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Journal of Experimental Psychology: Learning, Memory, and Cognition Retroactive Interference Effects in Implicit Memory Deborah K. Eakin and Robert Smith Online First Publication, February 13, 2012. doi: 10.1037/a0027208

CITATION Eakin, D. K., & Smith, R. (2012, February 13). Retroactive Interference Effects in Implicit Memory. Journal of Experimental Psychology: Learning, Memory, and Cognition. Advance online publication. doi: 10.1037/a0027208

Journal of Experimental Psychology: Learning, Memory, and Cognition 2012, Vol. ●●, No. ●, 000 – 000

© 2012 American Psychological Association 0278-7393/12/$12.00 DOI: 10.1037/a0027208

RESEARCH REPORT

Retroactive Interference Effects in Implicit Memory Deborah K. Eakin

Robert Smith

Mississippi State University

Florida State University

One source of evidence for separate explicit and implicit memory systems is that explicit but not implicit memory is impacted by interference (e.g., Graf & Schacter, 1987). The present experiment examined whether retroactive interference (RI) effects could be obtained in implicit memory when a strong test of RI was used. People studied an original list of word pairs (e.g., COTTON–PRIZE) using the typical RI paradigm. During the interpolated phase, participants studied either interference pairs for which the same cue was re-paired with a different target (e.g., COTTON–PRINT) or novel pairs (e.g., HOST–VASE). RI was tested with the modified opposition cued recall test (Eakin, Schreiber, & Sergent-Marshall, 2003). The original-list cue was presented along with the beginning stem of its target (e.g., COTTON–PRI–) and a hint (e.g., not PRINT). RI effects were obtained for explicit and implicit memory. Taken together with prior research finding proactive interference effects in implicit memory, the findings indicate that implicit memory is not immune from retroactive interference. Keywords: retroactive interference, implicit memory, retrieval blocking

original and an interpolated phase. Memory is tested for the target paired with the cue during the original phase; the degree to which memory is impaired by studying a second target during the interpolated phase is called the RI effect. The RI effect has been attributed to response competition: The targets from the original and interpolated phases compete for retrieval, given the cue (e.g., McGeoch, 1942; Melton & Irwin, 1940; Postman, 1971; Postman, Stark, & Fraser, 1968; Raaijmakers & Shiffrin, 1981). If RI effects are obtained in explicit but not in implicit memory, the finding would support a separate systems view (e.g., Schacter, 1987).

A long-debated issue in memory research concerns whether memory comprises a single system or multiple systems (see Squire, 2004, for a review). Tulving and Schacter (1990) identified separate declarative and nondeclarative memory systems, also called explicit and implicit memory, respectively (Polanyi, 1952; Tulving, 2002; for reviews, see Roediger, Guynn, & Jones, 1994; Schacter, 1987; Schacter, Chiu, & Ochsner, 1993). Explicit memory is conscious, intentionally accessed memory, whereas implicit memory is automatic and unconscious. Evidence for separate explicit and implicit memory systems arises from a variety of domains, such as patient studies (e.g., Schacter & Buckner, 1998; Shimamura & Squire, 1984; Tulving & Schacter, 1990) and studies of cognitive aging (e.g., Mitchell, 1989). In addition, a large body of research has suggested that retrieval interference impacts explicit but not implicit memory (Graf & Schacter, 1987; Jacoby, 1983, Experiment 4; Lustig & Hasher, 2001a; Nelson, Keelean, & Negrao, 1989; Pilotti, Chodorow, & Tan, 2004; Ratcliff & McKoon, 1997; Sloman, Hayman, Ohta, Law, & Tulving, 1988, Experiments 5 and 6; Winocur, Moscovitch, & Bruni, 1996; for a review, see Anderson & Neely, 1996). One type of retrieval interference, retroactive interference (RI), occurs when learning new information hinders the ability to retrieve previously learned information. The typical RI paradigm pairs two targets with a single cue in two study sessions: an

