http://www.jstor.org

Interference in Implicit Memory Caused by Processing of Interpolated MaterialAuthor(s): Sander Martens and Gezinus WoltersSource: The American Journal of Psychology, Vol. 115, No. 2, (Summer, 2002), pp. 169-185Published by: University of Illinois PressStable URL: http://www.jstor.org/stable/1423433Accessed: 23/07/2008 09:14

Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at

http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless

you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you

may use content in the JSTOR archive only for your personal, non-commercial use.

Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at

http://www.jstor.org/action/showPublisher?publisherCode=illinois.

Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed

page of such transmission.

JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We work with the

scholarly community to preserve their work and the materials they rely upon, and to build a common research platform that

promotes the discovery and use of these resources. For more information about JSTOR, please contact support@jstor.org.

Interference in implicit memory caused by processing of interpolated material SANDER MARTENS

University of Groningen

GEZINUS WOLTERS Leiden University

This study addresses the susceptibility of implicit memory to interference. Inter- ference is manipulated by presenting interpolated lists of words that do or do not have word stems in common with previously studied target words (e.g., tar-

get word paragraph followed by interpolated words such as paradise or vicinity). Interference in a word stem completion task occurred only when words had sim- ilar word stems (Experiment 1). Increasing the number of interpolated words with corresponding word stems (e.g., not only paradise but also parking, pardon, and parliament) produced increasing amounts of interference (Experiment 2). Interference in implicit memory appears to be a simple response competition phenomenon that occurs when cues simultaneously activate primed targets and primed competing responses. The amount of interference can be explained by a quantitative model of the relative strengths of target and competing responses.

Implicit and explicit memory tests are differentially affected by a large number of variables and manipulations (for reviews see Richardson- Klavehn & Bjork, 1988; Roediger & McDermott, 1993; Schacter, 1987). One potentially interesting dissociation effect is a differential sensitivi-

ty to interference caused by processing interpolated material. It is well known that explicit remembering is impaired by interference

manipulations (Crowder, 1976; Postman & Underwood, 1973). In con- trast, it has been claimed Ithat interference has little or no effect on

priming implicit memory tests. Jacoby (1983), for example, reported absence of interfering effects in implicit memory. In this study, subjects studied a different word list on each of five successive days. No evidence of either proactive or retroactive interference was found in a perceptu- al identification task. Sloman, Hayman, Ohta, Law, and Tulving (1988) investigated interference effects in implicit and explicit memory by manipulating the nature of interpolated activities (verbal or nonverbal) between study and tests. After the interpolated tasks, subjects complet- ed either a fragment completion test (an implicit memory test) or a two- alternative forced-choice recognition test (an explicit memory test). The nature of the interpolated activity had no reliable effect on the fragment

AMERICAN JOURNAL OF PSYCHOLOGY Summer 2002, Vol. 115, No. 2, pp. 169-185 ? 2002 by the Board of Trustees of the University of Illinois

completion test. This result is inconclusive because there was no effect on the recognition test, either. Graf and Schacter (1987) reported a real dissociation between implicit and explicit tests of memory performance as a function of interference. Subjects studied unrelated word pairs. The experimental group first studied a list of target word pairs (e.g., "shirt- window"), followed by an interpolated list of word pairs containing previous words (e.g., "shirt-finger"), or an interpolated list that had no words in common with the first list (control group). Interference was observed in a subsequent explicit cued-recall test but not in an implicit word stem completion test with the first word and the word stem of the second word (the target) given as cues.

These studies seem to indicate that, at least in some cases, explicit memory is susceptible to interference manipulations, whereas under the same conditions implicit memory remains unaffected. However, instead of concluding that implicit memory is not affected by interference from

interpolated material, there is a viable alternative explanation for these findings. Implicit tests of memory measure an increased probability of

generating a particular stimulus as a result of a prior presentation of that stimulus in the absence of intentional retrieval. If it is assumed that a cue in an implicit memory task automatically activates all correspond- ing representations, then interference in such a task can be expected but only when the interfering items also match the cue causing response competition.

Cues did not match the interpolated items in the studies mentioned earlier. For example, in the Graf and Schacter (1987) study, interfering words did not have the same word stems as target words. Therefore, these primed interfering words will not be activated by the cue, so no interference is to be expected. A similar argument holds for the stud- ies byJacoby (1983) and Sloman et al. (1988). In these studies comple- tion of word fragments and word identification were used as implicit memory tests, respectively. In both of these tasks the cues presented at test address unique responses. Therefore, the study of interpolated words not fitting these cues will not result in the priming of competi- tors, and no interference will occur in the implicit memory test.

Several researchers have suggested that interference in implicit mem-

ory tests may be a response competition phenomenon. Nelson, Keelean, and Negrao (1989, Experiments 2 and 3) found retroactive interference effects on word fragment completion tasks when the interpolated words were orthographically and phonemically related to the target words. In contrast, no interference effects were found when the interpolated words consisted of unrelated or semantically related words. They concluded that interference in data-driven implicit memory tests results from priming of

competing responses at a lexical level (the lexical search hypothesis).

