Consolidation effect of repeated processing of declarative knowledge in mental experiences during human sleep

Sleep may positively influence declarative memory through the processing, which transforms items of declarative knowledge into contents of mental sleep experience (MSE). A prediction from this general hypothesis is that the consolidation level should be higher for the output of items repeatedly processed and transformed into identical or very similar (so-called interrelated) contents of distinct MSEs of the same night rather than for the output of items presumably processed once (that is, all other, non-interrelated contents). Two experiments examined whether and how far the frequency and long-term retention of interrelated contents depend on the repeated processing of given items rather than on the experimental procedure applied for detection of interrelated contents. This procedure entails both multiple awakenings and a verbal report of MSE after awakening. Multiple awakenings could facilitate the re-access and elaboration of some contents into the subsequent (i.e. contiguous) MSE rather than non-contiguous MSEs; verbal reports could enhance the delayed recall of interrelated contents in as much as repeatedly encoded. The first experiment showed that interrelated contents were more frequent and better retained than both non-interrelated and pseudo-interrelated (i.e. by-chance similar or identical) contents, and even more in pairs of contiguous than non-contiguous MSEs collected from the first four periods of REM sleep on each experimental night. The second experiment showed that the frequency and retention rate of interrelated contents, while higher than those of non-interrelated and pseudo-interrelated contents, were not significantly different in pairs of MSEs which were verbally or mentally recalled after awakening provoked during the first four periods of REM sleep in each experimental night. Taken together, these findings indicate that the advantage provided by repeated processing during REM sleep for the consolidation of the output of items of declarative knowledge is conspicuous and largely independent from the experimental procedure, as this only slightly enhances the frequency and retention rate of interrelated contents.

Consolidation effect of the processing of declarative knowledge during human sleep: evidence from long-term retention of interrelated contents of mental sleep experiences

Sleep may partly exert a positive influence on memory through the processing underlying the transformation of items of declarative knowledge into contents of mental sleep experience (MSE). This hypothesis implies that the level of consolidation (and thus, long-term retention) should be enhanced for those items which are repeatedly processed and transformed into identical or very similar (so-called interrelated) contents of distinct MSEs developed over the same night. To test this prediction, we examined accessibility at delayed recall (i.e., the next morning) of the interrelated contents of MSEs reported (immediate recall) by 14 subjects who were awakened during the first four periods of rapid eye-movement (REM) sleep in two experimental nights. Interrelated contents were much more frequent, and at delayed recall much better retained, than other, non-interrelated contents in report pairs of MSEs. Moreover, they were also more frequent and better retained than by-chance similar or identical contents, as estimated in report pairs of MSEs of different subjects. These findings provide partial but coherent evidence in favour of the hypothesis that a generation effect occurs during sleep, with a further consolidation of the input and the output of MSE processing (respectively, the items of declarative knowledge and the contents of MSEs resulting from their elaboration during sleep).

Incorporation of presleep stimuli into dream contents: evidence for a consolidation effect on declarative knowledge during REM sleep?

Presleep stimuli to be retained for further recall is often incorporated into dream contents. To establish whether processing for insertion into dream contents may improve consolidation, we compared the retention rate at delayed recall of contents resulting from incorporation of presleep sentence-stimuli with those of other contents of the same dream experiences. We hypothesized that association with a cognitive task of recall facilitates access to recently acquired items of declarative knowledge such as presleep stimuli, and triggers the deep elaboration of their semantic features, which involves rehearsal. Twelve subjects were given a task of delayed recall for three nonsense sentences delivered once a time before each of the sleep (re-)onsets over an experimental night. After each awakening in rapid eye movement sleep, subjects were asked to report dream experience and recall the sentence to be retained. In the morning, after spontaneous awakening, subjects were unexpectedly requested to again report their dream experiences and to recall the stimuli. Two pairs of judges independently identified possible incorporations of the stimuli, and parsed dream reports into propositional content units. The proportion of night reports with at least one incorporation of the stimulus delivered (i.e. valid incorporations) was higher than that of reports with contents similar to a stimulus(-i) not yet delivered (forward pseudo-incorporations) or delivered prior to an earlier sleep period (backward pseudo-incorporations). The proportion of content units common to night and morning reports (considered to be better consolidated) was significantly higher for incorporated contents than for other contents, including pseudo-incorporated contents. Instead, the retention at morning recall of words of sentence-stimuli corresponding to incorporated contents was not significantly higher than that of other words. The better retention of incorporated contents provides a partial confirmation (that is, limited to the output of the processing) that a generation effect, which benefits retention of actively processed information, is operative during sleep as well as in waking.