Implicit Memory and RI Because implicit memory is a separate system from explicit memory, an explanation for the dissociation between implicit and explicit memory under RI is that each system has a unique retrieval process that is impacted by interference to different degrees. Squire and Cohen (1984) proposed that retrieval from explicit memory requires deliberate recollection. In particular, explicit memory requires the intentional use of the encoding context to identify an activated representation as belonging to a particular episode. At retrieval, explicit memory requires intentional retrieval of the representation of a specific target as it occurred within a specific context (e.g., paired with a given cue on the original list; Dorfman & Mandler, 1994; Graf & Mandler, 1984; Mandler, 1980). This intentional retrieval could produce RI when a cue has been associated with multiple targets. By contrast, retrieval from implicit memory is marked by unconscious, automatic processes that do not require effortful recollection, such as completing a word fragment on an indirect test with the first word that comes to mind. Dorfman and Mandler (1984) further suggested activation as the theoretical mechanism for automatic retrieval from implicit memory. They proposed that the representation of a target item was activated during encoding

Deborah K. Eakin, Department of Psychology, Mississippi State University; Robert Smith, Department of Sports Management, Florida State University. Correspondence concerning this article should be addressed to Deborah K. Eakin, Mississippi State University, Department of Psychology, P.O. Box 6161, Mississippi State, MS 39762. E-mail: deakin@psychology .msstate.edu 1

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and was readily available for retrieval when cued with its word fragment. Because intentional reliance on the context to distinguish the original from the interpolated phase is unnecessary for successfully completion of the word fragment, targets from the interpolated phase do not compete for retrieval on an indirect test. Therefore, interference effects are obtained only on tests of explicit memory that rely on successful discrimination between the original and the interpolated study phase. These theoretical predictions were supported by research finding that RI effects were not obtained in implicit memory. For instance, Graf and Schacter (1987) found RI effects in explicit but not implicit memory. In this experiment, participants studied unrelated word pairs (e.g., SHIRT–WINDOW) during the original and interpolated phases of the typical RI paradigm (half of the cues were re-paired with new targets during this phase; e.g., SHIRT– FINGER). Recall was prompted with the cue from the original study phase paired with the stem of its corresponding target (e.g., SHIRT–WIN—). For the direct test of explicit memory, participants were told to complete the stem with the target word from the original phase. For the indirect test of implicit memory, participants were told to complete the stem with the first word to come to mind. Graf and Schacter obtained the typical RI effect on the direct test, indicating that the interpolated list target interfered with explicit memory for the original list target; however, they did not obtain a RI effect on the indirect test. They concluded that interference impacted explicit but not implicit memory. Graf and Schacter (1987) interpreted this finding as evidence that implicit and explicit memory are separate memory systems (see also Jacoby, 1983; Sloman et al., 1988). However, methodological problems make this conclusion questionable. For instance, the target (e.g., FINGER) that presumably could have provided RI for retrieval of the original target (e.g., WINDOW) was not a potential candidate for completion on the word stem used to test memory (e.g., WIN—). The only possible candidate for completion was the target presented in the original phase, which made it difficult to examine the potential influence of interference from the target from the interpolated phase (for a discussion, see Lustig & Hasher, 2001b). Eliminating response competition also eliminated the key theoretical explanation for RI (e.g., McGeoch, 1942; Melton & Irwin, 1940; Postman, 1971; Postman et al., 1968; Raaijmakers & Shiffrin, 1981). RI may not have been obtained in implicit memory because the manipulation of interference was too weak to be induced on the indirect test.

