investigated. Results showed small but persistent error increases due to interference from the spatial secondary task. It is proposed that there is an advantage for this approach in revealing the basic characteristics of a visuospatial slave system in working memory.

1. Introduction

Within the context of exploring the characteristics of the working memory system (Baddeley, 1986), there has recently been an increasing emphasis on the visuospatial sketchpad (VSSP). This, it is claimed, is one of two 'slave' systems which in interaction with the controlling central executive underpin a wide range of cognitive processes occurring over short time intervals. Speech and reading based information is thought to be processed by the phonological loop system in

* Corresponding author.

0001-6918/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0001-6918(1995)00035-F

198 A. Barton et al. /Ac ta Psychologica 89 (1995) 197-216

association with the central executive, with the loop having quite well-defined and relatively unsophisticated characteristics which have been widely explored at an empirical level. Visuospatial information is thought to be processed in an analogous way with the visuospatial sketchpad interacting with central executive processes, but the range of empirical evaluations of this relationship is much more restricted, particularly with processes occurring over short time intervals. Addition- ally, the separation of slave system processes from central executive processes has been much less clearly achieved in this domain than in the speech-based investigations. Thus to date, the specification of the defining characteristics of the VSSP leaves something to be desired, perhaps partly due to the complexity of the tasks used in many of the empirical investigations.

A brief consideration of some of these tasks may be informative. Typical of many of these investigations is a sequential presentation of individual items which allows the construction of a spatially organised matrix. This takes time to construct and seems likely to require the use of central executive resources during the presentation interval as well as requiring the maintenance of individual items for different periods of time before the spatial array is complete (e.g. Brooks's task (Brooks, 1967), Corsi blocks task (Morris, 1987)). The memorial task has often included a demand that the order of presentation as well as the final array is required, so that the possible differential loss of 'order' information and spatial information has not been easy to isolate. If order of presentation information is required and is emphasised at recall, this may require the involvement of central executive resources at recall too. It also raises the possibility that secondary tasks, which, it has been claimed, affect only the visual slave system processes, might have had central executive effects, as Phillips and Christie (1977a,b) have argued and which Farmer et al. (1986) accepted as a possible explanation of the effects which they obtained, even though they emphasised an alternative explanation. It is noteworthy that Morris (1987) argued that spatial secondary tasks interfered only with the encoding and not the maintenance of information in experiments that did not require that recall reflected the order of input, and that Smyth and Pendleton (1990) using the Corsi blocks test showed interference from a secondary spatial tapping task on a recall measure which required the order of presentation to be maintained, but did not show a significant interference effect when a free recall measure was used.

If, as argued, central executive processes are involved in a substantial number of investigations which purport to be exploring the VSSP, it suggests that simpler tasks may be a potentially more useful investigative technique, with central executive demands reduced to a minimum.

A task which meets these requirements is the presentation of a complete spatial array of meaningless information for a short period of time, followed by either an empty or filled interval which lasts for a number of seconds. If the filled interval is occupied by a relatively automatic task that draws upon visuospatial resources but makes minimal demands on central executive resources, and recall is allowed in any order, information may be made available about the basic retention characteristics of the visuospatial sketchpad.

A. Barton et al. /Acta Psychologica 89 (1995) 197-216 199

The experiments to be reported here adopted this strategy. In addition, two other general considerations are worth noting. The first was a deliberate intention to reduce the tendency of subjects to guess if they were unable to complete their recall, so allowing an accurate estimation of retention capacity. The second was the adoption of a recall score which took account of the partial retention of information within a presented sequence, and so contributed to an accurate estimation of information retained.

2. Experiment 1

Experiment 1 investigates whether a spatial tapping task competes for processing resources with the maintenance of visuospatial information presented as a complete pattern.

As is well known, short-term memory studies are frequently contaminated by long-term memory effects. The choice of novel meaningless patterns as stimuli is an attempt to reduce this problem since such information cannot be refreshed from long-term memory. Similar contamination could also arise through too long exposure of the stimulus, resulting in efficient initial learning and disguising any detrimental effects of competing tasks at recall. On the other hand, too short an exposure could result in inadequate coding of the information with an unknown effect on rehearsal processes. It is therefore important to determine a range of stimulus durations which reveals the potential effects of spatial suppression during maintenance.

The initial choice of a stimulus duration which permitted efficient encoding into short-term memory yet reduced the risk of long-term memory involvement to a minimum was guided by evidence from Avons and Phillips (1980). They identified an exposure time of 200 msec as meeting the above requirements for complex matrices. This stimulus duration was selected for Experiment 1 and contrasted with a stimulus duration of 2 sec, an arbitrarily chosen value.

