The Effect of Sleep Before or After Learning on Memory

Transcription

1 Sleep, 7(2): Raven Press, New York The Effect of Sleep Before or After Learning on Memory Andrea Grosvenor and Leon C. Lack Tke Flinders University of South Australia, Bedford Park, South Australia, Australia Summary: Early studies in which it was found that learning followed by sleep was better remembered than learning followed by wakefulness were interpreted as giving support for the Interference Theory of Forgetting. More recent studies have shown better retention over the first half of the night's sleep (slow-wave sleep) than over the second half (REM sleep), and conclusions have been drawn that a Decay Theory of Forgetting is more strongly supported. Those studies, however, confounded the type of sleep following learning with sleep prior to learning. When prior sleep was controlled in the present study, there was no support for a first half-night sleep benefit, and, contrary to Decay Theory, there was a second half-night benefit for high imagery material. The strong detrimental effect of sleep prior to learning was inconsistent with the Interference Theory of Forgetting and suggested, instead, the importance of the consolidation process for long-term memory. Key Words: Sleep stages-long-term memory-forgetting theories-memory consolidation. In the present study we examined several variables which have, in the past, been shown to mediate the effects of sleep on memory. These are the post-learning condition (sleep or wakefulness), prior learning condition (sleep or wakefulness), physiological arousal, length of the retention interval, and type of material learned. Jenkins and Dallenbach (1) discovered that subjects who slept following learning remembered more than subjects who were awake for the same length of time. They used this finding as support for the Interference Theory of Forgetting, since it was assumed that interference learning would be reduced during sleep. Although the beneficial effect of sleep on memory has subsequently been confirmed (2-4), the effect of other variables on memory has raised some doubt regarding the Interference Theory. The finding that sleep is most beneficial to memory when it follows learning immediately rather than later (5-7) seems to implicate differential effects on a consolidation process of memory. Studies that have manipulated the level of arousal directly (8,9) Accepted for publication November Address correspondence and reprint requests to Dr. Leon Lack, School of Social Sciences, Flinders University, Bedford Park 5042, South Australia, Australia. 155

2 156 A. GROSVENOR AND L. C. LACK or naturalistically using different times of the day (10-13) have shown that high arousal at the time of learning is not usually beneficiai to immediate recall but does result in better long-term retention than does low arousal. These results also seem to implicate differences in a consolidation process following different levels of arousal during learning. The demonstration by Dement and Kleitman (14) that sleep can be divided into stages with distinctive physiological characteristics provided a major impetus to current sleep research, including the investigation of the relationship between sleep and memory. The most distinctive stages are stage-4 sleep, characterized by high-amplitude, low.frequency EEG waves, and REM sleep, characterized by a desynchronized "activated" EEG and rapid conjugate eye movements. REM sleep appears to be closer to wakefulness in terms of physiological arousal than does stage-4 sleep, which seems most physiologically distinct from wakefulness. These differences in sleep stages may be utilized to discriminate between theories of forgetting, since most stage-4 sleep occurs in the first half of the night's sleep and most REM sleep occurs in the second half (15). If learning is followed by the onset of a normal night's sleep and subjects are awakened to test their memory after 4 h of sleep, the retention interval will differ markedly from that of subjects awakened after 4 h of sleep to learn the material and allowed 4 h more sleep before testing. Using this method, Yaroush et al. (16), Barrett and Ekstrand (17), and Fowler et al. (18) found that learning followed by the first half of the night's sleep led to much better memory of neutral material than did learning followed by the second half ofthe night's sleep. The second-half condition was superior to a retention interval of wakefulness. These experimenters concluded that stage-4 sleep following learning was more beneficial to memory than REM sleep following learning. Ekstrand et al. (19) argued that this difference more strongly supports the Decay Theory than the Interference Theory of Forgetting since interfering learning should be absent in both stages of sleep, but the decay of memory should be more rapid when physiological arousal is higher, in REM sleep. However, in all these studies retention over the second half-night's sleep may have been detrimentally affected by the 4 h of sleep immediately prior to learning. Stones (20), Shearer (21), and Ekstrand et al. (19) have demonstrated that sleep prior to learning has a detrimental effect on long-term recall, although it has no effect on rate of learning or immediate recall of items. Ekstrand et al. (19) found the detrimental effect for prior sleep with sleep lengths from 0.5 h to as long as 6 h of prior sleep. Thus, the 4 h of sleep prior to learning in the second half-night condition would have detrimentally affected recall. Although Ekstrand et al. (19) suggested from indirect evidence that the prior sleep effect was not as strong as the difference between first half-night and second half-night retention, they add"... Nevertheless, the difference in magnitude of the two effects is not great and it could very well be that if there were a way to eliminate the confounding from the prior sleep, we would find no difference between the first and second half of the night (no difference between REM and stage- 4 sleep." (p. 437). The confounding of a prior sleep effect with the comparison of the second half-night and first half-night sleep was also noted more recently by Idzikowski (22), who concluded, "... It is difficult to assess Ekstrand's experiments, as most of his subjects and especially his 'second half of the night' subjects will undoubtedly have been suffering from the effects of 'prior sleep'." (p. 314). What is needed in the experimental design to assess the strength of the prior sleep effect is a fourth group that is awake before and after learning. This would complete Sleep. Vol. 7, No

