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Effects of post-cue interval on intentional forgetting.

Forgetting usually occurs imperceptibly and spontaneously with the passing of time and is often considered a major memory problem. In some circumstances, however, forgetting is exactly what we need to do to function efficiently (see Anderson & Schooler, 2000 for an adaptive analysis of forgetting). First, a key to accurate memorization is the ignoring or inhibition of irrelevant information. For orderly and purposeful memory function, information must be classified as relevant or irrelevant, memory must be constantly updated, and only those memories relevant to the task at hand should be retrieved. Second, people often try to forget unwanted memories, such as events in their lives that are painful or embarrassing.

The issues related to intentional forgetting and suppression have involved many areas in psychology, such as social, clinical and cognitive psychology. Nonetheless, so far, the mechanisms of forgetting/suppression remain poorly understood. Different areas of study often produced mixed results. Studies on directed forgetting, conducted mostly by cognitive psychologists, have shown that forgetting can be intentional and controllable (see MacLeod, 1998, for a review). On the other hand, studies on thought suppression, the concern of clinical and social psychologists, tend to suggest that people have difficulties in suppressing unwanted thoughts (see Wegner, 1994; Wenzlaff & Wegner, 2000, for reviews). Yet the notion of repressing unwanted memories is central to psychoanalytic theory. However, studies on repression sometimes support the idea that unwanted memories can be repressed and then recovered, and sometimes not (see Loftus & Ketcham, 1994). The purpose of this study was to find a possible factor that was related to both intentional forgetting and thought suppression by examining the effect of post-cue interval.

Substantial literature has been devoted to directed forgetting (see Johnson, 1994; MacLeod, 1998, for reviews). Two common procedures have been 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 item), such as 'forget', and for each to-be-remembered item (R item), such as 'remember'. The memory cue comes after the relevant item to ensure that the participant has registered the words. The list method only involves one cue, usually an instruction to forget all 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 R items, and sometimes proactive interference on the R items followed by the F items is reduced.

The occurrence of this effect depends on how memory is tested. When participants are tested with a recall test, both item and list method produce a directed forgetting effect. However, when participants are tested using a recognition test, rather than a recall test, only the item method produced 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 favouring the R items, whereas the list method promotes inhibition of the F items (Basden & Basden, 1998; Basden, Basden, & Gargano, 1993; MacLeod, 1999; Wilson & Kipp, 1998). In the item method, the F items are less well rehearsed than the R items and thus F items are not encoded as completely as R items. This phenomenon impairs both recognition and recall for F items. In the list method, both R items and F items are encoded and stored in memory. The cue to forget affects retrieval access to F items. Consequently, the directed forgetting effect was found 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).

Some studies using the item method have found results consistent with the retrieval inhibition account, however. Firstly, Weiner and Reed (1969) found higher recall in the unrehearsed remember condition than in the active forgetting condition, suggesting that the active forgetting was not a result of mere nonrehearsal. Roediger and Crowder (1972), nonetheless, found evidence suggesting that the forgetting effect was due to covert rehearsal in the nonrehearsal remember condition. Secondly, Geiselman and Bagheri (1985) found greater memory gains for the initial F items than the R items and proposed that this was evidence for the releasing of F items from retrieval inhibition. Findings from directed forgetting on indirect memory tests are also consistent with the notion of retrieval inhibition (MacLeod, 1989). On the other hand, Basden et al. (1993) suggest that MacLeod's result may have been contaminated by explicit retrieval and the evidence from Geiselman and Bagheri was not compelling; thus the role of retrieval inhibition in the item method of forgetting is negligible.

If directed forgetting is mediated by processes that enhance the rehearsal of remember items rather than by processes that suppress or inhibit the retrieval of forget items, then F items ought to be ignored after the forgetting cue. As a result, increasing the post-cue interval should improve memory only for R items and not for F items. Specifically, memory for R items should increase as a function of the immediate rehearsal period, whereas the retention for F items ought to remain stable over such a period. Although many studies have investigated the effect of pre-cue interval on directed forgetting (e.g. Timmins, 1974; Woodward, Bjork, & Jongeward, 1973), we found only one study in the literature testing this prediction. Allen and Vokey (1998) manipulated the time available to retrieve and rehearse an item following an instruction to remember or forget it. They found that the directed forgetting effect did not vary as a function of the time available (1 vs. 3 seconds) for rehearsal and occurred even for items immediately followed by a remember item (zero lag). Thus, they concluded that some process at the time of the first instruction to remember or to forget was responsible for the directed forgetting effect and that additional rehearsal opportunities played no significant role in directed forgetting.