used to test both implicit and explicit memory. Retrieval blocking occurs when the interpolated target is retrieved and must be disregarded in order to retrieve the original target. The use of the MOT (Eakin et al., 2003) allowed for an examination of retrieval blocking as the theoretical mechanism for any interference effects obtained. The MOT consisted of a cue plus target-beginning stem to prompt recall (e.g., COTTON–PRI—). A hint was also provided indicating a word that should not be used to complete the word stem (e.g., [not PRINT]); the hint was the competing word from the interpolated list. The presence of the hint on the MOT allowed for a strong test of RI by increasing the likelihood of response competition in the form of retrieval blocking. Reading the hint strengthens the association between the cue and the interpolated target, which was established when studying the interpolated-list pairs. The result of the strengthened association is an increased likelihood of retrieving the interpolated target, given the cue. Because the hint on the MOT ensures retrieval of the interpolated target, the likelihood of response competition due to retrieval blocking is high (Eakin, 2005; Eakin et al., 2003). Attempts to avoid providing the hint on the test leads to repeated retrieval of the interpolated target, which further strengthens the association between the cue and target from the interpolated list. Access to the original target is blocked, and the result is RI effects due to retrieval blocking (see Gillund & Shiffrin, 1984; Raaijmakers & Shiffrin, 1981). Using the MOT resolves the methodological concern that previous manipulations of interference were not strong enough to be measured in implicit memory. The use of the MOT sets up the strongest potential for response competition due to retrieval blocking, which we hypothesized should induce RI in memory. Additionally, the hint on the MOT guided participants toward the original list, which is especially necessary to test implicit memory. Even if the first word that came to mind was the target from the interpolated phase, the hint directed participants not to answer with that word, thereby implicitly directing them to the original list. Also, because the hint guided participants to the original list, using the MOT allowed that the test target-beginning stem could be completed with both the original and the interpolated target. Finally, we did not assume that the test of implicit memory was not influenced by explicit memory; rather we administered a postexperimental questionnaire to check for awareness that beginning word stems on the test referred to targets encountered during the study phase.

Current Study

Method

In order to fully test whether RI effects due to response competition are obtained in implicit memory, we tested both explicit and implicit memory in the current study. RI effects were examined in the current experiment with the typical RI paradigm. During the original phase, participants viewed unrelated cue–target pairs (e.g., COTTON–PRIZE) with the incidental instructions to rate their ability to mentally form an interactive image of each pair. During the interpolated phase, participants studied either interference pairs for which the same cue was re-paired with a different target (e.g., COTTON–PRINT) or novel pairs (e.g., HOST–VASE). The modified opposition test (MOT; Eakin, Schreiber, & SergentMarshall, 2003), which was designed to isolate retrieval blocking—a particularly strong form of response competition—was

Participants and Design The design was a 2 (Retroactive Interference: control, interference) ⫻ 2 (Memory Type: explicit, implicit) mixed factorial design, with RI manipulated within subjects and memory type manipulated between subjects. Participants were recruited from the student participant pool at Mississippi State University and received course credit in exchange for participation. The number of participants in each analysis varied, as discussed in the Results section; however, overall 186 students participated in the study. Five participants were removed from the study because they failed to recall even one control item.

