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Enhancing Human Performance: Background Papers, Learning During Sleep (1988)
Commission on Behavioral and Social Sciences and Education (CBASSE)

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sleep learning/4 SLEEP LEARNING: METHODOLOGY AND PHENOMENOLOGY As Aarons (1976) has observed, whether or not learning during sleep occurs depends on an intricate interplay of numerous psychological and physiological variables. In this section, I survey a selective sample of such variables-- ones that, in my opinion, have the most promise of being important moderators of sleep learning. For ease of exposition, the specific variables to be considered are classified according to four general types: sleep, item, task, and subject. Sleep Factors EEG Activation During and Following Item Presentation The research of Simon and Emmons revealed that alpha activity during the presentation of a target item was a necessary condition for the later recollection of that item. Evidence also exists which suggests a strong association between memory performance and the both the level and duration of EEG wakefulness or activation patterns that follow item input. Evidence of this sort has been

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sleep learning/5 supplied by a number of studies (e.g., Jus & Jus 1972; Koukkou & Lehmann 1968; Lehmann & Koukkou 1974; Oltman et al. 1977), one of which is described below for purposes of illustration. In the study by Koukkou and Lehmann (1968), short sentences were auditorily presented to subjects during slow wave (stage 2 or 3) sleep, and the duration of the EEG activation (alpha) pattern produced by the presentation of each sentence was measured. Upon awakening, the subjects completed a test of nominally noncued or "spontaneous" recall, which was succeeded by a test of old/new sentence-recognition memory. The results showed that the duration of EEG activation that followed the presentation of a given sentence was quite short (mean = 9 see) for sentences that were neither recalled nor recognized, significantly longer (mean = 26 see) for sentences that were recognized but not recalled, and longer still (mean = 165 see) for sentences that were spontaneously recalled verbatim (Koukkou & Lehmann 1968/Table JIB). These data clearly demonstrate that post-sleep recollection of sentences presented during slow wave sleep was related to the duration of EEG activation that occurred after presentation. (In later work, Lehmann and Koukkou (1974) demonstrated an analogous correlation between memory performance and the level (i.e., EEG wave frequency) of post-presentation activation.) The fact that intermediate durations of activation were associated with successful recognition, but unsuccessful recall, suggests that recognition may be a more sensitive measure of memory for sleep-presented material than is spontaneous recall--a point to which I will return later. In an effort to provide a theoretical rationale for their results, Koukkou and Lehmann (1968) proposed that the duration (and level: see Lehmann &

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sleep learning/6 Koukkou 1974) of EEG activation that occurs after the presentation of a target item reflects the time available for the long-term storage of that item. This proposal is reminiscent of the consolidation interpretation of sleep-learning problems put forth by Hebb (1949). According to Hebb's theory, there are two distinct forms of memorial representation: a short-term store in the form of reverberating neural circuits, and a long-term store involving the development of more permanent neural "knobs." It is the transformation or consolidation of information from a short- to a long-term representation that is assumed to be the process that is vulnerable to the absence of EEG activation. Several observations are compatible with the consolidation account (see Goodenough 1978; Lehmann & Koukkou 1974). For example, somnambulists can can carry out complex motor actions and respond appropriately to sensory input during very deep (stage 4) sleep, but cannot recall their actions and responses once they awaken (Jacobson et al. 1965); the apparent accuracy of dream recall is high if sleepers are awakened during stage 1 periods of rapid eye movement (REM) sleep--a stage characterized by a fairly active EEG--but without sleep interruption, dream recall decreases with increased time spent in slow wave sleep after the end of the REM period (Decent & Kleitman 1957); and a number presented during deep sleep that is not followed by appreciable EEG activation can be recalled if the subject is intentionally and rapidly awakened before the short-term trace of the digit ceases to exist (Oltman et al. 1977). Although much of the difficulty in recalling events that take place during sleep may reflect the impaired consolidation or long-term storage of these events, the possibility that recall difficulties may be due to deficient retrieval should not be overlooked. Within the last twenty years, several