Completeness and accuracy of morning reports after a recall cue: comparison of dream and film reports

Our goal was to test the efficiency and accuracy of a complementary morning report, after recall cue, of an experience (having a dream or viewing a film) made and first described during the night. Twenty participants were awakened 10 min after the onset of the second REM sleep. Upon awakening, on one night they described the dream they just had and on the other night they were presented a 4-min video, then had to describe it. A new description requested in the morning after a recall cue yielded an important amount of new information both for the film and the dreams, and for the film, where the accuracy could be checked, 86% of this new information was accurate. Some aspects of the results pointed to an effect of hypermnesia. In conclusion, the morning additional information after recall cue stems from a good access to the memory of the night experience.

Mental activity during sleep

Since remote antiquity humankind has believed in the supernatural origin of dreaming. The scientific approach to mental activity during sleep dates back to the eighteenth century. One hundred years ago, psychoanalysis introduced dream analysis for diagnostic and therapeutic purposes. Only 50 years ago psychophysiology made it possible to collect mental sleep activity by means of experimental awakenings while monitoring physiological variables; this approach encouraged investigation into the relationship between the features of sleep activity and sleep state (REM versus NREM). Advances in neurophysiology, in neurochemistry, and recently in brain imaging techniques, have shed light on the roles played by the different cerebral structures in determining specific characteristics of mental activity during sleep. The development of cognitive psychology has enabled investigation of dream generation in terms of output from a complex multilevel system of information processing. In addition to sleep state, other factors, such as the time of the night and the sequence of the NREM-REM cycles, have been shown to influence the characteristics of mental activity. The usefulness of investigation of mental sleep activity as a clinical tool is controversial. The psychophysiological approach to mental sleep activity in clinical contexts has enabled the exploration of adaptation processes and contributed to neuropsychological studies on focal and systemic brain pathology.

Accessing previous mental sleep experience in REM and NREM sleep

This study investigated the processes by which contents previously stored in memory are retrieved and inserted into mental sleep experience (MSE). MSE reports were collected from six subjects awakened three times on each of eight nights in two alternate sequences of awakenings (NREM-REM-NREM; REM-REM-REM). The occurrences of interrelations between contents of report pairs were scored using Clark's (1970) feature matching model. These were greater for same night pairs than for different night pairs, and did not differ with respect to sequence of awakenings or order of report pairs (first-second, second-third, first-third). Contents of previous MSEs, therefore, seem to be accessible in both sleep types for insertion into current MSE. The interrelated units were more frequently lexical than propositional, with more paradigmatic than syntagmatic relationships in report pairs from both sequences of awakenings. Thus, the re-elaboration of contents of previous MSEs seems to occur mainly at the level of single contents in both types of sleep, with similar modalities of processing.

The thematic continuity of mental experiences in REM and NREM sleep

In this study the characteristics of interrelated contents in paired reports of mental sleep experience (MSE) were analyzed to obtain insight as to the functioning of processes by which contents previously stored in memory are retrieved and inserted into MSE in rapid eye movements (REM) and non-REM (NREM) sleep. Ten subjects were awakened 3 times on each of 4 nights after 3 min of NREM sleep (in stage II or III before the first REM), of the first phase of REM sleep, and again of NREM sleep (in stage II or III after the first REM). The contents of all the possible pairings of reports were scored by using Clark's (1970) feature-matching model and compared with respect to the factors 'night' (same/different), 'type of report pairs' (1stNREM-REM/REM-2ndNREM/1stNREM-2ndNREM), 'unit interrelated' (lexical/propositional), 'interrelationship' (paradigmatic/syntagmatic). The occurrences of interrelations were greater for same night pairs than for different night pairs, but without significant differences between types of report pairs: these data provide support for the thematic continuity of MSE in both NREM and REM sleep. The units interrelated in pairs of reports were more frequent at a lexical than a propositional level, showing more paradigmatic than syntagmatic interrelationships: these data suggest that the re-elaboration of contents of previous MSEs occurs mainly at a lower level, and that the modality of processing previous contents by insertion into current MSE is similar in NREM and REM sleep.(ABSTRACT TRUNCATED AT 250 WORDS)