Human relational memory requires time and sleep

Does abnormal sleep impair memory consolidation in schizophrenia?

Although disturbed sleep is a prominent feature of schizophrenia, its relation to the pathophysiology, signs, and symptoms of schizophrenia remains poorly understood. Sleep disturbances are well known to impair cognition in healthy individuals. Yet, in spite of its ubiquity in schizophrenia, abnormal sleep has generally been overlooked as a potential contributor to cognitive deficits. Amelioration of cognitive deficits is a current priority of the schizophrenia research community, but most efforts to define, characterize, and quantify cognitive deficits focus on cross-sectional measures. While this approach provides a valid snapshot of function, there is now overwhelming evidence that critical aspects of learning and memory consolidation happen offline, both over time and with sleep. Initial memory encoding is followed by a prolonged period of consolidation, integration, and reorganization, that continues over days or even years. Much of this evolution of memories is mediated by sleep. This article briefly reviews (i) what is known about abnormal sleep in schizophrenia, (ii) sleep-dependent memory consolidation in healthy individuals, (iii) recent findings of impaired sleep-dependent memory consolidation in schizophrenia, and (iv) implications of impaired sleep-dependent memory consolidation in schizophrenia. This literature suggests that abnormal sleep in schizophrenia disrupts attention and impairs sleep-dependent memory consolidation and task automation. We conclude that these sleep-dependent impairments may contribute substantially to generalized cognitive deficits in schizophrenia. Understanding this contribution may open new avenues to ameliorating cognitive dysfunction and thereby improve outcome in schizophrenia.

REM, not incubation, improves creativity by priming associative networks

The hypothesized role of rapid eye movement (REM) sleep, which is rich in dreams, in the formation of new associations, has remained anecdotal. We examined the role of REM on creative problem solving, with the Remote Associates Test (RAT). Using a nap paradigm, we manipulated various conditions of prior exposure to elements of a creative problem. Compared with quiet rest and non-REM sleep, REM enhanced the formation of associative networks and the integration of unassociated information. Furthermore, these REM sleep benefits were not the result of an improved memory for the primed items. This study shows that compared with quiet rest and non-REM sleep, REM enhances the integration of unassociated information for creative problem solving, a process, we hypothesize, that is facilitated by cholinergic and noradrenergic neuromodulation during REM sleep.

The role of the hippocampus in transitive inference

Transitive inference (TI) is the ability to infer the relationship between items (e.g., A>C) after having learned a set of premise pairs (e.g., A>B and B>C). Previous studies in humans have identified a distributed neural network, including cortex, hippocampus, and thalamus, during TI judgments. We studied two aspects of TI using functional magnetic resonance imaging of subjects who had acquired the six-item sequence (A>B>C>D>E>F) of visual stimuli. First, the identification of novel pairs not containing end items (i.e., B>D, C>E, B>E) was associated with greater left hippocampal activation compared with the identification of novel pairs containing end items A and F. This demonstrates that the identification of stimulus pairs requiring the flexible representation of a sequence is associated with hippocampal activation. Second, for the three novel pairs devoid of end items we found greater right hippocampal activation for pairs B>D and C>E compared with pair B>E. This indicates that TI decisions on pairs derived from more adjacent items in the sequence are associated with greater hippocampal activation. Hippocampal activation thus scales with the degree of relational processing necessary for TI judgments. Both findings confirm a role of the hippocampus in transitive inference in humans.

The role of sleep in false memory formation

Memories are not stored as exact copies of our experiences. As a result, remembering is subject not only to memory failure, but to inaccuracies and distortions as well. Although such distortions are often retained or even enhanced over time, sleep's contribution to the development of false memories is unknown. Here, we report that a night of sleep increases both veridical and false recall in the Deese-Roediger-McDermott (DRM) paradigm, compared to an equivalent period of daytime wakefulness. But while veridical memory deteriorates across both wake and sleep, false memories are preferentially preserved by sleep, actually showing a non-significant improvement. The same selectivity of false over veridical memories was observed in a follow-up nap study. Unlike previous studies implicating deep, slow-wave sleep (SWS) in declarative memory consolidation, here veridical recall correlated with decreased SWS, a finding that was observed in both the overnight and nap studies. These findings lead to two counterintuitive conclusions - that under certain circumstances sleep can promote false memories over veridical ones, and SWS can be associated with impairment rather than facilitation of declarative memory consolidation. While these effects produce memories that are less accurate after sleep, these memories may, in the end, be more useful.