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sleep learning/7 retrieval-based accounts of sleep-learning problems have been advanced (see Foulkes 1966; Goodenough 1978). One of the more recent of these--the functional state-shift hypothesis of Lehmann and Koukkou (1983)--is framed around the concept of dissociative or state specific memory: the idea that what has been learned in a particular state of mind or brain is best remembered in that state (see Eich 1977, 1980i Overton 1973, 1982). According to Lehmann and Koukkou (1983), the forgetting of events that transpire during sleep (whether internally generated dreams or externally presented items) is a function of the magnitude of the difference between the functional (EEG defined) states in which storage and retrieval of the events take place. Their hypothesis accords well with a number of diverse findings, one of which is the aforementioned fact that if a transient period of wakefulness (as indicated by an increase in EEG activation) occurs soon after the presentation of a target item, then the subsequent recall of that item will be possible during full wakefulness. In addition, the state-shift hypothesis carries the intriguing implication that information acquired during sleep may be accessible for retrieval in later occasions of sleep, though not during intervening periods of wakefulness. Evidence pertinent to this implication will be examined shortly. But first, I would like to make one other point concerning the correlation between EEG activation and memory performance. As noted earlier, a number of Soviet and East European studies have reported success in producing relibale, sometimes robust, sleep learning effects. In these studies, presentations of the to-be-learned material are not regulated according to particular EEG patterns (as is customary in Western studies), but are timed to correspond with sleep onset, initial sleep, and early

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sleep learning/8 morning sleep--optimal times for eliciting EEG activations with alpha waves (Aarons 1976~. Thus, it is entirely possible, even probable, that participants in these studies are not "really asleep" when presentation occurs, but instead are in a rather drowzy--but nonetheless conscious--state. Is it any wonder, then, that so-called sleep learning is possible under such circumstances? The obvious answer, of course, is "no," but there is more to the story than that. Unlike their Western counterparts, Eastern researchers generally do not find the question, "Are subjects 'really asleep' during presentation of the learning material?" to be an important or meaningful one to ask. Their primary concern is not with the theoretical possibility of learning during deep sleep, but rather, with the practical purpose it serves to present learning material to superficially sleeping subjects. This is one of several salient differences (others will be discussed in due course) that distinguishes the prototypical Western study of sleep learning from the prototypical Eastern study. As Aarons (1976) has argued, these differences probably account for why Western researchers frequently fail to find evidence of sleep learning, while Eastern investigators often succeed. Sleep Specific Memory In 1910, Morton Prince conjectured that the reason many people have difficulty remembering their dreams is not that they do not want to remember--as Freud (1900/1953) and other psychodynamically oriented theorists of the day were claiming--but rather, that they cannot remember, due to the mismatch between

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sleep learning/9 the states of natural sleep and ordinary wakefulness. Intuitively, Prince's speculation seems plausible, and so does Lehmann and Koukkou's (1983) idea that failures of waking memory for experimentally devised materials (such as sentences) that had been presented during sleep are attributable to the shift from sleeping to waking states. Plausibility is one thing, however; proof quite another. What empirical evidence is there to support the proposition that memory for events occurring during sleep is specific to the sleep state? To my knowledge, only one study--described briefly by Evans et al. (1966), and more elaborately by Evans (1972) and Evans et al. (1969, 1970)--has sought tom secure such evidence. In this study, 18 student nurses slept in a laboratory for two or three nights. During the first night, suggestions of the form "Whenever I say the word 'itch,' your nose will feel itchy until you scratch it," were auditorily presented to subjects while they were in alpha-free stage 1 sleep. The suggestions were then tested immediately by saying a cue word ("itch," for instance) and observing the subjects' behavioral response, if any. Of the 18 subjects tested, 11 were able to perform the suggested responses while remaining in stage 1 sleep. After the subjects awakened, they did not remember the verbally presented suggestions, nor did they remember responding to them. In addition, when presented with the same cue words that had elicited an appropriate response during sleep, the subjects did not respond behaviorally when awake. Thus the subjects appeared to have a dense waking amnesia for events that had occurred during the prior night's sleep. That the absence of waking memory reflected amnesia rather than forgetting is implied by the observation that, of the 11 subjects who had responded to