3 SLEEP AND MEMORY 157 a 2 x 2 design such that sleep or wakefulness prior to learning can be compared with sleep or wakefulness following learning. Then, only if the difference in retention between the first and second half-night groups is larger than the prior sleep effect can a first half-night advantage be demonstrated to exist. Its statistical verification would be tested as the interaction between the main effects of prior learning condition (sleep or wakefulness) and post-learning condition (sleep or wakefulness). Although arousal at the time of learning affects long-term recall, Ekstrand et al. (19) discounted the idea that differences between prior sleep and prior awake groups could be due to differences in arousal level, since all subjects were fully awake, had performed an arousing warm-up task, and showed no differences in speed of learning. However, as mentioned earlier, a lower level of arousal does not necessarily result in a slower rate of learning. Although the prior sleep subjects may have been fully awake at the time of learning, physiological indices of arousal, such as body temperature, may still have been low and may have been a factor in long-term retention. The present study included the physiological arousal indicators of oral temperature, heart rate, and blood pressure. Research into the effects of sleep on memory has also shown differences in recall due to the type of material to be remembered. Grieser et al. (23) found that REMdeprived subjects recalled more "non-threatening" items than did non-rem-awakened subjects, whereas non-rem-awakened subjects remembered more "threatening" items than REM-deprived subjects. Tilley and Empson (24) found that the recall accuracy of stories was significantly poorer when followed by REM deprivation than when followed by stage-4 deprivation. In addition, there was less forgetting during REM recovery sleep than during stage-4 recovery sleep. The results of animal studies have shown that REM deprivation leads to poor recall of learned avoidance behavior (25). In studies in which slow-wave sleep has been assumed to be more beneficial to memory than REM sleep, neutral material such as nonsense syllables, digits, or word pairs has been used (e.g., Refs ). This suggests that REM sleep is important for memory of high association value and emotive material, whereas slow-wave sleep benefits retention of neutral material. In one experiment Fowler et al. (18) varied the imagery value of word pairs in a paired-associate learning task with the retention interval consisting of the first or second half of the night's sleep or wakefulness. They found a large effect of imagery on memory and a large effect of the retention interval condition, but no interaction. However, there may have been an interaction over longer retention intervals. Kleinsmith and Kaplan (26) found that paired responses to high arousal stimulus words were remembered poorly when retention was tested immediately after learning, but remembered well by groups tested over longer intervals. These results have been replicated by Kleinsmith and Kaplan (27), Walker and Tarte (28), and Butter (29). Corteen (30) found a positive correlation between evoked arousal of words to be learned and recall after no delay, 20 min, or 2 weeks, with the size of the correlation increasing with longer retention intervals. The present study included paired associates with stimulus words of high or low imagery value and response words of high or low imagery value, and extended the design of Fowler et al. (18) by including a long-term retention interval of 6 days in addition to the initial test 4 h after learning. The interaction of word imagery and type of retention interval, which Fowler et al. (18) did not find with a 4- h retention interval, may emerge over the longer retention interval of 6 days. If there is a first half-night sleep benefit for low imagery words, there may not be that advantage Sleep. Vol. 7. No