Selection of secondary tasks to be performed during the maintenance of the patterns demanded consideration of the problems of interpreting effects in concurrent task methodology. The function of the tasks is to suppress processing within a specific slave system. Secondary tasks thus have to be equated in difficulty and to be modality specific, requiring minimal central resources but demanding enough specialised resources to cause interference on the primary task. Because of the importance of this point in the interpretation of dual task results, some time will be taken here to justify the choice of secondary tasks.

Two such tasks were used by Farmer et al. (1986) in an attempt to resolve the issue of whether the working memory literature demonstrated a three component model comprising a central executive with separate verbal and visuospatial slave systems (Baddeley et al., 1975), or a two component model in which visuospatial information was processed by central executive activity (Phillips and Christie, 1977a,b). Phillips and Christie had suggested that the issue could be resolved by

200 A. Barton et al. /Acta Psychologica 89 (1995) 197-216

devising a task that placed heavy demands on the proposed visuospatial subsystem, but minimal demands on the central executive.

Farmer et al. (1986) devised such a task for use in a double dissociation design which included baseline conditions. In their study, subjects performed two primary tasks: a spatial reasoning task devised by Benson and Gedye (1963), and a verbal reasoning task devised by Baddeley (1986). Each task comprised both easy and difficult problems. Each was presented under single task conditions or concur- rently with a secondary task intended to suppress the activity of one of the proposed slave systems of working memory. The verbal secondary task consisted of continuous repetition of the digits 1-4. The spatial secondary task consisted of the continuous sequential tapping of four targets.

In their first experiment, the counting task was found to have slowed RTs for the difficult verbal reasoning problems, but RTs for the easy problems were unaffected. There was no effect on spatial reasoning. In their second experiment, the tapping task produced the opposite pattern of results. RTs were increased for the difficult spatial reasoning problems, but not for easy ones. There was no effect on verbal reasoning problems. Each secondary task has thus been shown to have a modality-specific effect on the primary tasks. If it is assumed that the primary tasks are equated in demands for central resources, then this cross-over effect indicates as clearly as is possible that the demands of each secondary task for general central resources are equivalent.

It does not seem reasonable to claim that there is no central executive involvement in these secondary tasks, but their simple and repetitive nature suggests that central involvement is likely to be minimal.

2.1. Error m e a s u r e m e n t

The experiments reported here demand reproduction of a pattern. Partial recall of pattern information could be revealed in various ways. For example, subjects might forget or misplace individual elements of an array, they might remember spatial relationships between pattern elements and yet displace them in the matrix, or they might remember the vague figural properties of a pattern outline but not the exact element specification.

Measurement of accuracy of pattern recall could be performed in several ways. The number of correct individual pattern elements emphasises units and not patterned information, and the number of totally correct patterns ignores partial pattern memory. A more informative measure was judged to be City Block metrics, and this method was adopted as a flexible, sensitive and effective way of reflecting partial pattern knowledge. City Block metrics assume that the difference between two (presented and recalled) stimuli within a matrix is the simple arithmetical sum of the differences on the individual horizontal and vertical dimensions. A diagonal error is a mistake on two dimensions and is therefore measured along both axes or by 'going round a corner'. The advantage of this scale is that it reflects the size and the direction of an error. It was judged desirable to emphasise diagonal errors since such errors include both vertical and horizontal deviations from the correct


position. City Block was chosen in preference to a Euclidean measure because of the greater weight attributed to diagonal error by the former.

The question of whether City Block is a way of measuring performance error that corresponds to any kind of psychological reality has been addressed by Attneave (1950), who observed that " . . . provided the range of stimuli is not excessive, the psychological differences between them may be conceptualised as distances in a non Euclidean space" (p. 551).

Error is thus calculated as the total number of moves across squares required to match each element of the response with its appropriate stimulus element. The appropriate s t imulus-response match for each pat tern is assumed to be that which affords the minimum total error.

For Experiments 1 and 2, the value of a single move was fixed at 0.2 (a fifth) for a five-item array and in Experiment 3 as 0.142 (a seventh) for a seven item array.

Approximate estimates of maximum and chance error were obtained from a randomly generated frequency distribution (n = 10,000). These were 4.8 (maximum) and 2.02 (chance) for a five item array and 4.43 (maximum) and 1.72 (chance) for a seven item array.

Participants were not instructed to complete each response pat tern since this would bias them in favour of guessing. Consequently a suitable error value had to be substituted for any missing elements. The constant selected was a value equivalent to three moves on the matrix, this being roughly the mean value that would be expected from one error matched to a stimulus element as near to the centre of the matrix as possible. Error for responses with missing pat tern elements was therefore calculated for five item patterns as Total E r r o r / X + (0.6 x n) and for seven item patterns as Total E r r o r / X + (0.42 x n) where X was the number of items present and n was the number of items missing.