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sleep 7earning/10 cue words during the first night, 7 responded to the same cues during the second night. Thus, in the majority of cases, successful second-night responding to cue words during sleep occurred even though the suggestions themselves were not readministered, and even though the subjects had no intervening waking recollection of the suggestions or their responses during sleep. After an interval of approximately five months, seven subjects were retested on a third sleep night. None of these subjects remembered the events of either earlier evening, and five of the seven had responded on both prior nights to the cue words of the initial night. These five subjects responded, while in stage 1 sleep, to cue words from the first night's sleep, even though the suggestions had not been reade~nistered and could not be consciously recalled in the intervening months. To summarize, the results of Evans' study suggest that at least some subjects can respond to suggestions for specific motor actions while they remain in stage 1 sleep. Further, these responses can be elicited during stage 1 sleep of a following night, and even in the same sleep stage several months later, without further reinstatement of the suggestion. This retention occurs even though the subjects, when awake, are unable to either verbalize their sleep experiences or perform the sleep-acquired responses. As I mentioned earlier, Evans' experiment is the only one of which I am aware that directly examined whether memory for events experienced during sleep is specific to the sleep state. Accordingly, his results, though strongly suggestive of sleep specific memory, should be viewed with caution. Why no efforts have evidently yet been made to replicate and extend Evans' findings is, to me, a mystery.

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sleep learning/11 There is one other aspect of the relationship between state specificity and sleep learning that deserves attention, and it concerns the asymmetric form in which dissociative or state specific effects frequently appear. In several studies involving alcohol or other depressant drugs (e.g., Goodwin et al. 1969), it has been shown that although events encoded in an intoxicated state are "dissociated" or difficult to retrieve under conditions of sobriety, events experienced in the drug-free state are not state specific, and can be accessed as efficiently in the presence of alcohol as in its absence. An analogous pattern of results has obtained in research involving stimulant drugs, such as nicotine (Peters & McGee 1982), as well as in experiments entailing alterations of affect or mood. Bartlett and Santrock (1979), for example, found that if preschoolers learned a list of common words while they were feeling especially happy, they remembered many more of these words when tested for recall in a happy than in a neutral mood. However, words studied in a neutral affective state were equally well recalled regardless of whether the children were tested in a neutral as opposed to a happy mood. The implication of these and other studies (see Eich 1986) is that information "transfers" more completely from an ordinary or typical state of mind or brain (such as sobriety or neutral affect) to an atypical or altered state (such as alcohol intoxication or extreme happiness) than is does in the reverse direction. The main point I wish to make now is that asymmetrical dissociation may also be implicated in sleep. That is to say, while it is evident that knowledge acquired during wakefulness is expressable during sleep (we do, after all, tend to dream about things we perceived while awake), events experienced during sleep are difficult, if not impossible, to access during

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sleep learning/12 wakefulness. Why asymmetric dissociation should occur in conjunction with sleep, or any other experiential state (such as intoxication or happiness) for that matter, is an open issue. One possible reason relates to the concept of cue overload: the idea that the effectiveness of a given retrieval cue is inversely related to the number of discrete events it subserves (Watkins 1979; also see Bartlett et al. 1982; Eich 1985). Since the vast majority of our perceptual experiences occur while we are awake, the state of wakefulness cannot act as an effective cue for the retrieval of these experiences--it is simply too overloaded. Sleep, in contrast, may constitute a much more salient or distinctive context for encoding, and thus may serve as a powerful cue for the retrieval of events that had been encoded in the sleep state. It remains to be seen whether this reasoning can be developed into a satisfying account of asymmetric dissociation as it appears in concert with sleep or other experiential states.