170 MARTENS AND WOLTERS

INTERFERENCE IN IMPLICIT MEMORY

A similar idea has been put forward by Bower (1996). He suggested that priming is the result of the prior activation and strengthening of

preexisting associations (see Graf & Mandler, 1984). This elevated

strength will decline over time, fading more rapidly when more com-

peting items are processed and strengthened. In other words, more interference will occur when there are more and stronger competitors. Results by Ratcliff, McKoon, and Verwoerd (1989) provide an example of such interference. In one condition of their experiments, a brief flash of a target word (e.g., "LIED") was followed by a forced choice between similar alternatives ("LIED" or "DIED"). Prior presentation of the tar-

get word (or the distractor) in a study list led to an increase in choos-

ing the target (or the distractor) on the test, showing either facilitation

(priming) or interference in identifying the target word. Strong inter- ference was reported also by Reingold (1995) for a letter deletion task after priming a competing response. For example, after being asked during study to delete either "E" or "D" in "PEDARL" (to get "PEARL"), responses were much slower when later asked to delete "A" or "R" in "PEDARL" (to get "PEDAL"). Presumably, the prior task strengthened the response "PEARL," and that response interfered with finding the correct solution in the later test. Smith and Tindell (1997) reported a similar type of interference in implicit memory. Priming incorrect com- petitors to word fragments (e.g., "ANALOGY" to the prime "A_L_GY') strongly reduced the completion rate of the correct target ("Al .LER- GY"). Because the primed and strengthened incorrect competitor partly fits the fragment, it apparently interferes with the correct completion.

Following Nelson et al. (1989) and Bower (1996), we suggest a re- sponse competition explanation for interference effects in implicit memory. Assuming that in data-driven implicit memory tests (such as word stem completion) the cues automatically activate all possible lex- ical completions, these possible completions will compete with each other as only one response is to be selected. Selection of the winner probably is based on the relative strengths of the competing respons- es. Therefore, if not only a target word but also interpolated words fitting the cue have been primed before, the chances of selecting the

target word will diminish, and interference (i.e., reduced priming) will occur.

The purpose of these two experiments was to analyze in more detail this response competition hypothesis for interference effects in implicit memory. Repetition priming was measured in word stem completion tests, and interference was manipulated by varying the number of in- terpolated words sharing the beginning word stem with words from a study list. Experiment 1 studied the basic effect by comparing the amount of interference of interpolated lists of words with or without the

171

same word stems as target words to a condition without interpolated list

learning. In Experiment 2 the number of interpolated words sharing the beginning word stem with target words was varied systematically.

EXPERIMENT 1

Experiment 1 was designed to examine interference effects in implicit memory by manipulating the nature of interpolated activities between

study and tests. All subjects first studied a list of target words. Subse-

quently, one group of subjects studied two additional lists of words that were orthographically related (i.e., each word in the list had a word stem in common with one of the target words). Another group studied two additional lists of words that were unrelated (no corresponding word stems to the target words), whereas a control group performed a non- verbal interpolated task. Implicit memory performance was measured in all groups using a word stem completion test, followed by a free re- call task for explicit memory. It was expected that explicit but not im-

plicit memory performance would be affected by the presence of un- related verbal material in the interpolated task relative to the condition with a nonverbal interpolated activity. In contrast, we expected to find interference effects (i.e., reduced priming) in the data-driven implicit memory test only when the interpolated task consisted of orthograph- ically related words.

To some extent this study is a replication of Nelson et al. (1989). However, in the study reported here, only beginning word stems were used as cues instead of various types of word fragments and word end-

ings. The latter types of cues may also involve conceptual processes (e.g., Weldon, 1991), and they may induce different search mechanisms (e.g., Nelson, Schreiber, & Holley, 1992). In addition, we will look not only at the interfering effect of interpolated items on targets but also at the mutual interference effects that result from priming multiple solutions to the cue used in a stem completion test. Generally, interference par- adigms specify the correct target, and interfering words are regarded as incorrect responses. In contrast, in this study both target and inter-

fering words are perfectly permissible "correct" responses to the word stem cues. This characteristic allows us to determine more precisely the mutual interference between targets and interpolated items.

METHOD

Design and subjects A 3 x 2 mixed factorial design was used. Type of interpolated activity (same

word stems, different word stems, or nonverbal control) was varied between

172 MARTENS AND WOLTERS

INTERFERENCE IN IMPLICIT MEMORY

subjects, and type of test (word stem completion or free recall) was a within-

subject factor. A total of 45 subjects, all undergraduate students at the Univer-

sity of Leiden, participated and were paid for their cooperation. They were

randomly assigned to each of the three groups, with 15 subjects per group. All

subjects had normal or corrected-to-normal eyesight.

