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Can intentional forgetting reduce false memory? Effects of list-level and item-level forgetting
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Acta Psychologica 127 (2008) 146–153 www.elsevier.com/locate/actpsy
Can intentional forgetting reduce false memory? Effects of list-level and item-level forgetting Yuh-shiow Lee
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Department of Psychology, National Chung-Cheng University, Chiayi 621, Taiwan, ROC Received 18 September 2006; received in revised form 20 March 2007; accepted 26 March 2007 Available online 1 May 2007
Abstract This study examined whether false memory produced by the learning of lists of categorized and associative materials could be reduced by directed forgetting. The number of items within a list that participants were asked to remember or forget was manipulated, while the length of the list remained constant. Experiment 1a used categorized lists and Experiment 1b used associative lists; the participants performed immediate free recall and cued recall tests. For both the categorized and associative lists, the rate of false recall increased upon increasing the proportion of ‘‘forget’’ (F) words. After removing the immediate recall test, Experiment 2 found that intentionally forgetting part of the studied list reduced false memory, whereas forgetting the whole studied list did not. The results are discussed in terms of the list-level-vs.-item-level inhibition in semantic activation and the role of monitoring in reducing false memory. Ó 2007 Elsevier B.V. All rights reserved. PsycINFO classification: 2340; 2343 Keywords: DRM paradigm; Directed forgetting; Semantic activation; Part-list cuing; Cued recall
1. Introduction Studies on directed forgetting have shown that forgetting can be intentional and controllable (see MacLeod, 1998, for a review). Can intentional forgetting also reduce false memory? The present study investigated this question using a list-learning paradigm to create false memory and an item method directed forgetting paradigm to induce forgetting. Two procedures are commonly used in directed forgetting research to present instructional cues to participants regarding items they should forget or remember. Under the item method, participants are given explicit cues for each to-be-forgotten item (F word), such as ‘‘forget’’, and each to-be-remembered item (R word), such as ‘‘remember.’’ The memory cue is given after the relevant item to ensure that the participant had registered the
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words. The list method involves only one cue, usually an instruction to forget all of the preceding items, given in the middle of the list. The directed forgetting effect describes the finding that F items are less well remembered than are R items, and sometimes interference on the R items followed by the F items is reduced. The occurrence of this effect depends on how memory is tested (e.g., Basden & Basden, 1998, Basden, Basden, & Gargano, 1993). When participants are tested with a recall test, both item and list methods produce a directed forgetting effect. When participants are tested, however, using a recognition test, rather than a recall test, only the item method produces such an effect (e.g., Block, 1971; MacLeod, 1975). Based on this and other findings, several studies have suggested that the item method fosters selective rehearsal favoring the R words, whereas the list method promotes inhibition of the F words (Basden & Basden, 1998, 1993; MacLeod, 1999; Wilson & Kipp, 1998). In the item method, the F words are less well rehearsed than are the R words and, thus, F words are not encoded as
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completely as are R words. This feature impairs both the recognition and recall of F words. In the list method, both the R and F words are encoded and stored in memory. The cue to forget affects retrieval access to F words. Consequently, the directed forgetting effect was found to occur only in the recall test and not in the recognition and indirect memory tests, in which active retrieval is not necessary (e.g., Basden et al., 1993). One way to produce false memory for words is to use the DRM paradigm, adopted originally by Deese (1959) and recently by Roediger and McDermott (1995). In a typical experimental procedure, participants study a list of semantically related words (e.g., bed, awake, and rest), all of which are related to a critical word that is not presented (e.g., sleep). Later, participants falsely recall or recognize the missing critical word. High levels of false memory for lure items (e.g., sleep) have been demonstrated in tests such as free recall (Deese, 1959; McDermott, 1996; Payne, Elie, Blackwell, & Neuschatz, 1996; Roediger & McDermott, 1995) and recognition (McDermott, 1996; Payne et al., 1996; Roediger & McDermott, 1995; Underwood, 1965) tests. In addition to associative list materials used in the DRM paradigm, categorized lists have also been used to produce false memories (e.g., Seamon, Luo, Schlegel, Greene, & Goldenberg, 2000; Smith, Gerkens, Pierce, & Choi, 2002). Several studies have used the list method to investigate the effect of directed forgetting on false memory. For example, Lee and Hsu (2000) used the standard list method to examine whether false recall still occurred when participants were instructed to forget the DRM lists. After studying an initial set of DRM lists, the forget-the-firstlist-and-remember-the-second-list (FR) group was asked to forget them, whereas the remember-both-lists (RR) group was asked to remember them. All participants were then required to study and remember a second set of the DRM lists. In the final recall stage, all participants were asked to recall both sets of studied words. Notably, no difference was found between the FR and RR groups in terms of their false recall of critical lures semantically associated with the initial F lists. Moreover, this finding was replicated in Seamon, Luo, Shulman, Toner, and Caglar (2002). Their study showed that list-level directed forgetting instructions decreased the recall of studied words, but not the false recall of critical lures. Additionally, using the list method, Kimball and Bjork (2002) found typical directed forgetting effects for studied items and an inverse pattern for critical-item intrusions; that is, they found that the forgetting instruction was associated with impaired retrieval of studied items and increased critical-item intrusions. Although the results of these studies were not identical, they all suggested that directed forgetting did not reduce false memory. The item method and list method differ not only in the procedures but also in the mechanisms that create the directed forgetting effect. The present study aimed to use the item method to examine the effect of intentional for-
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getting on false memory. In addition, this study also manipulated the number of F words within a list, including three types of study conditions: remembering the whole list (R list), forgetting the whole list (F list), and forgetting part of the list. The R list was used as a baseline. Manipulating the proportion of F words within a list allowed this study to investigate not only the effect of remembering vs. forgetting but also the effect of different degrees of forgetting on false memory. Moreover, forgetting the whole list, a list-level forgetting, might have a qualitatively different effect on false memory than forgetting part of the list, an item-level forgetting. It is important to note that false memory was influenced by the list length (Robinson & Roediger, 1997). To avoid this confounding, the list length remained constant across different types of study conditions. It is well established that studying list items leads to an unconscious, rapid, and automatic spread of activation in a semantic network (e.g., Collins & Loftus, 1975; Seamon, Luo, & Gallo, 1998). According to the activation/monitoring framework (Roediger, Watson, McDermott, & Gallo, 2001), studying semantically related words in the DRM paradigm should automatically activate representations for non-presented critical words. Similarly, based on the implicit associative response theory (Underwood, 1965), studying list words causes implicit associative responses that produce semantic associates of the studied words, such as the critical word, into conscious awareness. The former theory suggests that false memory for the critical word is produced through a high level of semantic activation of critical words in conjunction with source confusion or monitoring failure. The fuzzy trace theory (Reyna & Brainerd, 1995) explains false memory in terms of memory trace. According to this theory, memory judgments can be based on verbatim or gist traces. Verbatim traces represent the surface details of physical stimuli, whereas gist traces represent the meaning or theme of the stimuli. In the DRM procedure, correct memory of list items is based on verbatim traces, whereas false memory of critical words is based on gist traces. The critical word is subsequently remembered because it is consistent with the gist representation. The recollection rejection monitoring mechanism based on verbatim trace, on the other hand, could be used to reject or accept the critical word (Odegard & Lampinen, 2005). Based on the above theories, spread of activation among related items in a semantic network is an automatic process; thus, a cue to forget a list does not impair the semantic activation during study. However, a forget cue would impair access to episodic information that could be used to reject the critical-item. Thus, it was expected that forgetting the whole list (the F list) would not reduce and sometimes even increase false memory, a result similar to that obtained for the list method. As to the effect that forgetting part of the list would have on false memory, because all items within a list were semantically or thematically related, there might be within-list retrieval completion
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between R and F words. R words would gain a competitive advantage over F words, a phenomenon similar to the retrieval competition induced by the part-list cuing procedure (Rundus, 1973). If this is the case, then semantic activation as well as false memory should decrease upon increasing the number of F words within a list. In other words, there would be an item-level or within-list forgetting effect on false memory. On the other hand, if there is no competition between R and F words, then the number of F words within a list should not have an effect on false memory of the critical word. To test these two possibilities, this study reports two experiments using the item method in which, within a studied list, participants were asked to either (a) remember or forget the whole list or (b) remember some list words and forget others. The R and F words appeared randomly within the studied list. Participants received a memory cue immediately following each studied item. The proportion of F and R words in each studied list was also manipulated, but the length of the list remained constant. Although both categorized and associative list materials have been used to produce false memories, Smith et al. (2002) suggest that categorized lists produced false memories because of the use of semantic knowledge during testing, whereas associative lists produced false memories owing to semantic associative processes involved in studying list words. The current study however hypothesizes that the above-mentioned within-list retrieval competition between R and F words should occur for both categorized lists and associative lists. There are two reasons. First, as acknowledged by Smith et al. (2002) studying categorized lists, just as associative lists, can activate nonstudied category members based on their associative strength. Second, competition in semantic activation between R and F words should occur as long as list words are related, either semantically or thematically. To test this hypothesis, Experiment 1a used the categorized lists and Experiment 1b used the associative lists. In both experiments, after studying each list, participants were asked to immediately recall the R words. The immediate recall test aimed to ensure that participants followed the instructions to remember or to forget the words that they had studied and to ensure their ability to differentiate between the two types of words. This procedure has been used in previous studies (e.g., Woodward, Bjork, & Jongeward, 1973; Zacks, Radvansky, & Hasher, 1996). The main result of interest was, however, in the final cued recall. Cued recall, rather than free recall, was used to avoid a floor effect. 2. Experiment 1a 2.1. Method 2.1.1. Participants The participants were 40 students taking introductory psychology courses who participated voluntarily to fulfil part of their course requirements.