There were several problems with this study, however. Firstly, the difference between the post-cue intervals of 1 vs. 3 seconds might not be large enough to create a difference in the directed forgetting effect. This was very likely because the investigators also did not find an expected effect of rehearsal (post-cue interval) on R items (compare with Wetzel & Hunt, 1977). Secondly, even at the shortest rehearsal times (1 second), on the average R items still had more chances to be rehearsed than the F items because participants are very likely to take every instruction to forget as a period in which to retrieve and to rehearse prior R items. Thus, the single-retrieval hypothesis proposed by Allen and Vokey (1998) was not the only explanation.

Thirdly, their manipulation of training lag (the number of F items given prior to an R item) might not be effective, because the post-cue interval for F items might not all be used to rehearse the immediately prior remember items. Instead, it might be used to rehearse several prior remember items. This problem was similar to the one in within-list manipulation experiments of Wetzel and Hunt (1977), which found that varying the rehearsal time for items within a list was relatively ineffective, because participants might take every instruction to forget as a period in which to rehearse prior remember items. Furthermore, it seems counterintuitive that asking participants to rehearse prior R item while encountering F items did not improve later memory performance of that R item. It is possible that participants inadvertently spent some effort on the intervening F items and this reduced the effect of lag on rehearsal. Overall, there was possibly an ineffective manipulation of rehearsal in Allen and Vokey (1998). Moreover, the item method of directed forgetting may involve multiple mechanisms and the single-retrieval hypothesis may be just one of them.

The purpose of this study was to examine the effect of post-cue interval on directed forgetting. Instead of manipulating the training lag, Experiments 1 and 2 focussed on the time available after the R or F cue and increased the difference between short and long post-cue interval to 1 vs. 5 seconds. If the directed forgetting effect comes from selective rehearsal--that is, participants only rehearse the R items and ignore the F items after the cue--then R items will be remembered better than the F items. In addition, as the post-cue interval increases, memory for R items should improve, but memory for F items should remain stable. On the other hand, if memory for F items changes with an increase in post-cue interval, then participants must not just ignore the F items; some other processes must be involved. There are two possibilities. First, Anderson and Green (2001) recently found that increasing the number of suppression attempts led to worse recall in the subsequent memory test, suggesting that people can voluntarily forget information by willfully suppressing it. Based on this finding, it is possible that the longer the post-cue interval is for F items, the better participants can suppress the memory for F items. This would lead to decreasing memory for F items as the post-cue interval increases. On the other hand, based on thought suppression studies suggesting that people usually have difficulties in suppressing unwanted thoughts (Wenzlaff & Wegner, 2000) and the ironic process theory (Wegner, 1997), it is possible that as the post-cue interval increases, participants have more chances to be reminded of the F items, resulting in better memory for F items.

The item method of directed forgetting and the procedure used in Anderson and Green (2001) are very different. To obtain convergent evidence, Experiment 3 followed the procedure of Anderson and Green and manipulated both the number of suppressions and the duration of each suppression attempt. Manipulation of the duration of suppression is analogous to the manipulation of post-cue interval in Experiments 1 and 2.

EXPERIMENT I

Experiment 1 examined the effect of post-cue interval on directed forgetting by comparing memory for two different post-cue intervals: 1 vs. 5 seconds. The main focus was on how memory for F items is affected by the increase in post-cue interval.

Method

Participants and design

The participants were 32 students taking introductory psychology courses at National Chung-Cheng University, Taiwan, who participated voluntarily to fulfil part of their course requirements. This experiment was a 2 by 2 within-subjects design. The two factors manipulated were post-cue interval (1 vs. 5 seconds) and cue type (remember vs. forget).