Materials

Words were selected from a normative list for spontaneous completion fre-

quencies of Dutch words to two- or three-letter beginning word stems (Phaf & Wolters, 1991). The selection criterion was a low spontaneous completion fre-

quency (between 8 and 24 times out of 200 respondents). Starting with 110 different word stems, 150 words were selected. For 90 word stems only a single word was selected, and for each of the remaining 20 word stems three words were selected. For all word stems, at least 10 lexically correct completions were

possible. Semantic relationships between selected words were excluded as much as possible. Five lists of 20 words were created: one list of target words, two lists of interfering words each having a word stem in common with one of the tar-

get words (the same-word-stem condition), and two lists of words having a dif- ferent word stem than any of the target words (the different-word-stem condi- tion). In the same-word-stem condition, a target word such as "ELEment" would be followed by interpolated lists containing words such as "ELEphant" and

"ELEctricity." In the different-word-stem condition, "ELEment" would be fol- lowed by interpolated words such as "GIRaffe" and "COMputer." All lists con- sisted of words that on average had the same spontaneous completion rate (.06, SD= .02). In the control condition subjects judged the "friendliness" of 25

photographed faces instead of learning additional lists of interfering words. In the word stem completion test all subjects were given a form containing

70 word stems. Twenty of these word stems corresponded with the words of the

target list previously studied (and with words of the interpolated lists with the same word stems as targets). The other 50 word stems corresponded with 50 distractor words not shown previously. These distractor words had no word stems in common with any other words in the experiment, but they had the same average spontaneous completion rate. The completion of the distractor word stems provides a nonprimed base rate with which the completion of tar-

get words can be compared. The first 10 word stems in the completion task consisted of distractor word stems only, after which the other 40 distractor and 20 target word stems alternated randomly. To avoid sequence effects, two dif- ferent random orders were created for the word stem completion test (versions 1 and 2).

Procedure

Subjects were tested individually. First, all three groups studied the same list

consisting of 20 target words. Subjects were instructed to study the words in

anticipation of a recall test at the end of the experiment. Words were present- ed one at a time for 2.5 s with a 1.5-s interval. After a short break and addi- tional instructions, each group engaged in a different interpolated activity. The same-word-stem group studied two more lists of 20 words that had word stems in common with the previously studied target words. The different-word-stem

173

group also studied two more lists of 20 words, but these words did not have word stems in common with the target words. Presentation of target and interpolat- ed words was controlled by a BBC laboratory computer. The order of presen- tation of the words of all lists was randomized. The nonverbal control group judged the friendliness of 25 photographed faces after studying the target list.

After completing the interpolated tasks, in all groups implicit memory was measured with a word stem completion test. Subjects were asked to complete a form containing 70 word stems (20 target and 50 distractor word stems) with the first word that came to mind. The task was presented to the subjects as an unrelated interpolated activity before recall would be tested. No mention was made of any relationship with the previously studied lists. Upon questioning afterwards, none of the subjects indicated awareness of the nature of the im- plicit test, and all said they had used an unintentional retrieval strategy. There- fore, we believe the test adequately met the unintentional retrieval criterion (see Roediger & McDermott, 1993). Finally, explicit memory was tested by ask- ing subjects to freely recall as many words as possible from the target list only in 5 min. Both retention tests were paper-and-pencil tasks.

RESULTS AND DISCUSSION

For the implicit memory task, proportions of word stems completed to target words, interpolated list words (studied), and distractor words (nonstudied) were determined. All subjects completed all word stems to lexically correct words; there were no omissions. The proportion of correctly completed distractor word stems in each of the three groups (same-word-stem group, .06; different-word-stem group, .08; nonverbal control group, .07) did not differ statistically from the spontaneous completion rate (.06), which confirms the validity of the norm. This proportion formed the baseline in the experiment. Proportions of cor- rectly completed target words (same word stem group, .19; different word stem group, .28; nonverbal control group, .28) were all much high- er than the proportions of correctly completed distractor word stems (see Table 1). This indicates highly significant repetition priming effects in all groups (Fisher's z scores were 6.6, p < .01; 8.7, p < .01; and 9.1, p < .01 for the three groups, respectively). Table 1 shows the average pro- portion of target words generated in the word stem completion test and in the free recall test as a function of interpolated activity.