RETROACTIVE INTERFERENCE IN IMPLICIT MEMORY

Materials Manipulation check. In a postexperiment interview, participants in the implicit memory condition were asked whether they were aware of the association between the targets in the study phase and the target word stems on the final test. First, participants were asked, “Did you notice anything about the experiment that you would like to comment on? Yes/No.” If participants selected “yes,” they were invited to specify what they noticed. To avoid the “knew it all along” effect (Fischhoff, 1982), regardless of whether they indicated a “yes” or “no” response, we asked the second question on the back of the sheet: “Did you notice that the stems on the test could be completed with words from the interactive imagery portion of the experiment?” Participants chose between two possible responses: “I recognize that now that you mention it” and “I noticed that while I was completing the word stems.” They also had the option of not responding if neither of the statements were correct. Encoding phase items. The word list consisted of 96 cue– target word pairs. Pairs were chosen from the University of South Florida Free Association Norms (Nelson, McEvoy, & Schreiber, 1998). Of the 96 cue–target word pairs in the list, six pairs were used as practice items. The remaining 90 pairs were divided into two sets (A and B) of 30 pairs each, a control set of 15 pairs, and a set of 15 foil pairs. Sets A and B were each divided into two subsets (A1, A2, B1, B2). Each cue from a particular subset (i.e., A1) was re-paired with a target in the second subset (i.e., A2) that shared a beginning stem. A separate set of 15 cue–target pairs was selected to serve as control items to be used in the interpolated phase. Also, a set of 15 pairs was selected to serve as test foils in order to protect against awareness that the test items were actually studied during the original study phase. Table 1 shows the labeling schematic, as well as examples for each set. Each of the sets and the foil set were equated in terms of several characteristics that have been demonstrated to impact memory. Because all items were unrelated, the associative strength between cues and targets equaled zero in all of the sets. In addition, the number of shared associates was equated between subsets and across sets (Nelson et al., 1998). ListChecker Pro 1.2 (Eakin, 2010) tested for associations among words on the list. For the purpose of counterbalancing across each set and subset, the two sets (A and B) served equally often in the two RI experimental conditions (control and interference). However, as a control over the mean values across and within sets, items within a set and subset were always part of that set or subset only and were not randomly assigned to a set. The mean values of additional characteristics of words (e.g., printed word frequency, semantic set size, concreteness, mean connectivity) were equated both between the subsets and across the sets, including the control and foil sets.1

Table 1 Labeling Schematic Original phase

Interpolated phase

Test phase

Set A1 Set A2 COTTON–PRIZE COTTON–PRINT COTTON–PRI— (not PRINT) Set B1 Control FLAME–PILL HOST–VASE FLAME–PI— (not PINE)

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The subsets were counterbalanced such that each served in the original and interpolated phases equally often, with the control set always appearing during the interpolated study phase. Each participant was presented with a randomized combination of two subsets during each study phase, resulting in a total of 30 items for each session. These two subsets, however, could not be derived from the same set (e.g., A1 and A2 were never presented during the same phase). Target selection was determined by criteria set for test items. Test items. For the recall phase, the test item was the cue from the original study phase paired with the beginning stem of the target with which it was paired in the original study phase. Target selection had to meet several criteria. First, for interference items, each stem had to be able to be completed by both the original and the interpolated targets. Second, the beginning forward strength, or the probability that the target would be used to complete the beginning stem, had to have a mean of .10 within each subset. Therefore, the probability of completing the beginning stem with the original versus the interpolated target was equated, serving as the baseline for beginning-stem completion. Third, the mean beginning set size (Appendix B of the University of South Florida norms), or the number of potential targets that could complete the stem, was equated between the original and interpolated phases and across the sets. Because the MOT was used, the beginning stems were accompanied by a hint, which was the interpolated phase target. This hint indicated a word that should not be used to complete the stem (see Table 1). The set of 15 foils was randomly intermixed with the critical test items on the test in order to disguise the fact (on the indirect test) that the target beginning stems could be completed with studied items.

Procedure Participants were randomly assigned to one of the two betweensubjects conditions: explicit or implicit memory. The experiment was presented on standard PCs with E-Prime software. Participants in both memory conditions began the experiment with the original study phase. The cue–target pairs were presented on the computer screen for a period of 6 s each. Participants were given incidental instructions indicating that we were interested in their ability to form interactive images of the two words; clarity of the image was rated as vivid, neutral, or none (the rating was not collected). The interpolated study phase followed the same procedure as the original study phase. Due to a within-subjects manipulation, participants were presented with the items both from the control set and from the appropriate subset for the interference condition (depending on the counterbalancing scheme). Items were presented randomly during the original phase and in a different random order during the interpolated phase. During the recall phase, the cue from the original study phase was presented along with the beginning stem of the target with which it was paired during that phase (e.g., COTTON–PRI— [not PRINT]). Participants in the explicit memory condition received instructions to complete the beginning stem with the target they remembered being paired with the cue during the original study 1

The word lists and mean characteristics are available as an appendix at www.eakinmemorylab.psychology.msstate.edu

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phase. Participants in the implicit memory condition were instructed to complete the beginning stem with the first word that came to mind. Both groups were further instructed not to complete the beginning stem with the word presented in the hint. Participants were given unlimited time to type their responses. During filled intervals of 10 min placed between study phases and before the recall phase, participants engaged in a nonverbal, mental rotation task. Before they left, participants completed the postexperiment interview and were debriefed.