2.1.2. Materials Eight categorized lists were used, each containing 15 multiple-character Chinese words from the same taxonomic category. A different category was used for each list. The 15 words were ordered according to their output dominance (according to Jeng, Lai, & Liu, 1973). Each list consisted of a non-presented critical word, which had the highest output dominance. Thus, the experiment involved a total of eight critical words. All words in the same list were from the same taxonomic category as the critical word, which was not presented during the study phase, but was tested to indicate false recall. All words used in this study contained two to four characters and were tape-recorded by a female voice at an average rate of one word per second. A memory cue read by the same voice appeared one second after each studied word. Two seconds after the memory cue, the next studied word appeared. The eight lists were divided into four types, based on the proportion of forget words they contained: R0/F15, R5/F10, R10/F5, and R15/F0. For example, List R0/F15 contained no remember words and 15 forget words and List R5/F10 contained five remember words and 10 forget words. The R15/F0 and R0/F15 types of lists were used as controls and were equivalent to remember and forget lists, respectively. In particular, the R15/F0 type of list, which incorporated no forget words, was used to estimate the baseline of false recall. Each type contained two lists of words. The word number type and word list were counterbalanced, so that each word list had the same chance of appearing in each type of word number. Within each list, words were assigned randomly as remember or forget words. 2.1.3. Procedure Participants were tested individually. Each experimental session consisted of a study phase and a testing phase. During the study phase, participants listened to the eight word lists one by one. After listening to each list, including those in the R0/F15 condition, participants were given at least 30 s to write down all of the R words on that list. The presentation order of these eight lists was counterbalanced across the word number type. Specifically, participants were instructed to listen to the lists of words and the cues that followed each studied word. The participants were required to remember the studied words that were cued to ‘‘remember’’ and to forget those that were cued to ‘‘forget’’. After listening to each list, the participants were asked to write down as many R words as they could recall. This process ensured that the participants had listened carefully and could differentiate between the R and F words. After all eight word lists had been presented, as a recency control, participants were asked to mentally solve three two-digit addition problems. Participants were then given the cued recall sheet with the category names listed at the top. They were required to recall all of the words, including both R and F words that they had heard during the study phase, in the order
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of the listed category names. All participants received the same random order. The entire experiment lasted about 30 min.
2.2. Results Table 1 presents the results for the immediate recall and cued recall tests. In the R0/F15 (F list) condition, the immediate recall test contained no R words and, thus, participants did not have to recall. Furthermore, the R15/F0 (R list) condition contained no F words. For cued recall, the R0/F15 condition involved no R words and the R15/ F0 condition involved no F words. Participants were asked to recall only the R words during the immediate recall test. From Table 1, the rates of F word intrusions were very low (.03 and .06), indicating that participants had followed the instructions and could differentiate between the R and F words. One-way ANOVAs were conducted to examine the effect of the word number. For R words, there was a main effect [F(2, 78) = 46.13; MSe = .01; p < .001]. Bonferroni tests revealed that participants recalled a significantly higher rate of R words in the R5/F10 condition than in the R10/F5 and R15/F0 conditions, and that the difference between the R10/F5 and R15/F0 conditions was also significant (all values of p < .01). Regarding both F words and critical word intrusions, no differences were noted between the various conditions. The focus of analysis was on the final cued recall. A oneway ANOVA revealed a main effect of the word number [F(3, 117) = 3.15; MSe = .08; p < .05]. Further, pairwise comparisons were conducted to examine the differences between the four word number conditions (R0/F15 vs. R5/F10 vs. R10/F5 vs. R15/F0). The false recall of critical lures in the R0/F15 condition was significantly higher than both the R10/F5 and R15/F0 conditions (both values of p < .05). To determine whether a directed forgetting effect and/or an effect of word number existed, a two (word type: R vs. F words) by two (word number: R5/F10 vs. R10/F5) ANOVA was performed. The ANOVA revealed significant effects of both word type and word number, F(1, 39) = 382.96, MSe = .02, p < .001, and F(1, 39) = 10.80,
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MSe = .03, p < .01, respectively. Notably, the interaction effect was also significant, F(1, 39) = 6.63, MSe = .01, p < .05. Bonferroni tests revealed that participants recalled a significantly higher rate of R words in the R5/F10 than in the R10/F5 condition (p < .01). For F words, no significant differences were noted between these two conditions. These findings reveal two important patterns. First, the proportion of F words within a studied list affected false recall. Specifically, false recall increased upon increasing the proportion of F words. Second, consistent with the findings of previous studies using the item method, this study observed a directed forgetting effect in the cued recall test. 3. Experiment 1b Experiments 1a and 1b differed only in the materials used. In Experiment 1b, all of the words in a list were related semantically to a critical word. A pilot study was conducted in which a separate group of 40 participants was presented with critical words and then asked to write down all other words that came to mind for each critical word. Words produced earlier and by more participants were considered to have higher semantic relatedness. The words in each list were ordered based on their semantic relatedness. For the cued recall test, the first word in each list served as the cue (a reminder of that list) for the participants. Forty undergraduate students participated in this experiment voluntarily to fulfil part of their course requirements. 3.1. Results and discussion Table 2 presents the results of the immediate recall and cued recall tests. The participants were asked to recall only the R words during the immediate recall task. From Table 2, the rates of F word intrusions were very low (.01 and .05), indicating that the participants had followed the instructions and could differentiate between the R and F words. One-way ANOVAs were conducted to test the effect of the word number. The R words displayed a significant effect [F(2, 78) = 37.42; MSe = .02; p < .001]. Bonferroni tests revealed that the participants recalled a significantly higher rate of R words in the R5/F10 condition than in
Table 1 Mean rates of correct recall on the immediate recall and cued recall tests as a function of word type in experiment 1a (categorized lists) Word type
R0/F15 (F list)
R5/F10
R10/F5
R15/F0 (R list)
Immediate recall R words F word intrusions Critical word intrusions
– – –
.86 (.02) .03 (.01) .01 (.01)
.69 (.02) .06 (.01) .03 (.02)
.63 (.02) – .06 (.03)
Cued recall R words F words Critical words
– .21 (.02) .29 (.06)
.72 (.03) .19 (.02) .19 (.04)
.59 (.02) .24 (.03) .15 (.04)
.52 (.02) – .10 (.04)
Note: R0/F15: zero ‘‘remember’’ and 15 ‘‘forget’’ words. SDs are provided in parentheses.