If any element of a response was present, it was assumed that missing elements were the result of memory failure and error was calculated according to the above formula. If a response was totally missing, it was assumed that this was due to attentional failure or to equipment problems and it was treated as missing data.

By substituting a standard value for missing pat tern elements, uncertainty became apparent and guessing was controlled.

2.2. Method

Subjects

Subjects were 12 volunteers, five men and seven women aged 21-60, in employment at the R.A.F. Institute of Aviation Medicine.

Design A 2 (0.2 sec or 2 sec stimulus duration) × 3 (no suppression, verbal or spatial

suppression) repeated measures design was used. Verbal and spatial suppression tasks were also performed alone to provide a baseline for performance.

Subjects performed eight blocked conditions. These were tapping and counting alone, matrix patterns with no suppression for both 0.2 and 2 sec, matrix patterns


with counting for both 0.2 and 2 sec, and matrix patterns with tapping for both 0.2 and 2 sec.

The order of the suppression conditions (matrices alone, matrices with counting, matrices with tapping) was balanced in a Latin square. In each suppression condition, subjects performed two blocks of trials, each comprising six trials of 0.2 sec or six trials of 2 sec stimulus duration, preceded by two practice trials. Each condition thus comprised twelve experimental trials and four practice trials. The order of the 0.2 and 2 sec displays was balanced across suppression conditions.

Half the subjects performed the counting alone at the beginning of the experiment with the tapping alone at the end, and the other half performed these tasks in the reverse order. Each block consisted of six trials and two practice trials.

Equipment The experiment was controlled by an IBM PC AT computer, interfaced with (1)

a monitor for displaying instructions; (2) an Apple II computer for recording the tapping task; and (3) the matrix display.

Patterns were displayed by the illumination of subsets of 36 blue lights mounted on a console to form a 6 x 6 matrix of 8.5 x 8.5 cm. The console was situated on a bench in front of the subjects so that they observed it from a height of 43-50 cm. The matrix subtended an angle of 9.25-10.5 degrees.

For the tapping tasks, the equipment consisted of four metal plates each measuring 70 X 70 cm, arranged as a square and mounted on a horizontal board with a 20 cm separation between each plate. The plates were covered with conductive plastic, coated with conductive paint to reduce noise. A metal stylus was provided for tapping.

Performance on the verbal task was recorded using a microphone and cassette recorder. Recording was controlled by the experimenter.

Tasks

Matrix task. For each pattern display, all five elements of the pattern were illuminated simultaneously. To avoid the possibility of two adjacent items being encoded as a single item, constraints were imposed on item selection. No two adjacent lights in any direction (horizontal, vertical or diagonal) could be illuminated in any one pattern. Within these limits, item selection was random. Recall of the patterns was registered by pressing the appropriate lights, which were consequently illuminated. A mistake could be rectified by re-pressing the light. Subjects pressed an 'enter' key when they were satisfied with their response.

Tapping task. Tapping was performed with the dominant hand, the apparatus being moved to the appropriate side of the subject, who was required to tap the four plates with the stylus continuously, and as quickly and as accurately as possible, starting in the top left hand corner and working in a clockwise direction. Both speed and accuracy of performance were measured.

A. Barton et al. / Acta Psychologica 89 (1995) 197-216 203

Verbal task. Subjects were required to repeat the digits 1, 2, 3, 4 aloud, continuously and as quickly as possible.

Procedure Subjects sat in front of the matrix console with the tapping board on their right

or their left depending upon which hand was dominant. The experimenter demonstrated the suppression tasks. Detailed instructions were displayed on the monitor screen at the beginning of each condition. Subjects pressed a button on the console when ready to proceed to the practice trials and again to indicate readiness to start the experimental trials.

For the verbal and spatial secondary tasks performed alone, each trial consisted of ten seconds of tapping or counting. A single bleep signalled the start of each trial, and a double bleep signalled the end.

In the dual task conditions subjects performed the suppression tasks during a ten second delay interval following presentation of the stimulus. As the stimulus disappeared, a single bleep was the signal to start tapping or counting. After ten seconds, two bleeps were the signal to stop the suppression task and recall the pattern. In the single matrix task condition with no suppression, the single bleep was retained as a control and followed by an unfilled interval and the signal to recall. Recall of patterns was self-paced, subjects indicating readiness to proceed by pressing the button on the console when they were satisfied with their response. A four second interval was then followed by the next pattern or by a message to await the next instructions. Responses were entered with the non-dominant hand.

Subjects were asked to perform as quickly and as accurately as possible on all tasks.