Results Manipulation Check Of the 181 students remaining the in the analyses, 100 were in the explicit memory condition and 81 were in the implicit memory condition. In the implicit memory condition, five participants were eliminated because they failed to answer the postexperimental questionnaire; 12 participants were eliminated because they answered the first question by circling “yes,” indicating that they noticed something about the experiment, regardless of their subsequent response on the questionnaire. The following analyses included 100 participants in the explicit memory condition and 64 participants in the implicit memory condition, all of whom provided a “no” response to the first part of the postexperimental questionnaire.

Counterbalancing Check To ensure that the counterbalancing procedure did not impact the results, we conducted an analysis of variance (ANOVA) on probability of recall crossing list (1– 8) with interference condition and memory type. Probability of recall did not differ among the eight lists (F ⬍ 1.00); all subsequent analyses were collapsed across list.

A 2 (Interference Condition: control, interference) ⫻ 2 (Memory Type: explicit, implicit) repeated measures ANOVA was conducted on probability of recall. The means are reported in Table 2. Probability of recall was better in the control condition (M ⫽ .40, SEM ⫽ .02) than in the interference condition (M ⫽ .27, SEM ⫽ .02); significant RI effects were obtained overall, F(1, 162) ⫽ 58.06, p ⬍ .001, ␩2p ⫽ .26. The main effect of memory type was not significant (p ⫽ .11). Probability of recall was similar in the

Table 2 Probability of Recall for Each Experimental Condition Explicit

Initial analysis Control Interference Secondary analysis Control Interference

Secondary Manipulation Check Analysis Sometimes people who indicated on the first question of the postexperimental questionnaire that they did not notice anything changed their response on the second question. Although they selected the “no” response on the first question, indicating that they did not notice anything about which they wanted to comment, they indicated on the second question that they noticed that target word stems could be completed with words from the study phase while they were completing the recall test. Participants in the implicit memory condition were included in this secondary analysis only if they answered “no” on the first and the second questions on the postexperimental questionnaire. When this criterion was applied, an additional 32 participants were removed. The secondary analysis was then conducted on the remaining 32 participants. A second 2 (Interference Condition: control, interference) ⫻ 2 (Memory Type: explicit, implicit) repeated measures ANOVA was conducted on probability of recall. Probability of recall was better in the control condition (M ⫽ .40, SEM ⫽ .02) than in the interference condition (M ⫽ .26, SEM ⫽ .02); significant RI effects were obtained overall, F(1, 130) ⫽ 41.59, p ⬍ .001, ␩2p ⫽ .24. The main effect of memory type was not significant (p ⫽ .14). Again, probability of recall did not vary between memory conditions (M ⫽ .36, SEM ⫽ .02 and M ⫽ .31, SEM ⫽ .02 for explicit and implicit memory, respectively). Significant RI effects were obtained in explicit and implicit memory (see Table 2); the interaction between the interference and memory type conditions was not significant (p ⫽ .16).

Discussion

Probability of Recall

Condition

explicit and implicit memory condition (M ⫽ .36, SEM ⫽ .02 and M ⫽ .32, SEM ⫽ .02, respectively). Importantly, the interaction between the two factors was not significant (F ⬍ 1). Significant RI was obtained in explicit and implicit memory (see Table 2).