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Table 2 Mean rates of correct recall on the immediate recall and cued recall tests as a function of word type in experiment 1b (associative lists) Word type
R0/F15 (F list)
R5/F10
R10/F5
R15/F0 (R list)
Immediate recall R words F word intrusions Critical word intrusions
– – –
.89 (.02) .01 (.00) .04 (.02)
.66 (.03) .05 (.01) .10 (.03)
.64 (.02) – .13 (.03)
Cued recall R words F words Critical words
– .12 (.02) .38 (.06)
.70 (.03) .15 (.02) .23 (.05)
.48 (.02) .17 (.02) .16 (.04)
.48 (.02) – .19 (.04)
Note: R0/F15: zero ‘‘remember’’ and 15 ‘‘forget’’ words. SDs are provided in parentheses.
the R10/F5 and R15/F0 conditions (both values of p < .001). No significant difference was found between the results of the R10/F5 and R15/F0 conditions. For the F words, there was a significant difference between the R5/ F10 and R10/F5 conditions [F(1, 39) = 12.38; MSe = .002; p < .01]. The word number did not affect the critical word intrusions. For cued recall, a one-way ANOVA was conducted to test the effect of the word number (R0/F15 vs. R5/F10 vs. R10/F5 vs. R15/F0) on the false recall of critical lures. Pairwise comparisons revealed that the false recall of critical lures in the R0/F15 condition was significantly higher than the R5/F10, R10/F5 and R15/F0 conditions (all values of p < .05). To determine whether a directed forgetting effect and/or an effect of word number existed, a two (word type: R vs. F words) by two (word number: R5/F10 vs. R10/F5) ANOVA was performed. The ANOVA revealed significant effects of both word type and word number, F(1, 39) = 351.80, MSe = .02, p < .001, and F(1, 39) = 38.22, MSe = .02, p < .001, respectively. The interaction effect was also significant, F(1, 39) = 14.25, MSe = .02, p < .001. Bonferroni tests revealed that participants recalled a significantly higher rate of R words in the R5/ F10 than in the R10/F5 condition (p < .001). For F words, no significant differences were noted between these two conditions. Experiment 1b differed from Experiment 1a only in the materials used. This difference apparently did not cause any differences in the results. Experiment 1b replicated all of the major findings of Experiment 1a. First, false recall increased with the proportion of F words in the list. Second, a directed forgetting effect was found in cued recall. In both experiments, prior to the final cued recall, the participants had to recall and write down the R words for each list. Although the rate of immediate false recall was relatively low and in the opposite direction of the findings of the final recall test, the immediate recall test itself might have affected the final cued recall by strengthening the encoding of R words. In particular, the final cued recall of critical lure could have been reduced due to either increase in retrieval competition from R words or changes in monitoring criterion. To avoid this confounding, the immediate recall was removed in Experiment 2.
4. Experiment 2 The purpose of Experiment 2 was to examine the effect of intentional forgetting on false memory, without the confounding caused by the prior immediate recall of R words. Participants in this experiment were subjected only to the cued recall test. 4.1. Method Experiment 2 differed from the previous two experiments only in that it did not involve the immediate recall test. After the participants had listened to each studied list, the experimenter played the next list. The participants did not have to recall the R words. The interval between the lists was 2 s. The rest of the procedure was identical to those of Experiments 1a and 1b. Sixty undergraduate students participated in this experiment voluntarily to fulfil part of their course requirements. Half of the participants received the categorized lists and the other half received the associative lists. 4.2. Results and discussion Table 3 presents the results of the cued recall tests. Categorized lists. To determine whether a directed forgetting effect and/or an effect of word number existed, a two (word type: R vs. F words) by two (word number: R5/F10 vs. R10/F5) ANOVA was performed. The ANOVA revealed significant effects of both word type and word number, F(1, 29) = 98.05, MSe = .03, p < .001, and F(1, 29) = 4.62, MSe = .03, p < .05, respectively. The interaction effect was also significant, F(1, 29) = 5.39, MSe = .01, p < .05. Bonferroni tests revealed that participants recalled a significantly higher rate of R words in the R5/F10 than in the R10/F5 condition (p < .01). For F words, no significant differences were noted between these two conditions. As for the false recall of critical lures, a one-way ANOVA revealed a significant effect of word number (R0/F15 vs. R5/F10 vs. R10/F5 vs. R15/F0) on false recall [F(3, 87) = 3.29; MSe = .09; p < .05]. Pairwise comparisons revealed higher false recall rates in both the R0/F15 and
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Table 3 Mean rates of correct cued recall as a function of word type for categorized lists and associative lists in experiment 2 Word type
R0/F15 (F list)
R5/F10
R10/F5
R15/F0 (R list)
Categorized lists R words F words Critical words
– .20 (.02) .40 (.07)
.62 (.05) .26 (.02) .22 (.05)
.51 (.03) .28 (.03) .23 (.06)
.42 (.03) – .38 (.06)
Associative lists R words F words Critical words
– .10 (.01) .37 (.06)
.52 (.04) .17 (.03) .18 (.05)
.40 (.02) .17 (.03) .23 (.06)
.28 (.02) – .35 (.06)
Note: R0/F15: zero ‘‘remember’’ and 15 ‘‘forget’’ words. SDs are provided in parentheses.
R15/F0 conditions than in both the R5/F10 and R10/F5 conditions (all values of p < .05). There were no significant differences between the results obtained for the R0/F15 and R15/F0 conditions and between those for the R5/F10 and R10/F5 conditions. These findings indicate that the F list (R0/F15) and R list (R15/F0) produced the same rate of false recall. Because the participants were asked to remember or forget the whole list, these two study conditions were similar to those of the list method. These results are also consistent with the previous finding that list method directed forgetting did not reduce false memory. Associative lists. A two (word type: R vs. F words) by two (word number: R5/F10 vs. R10/F5) ANOVA was performed to examine whether there was a directed forgetting effect or an effect of word number, or both. The ANOVA revealed significant effects of both word type and the interaction between word type and word number, F(1, 29) = 65.01, MSe = .04, p < .001, and F(1, 29) = 3.92, MSe = .03, p < .058, respectively. The effect of word number was also significant, F(1, 29) = 7.55, MSe = .02, p < .05. Bonferroni tests revealed that participants recalled a significantly higher rate of R words in the R5/F10 than in the R10/F5 condition (p < .05). For F words, no significant differences were found between these two conditions. For the false recall of critical lures, a one-way ANOVA revealed a significant effect for the word number [R0/F15 vs. R5/F10 vs. R10/F5 vs. R15/F0: F(3, 87) = 2.90; MSe = .08; p < .05]. Pairwise comparisons revealed that the false recall rates in both the R0/F15 and R15/F0 conditions were higher than that in the R5/F10 condition (both values of p < .05). There were no significant differences between the R0/F15 and R15/F0 conditions and between the R5/F10 and R10/F5 conditions. After the immediate recall was removed, this study still detected a directed forgetting effect in cued recall. More importantly, for both the categorized and associative lists, the false recall rate decreased in relation to the proportion of F words in the list. When participants were asked to forget the whole list, forgetting did not reduce false memory. It appears that the immediate recall did have an effect on the final false recall, especially for the R15/F0 condition in which participants had to immediately recall all the studied words. When the immediate recall was removed, Exper-
iment 2 found that intentionally forgetting part of the list items reduced false recall. In contrast, forgetting the whole list (the R0/F15 condition), similar to the F list of the list method directed forgetting, did not have such an effect. 5. General discussion This study used the item method directed forgetting paradigm to examine the effect of intentional forgetting on false memory produced by the list-learning procedure. Both categorized and associative list materials were used. The proportion of F words in the studied list was manipulated, but the length of the list was maintained constant. In all experiments, directed forgetting effects were found in cued recall, suggesting that manipulation of the ‘‘remember’’ vs. ‘‘forget’’ instruction was effective. When there was immediate recall of R words, Experiments 1a and 1b found that false recall decreased upon decreasing the proportion of F words. This result apparently was caused by the confounding that encoding was strengthened only for the R words. When the immediate recall test was removed, the degree of false recall decreased upon increasing the proportion of F words, except for the condition in which participants were instructed to forget the whole list. These findings suggest that forgetting part of the studied list reduced the false recall of critical lures, whereas forgetting the whole studied list did not affect false recall. This study used two conceptually similar procedures – semantic associate and category associate lists – to produce false memory. The present findings seem to be reliable and robust, given that different processes of inducing false memory might be involved in these two kinds of materials (Smith et al., 2002). Moreover, as predicted, both categorized lists and associative lists evoked semantic activation of critical words during study and within-list forgetting occurred for both semantically and thematically related words. Kimball and Bjork (2002), using the list method of directed forgetting, found that participants were less likely to recall the studied item, but more likely to falsely recall the critical-item, for the F list than for the R list. When the part-list cuing procedure was used, however, retrieving part of the list reduced both recall of the studied item and