For the word stem completion data, an ANOVA on the number of cor- rectly completed target words confirmed a main effect of interpolated activity, F(2, 42) = 3.90, MSE = 4.15, p < .05, on implicit memory perfor- mance. A post hoc pairwise comparison of means (Newman-Keuls test) showed that the mean of the same-word-stem group differed significant- ly, p < .01, from the means of both the different-word-stem group and the nonverbal control group. There was no significant difference in

174 MARTENS AND WOLTERS

INTERFERENCE IN IMPLICIT MEMORY

Table 1. Proportions of completed target words in the word stem completion task and proportions of explicitly recalled target words as a function of interpolated activity between study and test

Test condition

Stem completion Free recall

Verbal related group (lists with same word stem)

M .19 .27 SD .10 .20

Verbal unrelated group (lists with different word stems)

M .28 .27 SD .10 .14

Nonverbal control group (nonverbal task)

M .28 .34 SD .11 .12

Note. Average proportion of correctly completed nonstudied items (i.e., distrac- tor words providing a base rate) was .07.

performance between the latter two groups. Compared with judging photographed faces (the nonverbal control task), interference in im-

plicit memory clearly does occur after studying an interpolated list of words having word stems in common with target words. No such inter- ference occurs after studying interpolated lists without overlapping word stems.

The results for the explicit memory test were analyzed as well, al-

though it should be realized that these results might be confounded by the preceding word stem completion test. No significant differences in

explicit recall were found between the three groups. However, it is in-

teresting to note that there is a trend suggesting that the nonverbal control group remembered more target words (i.e., suffered less inter- ference) than the two groups with a verbal interpolated activity (pro- portions correct are .34, .27, and .27, respectively).

The results of this study clearly show that interference in implicit memory tasks requires that primed interpolated items correspond to the cues given at test. Only if this condition is satisfied do they compete with the primed target items, reducing the amount of target repetition prim- ing. In this study, two competing interfering words were primed to in- duce such an interference effect. The priming effect of target items clearly was reduced by the presence of interfering words with the same stem (i.e., .19 vs .28 in the noninterfering conditions). Because the task

175

of completing word stems allowed both target and interfering words to be named, the mutual interference between target and interpolated words could also be determined. An analysis of the proportions of word stems completed with either the target words or the first or second in-

terfering words showed completion proportions of .19, .23, and .25, respectively. Although these figures may suggest that later presented words suffer less decay, such a conclusion is not warranted because the

proportions do not differ significantly. Experiment 2 was designed to

study the quantitative effect of multiple competing responses in an

implicit memory test in more detail. More specifically, we manipulated the number of interfering words that had the same word stem as a tar-

get word.

EXPERIMENT 2

In Experiment 1 each target word had a word stem in common with two interfering words (one from each list). This resulted in a significant but modest amount of interference. Presumably, an increasing number of matching word stems would produce an increasing amount of inter- ference. The purpose of Experiment 2 was to test this hypothesis and to analyze the amount of mutual interference between items from a

target and interfering lists in more detail.

METHOD

Design and subjects Two independent variables were combined in a 4 x 2 factorial design. Num-

ber of corresponding word stems in interpolated lists (0, 1, 3, or 5) and type of test (word stem completion or cued recall) were both varied within subjects. Thirty-two subjects, undergraduate students at the University of Leiden, par- ticipated in the experiment. None of them took part in Experiment 1.

Materials

From the same normative list (Phaf & Wolters, 1991) as used in Experiment 1, 458 words were selected with 138 different beginning word stems. Words were selected on the basis of having a low spontaneous completion frequency (2 to 38 times out of 200 respondents). For all word stems at least 10 alternative

completions were possible. All lists and sublists created had about the same

average spontaneous completion proportion of.06 (SD= .03). One list of 64 target words with different word stems was created. Each tar-

get word belonged to one of four categories. The first category (Category 0) contained 16 target words with a word stem that did not correspond to any other words presented later in the experiment. The 16 target words of Cate- gory 1 each had a word stem in common with one word in a second, interfer-

176 MARTENS AND WOLTERS

INTERFERENCE IN IMPLICIT MEMORY

ing list of words. The 16 target words in Categories 3 and 5 each had a word stem in common with three or five other words in this interfering list. Thus, each target word had a word stem in common with 0, 1, 3, or 5 other words in a second interfering list.

The interfering word list consisted of 144 (0 + (1 x 16) + (3 x 16) + (5 x 16)) words. To reduce item effects, four different versions of this interfering word list were created, counterbalancing target words and number of interfering words with the same word stem. These lists were also counterbalanced across the subjects. Consequently, for subject 1, a particular target word belonged to

Category 0, for instance, for subject 2 this word belonged to the next category (Category 1), and so on, and for subject 5 it belonged to Category 0 again.

Seventy-four words were used as distractor words in the word stem comple- tion task. These had no word stems in common with any other words in the

experiment. As in Experiment 1, the proportion of completed distractor words was used as base rate. The word stem completion task consisted of 138 differ- ent word stems. The first 10 were all distractor word stems, after which 64 dis- tractor and 64 target word stems alternated randomly. After the word stem

completion test, the 64 target word stems were presented again as cues in a cued recall explicit memory test for first list target words. Words and word stems in all study and test lists were presented on a PC in random order.