Implicit

M

SD

M

SD

.41 .30

.20 .20

.39 .24

.19 .16

.41 .30

.20 .20

.40 .22

.20 .14

Overall, memory was relatively poor. On average, only 33% of the items were correctly recalled, indicating the difficulty of the task. The word-stem completion rate overall, however, was over three times that of the mean completion rate of 10% achieved according to the norms, demonstrating that the study sessions impacted word-stem completion rates in the experiment. That RI effects were obtained in explicit memory was not surprising. As discussed, the finding of lower probability of recall in the interference than in the control condition is a robust phenomenon in explicit memory (Crowder, 1976; Eakin, 2005; McGeoch, 1942; McGeoch & McDonald, 1931; Melton & Irwin, 1940; Underwood, 1957). Because the MOT (Eakin et al., 2003) was used, these effects can be attributed to response competition, particularly retrieval blocking. The critical finding was that RI effects were obtained in implicit memory. The fact that RI also was found in explicit memory allays the criticism of prior research that failed to obtain RI in explicit memory (e.g., Sloman et al., 1988). In the present study, RI was obtained in both explicit and implicit memory with the same materials and participants. Two methodological features of our study— both achieved by using the MOT (Eakin et al., 2003)— could explain why we obtained RI effects in implicit memory

RETROACTIVE INTERFERENCE IN IMPLICIT MEMORY

when others have not. First, because the hint on the MOT consisted of the item that was most likely to cause RI— due to response competition in general and retrieval blocking in particular—the MOT provided a strong manipulation of interference. RI was obtained for explicit memory using the MOT, demonstrating that this test effectively induced retrieval blocking. RI also was obtained for implicit memory using the MOT, providing strong evidence that implicit memory is not resistant to strong manipulations of response competition. Second, the competing targets not only appeared to be candidates for completion of the test question; both could actually complete the word stem presented at recall because the competing targets shared a beginning stem. Using the MOT made this possible because the hint allowed for an implicit indication to complete the target beginning stem with the target from the original list. The few other studies that have obtained interference effects in implicit memory examined proactive interference (e.g., Lustig & Hasher, 2001a; Pilotti et al., 2004; Winocur et al., 1996) or had methodological problems that prevented a clear conclusion about the influence of interference on implicit memory (e.g., Nelson et al., 1989). Pilotti et al. found proactive but not retroactive interference in implicit memory. They attributed the difference to the strength of the representation of the to-be-remembered target; the target was proposed to be stronger when it was studied first in an RI paradigm as opposed to second in a proactive interference paradigm. If this was the case, we suggest, their manipulation of interference was not strong enough to elicit RI and override the differential strength of the target in a proactive versus retroactive interference paradigm. However, our view is that the more impactful factors explaining our findings versus prior research are methodological. As evidence, the MOT has been demonstrated to elicit interference even when the interfering information is presented prior to the tested information (see Eakin et al., 2003, Experiment 2). RI effects also could have been compromised because, as with Graf and Schacter (1987), the test fragments used by Pilotti et al. could not actually be completed by the interpolated target word. Tulving and Schacter (1990) cited a dissociation of memory performance across systems in terms of materials and task manipulations as a criterion of multiple systems. Previously, the absence of interference effects in implicit memory (e.g., Graf & Schacter, 1987) has been used to support the view that memory consists of multiple systems, such as explicit and implicit memory (e.g., Schacter, 1987). However, more recent findings that implicit and explicit memory are similarly impacted by PI (e.g., Lustig & Hasher, 2001a; Pilotti et al., 2004; Ratcliff & McKoon, 1997; Winocur et al., 1996) and by RI, as demonstrated in the present study, eliminate differential effects of interference from the list of evidence for separable systems. This finding alone does not—and is not intended to—invalidate the multiple systems view. Many other findings provide support for multiple memory systems, including a distinction between implicit and explicit memory (e.g., Graf & Mandler, 1984; Tulving, Schacter, & Stark, 1982). Immunity from interference simply does not appear to be one of them.

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Received June 16, 2010 Revision received December 15, 2011 Accepted January 2, 2012 䡲