Materials

Forty two-character Chinese words were used. They were common nouns and belong to different taxonomic categories. The 40 words were divided into two sets to serve as the study (target) items or distracters and were given to two different subgroups of participants. Across these two subgroups of participants, a word had the same chance of being a study item or a distracter. Each set of 20 words was further divided into four subsets and assigned to the four conditions (1 vs. 5 seconds post-cue interval and remember vs. forget cue type) manipulated in this experiment. The assignment of each subset of words to the four experimental conditions was counterbalanced. All 40 words were used in the recognition test, 20 of which were target items and the other 20 of which were distracters. The experiment was run on an IBM PC compatible microcomputer. The viewing distance was 50 cm and the size of the character was 3 x 3.5cm.

Procedure

Participants were tested individually. Each experimental session consisted of a study phase and two testing phases. During the study phase, participants read the 20 words one by one. After reading the studied word for 2 seconds and then a blank screen for 0.5 seconds, participants were given a cue to remember or forget the word. The cue remained on the screen for either 1 or 5 seconds, depending on the post-cue interval condition. The next studied word appeared after a 0.5-second blank screen. The presentation of the studied words was blocked by the post-cue interval. The order of two different post-cue interval conditions was counterbalanced across participants. Within each block, the presentation order of the studied words and cue words was randomly determined. However, the same type of cue word would not appear on more than three consecutive trials.

Participants were instructed to read each word and the cues that followed. They were required to remember the studied words that were cued to 'remember', and to forget those that were cued to 'forget'. After studying two blocks of 10 words, participants were asked to write down as many words as they could recall, including both R and F items that they had read during the study phase. After the 5-minute recall test, a recognition test was given to participants. The recognition test comprised 40 randomly presented words. Words were presented individually on a computer screen with self-paced timing. They were told to press the 'Y' key for words they remembered having read before, regardless of whether the words had been R or F items; otherwise, they were told to press the 'N' key.

Results and discussion

The mean rates of correct recall and recognition (the hit rate, the false alarm rate) and false alarm rates are presented in Table 1. For both recall and recognition, there was no block order effect and it also did not interact with other factors.

Recall

A 2 (1- vs. 5-second post-cue interval) by 2 (R vs. F cue type) ANOVA showed significant main effects of post-cue interval and cue type, F(1, 31) = 7.41, MSe = .061, p < .05, F(1, 31) = 195.30, MSe = .045, p < .001, respectively. The interaction effect was marginally significant, F(1, 31) = 3.57, MSe = .028, p = .06. The mean differences between 1- and 5-second intervals and 95% confidence intervals were: for R cue .18 [+ or -] .12 and for F cue .06 [+ or -] .09.

Recognition

A 2 (1- vs. 5-second post-cue interval) by 2 (R vs. F cue type) ANOVA showed significant main effects of post-cue interval and cue type, as well as a significant interaction effect, F(1, 31) = 10.19, MSe = .042, p < .01, F(1, 31) = 32.20, MSe = .055, p < .001 and F(1, 31) = 6.91, MSe = .033, p < .05, respectively. The mean differences between 1- and 5-second intervals and 95% confidence intervals were .03 [+ or -] .08 for R cue and for .20 [+ or -] .11 for F cue.

From the above analyses, the directed forgetting effect was found in both recall and recognition tests. In addition, for the recall test, the advantage of the 5-second post-cue interval condition over the 1-second condition was reliable for the R item. Increasing cue interval did not improve recognition of R items possibly because the recognition test was carried out after the recall test and this caused a ceiling effect in recognition. The recognition test, however, indicated that post-cue interval affected F items; the recognition rate increased as the post-cue interval increased. The finding that recall of R items increased with the increasing post-cue interval was consistent with the selective rehearsal account. On the other hand, the result that recognition for the F items in the 5-second condition was better than recognition for the F items in the 1-second condition suggests that participants did not stop processing F items during the post-cue period. It seems difficult for participants to ignore the F items. Memory for F items continuously increased as the post-cue interval increased. The result that increasing cue interval improved recognition and not recall of F items suggests that the extra post-cue interval for F items may only increase the familiarity and not conscious recollection of F items. Experiment 2 was designed to test this possibility.

EXPERIMENT 2

The purpose of Experiment 2 was to replicate the results of Experiment 1 with some improvements in the methods. First, Experiment 2 included more studied items in each condition. Second, in addition to the directed forgetting group, a control group who had to remember all the studied words was included in Experiment 2. Finally, Experiment 1 indicated a different pattern of results for recall and recognition. To avoid the influence of prior recall on recognition, recognition in Experiment 2 was tested on a different group of participants.