Sleep-dependent memory processing

While the functions of sleep remain largely unknown, one exciting hypothesis is that sleep contributes importantly to processes of memory and brain plasticity. Over the last decade, a large body of work has provided substantive evidence supporting this role of sleep in what is becoming known as sleep-dependent memory processing. This review offers a summary of these data, focusing specifically on the role of sleep in (1) memory encoding, (2) memory consolidation (along with the brain basis of this process), and (3) neural plasticity. The clinical ramifications of such findings are also explored.

From creation to consolidation: a novel framework for memory processing

Long after playing squash, your brain continues to process the events that occurred during the game, thereby improving your game, and more generally, enhancing adaptive behavior. Understanding these mysterious processes may require novel theories.

Sleep promotes generalization of extinction of conditioned fear

STUDY OBJECTIVE

To examine the effects of sleep on fear conditioning, extinction, extinction recall, and generalization of extinction recall in healthy humans.

DESIGN

During the Conditioning phase, a mild, 0.5-sec shock followed conditioned stimuli (CS+s), which consisted of 2 differently colored lamps. A third lamp color was interspersed but never reinforced (CS-). Immediately after Conditioning, one CS+ was extinguished (CS+E) by presentation without shocks (Extinction phase). The other CS+ went unextinguished (CS+U). Twelve hours later, following continuous normal daytime waking (Wake group, N=27) or an equal interval containing a normal night's sleep (Sleep group, N=26), conditioned responses (CRs) to all CSs were measured (Extinction Recall phase). It was hypothesized that the Sleep versus Wake group would show greater extinction recall and/or generalization of extinction recall from the CS+E to the CS+U.

SETTING

Academic medical center.

SUBJECTS

Paid normal volunteers.

MEASUREMENTS AND RESULTS

Square-root transformed skin conductance response (SCR) measured conditioned responding. During Extinction Recall, the Group (Wake or Sleep) x CS+ Type (CS+E or CS+U) interaction was significant (P = 0.04). SCRs to the CS+E did not differ between groups, whereas SCRs to the CS+U were significantly smaller in the Sleep group. Additionally, SCRs were significantly larger to the CS+U than CS+E in the Wake but not the Sleep group.

CONCLUSIONS

After sleep, extinction memory generalized from an extinguished conditioned stimulus to a similarly conditioned but unextinguished stimulus. Clinically, adequate sleep may promote generalization of extinction memory from specific stimuli treated during exposure therapy to similar stimuli later encountered in vivo.

Is awareness necessary for true inference?

In transitive inference, participants learn a set of context-dependent discriminations that can be organized into a hierarchy that supports inference. Several studies show that inference occurs with or without task awareness. However, some studies assert that without awareness, performance is attributable to pseudoinference. By this account, inference-like performance is achieved by differential stimulus weighting according to the stimuli's proximity to the end items of the hierarchy. We implement an inference task that cannot be based on differential stimulus weighting. The design itself rules out pseudoinference strategies. Success on the task without evidence of deliberative strategies would therefore suggest that true inference can be achieved implicitly. We found that accurate performance on the inference task was not dependent on explicit awareness. The finding is consistent with a growing body of evidence that indicates that forms of learning and memory supporting inference and flexibility do not necessarily depend on task awareness.

Nocturnal seizures and the effects of anticonvulsants on sleep

Epilepsy is a common condition that affects up to 1% of the population. Patients with epilepsy are particularly sensitive to the adverse effects of sleep disruption. Failure to recognize and treat sleep disturbances can lead to worsening of attention, cognitive functioning, and quality of life, and can increase seizures. Anticonvulsant agents have the potential to either improve or worsen sleep and sleep disorders in patients with epilepsy. Therefore, awareness of the implications of sleep disorders and anticonvulsants is critical for the optimal care of patients with epilepsy.

