Neuropsychology of dreams

Rev Neurol. 2012 Jul 16;55(2):101-10. 
Tirapu-Ustarroz J.

Dreams are a universal human experience and studying them from the point of view of neuroscience, consciousness, emotions and cognition is quite a challenge for researchers. Thus, dreams have been addressed from a number of different perspectives ranging from philosophy to clinical medicine, as well as psychiatry, psychology, artificial intelligence, neural network models, psychophysiology or neurobiology.

The main models are grounded on the biological function of dreams, especially those based on processes involving the consolidation of memory and forgetting, and models of simulation. Similarly, current models are developed upon the neurobiology and the neuropsychology of the REM phases of sleep and how they are differentiated from wakefulness. Thus, neurobiologically speaking, dreams are related to the role of acetylcholine and, neuropsychologically, to the activation of the limbic and paralimbic regions, the activation of the basal ganglia, the activation of cortical areas with a specific modality (especially Brodmann's areas 19, 22 and 37) and the deactivation of the ventromedial, parietal and dorsolateral prefrontal cortex and posterior cingulate.

Dreams can be considered a state of consciousness that is characterised by a reduced control over their content, visual images and activation of the memory, and which is mediated by motivational incentives and emotional salience.

The riddle of experience vs. memory

Nobel laureate and founder of behavioral economics Daniel Kahneman talks about happiness from a perspective of experience and remembering.

Sleep depotentiates prior emotional experiences/painful memories

REM Sleep Depotentiates Amygdala Activity to Previous Emotional Experiences
Current Biology, 23 November 2011 (Link)
Els van der Helm, Justin Yao, Shubir Dutt, Vikram Rao, Jared M. Saletin, Matthew P. Walker

Highlights:

• Sleep decreases amygdala activity to prior waking emotional experiences 
• The amygdala decrease is associated with reestablished prefrontal connectivity 
• These neural changes are accompanied by overnight reductions in subjective reactivity 
• Reductions in both brain and behavioral reactivity are associated with REM physiology 

Summary: Clinical evidence suggests a potentially causal interaction between sleep and affective brain function; nearly all mood disorders display co-occurring sleep abnormalities, commonly involving rapid-eye movement (REM) sleep. Building on this clinical evidence, recent neurobiological frameworks have hypothesized a benefit of REM sleep in palliatively decreasing next-day brain reactivity to recent waking emotional experiences. Specifically, the marked suppression of central adrenergic neurotransmitters during REM (commonly implicated in arousal and stress), coupled with activation in amygdala-hippocampal networks that encode salient events, is proposed to (re)process and depotentiate previous affective experiences, decreasing their emotional intensity. In contrast, the failure of such adrenergic reduction during REM sleep has been described in anxiety disorders, indexed by persistent high-frequency electroencephalographic (EEG) activity; a candidate factor contributing to hyperarousal and exaggerated amygdala reactivity. Despite these neurobiological frameworks, and their predictions, the proposed benefit of REM sleep physiology in depotentiating neural and behavioral responsivity to prior emotional events remains unknown. Here, we demonstrate that REM sleep physiology is associated with an overnight dissipation of amygdala activity in response to previous emotional experiences, altering functional connectivity and reducing next-day subjective emotionality.

Preferential benefits of sleep

Sleep-Dependent Facilitation of Episodic Memory Details
PLoS ONE 6(11): e27421. doi:10.1371/journal.pone.0027421 (Link)
Els van der Helm, Ninad Gujar, Masaki Nishida, Matthew P. Walker

While a role for sleep in declarative memory processing is established, the qualitative nature of this consolidation benefit, and the physiological mechanisms mediating it, remain debated. Here, we investigate the impact of sleep physiology on characteristics of episodic memory using an item- (memory elements) and context- (contextual details associated with those elements) learning paradigm; the latter being especially dependent on the hippocampus. Following back-to-back encoding of two word lists, each associated with a different context, participants were assigned to either a Nap-group, who obtained a 120-min nap, or a No Nap-group. Six hours post-encoding, participants performed a recognition test involving item-memory and context-memory judgments. In contrast to item-memory, which demonstrated no between-group differences, a significant benefit in context-memory developed in the Nap-group, the extent of which correlated both with the amount of stage-2 NREM sleep and frontal fast sleep-spindles. Furthermore, a difference was observed on the basis of word-list order, with the sleep benefit and associated physiological correlations being selective for the second word-list, learned last (most proximal to sleep). These findings suggest that sleep may preferentially benefit contextual (hippocampal-dependent) aspects of memory, supported by sleep-spindle oscillations, and that the temporal order of initial learning differentially determines subsequent offline consolidation.