More importantly, to further examine the effect of post-cue interval on different aspects of recognition, participants were also asked to make remember vs. know judgements during the recognition test. Asking participants to make these judgements allowed us to assess participants' subjective experience of their memory. According to Tulving (1985), remember and know judgements measure autonoetic and noetic consciousness, which is respectively characterized by awareness of one's self in relation to time and place, and by awareness of just event and not one's self in relation to time and place. Remember responses assess participants' conscious recollective experiences, whereas know responses reflect an automatic process of feeling of familiarity. The 'guess' responses were also included to assess other nonmnemonic influences of studied word.

Method

Participants and design

The participants were 48 students taking introductory psychology courses at National Chung-Cheng University, Taiwan, who participated voluntarily to fulfil part of their course requirements. Sixteen participants were assigned to the directed forgetting/recall (FR) group, 16 to the remember/recall (RR) group and 16 to the directed-forgetting/recognition group. Within each group, the experiment was a 2 by 2 within-subjects design. The two within-subjects factors were post-cue interval (1 vs. 5 seconds) and cue type (remember vs. forget).

Materials

The materials were the same as those used in Experiment 1. The 40 words were divided into four subsets and assigned to the four conditions (1- vs. 5-second post-cue interval and remember vs. forget cue type) manipulated in this experiment. The assignment of each subset of words to the four experimental conditions was counterbalanced. All 40 words were used in the recognition test as the targets. An additional 40 words were used as the distracters. The experiment was run on an IBM PC compatible microcomputer. The viewing distance was 50 cm and the size of the character was 3 x 3.5 cm.

Procedure

Participants were tested individually. The procedures were the same as those of Experiment 1, except for the following differences. First, in addition to the directed forgetting/recall (FR) group who was given both 'remember' and 'forget' cues, a remember/recall (RR) group was included. For this group the cue that followed the studied word was always 'remember'. Second, the FR and the RR groups were only asked to recall all the studied words, whereas the directed-forgetting/recognition group was only asked to carry out the recognition test. In the recognition test, participants were told to press the 'Y' key for any word that they had seen in the study phase, and press the 'N' key for any words they had not seen in the study phase. If participants pressed the 'Y' key, a prompt on the screen instructed them to make a 'remember' or 'know' or 'guess' judgement by pressing the 'R' or 'K' or 'G' key. Participants were told to press 'R' if they had a conscious recollection of the word from the study lists, such as a recollection of what the word looked like, or 'K' if they were sure that the word was presented, but they could not recollect its actual occurrence or any related details. If they were not sure that the word was presented and their response was just a guess, then they should press the 'G' key.

Results and discussion

The mean rates of correct recall and different types of responses on the recognition test are presented in Table 2. For all three groups, there was no block order effect and it also did not interact with other factors.

Recall

A 2 (FR vs. RR group) by 2 (1- vs. 5-second post-cue interval) by 2 (R vs. F cue type) ANOVA showed significant main effects of group, post-cue interval and cue type, F(1, 30) = 5.70, MSe = .028, p < .05, F(1, 30) = 21.65, MSe = .044, p < .001 and F(1, 30) = 42.84, MSe = .028, p < .001, respectively. More importantly, the interaction effects between group and cue type and between post-cue interval and cue type were significant, F(1, 30) = 54.51, MSe = .028, p < .001 and F(1, 30) = 5.53, MSe = .020, p < .05, respectively. The three-way interaction effect was also significant, F(1, 30) = 4.96, MSe = .020, p < .05. The mean differences between 1- and 5-second intervals and 95% confidence intervals were: for FR group/R cue .28 [+ or -] .15, for FR group/F cue .05 [+ or -] .06, for RR group/R cue .18 [+ or -] .16 and for RR group/F (R) cue .18 [+ or -] .14. The significant three-way interaction might rely on a floor effect of duration for F cue in the FR group.