4 158 A. GROSVENOR AND L. C. LACK for high imagery words, which should benefit more from REM sleep, particularly when retention is tested after the longer interval in the present study. METHODS SUbjects The subjects were 40 (19 female and 21 male) volunteers, who were students at Flinders University. The subjects were divided randomly into four groups, each containing 10 subjects, with an attempt made to keep the numbers of males and females balanced. Each group represented one of the four possible combinations of two different conditions (sleep or awake) prior to learning and post-learning. They were designated as the Awake-Learning-Sleep group (ALS), the Sleep-Learning-Sleep group (SLS), the Sleep-Learning-Awake group (SLA), and the Awake-Learning-Awake group (ALA). All subjects were naive to the purposes of the experiment. Two subjects who failed to reach the performance criterion during the learning task were discarded and replaced by two others. Learning material The learning material consisted of 16 pairs of nouns with frequencies of at least 50 per million (31). Four of these pairs were made up of high imagery stimulus and high imagery response words; four pairs were of high imagery stimulus and low imagery response words; four pairs were of low imagery stimulus and high imagery response words; and four pairs were of low imagery stimulus and low imagery response words. High imagery words were those having an imagery rating of at least 6.5, and low imagery words were those with an imagery rating of no more than 3.43 from the imagery scale of Paivio et al. (32). Each pair consisted of words with no association rating in the tables compiled by Postman and Keppel (33). The words, printed in block letters, were recorded on video tape to provide study and test trials. In the study trials the pairs were presented for 3 s each, with 1 s between each pair. In the test trials the stimulus words were presented for 3 s each, with 3 s between each word for responding. A total of 10 study trials alternating with 10 test trials, each with a different random sequence of word pairs or stimulus words, respectively, were tape recorded. During the experiment the learning material was presented on a National video monitor with a 25-cm screen. The laboratory The laboratory consisted of an experimenter's room and three sleep rooms connected by a corridor. The sleep rooms were carpeted, air-conditioned, and sound-attenuated; each contained a bed and a chair. The experimenter's room contained a table, six chairs, and the video equipment. Procedure The subjects were asked to try to maintain their normal sleep/waking pattern for 2 days prior to the experiment, not to sleep during the day of the experiment, and not to consume any alcohol or caffeine following their evening meal that night. On arrival at the laboratory at 2200 h the subjects were asked if they had followed these instructions. There were usually four or five subjects participating in the experiment on anyone night. At about 2230 h oral temperature, heart rate, and blood pressure were measured