2.3. Resul~

Matrix task

The major findings of this experiment are summarised in Table 1. There were one totally missing and three incomplete responses. Incomplete responses comprised 0.7% of the data set.

The results were subjected to ANOVA for the factors trial (1-6), presentation (0.2 or 2 sec), suppression (no suppression, verbal or spatial suppression) and subjects.

Table 1 Mean error estimates for Experiment 1 (CB error)

Stimulus duration

0.2 sec 2 sec

Baseline 0.659 0.325 Counting 0.687 0.459 Tapping 0.789 0.589


Table 2 Mean speeds in milliseconds and mean percentage error estimates for the tapping task in Experiment 1

Single Dual task

tapping 0.2 sec 2 sec

Speed 232.6 233.12 235.71 Error 20.67 21.63 22.21

A N O V A revealed a significant main effect of presentation, F (1 ,9 )= 30.985, p < 0.001, due to fewer errors with the longer than with the shorter presentation times, and a significant main effect of suppression, F(2,20) = 5.117, p < 0.05. Post hoc comparisons of the means of the pooled error scores for each suppression condition were made using simultaneous t-tests with appropriate adjustments to take account of the number of comparisons (Bonferroni bounds). These indicated that the main effect of suppression was caused by significantly increased error due to tapping compared with the baseline condition ( p < 0.05). Error increase due to tapping following the 2 sec stimulus duration was greater than that following the 0.2 sec stimulus duration but the interaction did not reach significance.

Tapping task Tapping performance was measured as mean speed and percent error (see

Table 2). Comparisons using t-tests were made between the single and each of the dual

conditions for each part of the experiment. There were no significant differences in performance for either speed or error, and it can be assumed that participants performed at a similar level in all conditions.

Counting task Comparisons of mean articulation rates showed that participants had performed

at a similar level in all conditions (see Table 3).

Summary These results indicate that spatial rehearsal processes occur during the reten-

tion of simultaneously presented patterns. Performance measures for the tapping and counting did not indicate any trade-offs due to inconsistent allocation of attention between tasks.

Table 3 Mean articulation rates (digits per second) for Experiment 1

Single Dual task counting 0.2 sec 2 sec

4.9 4.6 4.6


2.4. Discussion

The most important finding of this experiment is the significant effect of tapping compared with the baseline condition on the maintenance of novel patterns when all the elements have been presented simultaneously. Evidence for spatial rehearsal of sequentially presented patterns during a post-presentation maintenance period has been conflicting, with Morris (1987) finding no evidence for this but with Logie and Marchetti (1991) observing an effect.

The present experiment was designed to minimise central demands of both primary and secondary tasks compared with other experiments which have used verbal or sequential visual presentation of spatial information. The observation that tapping has a small but significant effect on maintenance in these circumstances is therefore noteworthy.

However, caution is advisable in the interpretation of these results. The difference between the effect of tapping and the effect of counting did not reach significance, which suggests that unless there is a small verbal interference effect due perhaps to subjects' attempts to recode verbally the visual information, then the effect of tapping is not much greater than the cost of concurrence.

A possible reason for the small effect might be the very small increase in error due to tapping following the 0.2 sec stimulus duration. It was predicted that a stimulus duration of 0.2 sec would maximise the need for potential rehearsal with which a spatial secondary task should interfere, compared with a longer stimulus duration. Inspection of the means shows the trend to be in the opposite direction; a result which appears to be counter-intuitive. One possible explanation of the small error increase due to tapping at the shorter presentation time is that performance was already at floor, although responses were well above chance level. An alternative possibility is that rehearsal processes differ following different encoding opportunities.

The significant main effect of stimulus duration shows that the longer exposure interval did indeed lead to more efficient learning. However, the mean error increases due to tapping suggest that more efficient learning did not apparently reduce the requirement for specialised resources during subsequent rehearsal compared with a shorter stimulus duration.

Experiment 2 extended the investigation over a wider range of stimulus durations to evaluate and explore the findings suggested by Experiment 1.

3. Experiment 2

The aim of Experiment 2 was to test the reliability of the small but significant effect of tapping on error rate observed in Experiment 1, and to explore the range of stimulus intervals which give rise to rehearsal processes that are sensitive to interference from a spatial secondary task.

The scope of the investigation was further extended by exploring whether processing effects and the interfering effects of secondary tasks vary with storage


time. For example, it is possible that a shorter maintenance period might provide less opportunity for verbal recoding, thus enhancing the interfering potential of the tapping task.

In case there was a floor effect following the 0.2 sec stimulus duration in Experiment 1, the shorter end of the range was represented by an interval of 0.5 sec, allowing more time for adequate encoding. The 2 sec duration had shown more sensitivity to interference and was repeated in Experiment 2. Intervals of 1 sec and 3 sec were added to complete the range.