Recognition

Several 2 (1- vs. 5-second post-cue interval) by 2 (R vs. F cue type) ANOVAs were performed on the percentages of 'remember', 'know', 'guess' and overall 'yes' responses on the recognition test. For the overall 'yes' responses, significant main effects of both post-cue interval and cue type were found, F(1, 15) = 24.39, MSe = .006, p < .001 and F(1, 15) = 33.22, MSe = .032, p < .001, respectively. There was no significant interaction effect. Participants gave more 'yes' responses to R items than F items, and more 'yes' responses for the 5-second cue interval than the 1-second cue interval condition. The same pattern of result was found for the 'remember' responses: there were significant main effects of both post-cue interval and cue type, F(1, 15) = 27.33, MSe = .014, p < .001 and F(1, 15) = 95.94, MSe = .039, p < .001, respectively. The mean differences between 1- and 5-second intervals and 95% confidence intervals were: overall response/R cue .06 [+ or -] .07, overall response/F cue .13 [+ or -] .08, remember response/R cue .18 [+ or -] .09 and remember response/F cue .13 [+ or -] .11.

Both 'know' and 'guess' responses showed the main effect of cue type, F(1, 15) = 5.81, MSe = .035, p < .05 and F(1, 15) = 20.35, MSe = .009, p < .001, respectively. Participants gave more 'know' and 'guess' responses to F items than R items. Neither the main effect of post-cue interval nor the interaction effect between post-cue interval and cue type was reliable. This might partly depend on a floor effect in the remember conditions. The mean differences between 1- and 5-second intervals and 95% confidence intervals were: know response/R cue -.07 [+ or -] .09, know response/F cue .02 [+ or -] .09, guess response/R cue -.05 [+ or -] .06 and guess response/F cue -.02 [+ or -] .08.

Since the RR group did not receive the F cue, they did not reveal a directed forgetting effect and only showed a beneficial effect of increasing post-cue interval on recall. The pattern of result for the FR group replicated the finding of Experiment 1. For the directed forgetting/recognition group, first, a directed forgetting effect was found. Second, increasing post-cue interval improved recognition of both R and F items. More importantly, based on participants' report of subjective experience, increasing post-cue interval did not increase judgements for familiarity, instead it increased responses of conscious recollection. In other words, increasing the post-cue period actually enhanced the conscious recollective experience of F items, suggesting that participants did not stop consciously processing F items after the presentation of the forget cue. The present results were also compatible with a single-process trace strength account (e.g. Donaldson, 1996): R items produced a stronger memory trace than F items, which had a higher rate of know response than R items. In addition as the post-cue interval increased, strength of memory trace for F items also increased.

EXPERIMENT 3

Experiments 1 and 2 found that recognition for the F items increased with increasing post-cue interval, suggesting continued processing of F items and failure to ignore F items during the post-cue period. This result was consistent with thought suppression research (e.g. Wegner, 1994) proposing that ignoring unwanted thoughts is difficult, and not with the finding of Anderson and Green (2001). To examine whether the same result could be obtained using Anderson and Green's paradigm, Experiment 3 adopted their procedure, manipulating both the number of response/suppression attempts and the duration of response/suppression time (3 vs. 5 seconds) for each word pair. The duration of response/suppression time was 4 seconds in Anderson and Green.

Method

Participants and design

The participants were 84 students taking introductory psychology courses at National Chung-Cheng University, Taiwan, who participated voluntarily to fulfil part of their course requirements. Forty-two participants were assigned to the short duration condition and the other 42 to the long duration condition. This experiment was a 2 by 2 by 3 mixed-subjects design. The between-subjects factor was the duration of response/suppression period (short vs. long). The two within-subjects factors were the type of response (response vs. suppression) and the number of response/suppression attempts (0 vs. 8 vs. 16 times). The counterbalance of the three different numbers of attempts for both response and suppression trials resulted in six versions of the task. Within each condition of exposure duration, seven participants were assigned to each of the six versions.

Materials

Forty two-character Chinese word pairs were used. All the stimulus words and response words were neutral words. The stimulus and response words in each pair were weakly associated. The 40 word pairs consisted of 30 critical pairs and 10 filler pairs. Half of the 30 critical pairs were assigned to the response trials and the other half were assigned to the suppression trials, and this factor was crossed with response/suppression attempts (0 vs. 8 vs. 16). Assignment of 30 critical pairs to suppression and response trials and three different numbers of response/suppression attempts was counterbalanced across participants, with five critical pairs in each condition.