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false recall of critical items. In other words, recall of both studied words and critical words declined with part-list cuing. Kimball and Bjork (2002) hypothesized that this difference occurred because of retrieval competition between lists in directed forgetting and within lists in part-list cuing. The difference between the R15/F0 and R0/F15 conditions in the present study was equivalent to the list method manipulation. The fact that the R0/F15 condition was not rehearsed as intensively as that of the R15/F0 condition led to the directed forgetting effect. As for the list method findings of Kimball and Bjork (2002), semantic activation of the R0/F15 condition was, however, unimpaired because no within-list competition involved. An unimpaired semantic activation combined with a decreased access to episodic information led to an increase of false recall found in Kimball and Bjork (2002). One possibility is that F words decreased access to the episodic memory, leading to a more liberal response criterion. Unlike Kimball and Bjork (2002), however, the present study did not find an increase in the degree of false recall. Several studies have found that F lists must be followed by R lists for a forgetting effect to occur (e.g., Bjork, 1989). The studied list of the R0/F15 condition was similar, but not identical, to the forget list in the list method. The differences were that the R0/F15 list was not necessarily followed by a remember list and that the forgetting instruction was given after each item. Because no remember list was present, the forgetting effect caused by an item-by-item instruction might not be sufficiently strong to impair episodic information, as found in the study reported by Kimball and Bjork (2002). When the participants had to forget only part of the list (the R5/F10 and R10/F5 conditions), the current results were similar to the findings obtained from part-list cuing in Kimball and Bjork (2002) study. There was within-list retrieval competition between F and R words. More F words in the R5/F10 and R10/F5 types of lists, compared with those of the R15/ F0 type, led to a lower level of semantic activation and, thus, reduced the degree of false recall of critical lures. The current results are consistent with the account proposed by Kimball and Bjork (2002), i.e., that item-level and list-level forgetting had different effects on false recall. Furthermore, the results of the current study suggest that intentional forgetting, just as in the case of unintentional forgetting caused by part-list cuing, could also reduce false recall of critical words. In addition, Marche, Brainerd, Lane, and Leohr (2005) suggest that in contrast to the list method, the item method impairs storage of both verbatim and gist traces during study. Their study using the standard item method directed forgetting demonstrated that item-level directed forgetting reduced false recall, a result similar to the current finding. The current study made an additional contribution by examining both the item-level and list-level directed forgetting in a single control experiment, and thus providing stronger support for the difference between item-level and list-level inhibition. There is also a limitation on the inter-
pretation of the current study. Since the number of F and R words within a list is confounded (negatively correlated), the current study cannot firmly determine whether the current pattern of result was caused by the changes in the proportion or absolute number of R and/or F words. Further research is needed to find the answer. In conclusion, the level of semantic activation caused by studying the list words is one of the main determining factors of false memory (e.g., Roediger et al., 2001; Underwood, 1965). The current study demonstrates that when participants were asked to forget a partial number of list items (an item-level forgetting), the item method of directed forgetting could reduce the semantic activation within a semantic-related list and further reduce false memory. On the other hand, semantic activation was not impaired when the participants were asked to forget the whole semanticrelated list. As a result, forgetting the whole studied list did not reduce the degree of false recall. Acknowledgements This research was supported by Research Grant NSC 91-2413-H-194-027 from the National Science Council of the Republic of China. The author thanks Mr. Huangmou Lee for assistance in administering part of the experiments and the reviewers for helpful comments and suggestions. Portions of this research were presented at the 13th Conference of the European Society of Cognitive Psychology (September 2003, Spain). References Basden, B. H., & Basden, D. R. (1998). Directed forgetting: A contrast of methods and interpretations. In J. M. Golding & C. M. MacLeod (Eds.), Intentional forgetting (pp. 139–172). Mahwah, NJ: Erlbaum. Basden, B. H., Basden, D. R., & Gargano, G. J. (1993). Directed forgetting in implicit and explicit memory tests: A comparison of methods. Journal of Experimental Psychology: Learning, Memory, & Cognition, 19, 603–616. Bjork, R. A. (1989). Retrieval inhibition as an adaptive mechanism in human memory. In H. L. Roediger, III & F. I. M. Craik (Eds.), Varieties of memory and consciousness: Essays in honour of Endel Tulving (pp. 309–330). Hillsdale, NJ: Lawrence Erlbaum. Block, R. A. (1971). Effects of instructions to forget in short-term memory. Journal of Experimental Psychology, 89, 1–9. Collins, A. M., & Loftus, E. F. (1975). A spreading-activation theory of semantic processing. Psychological Review, 82, 407–428. Deese, J. (1959). On the prediction of occurrence of particular verbal intrusions in immediate recall. Journal of Experimental Psychology, 58, 17–22. Jeng, C., Lai, M., & Liu, I. (1973). Category norms in Chinese and English from bilingual subjects. Acta Psychologica Taiwanica, 15, 81–153. Kimball, D. R., & Bjork, R. A. (2002). Influences of intentional and unintentional forgetting on false memories. Journal of Experimental Psychology: General, 131, 116–130. Lee, Y. & Hsu, Y.-C. (2000). False recall of to-be-forgotten information. In Poster presented at the 41st Annual Meeting of the Psychonomic Society, New Orleans, Louisiana, USA. MacLeod, C. M. (1975). Long-term recognition and recall following directed forgetting. Journal of Experimental Psychology: Human Learning & Memory, 1, 271–279.
Y. Lee / Acta Psychologica 127 (2008) 146–153 MacLeod, C. M. (1998). Directed forgetting. In J. M. Golding & C. M. MacLeod (Eds.), Intentional forgetting: Interdisciplinary approaches (pp. 1–57). Mahwah, NJ: Erlbaum. MacLeod, C. M. (1999). The item and list methods of directed forgetting: Test differences and the role of demand characteristics. Psychonomic Bulletin & Review, 6, 123–129. Marche, T. A., Brainerd, C. J., Lane, D. G., & Leohr, J. D. (2005). Itemlevel directed forgetting diminished false-memory. Memory, 13, 749–758. McDermott, K. B. (1996). The persistence of false memories in list recall. Journal of Memory & Language, 35, 212–230. Odegard, T. N., & Lampinen, J. M. (2005). Recollection rejection: Gist cuing of verbatim memory. Memory & Cognition, 33, 1422–1430. Payne, D. G., Elie, C. J., Blackwell, J. M., & Neuschatz, J. S. (1996). Memory illusion: Recalling, recognizing and recollecting events that never occurred. Journal of Memory & Language, 35, 261–285. Reyna, V. F., & Brainerd, C. J. (1995). Fuzzy trace theory: An interim synthesis. Learning & Individual Differences, 7, 1–75. Robinson, K. J., & Roediger, H. L. III. (1997). Associative processes in false recall and false recognition. Psychological Science, 8, 231–237. Roediger, H. L., III, & McDermott, K. B. (1995). Creating false memories: Remembering words not presented in lists. Journal of Experimental Psychology: Learning, Memory, & Cognition, 21, 803–814. Roediger, H. L., Watson, J. M., McDermott, K. B., & Gallo, D. A. (2001). Factors that determine false recall: A multiple regression analysis. Psychonomic Bulletin & Review, 8, 385–407.