Procedure

The procedure was the same as in Experiment 1, with the following modifi- cations. All subjects first studied a list of 64 target words in anticipation of a final recall test. Words were presented for 2 s with a 1-s interval. After a short break and instructions for a subsequent learning phase, subjects studied a sec- ond list containing 144 interfering words. This list was presented in blocks of 48 words, separated by brief pauses.

Subsequently, implicit memory performance was tested by asking subjects to

complete 138 word stems with the first word that came to mind. Again, the task was presented to the subjects as an unrelated interpolated activity before re- call would be tested. No mention was made of any relationship with the previ- ously studied lists. One by one, 138 word stems were presented in the middle of a 17-inch computer monitor. Subjects wrote down the first word that came to mind, pressed the spacebar, and proceeded to the next word stem. Although some of the subjects afterwards indicated occasional unintentional recollection of words studied previously, none of them indicated any explicit retrieval strat-

egies or awareness of the true nature of the test.

Finally, subjects were given a cued-recall explicit memory task. However, the results were not analyzed because of confounding influences by the previous word stem completion test.

RESULTS AND DISCUSSION

For the implicit memory task, proportions of word stems completed to target, interfering, and distractor (nonstudied) words were deter- mined. There were no omissions in the word stem completion test. The

177

proportion of correctly completed distractor word stems was .065, which is very similar to the spontaneous completion frequency norm (.06), thus confirming the validity of the norms. This proportion formed the baseline in the experiment. The proportions of correctly completed target words (Category 0, .20; Category 1, .17; Category 3, .14; Catego- ry 5, .14) were much higher than the proportions of correctly complet- ed distractor word stems, indicating highly significant priming effects for all target words. Fisher's z scores were 9.6, p < .01; 7.3, p < .01; 5.6, p< .01; and 5.7, p< .01, respectively.

Table 2 shows the average proportions of target and interfering words that were reproduced in response to the word stem completion task.

Implicit memory performance is shown for each of the four categories in which target words have a word stem in common with either 0, 1, 3, or 5 interfering words. We first analyzed the number of target words

given on the word stem completion task as a function of the number of interfering words with the same word stem. An ANOVA on the num- ber of correct target words confirmed a main effect of the number of

Table 2. Proportions of target words and average and total proportions of

interfering words completed in the word stem completion test as a function of number of overlapping word stems in an interpolated learning task

Type of word

Interfering Interfering Target (average) (total)

Category 0: no overlapping word stems

M .20 SD .11

Category 1: one overlapping word stem

M .17 .16 .16 SD .08 .10 .10

Category 3: three overlapping word stems

M .14 .12 .36 SD .08 .06 .17

Category 5: five overlapping word stems

M .14 .10 .49 SD .08 .02 .12

Note. Average proportion of correctly completed nonstudied items (i.e., distrac- tor words providing a base rate) was .06.

178 MARTENS AND WOLTERS

INTERFERENCE IN IMPLICIT MEMORY

interfering words with overlapping word stems, F(3, 93) = 3.89, MSE = 1.73, p < .05. A post hoc pairwise comparison of means with a Newman- Keuls test showed that the means of Category 3 and 5 target items dif- fered significantly, p < .05, from the mean of Category 0 target items. As can be seen in Table 2, it suggests that an increase in the number of

interfering words with the same word stem results in a decreasing pro- portion of target words generated in the word stem completion task.

Next we turn to the proportions of interfering words produced as a

response in the word stem completion test. Table 2 gives both the total and the mean proportions of interfering words generated in the stem

completion test for the various category conditions. The mean propor- tions are derived by dividing the total proportions by 1, 3, or 5, respec- tively.

An ANOVA was done on the mean numbers of interfering words that were generated to the word stem cue in Category 1, 3, and 5 conditions

(dividing the total number generated in each condition by the number of overlapping words in that condition). This analysis showed a signifi- cant main effect of the number of overlapping word stems, F(2, 62) = 8.79, MSE= 0.89, p = .01. A post hoc pairwise comparison of these means with a Newman-Keuls test showed that the mean of Category 1 differed

significantly from the mean of Category 3, p < .05, and Category 5, p < .01, but the latter two did not differ. As Table 2 shows, increasing the number of words with the same word stem reduces the chances for each of these words to be given as a completion in the word stem comple- tion task because of mutual competition. Clearly, the amount of inter- ference is a function of the number of words with the same word stems. The repetition priming effect (both for targets and interfering words) decreases as the number of primed words sharing the same word stem increases. Averaged over targets and interpolated words, repetition priming (i.e., completion proportion - base rate) decreases from .14

(target words only), .10 (target and one interpolated word with the same word stem), .06 (target and three interpolated words with the same word stem) to .04 (target and five interpolated words with the same word stem). Obviously, the limiting case is that interference is maximal (i.e., reducing priming to zero) if all words that are possible completions to a word stem would have been presented, and thus primed, before test-

ing. Interference between targets and interpolated words is bidirection- al. The amount of mutual interference (i.e., reduction of priming) is about the same for targets and interpolated words in each interference category. So there is as much proactive interference as retroactive in- terference.