Procedure

Participants were tested individually. Each experimental session consisted of a learning phase, a suppression-training phase and a test phase. First, during the learning phrase, each word pair was presented in the center of a computer screen for 5 seconds. Participants were required to remember each word pair and were told that a cued-recall test would be given. They were told that in the cued-recall, the stimulus word on the left would be given as a cue and they had to recall the response word on the right. After participants studied all the word pairs, a stimulus word was displayed in the center of the screen and participants were instructed to recall the corresponding response word and say it aloud as quickly as possible. Once participants responded, the next stimulus word appeared. If participants made an incorrect response, the correct word would appear on the screen as feedback. The percentage of correct responses for each participant was calculated. If the correct percentage was lower than 50% on the first assessment, then the test-feedback cycles continued until a minimum of 50% of word pairs were correctly recalled.

Secondly, during the suppression-training phase, participants were first presented with the 10 stimulus words from the to-be-suppressed pairs without their responses. They were told to remember these words so that they could identify them during the suppression phase. Participants were given as much time as they needed to learn these words. After a brief practice session on the response/suppression task, five stimulus words for responding eight or 16 times and five stimulus words for suppressing eight or 16 times were presented, for a total of 240 trials. In addition, 120 trials displayed stimulus words for responding with the 10 filler words, to create an overall tendency of responding. The order of 360 trials was randomly determined. At the start of each trial, a cross appeared in the center of the screen for 200 msec. Then the stimulus word appeared for either 3 seconds (short condition) or 5 seconds (long condition), or less if participants responded earlier. On response trials, participants were instructed to recall the response word aloud as quickly as possible. The experimenter recorded participants' answers. For the suppression trials, participants were asked to prevent the associated memory item from coming to mind. Any response to a stimulus word for suppression was recorded by the computer and would trigger a beep sound.

In the final test phase, participants were asked to recall the response word for each stimulus word, regardless of prior instructions. All stimulus word cues were presented individually in the center of the screen. Participants had to say the response word aloud via a microphone and their response times were recorded. No feedback was given.

Results and discussion

Final recall

The results are shown in Figure 1, separately for the short and long conditions. The 2 (respond vs. suppress) by 2 (long vs. short duration) by 3 (0 vs. 8 vs. 16 attempts) ANOVA revealed a significant main effect of response type, F(1, 82) = 14.37, MSe = .029, p < .001, and significant interaction effects between response type and duration, F(1, 82) = 4.97, MSe = .029, p < .05, and between response type and number of attempts, F(2, 164) = 6.03, MSe = .017, p < .01. To examine the effect of duration and number of response/suppression attempts on final cued recall, 2 (long vs. short duration) by 3 (0 vs. 8 vs. 16 attempts) ANOVAs were performed. For the suppression trials, there was an interaction effect between duration and number of attempts, F(2, 164) = 2.74, MSe = .018, p = .06. The interaction of duration with the linear trend was also significant, F(1, 82) = 4.24, MSe = .021, p < .05. The means and 95% confidence intervals were: for long duration .86 [+ or -] .059 (0 attempt), .89 [+ or -]. 050 (8 attempts) and .87 [+ or -]. 057 (16 attempts); for short duration .91 [+ or -] .048 (0 attempt), .86 [+ or -]. 052 (8 attempts) and .82 [+ or -]. 073 (16 attempts).

[FIGURE 1 OMITTED]

As far as the response trials are concerned, there were significant main effects of both duration and number of attempts, F(1, 82) = 6.35, MSe = .031, p < .05, F(2, 164) = 5.46, MSe = .014, p < .01. The interaction effect was not significant. The interaction between duration and linear trend was also not significant, whereas the overall linear trend was significant, F(1, 82) = 6.56, MSe = .019, p < .05. The means and 95% confidence intervals were: for long duration .86 [+ or -] .056 (0 attempt), .91 [+ or -] .046 (8 attempts) and .92 [+ or -] .039 (16 attempts); for short duration .91 [+ or -] .046 (0 attempt), .97 [+ or -] .029 (8 attempts) and .97 [+ or -] .41 (16 attempts).

The short duration condition replicated the results of Anderson and Green (2001): as the number of suppression attempts increased, recall decreased. The long duration did not show such an effect. However, it remains possible that this was caused by the baseline differences between the short and long duration. The long duration had a longer retention interval between the initial study and final testing than the short duration. This might lead to the baseline differences between the two duration conditions. Further, recall in the response trials was better for short than long durations, again possibly because the short duration condition had a shorter retention interval and presumably less interference from the suppression trials on the final recall. The baseline recall rates were also quite high in the current experiment. To provide convergent evidence, future studies using a different set of materials could examine whether the results can be replicated when the baseline recall rates are lower and comparable between the two duration conditions.