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sleep learning/13 Item Factors Methods of Item Presentation Although analyses of dream reports indicate that sleep mentation can be reliably and systematically modified by the external presentation of either visual or factual stimuli (Arkin & Antrobus 1978), it is principally through audition that sleepers maintain contact with the external environment (Aarons 1976). For this reason, and in the interests of practicality, audition has been the sensory channel of choice in all studies of sleep learning reported to date. Two methods of transmitting auditory information are available to the sleep- learning researcher: air conduction (loud speaker; e.g., Lehmann & Koukkou 1974; Simon & Emmons 1956) and bone conduction (pillow speaker; e.g., Bruce et al. 1970; Zukhar' et al. 1965/1968). Although the former method has been used more often in past research, there is reason to think that the latter may be more conducive to the demonstration of sleep-learning effects. As Aarons (1976) has noted, bone transmits mainly in the low frequency range of speech, which includes the fundamental frequency of the speaker's voice, and may therefore enhance the fidelity of the spoken message. Moreover, bone conduction has the curious effect of shifting the phenomenal source of speech from the outside to the inside of one's head. That this may be beneficial for sleep learning is suggested by the idea (Foulkes 1966) that the extent

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sleep learning/14 to which external stimuli are ignored during sleep is reciprocally related to the sleepers' preoccupation with their own internal mentation. Thus, it is possible that sleepers may be more receptive to, and hence more retentive of, information that seems to originate in their own minds than in the outside world. Whether this possibility is real or remote is a matter that merits, but has not yet received, serious consideration. Characteristics of the Target Items The list of variables that have a significant impact on the leering of verbal items in the waking state is extremely long, and includes such factors as the frequency and spacing of item presentations, as well as the meaningfulness and familiarity of the items themselves (see Adams 1980; Baddeley 1976). Unfortunately, and almost unbelievably, the effect that these and other variables have on the efficiency of verbal learning during sleep is virtually unknown. As regards the frequency of item presentation, Simon and Emmons (1955) asserted that sleep learning, if it is to occur at all, may require that a massive number of item presentations take place, but they did not offer any clean empirical evidence to back their claim. Bliznitchenko (1968; also cited in Aarons 1976), a pioneer in applied Soviet research on sleep learning, argued that repeated item presentations in the same sequence is a prerequisite to improvements in learning during sleep, but he too supplied no solid supporting data. With respect to the spacing of item presentations, an early experiment by

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sleep learning/24 Subject Factors Age Several authors have speculated about whether the ability to learn during sleep is dependent upon age, but no one has to date provided any telling data. Svyadoshch (1962/1968), for example, employed subjects ranging in age from 10 to 60 years in a series of studies concerning the reproduction of stories presented during sleep. Although Svyadoshch asserted that the majority of his subjects--irrespective of their age--demonstrated a high level of text reproduction (arbitrarily defined as 66% of more of the story material), he did not provide a breakdown of reproduction scores by age group. Svyedoshch also offered no hard numbers to support a second assertion concerning the relationship between sleep learning and age--one that is seemingly at odds with the first: specifically, that the ability to assimilate speech during sleep can be acquired "artificially" by means of suggestions delivered in the context of either deep hypnosis or ordinary wakefullness, and that children and adolescents, being more suggestible by nature than older adults, are especially adept at developing sleep-learning abilities.

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sleep learning/25 Interestin9ly. the idea that an optimal period for learning how to learn during sleep may arise at an early age has also occurred to Aarons (1976), but for different, and more defensible, reasons. These include the observation that (a) even as early as three days after birth, the human voice and its fundamental frequency are more effective than other sounds in eliciting behavioral and physiological reactions during alert, relaxed, and somnolent states (Huts et al. 1968), (b) children appear to acquire second languages more readily than do adults, which suggests a greater facility in phonetic processsing during wakefulness that could conceivably carry over to sleep, and (c) in comparison with older children, younger children devote more attention to and are more likely to remember auditorily rather than visually presented information (Hallahan et al. 1974). Although the foregoing observations are compatible with the developmental hypothesis advanced by Aarons (1976), more direct evidence is clearly needed. Obtaining such evidence would doubtless be a difficult and demanding task, but potentially a rewarding one as well. Health Given that (a) between five and ten percent of otherwise healthy medical students suffer frown chronic sleep disturbances that range from mild to moderate in severity (Johns et al. 1971), (b) emotional stress disrupts the natural sleep cycles of men and women alike (Breger et al. 1971), and (c) both mentation and physiological processes during sleep are influenced by