3.1. Method

Subjects These were 12 right handed volunteers between the ages of 21 and 45. Five

were RAF employees at the Institute of Aviation Medicine and received experimental pay. Five were seconded as students to the Institute, and two were flying officers. None of them had taken part in the previous experiment.

Design A 2 (five or ten second delay) x 3 (no suppression, verbal or spatial suppression)

x 4 (0.5, 1, 2 or 3 sec stimulus duration) repeated measures design was used. Verbal and spatial secondary tasks were also performed alone.

The experiment was divided into two parts. Recall was delayed for five seconds in part a and for ten seconds in part b. Half the subjects started with part a and half with part b.

Each part comprised fourteen conditions. These were tapping alone, counting alone, matrices alone for each of four display times, matrices followed by the verbal task for each of four display times and matrices followed by the spatial task for each of four display times. The order of the three suppression conditions was balanced in a Latin square. Display times were blocked and the order balanced across suppression conditions. Each block consisted of two practice trials followed by four experimental trials. Each suppression condition thus consisted of a total of eight practice trials and sixteen experimental trials.

For the single tapping and counting, subjects performed two practice trials and four experimental trials. The trials lasted for five seconds in part a and for ten seconds in part b. Half the subjects performed the spatial task alone at the beginning of the experiment and the counting alone at the end. The other half performed them in the reverse order.

Tasks Five item patterns were presented, with items displayed simultaneously. The

same constraints were imposed on item selection as in the previous experiment. Patterns were displayed for periods of 0.5, 1, 2 and 3 sec. Subjects were required to remember them for intervals of five or ten seconds before recalling them. The secondary tasks were performed as in Experiment 1.


0 L_

1.1.1

In 0

0.6i

0.5

0.4

0.3

0.2

0.1

0.0 single task counting tapping

suppression Fig. 1. Mean error estimates for Experiment 2 (five sec maintenance period).

• 0.5 sec

[ ] 1 sec

2 sec

[ ] 3 sec

Procedure Subjects at tended on two consecutive days. The suppression tasks in the dual

task conditions were performed during the post-stimulus retention period. The general procedure was the same as for the previous experiment.

3.2. Results

Matrix task

The major findings of Experiment 2 are summarised in Fig. 1 and Fig. 2, There were two missing responses and 36 incomplete responses. Incomplete

responses comprised 3.1% of the data set. The data were analysed using a repeated measures ANOVA for the following factors: delay (5 or 10 sec), presentation (0.5, 1, 2 or 3 sec), suppression (no suppression, verbal or spatial suppression), trial (1-4) and subjects.

Analysis revealed a significant main effect of presentation, F(3,31)= 7.038, p < 0,001, and a significant main effect of suppression, F(2,20) = 21.794, p < 0.001. Post-hoc comparisons of the means of the pooled error scores for each stimulus duration and of the pooled error scores for each suppression condition were performed using simultaneous t-tests with appropriate adjustments to take account of the number of comparisons (Bonferroni bounds). These revealed that the main effect of presentation time was due to a lower error level for the 3 sec presentation compared with both the 0.5 sec (p < 0.001) and the 1 sec presentation (p < 0.05). The main effect of suppression was due to a significant increase in error caused by tapping compared with both counting (p < 0.001) and the baseline condition (p < 0.001). The interaction of these two factors was not significant.

208 A. Barton et aL /Acta Psychologica 89 (1995) 197-216

0 L .

ILl

m

0

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0 single task counting tapping

suppress ion

Fig. 2. Mean error estimates for Experiment 2 (ten sec maintenance period).

• 0.5 sec

• 1 sec • 2 sec

[ ] 3 sec

The main effect of delay did not reach significance and there was no significant interaction of this factor with any other factor.

The differences in mean error increase for different presentation intervals in Experiment 1 supported the idea that there could be a differential effect of this variable on subsequent rehearsal processes. The interaction of suppression with stimulus duration in Experiment 2 was not significant but the mean error increases due to tapping were not in fact consistent. Some stimulus durations appear to give clearer evidence of spatial interference effects than others. So as to understand the nature of this interaction, it was judged appropriate in this case to examine specific families of differences independent of whether the particular interaction term in the overall analysis was significant (Ferguson, 1959).

It was found that when the secondary tasks were performed during the five second retention period, error increases due to tapping compared with the baseline condition were observed for all stimulus durations but reached significance following the 2 sec presentation only (p < 0.05). When the secondary tasks were performed during the ten second delay interval, there was an increase in error due to tapping compared with the baseline condition for all presentations, reaching significance following the 0.5 sec (p < 0.001) and the 2 sec (p < 0.001) stimulus durations. Tapping also caused significantly greater error than counting both at the 0.5 sec (p < 0.1) and at the 2 sec (p < 0.05) stimulus durations.