Response times for the final recall

The 2 (respond vs. suppress) by 2 (long vs. short duration) by 3 (0 vs. 8 vs. 16 attempts) ANOVA revealed significant main effects of response type, F(1, 82) = 49.08, MSe = 152,982, p < .001, number of attempts, F(2, 164) = 22.29, MSe = 135, 561, p < .001 and significant interaction effect between response type and number of attempts, F(2, 164) = 6.64, MSe = 176,110, p < .01. To find the effects of duration and number of response/suppression attempts on the response latency of final recall, we also carried out 2 (long vs. short duration) by 3 (0 vs. 8 vs. 16 attempts) ANOVAs. For the suppression trials, neither the main effects nor the interaction effect was significant. The means and 95% confidence intervals were: for long duration 1,432 [+ or -] 151 (0 attempt), 1,401 [+ or -] 174 (8 attempts) and 1,246 [+ or -] 156 (16 attempts); for short duration 1,580 [+ or -] 240 (0 attempt), 1,507 [+ or -] 149 (8 attempts) and 1,471 [+ or -] 173 (16 attempts). For the response trials, there was a significant main effect of number of attempts, F(2, 164) = 33.42, MSe = 111,468, p < .001. The linear trend was also significant, F(1, 82) = 32.83, MSe = 150,014, p < .001. The means and 95% confidence intervals were: for long duration 1,389 [+ or -] 154 (0 attempt), 1,046 [+ or -] 107 (8 attempts) and 1,007 [+ or -] 83 (16 attempts); for short duration 1,485 [+ or -] 111 (0 attempt), 1,061 [+ or -] 66 (8 attempts) and 1,182 [+ or -] 173 (16 attempts).

The above analyses suggest that for the suppression trials, the suppression effect is mainly reflected in the result that participants' correct recall rates decreased as the number of suppression attempts increased. There was no difference in the response time. Regardless of the number of suppression attempts, participants took about 1.4 seconds on average to recall the suppressed words, which was longer than the response words. On the other hand, for the response trials, increasing response attempts not only increased percentages of correct response but also decreased the response times (Figure 2).

During the response/suppression phase, on response trials, participants were asked to recall the response word aloud as quickly as possible. Contrary to the study by Anderson and Green (2001), participants in the present study did not receive the correct word as feedback when they responded incorrectly on a response trial. This was the reason that in Anderson and Green, the recall of participants in response trials associated with 16 response attempts was almost perfect, as compared with the lower recall rate in the current study. Providing participants with correct words as feedback gave them another chance to learn the word pairs. The recall rate of the response trials in our study measured only the effect of repeated response, without confounding this with additional learning of word pairs. Even with this modification, the basic pattern of results remained the same for the short duration condition.

This experiment showed that increasing suppression duration and increasing the number of suppression attempts had an opposite effect on the memory of the to-be-suppressed items. The paradigms used in Experiments 3 and 2 were very different, but they revealed a consistent result that increasing suppression time or post-cue interval made suppression or ignoring of the to-be-forgotten words more difficult.

GENERAL DISCUSSION

The current experiments revealed two main findings. Firstly, increasing post-cue interval did not increase the forgetting rate of the to-be-forgotten information; on the contrary, it increased its retention. Secondly, increasing the number of suppression attempts led to a lower rate of final recall, as found in Anderson and Green (2001). However, increasing the duration of each suppression attempt reduced such an effect. These findings were consistent with both directed forgetting and thought suppression studies, suggesting that continued ignoring or suppressing the to-be-forgotten information was not an effective way to forget.