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sleep learning/26 menstruation in women (Sheldrake & Conmack 1974), the need to screen subjects for sleep-learning research on the basis of specific criteria related to their physical and psychological health seems clear. Yet apart from the research of Zukhar' and his associates (1965/1968), in which people with histories of sleep disturbance were specifically excluded from participation, health- related variables have not been taken into account in prior studies of sleep learning. Instead, researchers have simply assumed that their subjects are in generally good health and have normal hearing. As Aarons (1976) has remarked, information on personal health and sleep habits would aid investigators in - determining the suitability of a particular person to a particular sleep-learning intervention, and it is therefore hoped that the gathering of such information will become a standard practice in future studies of sleep learning. Capacity to Learn While Awake , According to Simon and Emmons (1955), sleep-learning researchers would be well advised to select as their subjects people who are particularly proficient at learning in the waking state, since the effects of presenting material during sleep may be so subtle that its benefits will be evident only in highly intelligent individuals. In consideration of Simon and Emmon's conjecture,

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sleep learning/27 four points are worth making. First, there is no a priori reason to assume that general intelligence plays a more prominent role in learning during sleep that it does in wake instruction (Aarons 1976). Second, it seems plausible to think that positive effects of sleep learning might be more readily demonstrated in individuals who are deficient rather than proficient in wake-state acquisition, in much the same manner as the memory-enhancing effects of nootropic drugs, such as oxiracetam (Itil et al. 1982), may be more likely to obtain in memory impaired patients (e.g., those with senile dementia of the Alzheimer's type) than in cognitively intact controls. Third, there is no empirical evidence to support Simon and Emmon's position, and fourth, what little evidence does exist--and it is indeed little, as I will emphasize momentarily--runs counter to Simon and Emmon's conjecture. The pertinent evidence comes from an early experiment by Elliott (1947/1968~. All 40 of the subjects in Elliott's study first learned one list of words (List A) to criterion in the waking state. Subsequently, a second list (B) was presented to 20 subjects while they slept (the experimental group), but not to the other 20 (the control group). The following morning, all 40 subjects learned List 8 to criterion. The key finding was that the percentage of savings in learning list B (i.e., SB = (NA ~ NB/NA) x 100, where NA and NB designate the number of trials required by a given subject to learn Lists A and B) was significantly greater for experimental than for control subjects--a finding that Elliott interpreted as evidence of sleep learning. For present purposes, a more interesting finding concerns the correlation between the values of NA and SB for each group of subjects. For purely statistical reasons, one would expect to

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sleep Jearning/28 observe a positive correlation between these measures for either group (since if any two subjects take the same number of trials to learn List B. the one who required more trials to learn List A will necessarily obtain a higher savings score). However, if Simon and Emmon's speculation that the benefits of sleep learning are more likely to be detected in good than in poor wake-state learners, then one would also expect to find a smaller correlation between the NA and SB scores of experimental subjects than between those of control subjects. That is to say, good learners in the experimental group (those who required relatively few trials to master List A) ought to show more savings in their learning of List B than should poor learners in the same group (those who required relatively many trials to learn List A). In fact, the correlation between NA and SB is somewhat greater among the experimental subjects (r = +.37) than it is among the control subjects (r = +.21). (These correlations were calculated from the data presented in Table III of Elliott (1968, p 13).) Thus, the advantage of having been presented with List B during sleep on later learning of that list appears to have accrued more to the poor than to the good wake-state learners in Elliott's study--the opposite of what would have been anticipated on the basis of Simon and Emmon's account. I emphasize the word "appears" because Elliott's experiment was not free of methodological flaws (for one thing, he did not continuously monitor sleep using EEG; see Simon and Emmons (1955)). More rigorous research will need to be performed before the relationship between learning capacities in waking and sleeping states can be stated with any degree of precision.