The results from the explicit memory test (cued recall) will not be discussed here because these data may be confounded by the preced-

179

ing word stem completion test and by the instruction that encouraged guessing. Instead, a separate study was performed with 24 subjects us- ing exactly the same material and procedures, which was followed im- mediately by a standard cued recall task requiring recall of target list items only using word stems as cues. This task is reminiscent of the clas- sic A-B, A-C paradigm for studying retroactive interference. Instead of newly learned stimulus-response pairs, however, preexisting lexical units were presented and strengthened. An ANOVA on the number of correctly recalled target words showed a significant effect of the number of in-

terpolated words, F(3, 92) = 3.54, MSE = 3.37, p < .05. The proportions of correctly recalled target words were .19, .20, .11, and .12 with 0, 1, 3, and 5 interpolated words having the same word stem, respectively. Clear-

ly, performance in this cued recall test also suffers from interference, increasing with more interpolated words with similar word stems. A small number of intrusion errors, increasing with the number of inter-

polated words with the same word stems, indicates that this too may be a response competition effect.

GENERAL DISCUSSION

The concept of interference refers to the phenomenon that learning preceding or subsequent lists has a negative effect on the memory per- formance of a target list. In two experiments we have tested and refut- ed the claim that implicit memory tests are not susceptible to interfer- ence effects. Interference does occur in an implicit word stem

completion test, but only when interpolated words fit the test cue, that is, have word stems in common with target words. Increasing the num- ber of interpolated words with the same word stems produced an in-

creasing amount of interference. The interference effects reported here are highly specific. They are limited to the situation in which cues share lexical (and possibly perceptual) properties with both target and inter-

polated stimuli. This finding seems consistent with the characterization of implicit memory as hyperspecific (e.g., Tulving & Schacter, 1990).

The results provide strong evidence that interference in word stem completion is a response competition phenomenon, probably occurring at a lexical level (see Nelson et al., 1989; Weldon, 1991). Following Bower (1996), and Graf and Mandler (1984), we suggest that repetition priming results from the automatic strengthening of representations (i.e., intraitem associations) during original processing. If more repre- sentations with the same word stem are strengthened at a lexical level, they all get a higher probability of being accessed and selected in a word stem completion task. Because only one response is allowed, this will

180 MARTENS AND WOLTERS

INTERFERENCE IN IMPLICIT MEMORY

inevitably result in an increase of mutual response competition, caus-

ing an interference effect. If completions of interpolated words increase, completions for the target must necessarily go down. No such response competition at a lexical level is to be expected from strengthened word representations that do not match the cue presented at test. This ex-

plains the absence of interference effects in word stem and word frag- ment completion tasks using interpolated lists of words not fitting the cues given at test.

The argument that interference in implicit memory tests is to be

expected only if the interpolated material matches the cue information may also be used to explain the large variability in the decline of repe- tition priming over retention intervals. If it is assumed that the decline of priming over retention intervals is caused by interference from com- petitors, then a faster decline might be expected in tasks using cues that correspond to a greater number of possible responses. At least some experimental findings seem to support this reasoning. Long-lasting rep- etition priming effects seem to be found mainly in tests using unique cues, as in picture naming (Mitchell & Brown, 1988), unique word stem

completion (Warrington & Weiskrantz, 1978), and unique word frag- ment completion (Roediger & Blaxton, 1987; Sloman et al. 1988; Tulv- ing, Schacter, & Stark, 1982). In contrast, studies using word stem com- pletion tests and stems with many possible completions have often found a complete loss of priming after 2 hours or less (e.g., Chen & Squire, 1990; Graf & Mandler, 1984; Graf, Squire, & Mandler, 1984; Mandler, Graf, & Kraft, 1986; but see also Roediger, Weldon, Stadler, & Riegler, 1992, Experiments 1 and 2). If word stems are used that have many possible completions, chances are that over time other words fitting the cues are activated and strengthened (both intraexperimentally and extraexperimentally), and the accumulated interference would explain the faster loss of priming.

We suggest that the probability of generating a particular word as a response in a word stem completion test is based on the activation strength of that word relative to the total activation of all other match- ing words. This implies that the interference effect observed here is a pure response competition phenomenon that may be described using Luce's choice axiom (Luce, 1959). To see whether such a model can account for the data, we need a few simple assumptions. First, assume that each stem has an average of N completions, all having equal re- sponse strength. With a normative completion rate of 0.06 (as used throughout the experiments), this means N= 1/0.06, or 17 responses on average. Second, assume that each response has an initial relative strength of 1.0 and that this relative strength increases to S (S > 1) when a word is presented and primed. Third, assume that when asked to com-

181

plete a word stem, subjects choose one completion according to Luce's axiom (i.e., with a probability that equals the strength of one comple- tion relative to the sum of the strengths of all others). This means that if only a target word is primed with strength S, it will be chosen with

probability S/(S + 16). If not only the target word but also Idifferent

competing words with the same stem are strengthened to the same lev- el S, the probability of choosing the target word becomes S/(S + IS + 16 - 1). Moreover, this is also the probability that any of the competing words will be given.