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Rundus, D. (1973). Negative effects of using list items as recall cues. Journal of Verbal Learning & Verbal Behavior, 12, 43–50. Seamon, J. G., Luo, C. R., & Gallo, D. A. (1998). Creating false memories of words with or without recognition of list items: Evidence for nonconscious processes. Psychological Science, 9, 20–26. Seamon, J. G., Luo, C. R., Schlegel, S. E., Greene, S. E., & Goldenberg, A. B. (2000). False memory for categorized picture and words: The category associates procedure for studying memory errors in children and adults. Journal of Memory & Language, 42, 120–146. Seamon, J. G., Luo, C. R., Shulman, E. P., Toner, S. K., & Caglar, S. (2002). False memories are hard to inhibit: Differential effects of directed forgetting on accurate and false recall in the DRM procedure. Memory, 10, 225–237. Smith, S. M., Gerkens, D. R., Pierce, B. H., & Choi, H. (2002). The role of associative responses at study and semantically guided recollection at test in false memory: The Kirkpatrick and Deese hypotheses. Journal of Memory & Language, 47, 436–447. Underwood, B. J. (1965). False recognition produced by implicit verbal responses. Journal of Experimental Psychology, 70, 122–129. Wilson, S. P., & Kipp, K. (1998). The development of efficient inhibition: Evidence from directed forgetting tasks. Developmental Review, 18, 86–123. Woodward, A. E., Bjork, R. A., & Jongeward, R. H. (1973). Recall and recognition as a function of primary rehearsal. Journal of Verbal Learning & Verbal Behavior, 12, 608–617. Zacks, R. T., Radvansky, G., & Hasher, L. (1996). Studies of directed forgetting in older adults. Journal of Experimental Psychology: Learning, Memory, & Cognition, 22, 143–156.
Acta Psychologica 127 (2008) 146–153 www.elsevier.com/locate/actpsy
Can intentional forgetting reduce false memory? Effects of list-level and item-level forgetting Yuh-shiow Lee
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Department of Psychology, National Chung-Cheng University, Chiayi 621, Taiwan, ROC Received 18 September 2006; received in revised form 20 March 2007; accepted 26 March 2007 Available online 1 May 2007
Abstract This study examined whether false memory produced by the learning of lists of categorized and associative materials could be reduced by directed forgetting. The number of items within a list that participants were asked to remember or forget was manipulated, while the length of the list remained constant. Experiment 1a used categorized lists and Experiment 1b used associative lists; the participants performed immediate free recall and cued recall tests. For both the categorized and associative lists, the rate of false recall increased upon increasing the proportion of ‘‘forget’’ (F) words. After removing the immediate recall test, Experiment 2 found that intentionally forgetting part of the studied list reduced false memory, whereas forgetting the whole studied list did not. The results are discussed in terms of the list-level-vs.-item-level inhibition in semantic activation and the role of monitoring in reducing false memory. Ó 2007 Elsevier B.V. All rights reserved. PsycINFO classification: 2340; 2343 Keywords: DRM paradigm; Directed forgetting; Semantic activation; Part-list cuing; Cued recall
1. Introduction Studies on directed forgetting have shown that forgetting can be intentional and controllable (see MacLeod, 1998, for a review). Can intentional forgetting also reduce false memory? The present study investigated this question using a list-learning paradigm to create false memory and an item method directed forgetting paradigm to induce forgetting. Two procedures are commonly used in directed forgetting research to present instructional cues to participants regarding items they should forget or remember. Under the item method, participants are given explicit cues for each to-be-forgotten item (F word), such as ‘‘forget’’, and each to-be-remembered item (R word), such as ‘‘remember.’’ The memory cue is given after the relevant item to ensure that the participant had registered the
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Tel.: +886 5 2720411x32210; fax: +886 5 2720857. E-mail address: [email protected]
0001-6918/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.actpsy.2007.03.004
words. The list method involves only one cue, usually an instruction to forget all of the preceding items, given in the middle of the list. The directed forgetting effect describes the finding that F items are less well remembered than are R items, and sometimes interference on the R items followed by the F items is reduced. The occurrence of this effect depends on how memory is tested (e.g., Basden & Basden, 1998, Basden, Basden, & Gargano, 1993). When participants are tested with a recall test, both item and list methods produce a directed forgetting effect. When participants are tested, however, using a recognition test, rather than a recall test, only the item method produces such an effect (e.g., Block, 1971; MacLeod, 1975). Based on this and other findings, several studies have suggested that the item method fosters selective rehearsal favoring the R words, whereas the list method promotes inhibition of the F words (Basden & Basden, 1998, 1993; MacLeod, 1999; Wilson & Kipp, 1998). In the item method, the F words are less well rehearsed than are the R words and, thus, F words are not encoded as
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completely as are R words. This feature impairs both the recognition and recall of F words. In the list method, both the R and F words are encoded and stored in memory. The cue to forget affects retrieval access to F words. Consequently, the directed forgetting effect was found to occur only in the recall test and not in the recognition and indirect memory tests, in which active retrieval is not necessary (e.g., Basden et al., 1993). One way to produce false memory for words is to use the DRM paradigm, adopted originally by Deese (1959) and recently by Roediger and McDermott (1995). In a typical experimental procedure, participants study a list of semantically related words (e.g., bed, awake, and rest), all of which are related to a critical word that is not presented (e.g., sleep). Later, participants falsely recall or recognize the missing critical word. High levels of false memory for lure items (e.g., sleep) have been demonstrated in tests such as free recall (Deese, 1959; McDermott, 1996; Payne, Elie, Blackwell, & Neuschatz, 1996; Roediger & McDermott, 1995) and recognition (McDermott, 1996; Payne et al., 1996; Roediger & McDermott, 1995; Underwood, 1965) tests. In addition to associative list materials used in the DRM paradigm, categorized lists have also been used to produce false memories (e.g., Seamon, Luo, Schlegel, Greene, & Goldenberg, 2000; Smith, Gerkens, Pierce, & Choi, 2002). Several studies have used the list method to investigate the effect of directed forgetting on false memory. For example, Lee and Hsu (2000) used the standard list method to examine whether false recall still occurred when participants were instructed to forget the DRM lists. After studying an initial set of DRM lists, the forget-the-firstlist-and-remember-the-second-list (FR) group was asked to forget them, whereas the remember-both-lists (RR) group was asked to remember them. All participants were then required to study and remember a second set of the DRM lists. In the final recall stage, all participants were asked to recall both sets of studied words. Notably, no difference was found between the FR and RR groups in terms of their false recall of critical lures semantically associated with the initial F lists. Moreover, this finding was replicated in Seamon, Luo, Shulman, Toner, and Caglar (2002). Their study showed that list-level directed forgetting instructions decreased the recall of studied words, but not the false recall of critical lures. Additionally, using the list method, Kimball and Bjork (2002) found typical directed forgetting effects for studied items and an inverse pattern for critical-item intrusions; that is, they found that the forgetting instruction was associated with impaired retrieval of studied items and increased critical-item intrusions. Although the results of these studies were not identical, they all suggested that directed forgetting did not reduce false memory. The item method and list method differ not only in the procedures but also in the mechanisms that create the directed forgetting effect. The present study aimed to use the item method to examine the effect of intentional for-
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getting on false memory. In addition, this study also manipulated the number of F words within a list, including three types of study conditions: remembering the whole list (R list), forgetting the whole list (F list), and forgetting part of the list. The R list was used as a baseline. Manipulating the proportion of F words within a list allowed this study to investigate not only the effect of remembering vs. forgetting but also the effect of different degrees of forgetting on false memory. Moreover, forgetting the whole list, a list-level forgetting, might have a qualitatively different effect on false memory than forgetting part of the list, an item-level forgetting. It is important to note that false memory was influenced by the list length (Robinson & Roediger, 1997). To avoid this confounding, the list length remained constant across different types of study conditions. It is well established that studying list items leads to an unconscious, rapid, and automatic spread of activation in a semantic network (e.g., Collins & Loftus, 1975; Seamon, Luo, & Gallo, 1998). According to the activation/monitoring framework (Roediger, Watson, McDermott, & Gallo, 2001), studying semantically related words in the DRM paradigm should automatically activate representations for non-presented critical words. Similarly, based on the implicit associative response theory (Underwood, 1965), studying list words causes implicit associative responses that produce semantic associates of the studied words, such as the critical word, into conscious awareness. The former theory suggests that false memory for the critical word is produced through a high level of semantic activation of critical words in conjunction with source confusion or monitoring failure. The fuzzy trace theory (Reyna & Brainerd, 1995) explains false memory in terms of memory trace. According to this theory, memory judgments can be based on verbatim or gist traces. Verbatim traces represent the surface details of physical stimuli, whereas gist traces represent the meaning or theme of the stimuli. In the DRM procedure, correct memory of list items is based on verbatim traces, whereas false memory of critical words is based on gist traces. The critical word is subsequently remembered because it is consistent with the gist representation. The recollection rejection monitoring mechanism based on verbatim trace, on the other hand, could be used to reject or accept the critical word (Odegard & Lampinen, 2005). Based on the above theories, spread of activation among related items in a semantic network is an automatic process; thus, a cue to forget a list does not impair the semantic activation during study. However, a forget cue would impair access to episodic information that could be used to reject the critical-item. Thus, it was expected that forgetting the whole list (the F list) would not reduce and sometimes even increase false memory, a result similar to that obtained for the list method. As to the effect that forgetting part of the list would have on false memory, because all items within a list were semantically or thematically related, there might be within-list retrieval completion