5 SLEEP AND MEMORY 159 TABLE 1. Experimental design Treatment conditions and times Prior learning Treatment group Postlearning Days later I. ALS Awake Learn 2. SLS Sleeping Learn 3. SLA Sleeping Learn 4. ALA Awake Learn Sleeping Sleeping Awake Awake Test Test Test Test Retest Retest Retest Retest for all subjects. They were then randomly assigned to groups. The experimental design is summarized in Table 1. At about 2300 h the subjects in the SLS and SLA groups went to bed, while subjects in the ALS and ALA groups stayed in the experimenter's room. Awake subjects spent the time in the experimenter's room talking, playing board games or cards, reading, or studying, and had physiological measures taken hourly. No stimulants, depressants, or other drugs were permitted during the experiment. At 0230 h subjects in the SLS and SLA groups were roused and joined the others in the experimenter's room. Physiological measures were taken for all subjects. They were then seated around the table so that they all could see the video monitor clearly. The mean time for the start of learning was 0245 h. The learning instructions were read and explained. The subjects learned the paired-associate list by the study-test method until the criterion of 12 out of 16 correct responses in one test trial was reached. Physiological measures were taken when the subjects completed the learning task, and then those in the SLS and ALS groups went to bed at a mean time of 0325 h. Those in the ALA and SLA groups remained in the experimenter's room for the rest of the night, and their physiological measures were taken hourly. At 0730 h subjects in the ALS and SLS groups were awakened and joined the others in the experimenter's room. Physiological measurements were then taken on all subjects. A single, paced recall test trial was given. Following this a matching test was given in which response words in a randomly arranged list had to be matched to the appropriate stimulus words. The purpose of this was to include a test of recognition memory. Six days after this test the subjects returned to the laboratory and at a mean time of 1035 h were unexpectedly given a paced recall retest followed by a matching retest in which the order of the stimulus and response words was randomly rearranged from that of the initial test. RESULTS Prior and post-learning condition effects Two-way Analyses of Variance (ANOVA) were carried out with prior condition (sleep or awake before learning) and post-learning condition (sleep or awake in the 4 h following learning) as the independent variables. As illustrated in Fig. 1, the initial degree of learning was comparable for all groups. No significant (p < 0.05) effects of prior or post condition were found for the number of words correct on the criterion trial. Furthermore, the absence of significant effects in the number of trials to the criterion indicated that all groups had comparable rates of learning.

6 160 A. GROSVENOR AND L. C. LACK 16 AWAKE - SLEEP SLEEP - SLEEP o SLEEP - AWAKE o AWAKE - AWAKE 12 FIG.!. Mean for the four groups of the number of words correctly recalled in a test trial (out of 16) on the criterion learning trial (at least 12 correct), on the initial paced recall 4 h after learning and recall retest 6 days after learning. CRITERION TRIAL INITIAL TEST RE-TEST To compensate for any individual differences in the number of words correct on the criterion learning trials, the percent loss from criterion trial to recall trial was calculated for each subject as the amount of material forgotten. Table 2 shows the mean percent loss from the number of words correct on the criterion trials to the initial paced recall test. The ANOVA on these data showed a significant main effect for the prior condition [F(l,36) = 14.77, p < 0.01] in which prior awake groups (circles in Fig. 1) recalled better than prior sleep groups (squares). There was also a significant effect of postlearning condition [F(1,36) = 5.08, p < 0.05] in which "post" sleep groups (filled symbols) recalled better than "post" awake groups (open symbols). There was no significant interaction [F(1,36) = 1.912, p > 0.05], between prior and post-learning conditions. There were no significant condition effects for performance on the initial TABLE 2. Mean percent loss from the criterion trial to the initial paced recall test 4 h after learning and to the retest of paced recall 6 days after learning, and mean percentage correct on the matching retest Group Memory test ALA SLA ALS SLS 4-h Recall loss 6-Day recall loss Mean matching score Downloaded Sleep, from Vol. 7, No.2, 1984