Implicit analogy: New direct evidence and a challenge to the theory of memory

The authors propose that analogical reasoning may be achieved without conscious or explicit deliberation. The argument would be strengthened by more convincingly demonstrating instances of analogy that do not require explicit deliberation. Recent findings demonstrate that deliberative or explicit strategies are not necessary for flexible expression under novel circumstances (Greene et al. 2001) to include analogical transfer (Gross & Greene 2007). This issue is particularly critical because the existence of relational priming poses a serious challenge to the widely held notion that flexible expression of learned relations requires deliberative processes.

Shifting from implicit to explicit knowledge: different roles of early- and late-night sleep

Sleep has been shown to promote the generation of explicit knowledge as indicated by the gain of insight into previously unrecognized task regularities. Here, we explored whether this generation of explicit knowledge depends on pre-sleep implicit knowledge, and specified the differential roles of slow-wave sleep (SWS) vs. rapid eye movement (REM) sleep in this process. Implicit and explicit knowledge (insight) related to a hidden regularity were assessed in an associative motor-learning task (number reduction task, NRT), which was performed in two sessions (initial practice and retest) separated by 3 h of either early-night sleep, rich in SWS, or of late-night sleep, rich in REM sleep. About half of the participants developed signs of implicit rule knowledge (i.e., speeded reaction times for responses determined by the hidden regularity) at initial practice preceding early or late sleep. Of these, half developed explicit knowledge across early-night sleep, significantly more than across late-night sleep. In contrast, late-night subjects preferentially remained on the level of implicit rule knowledge after sleep. Participants who did not develop implicit knowledge before sleep had comparable rates of transition to implicit or explicit knowledge across early and late sleep. If subjects gained explicit knowledge across sleep, this was associated with lower amounts of REM sleep, specifically in the late-night group. SWS predominant during the early night may restructure implicit memory representations in a way that allows creating an explicit representation afterward, whereas REM sleep in the late night appears to stabilize them in their implicit form.

Analogical inference: the role of awareness in abstract learning

The acquisition and flexible expression of complex relations is often attributed to declarative memory processes. The extent to which such tasks may be done implicitly has not been sufficiently explored. We report that analogical or transfer processes may be accomplished implicitly. Our analogy task requires acquisition of a transverse patterning set, and then tests for transfer on an unrelated set. Participants learn the relations A>B (given a choice between A and B choose A) and B>C and the unrelated set X>Y and Y>Z. Only the experimental group was trained on the transverse pair C>A. At test all trials are unreinforced: A?B, B?C, A?C, X?Y, Y?Z, X?Z. Analogy was observed when the experimental group chose Z>X at greater frequency than controls who uniformly chose X>Z. Analogy occurred with or without awareness of the transfer. The capacity to transfer relations to an analogous set demonstrates a level of flexibility and abstraction not generally thought to be possible for implicit processes.

The dream as a model for psychosis: an experimental approach using bizarreness as a cognitive marker

Many previous observers have reported some qualitative similarities between the normal mental state of dreaming and the abnormal mental state of psychosis. Recent psychological, tomographic, electrophysiological, and neurochemical data appear to confirm the functional similarities between these 2 states. In this study, the hypothesis of the dreaming brain as a neurobiological model for psychosis was tested by focusing on cognitive bizarreness, a distinctive property of the dreaming mental state defined by discontinuities and incongruities in the dream plot, thoughts, and feelings. Cognitive bizarreness was measured in written reports of dreams and in verbal reports of waking fantasies in 30 schizophrenics and 30 normal controls. Seven pictures of the Thematic Apperception Test (TAT) were administered as a stimulus to elicit waking fantasies, and all participating subjects were asked to record their dreams upon awakening. A total of 420 waking fantasies plus 244 dream reports were collected to quantify the bizarreness features in the dream and waking state of both subject groups. Two-way analysis of covariance for repeated measures showed that cognitive bizarreness was significantly lower in the TAT stories of normal subjects than in those of schizophrenics and in the dream reports of both groups. The differences between the 2 groups indicated that, under experimental conditions, the waking cognition of schizophrenic subjects shares a common degree of formal cognitive bizarreness with the dream reports of both normal controls and schizophrenics. Though very preliminary, these results support the hypothesis that the dreaming brain could be a useful experimental model for psychosis.