These equations can then be applied to the implicit stem completion results of both experiments. From the target only conditions S can be estimated, and this can be used to predict the probability of generat- ing the target (and the interpolated) words. For example, in Experiment 1 p(target) = .28 = S/(S+ 16), so S is estimated to be 6.22. With two

competing responses strengthened, p(target) = S/(S+ 2S+ 14) = .19. This is exactly the proportion observed. The predicted proportions for the interfering items are also .19, which is actually somewhat lower than the observed proportions (.23 and .25).

For the data of Experiment 2 the predicted proportions of giving a

target (or an interfering word) when there are 1, 3, or 5 interfering items are .17, .14, and .11, respectively. Comparing these proportions with the observed proportions (see Table 2) shows a remarkably good fit given estimation of only two parameters (Nand S), which indicates that the results can be described quite well with a response competition process.

In implicit memory (at least as measured in a word stem completion test) interference seems to be exclusively a response competition phe- nomenon, whereas in explicit memory this is only one of several factors

contributing to interference and memory-blocking phenomena (e.g., Smith & Tindell, 1997). What causes interference and retrieval inhibi- tion is a longstanding question in memory research (e.g., Anderson &

Bjork, 1994; Barnes & Underwood, 1959; Crowder, 1976). Generally, it is assumed that associations never go away once learned but merely become inaccessible. However, the possibility of actual unlearning and decrements of associative strength has been suggested (e.g., Melton & Irwin, 1940; Anderson & Green, 2001), and its plausibility has increased

by recent neurophysiological findings indicating the presence of a mechanism that may reduce synaptic efficacy (Stanton & Sejnowski, 1989). It may be noted also that such a mechanism is often assumed in the learning rules applied in neural network models. The fact that only response competition may be responsible for interfering effects in word stem completion raises the interesting possibility to see if there is an ab-

182 MARTENS AND WOLTERS

INTERFERENCE IN IMPLICIT MEMORY

solute or a relative loss of intraitem associative strength of primed words as partly overlapping words are subsequently presented. To test this ques- tion, subjects should be allowed to generate not one but several com-

pletions in a word stem completion task (making the task comparable with the classic modified-modified-free-recall task). If there is only a relative loss of strength, interfering effects should disappear. However, if presentation of subsequent words causes an absolute loss of strength, considerable interference should still occur. Further studies addressing this question are under way.

It is still widely believed that implicit memory is resistant to interfer- ence caused by interpolated material. The current experiments show that this is not true. The main reason for failures to find such interfer- ence in the past is that the conditions for interference were not set in

place: Namely, for interference to occur, several active responses should fit the same cue and compete for selection.

Notes

This research was partly supported by a grant from the Dutch Organization for Scientific Research. We thank Carolien van Ammers, Robert Berg, Carolien van den Hoek, and Marijn Peeters for assistance in running the experiments. We also want to thank G. H. Bower, D. E. Dulany, P. Graf, L. L. Jacoby, N. W.

Mulligan, J. H. Neely, D. L. Nelson, S. M. Smith, and M. S. Weldon for their useful comments on earlier versions of this manuscript. We are especially grate- ful to G. H. Bower for suggesting the quantitative description of the results with a response competition model.

Correspondence about this article should be addressed to Gezinus Wolters, Department of Psychology, Leiden University, P.O. Box 9555, 2300 RB Leiden, the Netherlands (e-mail: wolters@fsw.leidenuniv.nl). Received for publication March 22, 2000; revision received October 2, 2000.

References

Anderson, M. C., & Bjork, R. A. (1994). Mechanisms of inhibition in long-term memory. In D. Dagenbach & T. H. Carr (Eds.), Inhibitory processes in atten- tion, memory, and language (pp. 265-325). San Diego: Academic Press.

Anderson, M. C., & Green, C. (2001). Suppressing memories by executive con- trol. Nature, 410, 366-369.

Barnes,J. M., & Underwood, B.J. (1959). "Fate" of first-list associations in trans- fer theory. Journal of Experimental Psychology, 58, 97-105.

Bower, G. H. (1996). Reactivating a reactivation theory of implicit memory. Consciousness and Cognition, 5, 27-72.

Chen, K S., & Squire, L. R. (1990). Strength and duration of word-completion priming as a function of word repetition and spacing. Bulletin of the Psy- chonomic Society, 28, 97-100.

183

Crowder, R. G. (1976). Principles of learning and memory. Hillsdale, NJ: Erlbaum. Graf, P., & Mandler, G. (1984). Activation makes words more accessible, but not

necessarily more retrievable. Journal of Verbal Learning and Verbal Behavior, 23, 553-568.