7 SLEEP AND MEMOR Y 161 matching test, but the very high scores for all groups (mean = 90% correct) on this test indicated the presence of a ceiling effect. Mean recall performance for the four groups 6 days after learning is illustrated in Fig. 1. The percent loss from the criterion trial to the recall retest (see Table 2) showed only a significant main effect of prior condition: F(l,36) = 20.47, P < Neither the post-learning condition (F < 1.0) nor.the interaction (F < 1.0) was significant. On the matching retest scores, prior condition was also a significant main effect: F(1,31) = 4.66, p < Neither the post condition (F < 1.0) nor the interaction (F < 1.0) was significant in the matching retest. In summary, sleep following learning benefited recall performance only over the short interval (4 h), but not in long-term (6 days) recall or recognition. On the other hand, sleep prior to learning, although it did not affect the rate of learning, did result in more forgetting over the short and long periods of retention. The absence of significant interactions between prior and post-learning conditions suggested that where there was a benefit of post-learning sleep, the ALS group (first half-night sleep) compared with its appropriate control group, ALA, was not benefited more than the SLS group (second half-night sleep) in comparison with its control group, SLA. In other words, there was no evidence of an advantage of first half-night sleep over second half-night sleep when prior condition was controlled. Word imagery effects Although word imagery value of stimulus words and response words had an effect on the number of correct responses on the criterion trial, the focus of interest in the present study was the differential forgetting between groups and between word pair types from criterion to test trials. Stimulus word imagery and response word imagery had no significant effect on the number of words lost from the criterion trial to the initial paced-recall test or the retest of recall. However, in the matching retest 6 days after learning, low imagery response words were correctly matched significantly less well (F(1,144) = 59.41, p < 0.01] than were high imagery response words. Since the initial matching test 4 h after learning showed generally high performance (mean = 90%) and no differences between the groups or word pair types, the subsequent differences after 6 days suggested a greater amount of forgetting of low imagery response word pairs, at least in recognition memory. This was confirmed in a direct test of the difference between high and low imagery response words in the decrease of correct matches from the initial test to the retest session: t(34) = 2.66, p = Also in the matching retest, a significant prior-condition effect [F(1,144) = 10.21, P < 0.01] and interaction of prior condition by response word imagery [F(1,144) = 5.49, p < 0.05] suggested that prior sleep was particularly detrimental to long-term recognition of paired associates with low imagery response words. These effects are illustrated in Fig. 2. Because response word imagery seems to determine retention, two-way ANOVAs between the groups were carried out separately for the high imagery response word pairs and for the low imagery response pairs. With high imagery response pairs, analysis of the number of words lost from the criterion trial to the initial paced test showed only a significant main effect of prior condition: F(1,36) = 6.42, p < The number of words lost from the criterion trial to the paced retest, however, showed significant effects for prior condition [F(1,31) = 67.83, p < 0.01] and a significant interaction of prior and post-learning conditions [F(1,31) = 6.05, p < 0.05], as illustrated in Fig. 3.

8 162 A. GROSVENOR AND L. C. LACK o PRIOR AWAKE \:::::::::::::\ PRIOR SLEEP FIG. 2. Mean percentage correctly matched by the prior learning awake groups (ALA and ALS) and also by the prior learning sleep groups (SLA and SLS) analyzed separately for the high imagery response words and low imagery response words in the matching retest 6 days after learning. This interaction indicated a smaller difference in forgetting between the post-learning sleep groups than in the post-learning awake groups. If the difference between the post-learning awake groups (ALA and SLA) is taken as the baseline measurement of the effect of prior condition, then the smaller difference between the post-learning sleep groups (ALS and SLS) would suggest that second half of the night sleep had a beneficial effect and/or the first half of the night sleep had a detrimental effect on longterm recall of high imagery response words. Scores on the matching retest showed only a significant effect of prior condition: F(1,31) = 7.49, p < Z lj.j t- O ~ f2 ~ a: ~ u.. o a: lj.j ID ~l z z «lj.j ~ O.:'--~:-:-'-~ o PRIOR AWAKE h:,,:}::) PRIOR SLEEP ALA SLA ALS SLS AWAKE SLEEP POST LEARNING CONDITION FIG. 3. Mean for the four groups of the number of high imagery response words forgotten (out of 8) in the paced recall retest 6 days after learning, Downloaded Sleep. from Vol. 7, No.2, 1984