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sleep learning/29 Suggestibility: Hypnotic Susceptibility and Learning Set According to several Soviet accounts (e.g., Svyadoshch 1962/1968; Zavalova et al. 1964/1968i also see Aarons 1976; Hoskovec 1966), learning during sleep is possible provided that the learners are suggestible. As a rule, however, Russian researchers have not been either clear or consistent in their usage of the term "suggestible"--at times the term appears to imply susceptibility to hypnosis, at other times it refers to a strong waking set that is induced in the subjects to convince them that sleep learning is a bona fide phenomenon, and on still other occasions the term connotes both of these senses--and the evidence they have presented to support their position cannot be regarded as compelling. Consider, for example, the work of Kulikov (1964/1968). Subjects in his studies numbered 21 grade school and 15 college students, all of whom were highly susceptible to hypnosis (as tested by the method of hand gripping). The subjects were (randomly?) separated into three groups of 12, each composed of 7 children and 5 adults. Subjects in the first group were repeatedly presented during natural sleep with a narrative (a Tolstoy story for the children; a description of nervous system functions for the adults), and were tested for recall of the text when they awoke. These subjects were not, as Kulikov put it, "prepared" for sleep learning; that is, they had received no specific suggestions for assimilation and retention of the text prior to its presentation. Kulikov did not specify the number of times the text was presented, the precise forth of the recall test (i.e., whether it was spontaneous or prompted), or the duration of the

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sleep learning/30 retention interval. Further, it is not clear from Kulikov's account exactly when the text was presented; the only procedural remarks he makes in this regard is that the text was presented, via tape recorder, at a volume that was below the threshold of hearing in the waking state, and that sleep was monitored by taking activity records (absence of motor movements) and pneumographic tracings (absence of marked respiratory reaction). Be that as it may, Kulikov found that only one of the 12 subjects in this group had any waking recollection of the text, and the one exceptional subject was a boy who had taken part in previous studies in which hypnopedic suggestions had been delivered. Testing of subjects in the second group started by establishing contact with them while they slept. After the subjects had been sleeping for one or two hours, tape-recorded suggestions were presented to the effect: "You are sleeping peacefully, do not wake up," and "your breathing is becoming deeper and deeper." Having made contact with the sleeping subjects in this manner, the suggestion was given: "Now you will hear a story, listen to what is said, try and memorize it as much as possible, you will remember this all your life, and whenever wanted you will be able to relate it." The text was then presented (an unspecified number of times), and was followed by additional suggestions to remember the text and to sleep soundly. The impact these suggestions had on the subjects waking-recall performance appears to have been profound. Among the 12 subjects in the second group who had been "prepared" with a suggested set to learn while asleep, the percentage of idea units contained in the text that were recalled averaged 64t, and ranged from 47% to 87%; there was no appreciable difference in the performance

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sleep learning/31 of the children and the adults. These figures are remarkably similar to those yielded by the third group of subjects, who were awake at the time of text presentation (mean recall: 66%, range: 44% to 80%~. Taken at face value, Kulikov's data indicate that learning during sleep is possible in hypnotically suggestible subjects when a suggested set to register and retain the learning material is involved. Moreover, his data suggest that the capacity to learn during sleep is comparable to that of the waking state. Kulikov's results are not beyond reproach, however. For one thing, the suggested set that was imparted to subjects in the second group was evidently" not induced in subjects representing the third group, thus preculding a valid comparison of effectiveness of sleep v. wake learning. For another, it is possible that the striking difference in recall performance found between the first and second groups does not demonstrate the importance of preparing subjects for sleep learning, but rather reflects the fact that only the second group of subjects received any suggestions at all. A more meaningful contrast would have been between groups receiving suggestions that either were or were not relevant to the specific learning task at hand. Although Kulivov's (1964/1968) studies have some serious shortcomings, his contention--one shared with other Soviet researchers (see Hoskovec 1966)--that sleep learning is possible in hypnotically susceptible subjects who have acquired an appropriate set to learn finds support in a small study by Evans (1972), an American-based investigator. Nine of the subjects in Evans' experiment were people of varying levels of hypnotic susceptibility, all of whom could respond, while remaining asleep, to suggestions for specific motor actions (e.g., "Whenever I say the word 'pillow,' your pillow will