Graf, P., & Schacter, D. L. (1987). Selective effects of interference on implicit and explicit memory for new associations. Journal of Experimental Psycholo- gy: Learning, Memory, and Cognition, 13, 45-53.

Graf, P., Squire, L. R., & Mandler, G. (1984). The information that amnesic

patients do not forget. Journal of Experimental Psychology: Learning, Memory, and Cognition, 10, 164-178.

Jacoby, L. L. (1983). Perceptual enhancement: Persistent effects of an experi- ence. Journal of Experimental Psychology: Learning, Memory, and Cognition, 9, 21-38.

Luce, R. D. (1959). Individual choice behaviour: A theoretical analysis. New York:

Wiley. Mandler, G., Graf, P., & Kraft, D. (1986). Activation and elaboration effects in

recognition and word priming. Quarterly Journal of Experimental Psychology, 38A, 645-662.

Melton, A. W., & Irwin, J. M. (1940). The influence of degree of interpolated learning on retroactive inhibition and the overt transfer of specific respons- es. American Journal of Psychology, 53, 173-203.

Mitchell, D. B., & Brown, A. S. (1988). Persistent repetition priming in picture naming and its dissociation from recognition memory. Journal of Experimen- tal Psychology: Learning, Memory, and Cognition, 14, 213-222.

Nelson, D. L., Keelean, P. D., & Negrao, M. (1989). Word-fragment cuing: The lexical search hypothesis. Journal of Experimental Psychology: Learning, Mem- ory, and Cognition, 15, 388-397.

Nelson, D. L., Schreiber, T. A., & Holley, P. E. (1992). The retrieval of controlled and automatic aspects of meaning on direct and indirect tests. Memory &

Cognition, 20(6), 671-684. Phaf, R. H., & Wolters, G. (1991). Spontaneous completion frequencies for 168 two

and three letter word stems. Internal report, Leiden University. Postman, L., & Underwood, B.J. (1973). Critical issues in interference theory.

Memory & Cognition, 1, 19-40. Ratcliff, R., McKoon, G., & Verwoerd, M. (1989). A bias interpretation of facil-

itation in perceptual identification. Journal of Experimental Psychology: Learn- ing, Memory, and Cognition, 15, 378-387.

Reingold, E. M. (1995). Facilitation and interference in direct/implicit mem-

ory tests in the process dissociation paradigm: The letter insertion and the letter deletion tasks. Consciousness and Cognition, 4, 459-482.

Richardson-Klavehn, A., & Bjork, R. A. (1988). Measures of memory. Annual Review of Psychology, 39, 475-543.

Roediger, H. L., & Blaxton, T. A. (1987). Effects of varying modality, surface features, and retention interval on priming in word fragment completion. Memory & Cognition, 15, 379-388.

Roediger, H. L., & McDermott, K. B. (1993). Implicit memory in normal hu-

184 MARTENS AND WOLTERS

INTERFERENCE IN IMPLICIT MEMORY

man subjects. In F. Boiler & I. Grafman (Eds.), Handbook of neuropsychology (Vol. 8). Amsterdam: Elsevier.

Roediger, H. L., Weldon, M. S., Stadler, M. L., & Riegler, G. L. (1992). Direct

comparison of two implicit memory tests: Word fragment and word stem completion. Journal of Experimental Psychology: Learning, Memory, and Cogni- tion, 18, 1251-1269.

Schacter, D. L. (1987). Implicit memory: History and current status. Journal of Experimental Psychology: Learning, Memory, and Cognition, 13, 501-518.

Sloman, S. A., Hayman, C. A. G., Ohta, N., Law,J., & Tulving, E. (1988). For- getting in primed fragment completion. Journal of Experimental Psychology: Learning, Memory, and Cognition, 14, 223-239.

Smith, S. M., & Tindell, D. R. (1997). Memory blocks in word fragment com- pletion caused by involuntary retrieval of orthographically related primes. Journal of Experimental Psychology: Learning, Memory, and Cognition, 23, 355- 370.

Stanton, P. K., & Sejnowski, T.J. (1989). Associative long-term depression in the hippocampus induced by Hebbian covariance. Nature, 339, 215-218.

Tulving, E., & Schacter, D. L. (1990). Priming and human memory systems. Science, 247, 301-306.

Tulving, E., Schacter, D. L., & Stark, H. A. (1982). Priming effects in word-frag- ment completion are independent of recognition memory. Journal of Ex- perimental Psychology: Learning, Memory, and Cognition, 8, 336-342.

Warrington, E. K., & Weiskrantz, L. (1978). Further analysis of the prior learn- ing effect in amnesic patients. Neuropsychologia, 16, 169-177.

Weldon, M. S. (1991). Mechanisms underlying priming on perceptual tests. Journal of Experimental Psychology: Learning, Memory, and Cognition, 17, 526- 541.

185