9 SLEEP AND MEMORY 163 With low imagery response pairs the analysis of the number of words lost from the criterion trial to the initial paced test showed significant main effects for both prior condition [F(1,36) = 29.43, P < 0.01] and post-learning condition [F(1,36) = 14.3, P < 0.01], but no interaction between these main effects. However, in the long term, as measured by the paced recall retest and matching retest, there were no differences between the groups in the amount of forgetting of low imagery response pairs. Physiological measures Heart rate and blood pressure measures showed no clear patterns or differences throughout the night. For all groups combined, oral temperature dropped significantly [t(39) = 6.79, p < 0.01] from a maximum mean of C at 2230 h to a minimum mean of C at 0230 h. Temperature was increased after learning [t(39) = 3.89, p < 0.01] and then showed no significant change throughout the rest of the night. Comparing the prior learning conditions, the prior sleep groups (SLA and SLS) showed a greater drop in oral temperature from 2230 h to the time of learning (0230 h) [t(38) = 2.29, p < 0.05] and had a lower mean oral temperature at 0230 h [t(38) = 2.08, p < 0.05] than did the prior awake groups (ALA and ALS). The relationship between oral temperature and performance measures was examined more directly with Pearson Product-Moment correlation coefficients. Although there was no difference in the number of trials to the criterion between prior sleep and prior awake groups, within the prior sleep groups only there was a significant correlation between the drop in oral temperature to the time of learning and the number of trials to criterion [r(18) = , P < 0.05]. However, all other correlations of drop in oral temperature or temperature at the time of learning with measures of retention were not significant (p < 0.05). DISCUSSION Because EEG recording was not taken on the subjects in the sleep condition of the present study, there was no objective verification of the presence, extent, or type of sleep in the present study. However, the procedures used in the present study were identical to those used by Barrett and Ekstrand (17), who confirmed the prevalence of slow-wave sleep in the first half of the night's sleep, whether it occurred before or after learning, and of REM sleep in the second half-night condition. Without the slight discomfort of EEG electrodes, and with university students who tend to have relatively short sleep latencies (34), it seems reasonable to assume that sleep prevailed in the sleep conditions of the present study. This was, in fact, reported by all subjects in the sleep conditions. The detrimental effect of prior sleep on memory is consistent with earlier findings (19-21). In this study we found that prior sleep still has a strong effect after a retention interval of 6 days when tested by either recall or recognition. It was suggested earlier that the prior sleep effect may be due to lower arousal at the time of learning despite the lack of a difference in rate of learning. It has been shown that sleep does reduce physiological arousal (35). Consistent with this finding, the prior sleep groups (SLA and SLS) showed a greater drop of oral temperature from 2230 h to 0230 h than did the prior awake groups (ALA and ALS), and thus had lower mean temperature at the time of learning. However, when individual drops of oral temperature before learning,

10 164 A. GROSVENOR AND L. C. LACK or absolute temperatures at the time of learning, were correlated with subsequent memory loss, there were no significant correlations-for all groups combined, or the prior sleep or prior awake groups separately. Therefore, although prior sleep reduces oral temperature and retention, there was no direct relation between oral temperature and retention. Either oral temperature is not an accurate indicator of arousal, or arousal is not a very useful explanation for the prior sleep effect in the present study. The post-learning sleep benefit in the present study is consistent with past findings and replicates the study of Barrett and Ekstrand (17), in which the time-of-day factor was also controlled by setting the retention interval for all groups from 0250 h to 0650 h. However, the post-sleep benefit over the first 4 h of retention was not strong [F(1,36) = 5.08, p < 0.05] and was not apparent over 6 days retention in either recall or recognition tests of memory. This finding contradicts those of Graves (5) and Richardson and Gough (6), who found that post-learning sleep did not lead to better retention over intervals of less than 48 h, but did for greater retention intervals. Because the time-of-day effect was not controlled in those studies, it is difficult to evaluate their finding of a delayed beneficial effect of post-learning sleep. We have concluded from the present study that the benefit of sleep compared with wakefulness following learning is neither great nor long-lasting. Consistent with Fowler et al. (18), there was no interaction between the type of postlearning condition and stimulus word imagery or response word imagery over the 4-h retention interval. Because the post-learning benefit did not remain over the 6-day retention interval, the lack of a significant interaction with stimulus or response word imagery over this longer interval was not surprising. The prior learning condition effect was generally stronger than that of the post -learning condition and was also manifested after the 6-day retention interval. Although there was no interaction with stimulus or response word imagery after the 4-h retention interval, there was an interaction between the prior learning condition and response word imagery in the matching retest. Prior sleep was particularly detrimental to the long-term retention of low imagery response words. Baddeley (36) suggests that high imagery words are easy to learn in part because they have both a verbal and a visual code. It was assumed that pairs containing high imagery words, particularly high imagery response words, would lend themselves to the mnemonic device of visual image formation to learn the paired associate, whereas low imagery pairs would more likely be learned by rote memorization. Most of the subjects reported this to be the case. It is also a plausible explanation for the better retention, over the long-term interval, of the high imagery response words. When performance was analyzed separately for low imagery response word pairs, it was found that they were affected detrimentally by prior sleep and beneficially by post-learning sleep over the 4-h retention interval, but not over the 6-day interval. This result could be due to a floor effect, since recall of low imagery response pairs was very poor for all groups in the retest. The absence of a prior and post-learning condition interaction did not support a first half-night advantage with the low imagery response words. Therefore, even with material that would be considered relatively neutral, there appears to be no advantage of slow-wave as compared with REM sleep in the retention interval. High imagery response pairs showed a detrimental effect of prior sleep in recall tests at both retention intervals and in the longer interval recognition test. The interactions