Tapping task For the five second delay, tapping was performed more slowly in all the dual

task conditions than in the baseline condition, the difference reaching significance for the 0.5 sec (p < 0.01), 2 sec (p < 0.05) and 3 sec (p < 0.05) stimulus durations. Error rates in each of the dual task conditions were less than in the single

A. Barton et al. / Acta Psychologica 89 (1995) 197-216 209


Single Dual task tapping 0.5 sec 1 sec 2 sec 3 sec

Five sec delay Speed 239.61 261.17 252.85 257.19 258.32 Error 9.72 5.66 8.19 6.64 7.11

Ten sec delay Speed 247.83 258.55 260.46 257.97 263.71 Error 11.5 9.08 8.43 9.03 10.14

condition. The consistency of the error decrease in the dual task conditions suggests a trend that might indicate a within-task s p e e d / e r r o r trade-off in all four dual task conditions. However, the decreases in error were not significant. Slower tapping speed in a dual task condition without a significant decrease in tapping error to indicate a within-task trade-off might suggest that greater attention was being paid to the primary task in that condition, thus possibly reducing impairment on primary task performance due to secondary task interference. The pat tern of tapping performance results does not convincingly suggest that any absence of significant impairment on the matrix task due to tapping can be attributed to an attentional trade-off between tasks. (See Table 4.)

Although the decrease in speed from the baseline to the dual task condition reached significance in only three out of four dual task conditions, the trend was similar in all conditions. Moreover, the 2 sec stimulus duration condition, which resulted in significant impairment due to tapping, was one of the conditions in which tapping performance significantly slowed, and the 1 sec stimulus duration condition, which did not result in significant impairment due to tapping, was the only condition which did not show a significant decrease in speed of tapping. The pat tern of results does not suggest that the inconsistent reliability of the effect of tapping following the different stimulus durations can be attributed to attentional trade-offs between primary and secondary tasks. For the ten second delay, there were no significant differences in tapping performance between the baseline and any of the dual task conditions for either speed or error. There is no indication of a trade-off in performance of this task when it is performed with the primary task.

Counting task

In the counting task subjects were found to have performed at a similar level in all conditions (see Table 5).

Summary

There was a consistent increase in error due to tapping. This did not reach significance at all presentat ion times. There was a consistently larger increase in error due to tapping for the longer retention interval but the interaction between suppression and length of delay was not significant.



Single Dual task counting 0.5 sec 1 sec 2 sec 3 sec

Five sec 6.0 5.7 5.9 5.5 6.0 Ten sec 5.4 5.2 5.3 5.4 5.0

3.3. Discussion

Experiment 2 has provided further evidence that a non-visual spatial task competes with the short-term maintenance of visuospatial information. This confirms the findings of Experiment 1 and is interpreted as indicating the use of active spatial rehearsal processes. The significant difference between the effects of tapping and the effects of counting in Experiment 2 are a more convincing demonstration of the specific interfering effect of tapping.

Information loss over both the delay periods in this experiment was very small and confirms previous findings by other authors that there is little loss from simple matrices. Moreover, the results of this experiment do not suggest that the length of the maintenance period is an important variable affecting rehearsal, at least for the times used here.

The post-hoc investigation of the effect of stimulus duration on subsequent processing sensitivity to tapping has shown that this effect is not consistent for all stimulus durations. It is difficult to account theoretically for this. It may be that five item patterns are on the threshold of the information load that requires active processing during short-term storage and hence leads to effects which are not reliable. Wilson et al. (1987) have proposed that active rehearsal may operate only on sub-span loads. However, the question of whether there might be a minimum load that can be held without active rehearsal has yet to be addressed.

Experiment 3 investigated the effect of increasing the information load of the patterns.

4. Experiment 3

Although increasing the number of items in a pattern is one way of making them more difficult, a concomitant increase in redundancy as a result of increasing the number of items within the same size of matrix might have the opposite effect. Seven item patterns were used as an appropriate compromise. It was difficult either to generate randomly or to construct deliberately seven-item patterns within the constraints imposed on previous pattern design. The constraints were therefore relaxed to allow adjacent diagonal squares to be illuminated together. Horizontal and vertical juxtapositions were still not permitted. The patterns were constructed


by the experimenters rather than randomly generated so as to avoid unwanted 'figural goodness' that might emerge according to the Gestalt principles of percep- tual organisation, which might increase the redundancy of the patterns.