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sleep learning/32 feel uncomfortable until you move it."), without a presleep "set" to perform these actions; none of the subjects had any waking recollection of these suggestions. A strong waking set was then instilled that sleep learning is possible. Specifically, the subjects were told that, unlike most people, they were able to respond to suggestions presented during sleep, and that this made them particularly promising canadidates for sleep learning. Further, the subjects were informed about successful Soviet demonstrations of sleep learning, and so the subjects were motivated both by their own special qualifications and by the competitive aim to duplicate the Russian results. In addition to these nine ~lih;~tc e=~/~1 ^~- ... :__..' ~ . ... set. ,_~__~_, ,. w~`I=l-a warm Inclucea wno ala not receive the suggested Material of the form "A is for Apple," "P is for Palace," was presented to the subjects during sleep stages REM, 2, and 4. Any letter-word pair whose presentation was accompanied simultaneously by alpha was excluded from subsequent analyses of retention. Eight target words, each beginning with a different letter, were presented twice to each subject; at least two different words were presented during each sleep stage. Waking retention was tested by having the subjects check any familiar word on a list of 10 words beginning with "A," and again from 10 words beginning with "P." etc.; two similar "dummy" lists, containing words that had not been presented during sleep, were also administered. Thus, the conservative probability of recognizing a target word by guessing was .10 for each of the eight relevant lists. Three main findings emerged from the recognition test. First, subjects who had not received the set to learn during sleep recognized none of the target

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sleep learning/33 words from any sleep stage. Second, subjects who had received the set recognized, on the average, .28, .10, and .00 of the words that had been presented during stages REM, 2, and 4, respectively; none of these subjects ever claimed to recognize a word that was not a true target. Thus, only those words that had been presented to "set" subjects during REM sleep were recognized at a better- than-chance level. Third, among the suggested-set subjects, those who had a relatively high level of hypnotic susceptibility (as indexed by the Stanford Hypnotic Susceptibility Scale, among other instruments) tended to recognize more stage REM targets than did subjects who had a relatively low level (r = .49). Viewed as a whole, the results of Evans' (1972) experiment seem to square with the Soviet position that sleep learning is possible in hypnotically susceptible subjects in whom a strong set to learn has been established. As such, Evans' results illuminate a number of interesting issues for future research. By way of example, consider first the concept of suggested set. Intuitively, it seems reasonable to suppose that what the induction of a set does is increase the subjects' motivation to learn while they sleep. If motivation is indeed one of the keys to successful sleep learning, then the odds of observing significant sleep-learning effects should be improved by offering subjects a substantial monetary reward for good retention performance (e.g., Levy et al. 1972), by ensuring that the material to be learned during sleep is pertinent to the subjects' personal needs or educational goals (e.g., Balkhasov 1965/1968), or by restricting the subject sample to individuals who have a strong interest in the research (e.g., Svyadoshch 1962/1968~. Turning now to the role of hypnotic susceptibility in sleep learning, research reviewed by Hilgard (1979) indicates that high hypnotizables are able to process

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sleep Jearning/34 information outside of conscious awareness more effectively and completely than are low hypnotizables. A striking example of this "splitting" of consciousness, a process termed dissociation, is when a person discovers that he or she is reacting, in an apparently automatic or involuntary manner, to a suggestion implanted previously under hypnosis. Owing to their greater dissociative abilities, high hypnotizables may be able to selectively attend and process incoming information without consciousness awareness after they have fallen asleep. Lacking this ability, low hypnotizables have to awaken to process similar information, and are therefore incapable of learning while they sleep-. Although this hypothesis is as speculative as it is sketchy, it does seem to fit with the findings that, in comparison with low hypnotizables, high hypnotizables are (a) less likely to awaken either spontaneously or following verbal stimulation (Evans 1972), (b) more likely to respond to behavioral suggestions administered during sleep (Evans et al. 1966, 1969), and (c) more adept at changing their dream experiences to conform with specific presleep instructions (Belicki & Bowers 1982). These findings, in addition to the others mentioned earlier in this section, suggest that the relations among hypnotizability, dissociability, and sleep learning represent an inviting target for future research.

Representative terms from entire chapter:

eeg activation