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between R and F words. R words would gain a competitive advantage over F words, a phenomenon similar to the retrieval competition induced by the part-list cuing procedure (Rundus, 1973). If this is the case, then semantic activation as well as false memory should decrease upon increasing the number of F words within a list. In other words, there would be an item-level or within-list forgetting effect on false memory. On the other hand, if there is no competition between R and F words, then the number of F words within a list should not have an effect on false memory of the critical word. To test these two possibilities, this study reports two experiments using the item method in which, within a studied list, participants were asked to either (a) remember or forget the whole list or (b) remember some list words and forget others. The R and F words appeared randomly within the studied list. Participants received a memory cue immediately following each studied item. The proportion of F and R words in each studied list was also manipulated, but the length of the list remained constant. Although both categorized and associative list materials have been used to produce false memories, Smith et al. (2002) suggest that categorized lists produced false memories because of the use of semantic knowledge during testing, whereas associative lists produced false memories owing to semantic associative processes involved in studying list words. The current study however hypothesizes that the above-mentioned within-list retrieval competition between R and F words should occur for both categorized lists and associative lists. There are two reasons. First, as acknowledged by Smith et al. (2002) studying categorized lists, just as associative lists, can activate nonstudied category members based on their associative strength. Second, competition in semantic activation between R and F words should occur as long as list words are related, either semantically or thematically. To test this hypothesis, Experiment 1a used the categorized lists and Experiment 1b used the associative lists. In both experiments, after studying each list, participants were asked to immediately recall the R words. The immediate recall test aimed to ensure that participants followed the instructions to remember or to forget the words that they had studied and to ensure their ability to differentiate between the two types of words. This procedure has been used in previous studies (e.g., Woodward, Bjork, & Jongeward, 1973; Zacks, Radvansky, & Hasher, 1996). The main result of interest was, however, in the final cued recall. Cued recall, rather than free recall, was used to avoid a floor effect. 2. Experiment 1a 2.1. Method 2.1.1. Participants The participants were 40 students taking introductory psychology courses who participated voluntarily to fulfil part of their course requirements.
2.1.2. Materials Eight categorized lists were used, each containing 15 multiple-character Chinese words from the same taxonomic category. A different category was used for each list. The 15 words were ordered according to their output dominance (according to Jeng, Lai, & Liu, 1973). Each list consisted of a non-presented critical word, which had the highest output dominance. Thus, the experiment involved a total of eight critical words. All words in the same list were from the same taxonomic category as the critical word, which was not presented during the study phase, but was tested to indicate false recall. All words used in this study contained two to four characters and were tape-recorded by a female voice at an average rate of one word per second. A memory cue read by the same voice appeared one second after each studied word. Two seconds after the memory cue, the next studied word appeared. The eight lists were divided into four types, based on the proportion of forget words they contained: R0/F15, R5/F10, R10/F5, and R15/F0. For example, List R0/F15 contained no remember words and 15 forget words and List R5/F10 contained five remember words and 10 forget words. The R15/F0 and R0/F15 types of lists were used as controls and were equivalent to remember and forget lists, respectively. In particular, the R15/F0 type of list, which incorporated no forget words, was used to estimate the baseline of false recall. Each type contained two lists of words. The word number type and word list were counterbalanced, so that each word list had the same chance of appearing in each type of word number. Within each list, words were assigned randomly as remember or forget words. 2.1.3. Procedure Participants were tested individually. Each experimental session consisted of a study phase and a testing phase. During the study phase, participants listened to the eight word lists one by one. After listening to each list, including those in the R0/F15 condition, participants were given at least 30 s to write down all of the R words on that list. The presentation order of these eight lists was counterbalanced across the word number type. Specifically, participants were instructed to listen to the lists of words and the cues that followed each studied word. The participants were required to remember the studied words that were cued to ‘‘remember’’ and to forget those that were cued to ‘‘forget’’. After listening to each list, the participants were asked to write down as many R words as they could recall. This process ensured that the participants had listened carefully and could differentiate between the R and F words. After all eight word lists had been presented, as a recency control, participants were asked to mentally solve three two-digit addition problems. Participants were then given the cued recall sheet with the category names listed at the top. They were required to recall all of the words, including both R and F words that they had heard during the study phase, in the order
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of the listed category names. All participants received the same random order. The entire experiment lasted about 30 min.
2.2. Results Table 1 presents the results for the immediate recall and cued recall tests. In the R0/F15 (F list) condition, the immediate recall test contained no R words and, thus, participants did not have to recall. Furthermore, the R15/F0 (R list) condition contained no F words. For cued recall, the R0/F15 condition involved no R words and the R15/ F0 condition involved no F words. Participants were asked to recall only the R words during the immediate recall test. From Table 1, the rates of F word intrusions were very low (.03 and .06), indicating that participants had followed the instructions and could differentiate between the R and F words. One-way ANOVAs were conducted to examine the effect of the word number. For R words, there was a main effect [F(2, 78) = 46.13; MSe = .01; p < .001]. Bonferroni tests revealed that participants recalled a significantly higher rate of R words in the R5/F10 condition than in the R10/F5 and R15/F0 conditions, and that the difference between the R10/F5 and R15/F0 conditions was also significant (all values of p < .01). Regarding both F words and critical word intrusions, no differences were noted between the various conditions. The focus of analysis was on the final cued recall. A oneway ANOVA revealed a main effect of the word number [F(3, 117) = 3.15; MSe = .08; p < .05]. Further, pairwise comparisons were conducted to examine the differences between the four word number conditions (R0/F15 vs. R5/F10 vs. R10/F5 vs. R15/F0). The false recall of critical lures in the R0/F15 condition was significantly higher than both the R10/F5 and R15/F0 conditions (both values of p < .05). To determine whether a directed forgetting effect and/or an effect of word number existed, a two (word type: R vs. F words) by two (word number: R5/F10 vs. R10/F5) ANOVA was performed. The ANOVA revealed significant effects of both word type and word number, F(1, 39) = 382.96, MSe = .02, p < .001, and F(1, 39) = 10.80,
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MSe = .03, p < .01, respectively. Notably, the interaction effect was also significant, F(1, 39) = 6.63, MSe = .01, p < .05. Bonferroni tests revealed that participants recalled a significantly higher rate of R words in the R5/F10 than in the R10/F5 condition (p < .01). For F words, no significant differences were noted between these two conditions. These findings reveal two important patterns. First, the proportion of F words within a studied list affected false recall. Specifically, false recall increased upon increasing the proportion of F words. Second, consistent with the findings of previous studies using the item method, this study observed a directed forgetting effect in the cued recall test. 3. Experiment 1b Experiments 1a and 1b differed only in the materials used. In Experiment 1b, all of the words in a list were related semantically to a critical word. A pilot study was conducted in which a separate group of 40 participants was presented with critical words and then asked to write down all other words that came to mind for each critical word. Words produced earlier and by more participants were considered to have higher semantic relatedness. The words in each list were ordered based on their semantic relatedness. For the cued recall test, the first word in each list served as the cue (a reminder of that list) for the participants. Forty undergraduate students participated in this experiment voluntarily to fulfil part of their course requirements. 3.1. Results and discussion Table 2 presents the results of the immediate recall and cued recall tests. The participants were asked to recall only the R words during the immediate recall task. From Table 2, the rates of F word intrusions were very low (.01 and .05), indicating that the participants had followed the instructions and could differentiate between the R and F words. One-way ANOVAs were conducted to test the effect of the word number. The R words displayed a significant effect [F(2, 78) = 37.42; MSe = .02; p < .001]. Bonferroni tests revealed that the participants recalled a significantly higher rate of R words in the R5/F10 condition than in
Table 1 Mean rates of correct recall on the immediate recall and cued recall tests as a function of word type in experiment 1a (categorized lists) Word type
R0/F15 (F list)
R5/F10
R10/F5
R15/F0 (R list)
Immediate recall R words F word intrusions Critical word intrusions
– – –
.86 (.02) .03 (.01) .01 (.01)
.69 (.02) .06 (.01) .03 (.02)
.63 (.02) – .06 (.03)
Cued recall R words F words Critical words
– .21 (.02) .29 (.06)
.72 (.03) .19 (.02) .19 (.04)
.59 (.02) .24 (.03) .15 (.04)
.52 (.02) – .10 (.04)
Note: R0/F15: zero ‘‘remember’’ and 15 ‘‘forget’’ words. SDs are provided in parentheses.