Stimulus intervals of 0.2 sec, 0.3 sec and 2 sec were selected to test the reliability of the effects observed so far following the 2 sec duration, and to increase the sensitivity of the investigation in the shorter stimulus range. A maintenance period of five seconds was used.

4.1. Method

Subjects These were 12 right handed male volunteers aged 18-23. Eight were engineer-

ing students seconded to the Royal Aerospace Establishment, three were flying officers and one was an employee at the RAF Institute of Aviation Medicine. None of the subjects had participated in a previous experiment.

Design A 3 (0.2, 0.3 or 2 sec stimulus duration)× 3 (no suppression, verbal or spatial

suppression) repeated measures design was used. Counting and tapping were also performed alone. Suppression tasks were performed during the five second delay following stimulus duration.

Subjects performed eleven conditions. These were tapping alone, counting alone, matrices followed by no suppression for 0.2, 0.3 and 2 sec stimulus intervals, matrices followed by counting for 0.2, 0.3 and 2 sec stimulus intervals and matrices followed by tapping for 0.2, 0.3 and 2 sec stimulus intervals. Each matrix condition comprised four trials preceded by three practice trials. The order of the suppression conditions was balanced in a Latin square, and presentation intervals were balanced across suppression conditions. Tapping and counting on their own were both performed at the beginning of the experiment. Half the subjects performed the tapping first and half performed the counting first.

A stimulus pool of twelve patterns was designed as described above and repeated for each suppression condition. Subjects therefore recalled each pattern three times in the course of the experiment.

Tasks' and procedure The tasks and general procedure were the same as in the two previous

experiments. The order of the patterns was randomised before each suppression condition.

Nine different patterns were also stored for the practice trials. A three second inter-trial interval preceded the stimulus display. Immediately following the stimulus, a single bleep instructed the subjects to maintain the patterns with or without suppression. After a delay of five seconds two bleeps were the signal to stop counting or tapping and to recall the patterns in the usual way.


Table 6 Mean error estimates for Experiment 3 (CB error)

Stimulus duration

0.2 sec 0.3 sec 2 sec

Baseline 0.58 0.59 0.36 Counting 0.55 0.57 0.40 Tapping 0.83 0.79 0.62

4.2. Results

The main findings of this experiment are summarised in Table 6. There were one missing response and 27 incomplete responses which comprised 6.25% of the data set. Of the incomplete responses, only one had more than one element missing. The incomplete data could be attributed to four subjects and were distributed across all conditions.

A square root transform was selected using the maximum likelihood method of Box and Cox. Homogeneity of variance over contrasts, i.e. sphericity, was investigated both by searching for appropriate transformations and by examining the differential variance associated with contrasts among appropriate measures, and no indication of violations was found.

There was no evidence that re-use of the same stimuli had led to a learning effect over the three suppression conditions. There were significant main effects of stimulus duration, F(2,22) = 13.49, p < 0.001, and of suppression, F(2,22) = 14.62, p < 0.001. Comparisons between means using Neuman-Keuls test showed that error at the 2 sec duration was significantly smaller than at both the 0.2 and the 0.3 sec durations (p < 0.001).

Tapping caused a significant error increase compared with both the baseline (p < 0.001) and counting (p < 0.001). Comparison of the mean error in the baseline and counting conditions indicated clearly that there was no interference due to counting. In the clear absence of an interaction (F(4,44) = 0.421) between the two main variables, stimulus duration and suppression type, there seemed to be no justification for further post hoc investigation.

Tapping task No significant difference was found for either speed or error between perfor-

mance in the single tapping condition and performance in any of the dual task conditions (see Table 7).


Single Dual task

tapping 0.2 sec 0.3 sec 2 sec

Speed 305.1 291.3 281.2 273.2 Error 18.93 19.00 20.41 18.42

A. Barton et al. / Acta Psychologica 89 (1995) 197-216


213

Single Dual task counting 0.2 sec 0.3 sec 2 sec

4.9 4.5 4.7 4.4

Counting task Comparison of each of the dual task conditions with the single condition did not

reveal any significant differences (see Table 8).

Summary

The results showed a clear increase in error due to tapping but none due to counting during the maintenance of simultaneously presented patterns. Stimulus duration, at least for the values used here, does not appear to affect the pattern of interference.

4.3. Discussion

The results of Experiment 3 provided more evidence that the tapping task competes for resources with the maintenance of simultaneous displays of spatial information, and the overall pattern of error due to the secondary tasks again indicated specific interference due to tapping. The picture with respect to the effect of stimulus duration has become a little clearer. Of particular note is that the increase in error due to spatial suppression following a 0.2 sec stimulus display was similar to that following the other stimulus durations, suggesting that the very small size of this effect in a previous experiment was due to error being already at ceiling. There was no evidence of a differential effect of stimulus duration on the processing of the patterns in this experiment.