11 SLEEP AND MEMORY 165 between prior and post-learning conditions in the recall retest suggest that the detrimental effect of prior sleep was reduced when learning was followed by sleep in the second half of the night. Therefore, when prior learning condition is controlled, the second half of the night's sleep after learning appears to be beneficial to long-term recall of material that could be considered more emotive, or having a greater number of associations. This is consistent with the notion that REM sleep benefits the consolidation of emotive material (23), high association value material (24), or material that calls more for divergent processing (37). Alternatively, the different sleep stages may differentially affect the retention of mnemonically learned material but not rotelearned material. An obvious suggestion from the present study would be to investigate whether the benefit of the second half-night's sleep occurs for emotive and high imagery material per se or whether the effect is due to the mnemonic method of learning. Ekstrand et al. (19) interpreted- their results and the earlier ones of Ekstrand (2) Fowler et al. (18), and Yaroush et al. (16) as supporting a first half-night sleep advantage over a second half-night sleep retention interval. Because slow-wave sleep predominates in the first half of the night and REM sleep predominates in the second half, they suggested that these results supported a Decay Theory of Forgetting based on the assumption of more rapid decay of memory in REM sleep than in slow-wave sleep. However, when the confounding effect of prior sleep was controlled in the present study, the absence of an interaction between the post-learning and prior learning conditions for all learning material combined suggested that there was no difference in the type of sleep following learning. Therefore, with regard to the effect of sleep following learning, the present findings do not support a Decay Theory, but they are consistent with the Interference Theory of Forgetting. The disappearance of a significant postlearning sleep advantage over the 6-day interval may reflect the proportionally decreasing size of the interference-free interval after learning as compared with the size of the retention interval. However, it should be kept in mind when considering theories of forgetting that the strongest effect in the present study is the prior sleep effect. It was stronger than the post-learning sleep effect over the 4-h retention interval, and it was still present (p < 0.01) after the 6-day retention interval, when the post-learning effect was no longer present. Decay Theory can be neither supported nor rejected by the prior sleep effect, since memory decay is conceived as a process following learning. Interference theory would also be considered unrelated to the prior sleep effect if it is assumed that interference learning can only follow the original learning. However, it is well known that proactive interference as well as retroactive interference affects memory (38,39). Proactive interference learning should be reduced by sleep and result in a benefit of prior sleep. Thus the strong detrimental effect of sleep prior to learning can only disconfirm the Interference Theory of Forgetting. The results of the present study seem to support most strongly a memory Consolidation Theory. Consolidation is drastically reduced for material presented during sleep, particularly slow-wave sleep (40-42). Whether the reason for this is the presence of a biochemical substance, such as growth hormone, as suggested by Ekstrand et al. (19) and Hoddes (43), or a decrease in some aspect of physiological arousal, it seems plausible that the effect may still be present to some extent after waking, thus impairing the consolidation in long-term memory of material learned at this time. The presence of a post-learning sleep benefit is also consistent with a Consolidation Theory. In Sleep. Vol_ 7. No

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