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Table 2 Mean rates of correct recall on the immediate recall and cued recall tests as a function of word type in experiment 1b (associative lists) Word type
R0/F15 (F list)
R5/F10
R10/F5
R15/F0 (R list)
Immediate recall R words F word intrusions Critical word intrusions
– – –
.89 (.02) .01 (.00) .04 (.02)
.66 (.03) .05 (.01) .10 (.03)
.64 (.02) – .13 (.03)
Cued recall R words F words Critical words
– .12 (.02) .38 (.06)
.70 (.03) .15 (.02) .23 (.05)
.48 (.02) .17 (.02) .16 (.04)
.48 (.02) – .19 (.04)
Note: R0/F15: zero ‘‘remember’’ and 15 ‘‘forget’’ words. SDs are provided in parentheses.
the R10/F5 and R15/F0 conditions (both values of p < .001). No significant difference was found between the results of the R10/F5 and R15/F0 conditions. For the F words, there was a significant difference between the R5/ F10 and R10/F5 conditions [F(1, 39) = 12.38; MSe = .002; p < .01]. The word number did not affect the critical word intrusions. For cued recall, a one-way ANOVA was conducted to test the effect of the word number (R0/F15 vs. R5/F10 vs. R10/F5 vs. R15/F0) on the false recall of critical lures. Pairwise comparisons revealed that the false recall of critical lures in the R0/F15 condition was significantly higher than the R5/F10, R10/F5 and R15/F0 conditions (all values of p < .05). To determine whether a directed forgetting effect and/or an effect of word number existed, a two (word type: R vs. F words) by two (word number: R5/F10 vs. R10/F5) ANOVA was performed. The ANOVA revealed significant effects of both word type and word number, F(1, 39) = 351.80, MSe = .02, p < .001, and F(1, 39) = 38.22, MSe = .02, p < .001, respectively. The interaction effect was also significant, F(1, 39) = 14.25, MSe = .02, p < .001. Bonferroni tests revealed that participants recalled a significantly higher rate of R words in the R5/ F10 than in the R10/F5 condition (p < .001). For F words, no significant differences were noted between these two conditions. Experiment 1b differed from Experiment 1a only in the materials used. This difference apparently did not cause any differences in the results. Experiment 1b replicated all of the major findings of Experiment 1a. First, false recall increased with the proportion of F words in the list. Second, a directed forgetting effect was found in cued recall. In both experiments, prior to the final cued recall, the participants had to recall and write down the R words for each list. Although the rate of immediate false recall was relatively low and in the opposite direction of the findings of the final recall test, the immediate recall test itself might have affected the final cued recall by strengthening the encoding of R words. In particular, the final cued recall of critical lure could have been reduced due to either increase in retrieval competition from R words or changes in monitoring criterion. To avoid this confounding, the immediate recall was removed in Experiment 2.
4. Experiment 2 The purpose of Experiment 2 was to examine the effect of intentional forgetting on false memory, without the confounding caused by the prior immediate recall of R words. Participants in this experiment were subjected only to the cued recall test. 4.1. Method Experiment 2 differed from the previous two experiments only in that it did not involve the immediate recall test. After the participants had listened to each studied list, the experimenter played the next list. The participants did not have to recall the R words. The interval between the lists was 2 s. The rest of the procedure was identical to those of Experiments 1a and 1b. Sixty undergraduate students participated in this experiment voluntarily to fulfil part of their course requirements. Half of the participants received the categorized lists and the other half received the associative lists. 4.2. Results and discussion Table 3 presents the results of the cued recall tests. Categorized lists. To determine whether a directed forgetting effect and/or an effect of word number existed, a two (word type: R vs. F words) by two (word number: R5/F10 vs. R10/F5) ANOVA was performed. The ANOVA revealed significant effects of both word type and word number, F(1, 29) = 98.05, MSe = .03, p < .001, and F(1, 29) = 4.62, MSe = .03, p < .05, respectively. The interaction effect was also significant, F(1, 29) = 5.39, MSe = .01, p < .05. Bonferroni tests revealed that participants recalled a significantly higher rate of R words in the R5/F10 than in the R10/F5 condition (p < .01). For F words, no significant differences were noted between these two conditions. As for the false recall of critical lures, a one-way ANOVA revealed a significant effect of word number (R0/F15 vs. R5/F10 vs. R10/F5 vs. R15/F0) on false recall [F(3, 87) = 3.29; MSe = .09; p < .05]. Pairwise comparisons revealed higher false recall rates in both the R0/F15 and
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Table 3 Mean rates of correct cued recall as a function of word type for categorized lists and associative lists in experiment 2 Word type
R0/F15 (F list)
R5/F10
R10/F5
R15/F0 (R list)
Categorized lists R words F words Critical words
– .20 (.02) .40 (.07)
.62 (.05) .26 (.02) .22 (.05)
.51 (.03) .28 (.03) .23 (.06)
.42 (.03) – .38 (.06)
Associative lists R words F words Critical words
– .10 (.01) .37 (.06)
.52 (.04) .17 (.03) .18 (.05)
.40 (.02) .17 (.03) .23 (.06)
.28 (.02) – .35 (.06)
Note: R0/F15: zero ‘‘remember’’ and 15 ‘‘forget’’ words. SDs are provided in parentheses.