[FIGURE 2 OMITTED]

Previous studies (e.g. Bjork, 1989) using the list-method direct forgetting paradigm have found that F lists must be followed by the R lists in order for a forgetting effect to occur. In addition, the forgetting effect was reduced by a secondary task during the second to-be-remembered list learning (Conway, Harries, Noyes, Racsmany, & Frankish, 2000). These findings were consistent with the above proposal that ignoring to-be-forgotten information without processing other information is difficult, if not impossible. Processing other information seems crucial for forgetting. In the response/suppression paradigm, it is the act of stopping retrieval of unwanted memory that reduced its later retention. This attempt needed to be reinitiated for successful suppression because attention also needs to be directed to other information in order to inhibit the target information. Presumably, individuals switched attention to something else after each attempt at excluding associated memory from consciousness. After that, another attempt at suppression was again initiated. Continued suppression without switching attention to other information made the effect of excluding memory from consciousness became less and less effective. This point of view is consistent with the finding on thought substitution using response/suppression paradigm (Hertel & Calcaterra, 2005) and the finding that directing attention to a specific stimulus causes the simultaneous inhibition of the competing stimulus from both retrieval-induced forgetting paradigm (e.g. Anderson, Bjork, & Bjork, 1994) and attention studies (Pashler, 1998).

Active forgetting involves executive control processes (Anderson & Green, 2001); thus, attentional resources are needed to sustain such processes. One line of evidence is that the ability to stop processing no longer relevant information is impaired in elderly adults (Zacks, Radvansky, & Hasher, 1996), who have been shown to have reduced attentional resources (e.g. Craik, 1986; for a review, see Light, 1991). The efficiency of active forgetting weakens over time because constant attentional resources are necessary for successful intentional forgetting and suppression. Based on the ironic process theory of thought suppression (Wegner, 1997), the operating process, which promotes the preferred state of suppression, is effortful and attention-demanding. The monitoring process remains vigilant for occurrences of the unwanted thought and ironically leads to more thought intrusions, when capacity for suppression operation is limited.

Both Experiments 1 and 2 found that retention of R items was better than F items and increased with increased post-cue interval (and thus the chances of rehearsal). These findings supported the selective rehearsal account of the item-method directed forgetting. However, even though participants were supposed to stop rehearsing the F items after receiving the forget cue, their memory of F items as measured by the recognition test still improved over time. Moreover, it seems that active rehearsal was necessary for successful recall; thus, increasing post-cue interval did not improve recall of F items. However, continued exposure to F items did increase conscious recollection and not just familiarity of these words, or it increased strength of the memory trace for F items. In either case, this result could come from an automatic activation of F items which then became consciously assessable and/or a failure of suppression as the post-cue interval increased--a thought suppression phenomenon. The finding of suppression failure at the longer duration is also consistent with the memory consolidation theory (e.g. McGaugh, 2000), suggesting that memory becomes less vulnerable over time. Nonetheless, the present study did not provide direct evidence for inhibition in the item method of directed forgetting. Further research is needed to examine this issue.

Acknowledgements

We thank Dr Wen-chi Chiang for suggesting the remember-know data collection in Experiment 2. We are also grateful to Dr Baguley and two anonymous reviewers for their comments and suggestions.

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Received 20 December 2005; revised version received 29 May 2006

Yuh-shiow Lee*, Huang-mou Lee and Sheng-hsiung Tsai

National Chung-Cheng university, Taiwan, ROC

*Correspondence should be addressed to Yuh-shiow Lee, Department of Psychology, National Chung-Cheng University, Chiayi 621, Taiwan, ROC (e-mail: psyysl@ccu.edu.tw).
Table 1. Mean rates of correct recall and recognition (hit-false alarm)
as a function of cue type and post-cue interval in Experiment 1

                    Forget            Remember
             1-second  5-second  1-second  5-second

Recall       .16       .22       .63       .80
Recognition  .51       .71       .83       .86
False alarm  .06       .06       .05       .08

Table 2. Mean rates of correct recall and different types of response on
the recognition test as a function of cue type and post-cue interval in
Experiment 2

                                       Forget              Remember
Group                            1-second  5-second  1-second  5-second

Directed forgetting/recall       .09       .14       .39       .68
Remember/recall                  .32       .50       .29       .48
Directed forgetting/recognition
  Hit
    Remember response            .21       .34       .67       .85
    Know response                .21       .23       .14       .07
    Guess response               .16       .14       .07       .02
    Overall                      .58       .71       .88       .94
  False alarm
    Remember response            .01
    Know response                .03
    Guess response               .11
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Author:Lee, Yuh-shiow; Lee, Huang-mou; Tsai, Sheng-hsiung
Publication:British Journal of Psychology
Date:May 1, 2007
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