How Best to Optimize Memory

To sleep, to strive, or both: how best to optimize memory
PLoS One. 2011;6(7):e21737. Epub 2011 Jul 20. (Link)
Matthew A. Tucker, Sunny X. Tang, Amaka Uzoh, Alexandra Morgan, Robert Stickgold

While numerous studies have shown that a night of sleep profits memory relative to wake, we still have little understanding about what factors mediate this effect of sleep. A clear understanding of the dynamics of this effect of sleep beyond the initial night of sleep is also lacking. Here, we examined the effect of extrinsic rewards on sleep-dependent declarative memory processing across 12 and 24 hr training-retest intervals. Subjects were either paid based on their performance at retest ($1 for each correct answer), or received a flat fee for participation. After a 12 hr interval we observed pronounced benefits of both sleep and reward on memory. Over an extended 24 hr interval we found 1) that an initial night of sleep partially protects memories from subsequent deterioration during wake, and 2) that sleep blocks further deterioration, and may even have a restorative effect on memory, when it follows a full day of wake. Interestingly, the benefit imparted to rewarded (relative to unrewarded) stimuli was equal for sleep and wake subjects, suggesting that the sleeping brain may not differentially process rewarded information, relative to wake. However, looking at the overall impact of sleep relative to reward in this protocol, it was apparent that sleep both imparted a stronger mnemonic boost than reward, and provided a benefit to memory regardless of whether it occurred in the first or the second 12 hrs following task training.

The memory function of sleep

Nature Reviews Neuroscience 11, 114-126 (February 2010) 
Susanne Diekelmann & Jan Born

Sleep has been identified as a state that optimizes the consolidation of newly acquired information in memory, depending on the specific conditions of learning and the timing of sleep. Consolidation during sleep promotes both quantitative and qualitative changes of memory representations. Through specific patterns of neuromodulatory activity and electric field potential oscillations, slow-wave sleep (SWS) and rapid eye movement (REM) sleep support system consolidation and synaptic consolidation, respectively. During SWS, slow oscillations, spindles and ripples - at minimum cholinergic activity - coordinate the re-activation and redistribution of hippocampus-dependent memories to neocortical sites, whereas during REM sleep, local increases in plasticity-related immediate-early gene activity - at high cholinergic and theta activity - might favour the subsequent synaptic consolidation of memories in the cortex.

Sleep deprivation impairs the accurate recognition of human emotions

Sleep. 2010 Mar 1;33(3):335-42.
van der Helm E, Gujar N, Walker MP.
Sleep and Neuroimaging Laboratory, Department of Psychology, University of California, Berkeley, CA 94720-1650, USA.

Study objectives: Investigate the impact of sleep deprivation on the ability to recognize the intensity of human facial emotions.

Methods: Randomized total sleep-deprivation or sleep-rested conditions, involving between-group and within-group repeated measures analysis. Thirty-seven healthy participants, (21 females) aged 18-25 y, were randomly assigned to the sleep control (SC: n = 17) or total sleep deprivation group (TSD: n = 20). Participants performed an emotional face recognition task, in which they evaluated 3 different affective face categories: Sad, Happy, and Angry, each ranging in a gradient from neutral to increasingly emotional. In the TSD group, the task was performed once under conditions of sleep deprivation, and twice under sleep-rested conditions following different durations of sleep recovery. In the SC group, the task was performed twice under sleep-rested conditions, controlling for repeatability.

Results: In the TSD group, when sleep-deprived, there was a marked and significant blunting in the recognition of Angry and Happy affective expressions in the moderate (but not extreme) emotional intensity range; differences that were most reliable and significant in female participants. No change in the recognition of Sad expressions was observed. These recognition deficits were, however, ameliorated following one night of recovery sleep. No changes in task performance were observed in the SC group.

Conclusions: Sleep deprivation selectively impairs the accurate judgment of human facial emotions, especially threat relevant (Anger) and reward relevant (Happy) categories, an effect observed most significantly in females. Such findings suggest that sleep loss impairs discrete affective neural systems, disrupting the identification of salient affective social cues.