R15/F0 conditions than in both the R5/F10 and R10/F5 conditions (all values of p < .05). There were no significant differences between the results obtained for the R0/F15 and R15/F0 conditions and between those for the R5/F10 and R10/F5 conditions. These findings indicate that the F list (R0/F15) and R list (R15/F0) produced the same rate of false recall. Because the participants were asked to remember or forget the whole list, these two study conditions were similar to those of the list method. These results are also consistent with the previous finding that list method directed forgetting did not reduce false memory. Associative lists. A two (word type: R vs. F words) by two (word number: R5/F10 vs. R10/F5) ANOVA was performed to examine whether there was a directed forgetting effect or an effect of word number, or both. The ANOVA revealed significant effects of both word type and the interaction between word type and word number, F(1, 29) = 65.01, MSe = .04, p < .001, and F(1, 29) = 3.92, MSe = .03, p < .058, respectively. The effect of word number was also significant, F(1, 29) = 7.55, MSe = .02, p < .05. Bonferroni tests revealed that participants recalled a significantly higher rate of R words in the R5/F10 than in the R10/F5 condition (p < .05). For F words, no significant differences were found between these two conditions. For the false recall of critical lures, a one-way ANOVA revealed a significant effect for the word number [R0/F15 vs. R5/F10 vs. R10/F5 vs. R15/F0: F(3, 87) = 2.90; MSe = .08; p < .05]. Pairwise comparisons revealed that the false recall rates in both the R0/F15 and R15/F0 conditions were higher than that in the R5/F10 condition (both values of p < .05). There were no significant differences between the R0/F15 and R15/F0 conditions and between the R5/F10 and R10/F5 conditions. After the immediate recall was removed, this study still detected a directed forgetting effect in cued recall. More importantly, for both the categorized and associative lists, the false recall rate decreased in relation to the proportion of F words in the list. When participants were asked to forget the whole list, forgetting did not reduce false memory. It appears that the immediate recall did have an effect on the final false recall, especially for the R15/F0 condition in which participants had to immediately recall all the studied words. When the immediate recall was removed, Exper-
iment 2 found that intentionally forgetting part of the list items reduced false recall. In contrast, forgetting the whole list (the R0/F15 condition), similar to the F list of the list method directed forgetting, did not have such an effect. 5. General discussion This study used the item method directed forgetting paradigm to examine the effect of intentional forgetting on false memory produced by the list-learning procedure. Both categorized and associative list materials were used. The proportion of F words in the studied list was manipulated, but the length of the list was maintained constant. In all experiments, directed forgetting effects were found in cued recall, suggesting that manipulation of the ‘‘remember’’ vs. ‘‘forget’’ instruction was effective. When there was immediate recall of R words, Experiments 1a and 1b found that false recall decreased upon decreasing the proportion of F words. This result apparently was caused by the confounding that encoding was strengthened only for the R words. When the immediate recall test was removed, the degree of false recall decreased upon increasing the proportion of F words, except for the condition in which participants were instructed to forget the whole list. These findings suggest that forgetting part of the studied list reduced the false recall of critical lures, whereas forgetting the whole studied list did not affect false recall. This study used two conceptually similar procedures – semantic associate and category associate lists – to produce false memory. The present findings seem to be reliable and robust, given that different processes of inducing false memory might be involved in these two kinds of materials (Smith et al., 2002). Moreover, as predicted, both categorized lists and associative lists evoked semantic activation of critical words during study and within-list forgetting occurred for both semantically and thematically related words. Kimball and Bjork (2002), using the list method of directed forgetting, found that participants were less likely to recall the studied item, but more likely to falsely recall the critical-item, for the F list than for the R list. When the part-list cuing procedure was used, however, retrieving part of the list reduced both recall of the studied item and
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false recall of critical items. In other words, recall of both studied words and critical words declined with part-list cuing. Kimball and Bjork (2002) hypothesized that this difference occurred because of retrieval competition between lists in directed forgetting and within lists in part-list cuing. The difference between the R15/F0 and R0/F15 conditions in the present study was equivalent to the list method manipulation. The fact that the R0/F15 condition was not rehearsed as intensively as that of the R15/F0 condition led to the directed forgetting effect. As for the list method findings of Kimball and Bjork (2002), semantic activation of the R0/F15 condition was, however, unimpaired because no within-list competition involved. An unimpaired semantic activation combined with a decreased access to episodic information led to an increase of false recall found in Kimball and Bjork (2002). One possibility is that F words decreased access to the episodic memory, leading to a more liberal response criterion. Unlike Kimball and Bjork (2002), however, the present study did not find an increase in the degree of false recall. Several studies have found that F lists must be followed by R lists for a forgetting effect to occur (e.g., Bjork, 1989). The studied list of the R0/F15 condition was similar, but not identical, to the forget list in the list method. The differences were that the R0/F15 list was not necessarily followed by a remember list and that the forgetting instruction was given after each item. Because no remember list was present, the forgetting effect caused by an item-by-item instruction might not be sufficiently strong to impair episodic information, as found in the study reported by Kimball and Bjork (2002). When the participants had to forget only part of the list (the R5/F10 and R10/F5 conditions), the current results were similar to the findings obtained from part-list cuing in Kimball and Bjork (2002) study. There was within-list retrieval competition between F and R words. More F words in the R5/F10 and R10/F5 types of lists, compared with those of the R15/ F0 type, led to a lower level of semantic activation and, thus, reduced the degree of false recall of critical lures. The current results are consistent with the account proposed by Kimball and Bjork (2002), i.e., that item-level and list-level forgetting had different effects on false recall. Furthermore, the results of the current study suggest that intentional forgetting, just as in the case of unintentional forgetting caused by part-list cuing, could also reduce false recall of critical words. In addition, Marche, Brainerd, Lane, and Leohr (2005) suggest that in contrast to the list method, the item method impairs storage of both verbatim and gist traces during study. Their study using the standard item method directed forgetting demonstrated that item-level directed forgetting reduced false recall, a result similar to the current finding. The current study made an additional contribution by examining both the item-level and list-level directed forgetting in a single control experiment, and thus providing stronger support for the difference between item-level and list-level inhibition. There is also a limitation on the inter-
pretation of the current study. Since the number of F and R words within a list is confounded (negatively correlated), the current study cannot firmly determine whether the current pattern of result was caused by the changes in the proportion or absolute number of R and/or F words. Further research is needed to find the answer. In conclusion, the level of semantic activation caused by studying the list words is one of the main determining factors of false memory (e.g., Roediger et al., 2001; Underwood, 1965). The current study demonstrates that when participants were asked to forget a partial number of list items (an item-level forgetting), the item method of directed forgetting could reduce the semantic activation within a semantic-related list and further reduce false memory. On the other hand, semantic activation was not impaired when the participants were asked to forget the whole semanticrelated list. As a result, forgetting the whole studied list did not reduce the degree of false recall. Acknowledgements This research was supported by Research Grant NSC 91-2413-H-194-027 from the National Science Council of the Republic of China. The author thanks Mr. Huangmou Lee for assistance in administering part of the experiments and the reviewers for helpful comments and suggestions. Portions of this research were presented at the 13th Conference of the European Society of Cognitive Psychology (September 2003, Spain). References Basden, B. H., & Basden, D. R. (1998). Directed forgetting: A contrast of methods and interpretations. In J. M. Golding & C. M. MacLeod (Eds.), Intentional forgetting (pp. 139–172). Mahwah, NJ: Erlbaum. Basden, B. H., Basden, D. R., & Gargano, G. J. (1993). Directed forgetting in implicit and explicit memory tests: A comparison of methods. Journal of Experimental Psychology: Learning, Memory, & Cognition, 19, 603–616. Bjork, R. A. (1989). Retrieval inhibition as an adaptive mechanism in human memory. In H. L. Roediger, III & F. I. M. Craik (Eds.), Varieties of memory and consciousness: Essays in honour of Endel Tulving (pp. 309–330). Hillsdale, NJ: Lawrence Erlbaum. Block, R. A. (1971). Effects of instructions to forget in short-term memory. Journal of Experimental Psychology, 89, 1–9. Collins, A. M., & Loftus, E. F. (1975). A spreading-activation theory of semantic processing. Psychological Review, 82, 407–428. Deese, J. (1959). On the prediction of occurrence of particular verbal intrusions in immediate recall. Journal of Experimental Psychology, 58, 17–22. Jeng, C., Lai, M., & Liu, I. (1973). Category norms in Chinese and English from bilingual subjects. Acta Psychologica Taiwanica, 15, 81–153. Kimball, D. R., & Bjork, R. A. (2002). Influences of intentional and unintentional forgetting on false memories. Journal of Experimental Psychology: General, 131, 116–130. Lee, Y. & Hsu, Y.-C. (2000). False recall of to-be-forgotten information. In Poster presented at the 41st Annual Meeting of the Psychonomic Society, New Orleans, Louisiana, USA. MacLeod, C. M. (1975). Long-term recognition and recall following directed forgetting. Journal of Experimental Psychology: Human Learning & Memory, 1, 271–279.
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