5. General discussion

Logie (1989) has suggested that by drawing a cautious analogy between the VSSP and the articulatory loop a useful methodological framework might be provided for further exploration of the visuospatial slave system. There has been some evidence for the obligatory access of unattended visual information into a specialised store (Logie, 1986). This, together with the effects of tracking or tapping tasks, has been interpreted as indicating a passive store in which visual information is maintained by spatial operations. Can any support for such an analogy be provided by the findings reported here?

The results of all three experiments have clearly shown a specific effect of tapping on the short-term storage of simple patterns which indicates that they are rehearsed using spatial processes. Increasing the stimulus duration led to consis-


tently better recall. It is possible that the longer stimulus interval allows time for the pattern to be organised or 'chunked' through making interconnections between the pattern elements, thus reducing the information load in a way analogous to short-term verbal processes. Rehearsal under these conditions might also require fewer of the (presumably limited) spatial resources available within the sub-system; there would be less competition with resources needed to monitor the tapping task and hence less error due to tapping. It can be predicted that very efficient organisation of the information would lead to both rehearsal and tapping being performed together perfectly within resource limits. This argument might explain the results of Experiment 2, in which the smaller error increase caused by the spatial secondary task following the 3 sec stimulus interval points to a possible trend in which increasing the stimulus duration beyond a certain point leads not only to better recall but also to less error due to tapping. The results could be seen as supporting a model in which spatial rehearsal processes act on an underlying representation with both the stored information and the rehearsal processes influenced by variation in stimulus duration.

Other investigators have presented evidence which is beginning to suggest that the visuospatial subsystem might be more clearly defined as two separate visual and spatial systems (Farah et al., 1988; Hanley et al., 1991). How these might functionally interrelate is not clear. Baddeley (in press) identifies colour and shape as examples of visually processed information, and stimuli involving location points as examples of spatially processed information.

The term 'spatial' in the working memory context has always been vaguely understood, and the tendency has been to accept an operational definition of 'spatial processing' as that which interferes with or is interfered with by tracking tasks. In the experiments in this paper, stimuli have been used which in most circumstances have been found to be sensitive to interference from tracking, but on some occasions (such as following a longer stimulus duration) are less sensitive. We have proposed that the evidence fits Logie's model of the visuospatial sketchpad in which well-learned visuospatial information can be held passively, without the need for active 'spatial' rehearsal, and in which active rehearsal processes are sensitive to tracking interference. It appears, therefore, that there are two models emerging from the literature. The question of how, if at all, the idea of passive storage (requiring no spatial resources) and active storage (requiring spatial resources) relates to the idea of visual (processing colour and shape) and spatial (processing location information) systems which are separate but interrelated, would seem to deserve some thought.

In the experiments reported here, demand for central resources by a visuospatial primary task was minimised by eliminating potential temporal characteristics of the task at presentation and recall. In addition, a conservative estimate of error was used which gave credit for partial memory. Even with these restrictions, a differential effect of tapping and counting during the maintenance of the matrix patterns was still observed and the pattern of interference from the secondary tasks was similar across all three experiments.

This evidence supports the idea that the construct of a basic visuospatial slave


system is a useful one, and is more convincing than that provided by researchers who have seriously confounded central executive and VSSP functions by including sequential temporal characteristics at various stages of their tasks. The close similarity of error rates in the control and counting conditions (with an exception of the 2 sec exposure interval in Experiment 1) provides further support for the view that only VSSP characteristics are being used in encoding and retrieval in this kind of task.

Enquiry into the range of stimulus durations which gives rise to rehearsal processes that are detectable by concurrent task methods has identified a potential problem. Short exposure intervals may result in floor effects, especially with few-item patterns. Taking this restriction into account, it is clear that the use of simple simultaneous displays of meaningless information such as those described here could help to reveal the basic properties of the proposed visuospatial slave system.

An issue increasingly under investigation is that of the nature of the hitherto ill-defined central executive (Baddeley, 1990). The justification for proceeding in this direction depends on a demonstration that the working memory system is indeed fractioned into sub-systems. The evidence presented here adds further weight to the argument in favour of fractionation into modality-based sub-components.

Acknowledgements

This research was carried out as part of the first author's doctoral thesis. We wish to thank the RAF Institute of Aviation Medicine, Farnborough, for use of facilities and Sgt. Adrian Johnson for help with equipment and programming of experiments. The authors would like to thank J. Cauraugh and an anonymous reviewer for their helpful comments on an earlier draft.

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revealing the basic properties of the visuospatial sketchpad: the use of complete spatial arrays

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