Complete and elaborate summary of the sixth and last task of the elective course PSY3349 - Sleep and sleep disorders. Summary contains all resources on the reference list, including figures.
Information is first processed by sensory memory, which is a brief period of time (fractions of a
second to a few seconds) that the initial sensation of environmental stimuli is initially remembered. It
allows the individual to retain the experience of the sensation slightly longer than the original
stimulus. Sensory memory is often experienced as a brief period in which sensory experiences can be
remembered as repeating or ‘echoing’.
Only a small fraction of information passes from sensory memory to the second stage, short-term or
working memory. This stage is longer than sensory memory, but still limited to seconds or minutes;
its capacity is limited to a few items. The length of the short-term memory can be extended through
rehearsal. The
capacity of short-
term memory can be
expanded through
techniques such as
chunking (grouping
pieces of information
together).
The third and final stage of memory is long-term memory. Information that will be retained from
short-term memory is consolidated into long-term memory. Again, not all information from short-
term memory makes it to long-term memory. Long-term memory consists of two major categories.
Nondeclarative memory (implicit memory) includes memories that we are not necessarily conscious
of and appear to operate automatically. They do not seem to require memorization or include facts or
experiences; instead, they control behaviors; the acquisition of specific motoric behaviors and skills is
probably the most important form of nondeclarative memory.
The other category of LTM, declarative memory (explicit memory) is memory of events and facts
that we can think and talk about. Declarative memories are not simply verbal memories; for example,
think about the last birthday. You
remember where you were, when the
event occurred, what other people were
present, what happened, and so on.
Declarative memory includes episodic
memories (which involve context;
information when and under what
conditions something occurred) and
semantic memories; facts, but without
information about the context in which
the facts were learned.
1
,PSY3349 Sleep and Sleep Disorders
THE ROLE OF SLEEP IN COGNITION AND EMOTION
Source: Walker (2009)
Within the human sleep cycle, NREM and REM sleep cycle every 90 minutes in an ultradian manner,
while the ratio of NREM to REM shifts. During the first half of the night, NREM stages 3 and 4
(SWS) dominate, while stage 2 NREM and REM sleep prevail in the latter half of the night. The
functional reason for this organizing principal (deep NREM early in the night, stage 2 NREM and
REM late in the night) remains unknown.
As NREM sleep progresses, EEG activity begins to slow in frequency. Throughout stage-2 NREM
there is the presence of phasic electrical events including K-complexes and sleep spindles. The deepest
stages of NREM, stage 3 and 4, are often grouped together as slow wave sleep, reflecting the
occurrence of low frequency waves, which have been termed slow-wave activity (SWA), representing
an expression of underlying mass cortical synchrony. During REM, waveforms change again in their
composition, associated with oscillatory activity in the theta range. Periodic bursts of REM also take
place, associated with the occurrence of phasic endogenous waveforms expressed in the pons (P),
lateral geniculate nuclei of the thalamus (G) and the occipital cortex (O), which have been termed
PGO waves.
In NREM sleep, subcortical cholinergic systems in the brain stem and forebrain become less active,
while firing rates of serotonergic raphe neurons and noradrenergic locus coeruleus neurons are also
reduced relative to waking levels. During REM sleep, both these aminergic populations are strongly
inhibited, while cholinergic systems become as/more active compared to what they are during wake,
resulting in a brain state largely devoid of aminergic modulation and dominated by acetylcholine.
During NREM SWS, rostral brain-stem regions, thalamic nuclei, basal ganglia, hypothalamus,
prefrontal cortex, cingulate cortices, and medial regions of the temporal lobe all show reduced
activity. However, during REM sleep significant elevations in activity have reported in the pontine
tegmentum, thalamic nuclei, occipital cortex, mediobasal prefrontal lobes, and associated limbic
groups, including amygdala, hippocampus and anterior cingulate cortex. In contrast, the dorsolateral
PFC, posterior cingulate and parietal cortex appear least active in REM sleep.
Memory processing and brain plasticity
The conception of a memory begins with the process of encoding, resulting in a stored representation
of an experience within the brain. It is understood that a vast number of post-encoding memory
processes can take place. For memories to persist over the longer time course, an offline, nonconscious
operation of event consolidation appears to be necessary, affording memories greater resistance to
decay, or even improved recollection. Sleep has been implicated in both the encoding and
consolidation of memory.
2
, PSY3349 Sleep and Sleep Disorders
Sleep and memory encoding
One study investigated the effects of 35h of total sleep deprivation on verbal learning. In those who
were sleep deprived, regions of the medial temporal lobe were significantly less active during learning,
while the prefrontal cortex actually expressed greater activation. Most interesting, the parietal lobes,
which were not activated in the control group during learning, were significantly active in the
deprivation group. This suggests that inadequate sleep prior to learning produced bidirectional
changes in episodic encoding activity, involving the inability of the medial temporal lobe to engage
normally during learning, combined with potential compensation attempts by prefrontal regions,
which in turn may facilitate recruitment of parietal lobe function.
The impact of sleep deprivation on memory formation may be especially pronounced for emotional
material. Walker investigated the impact of sleep deprivation on the encoding of emotionally negative,
positive and neutral words. When combined, subjects in the sleep-deprived group exhibited a striking
40% reduction in the ability to form new memories under conditions of sleep deprivation (figure).
When these data were separated into the three affective categories (negative, positive, neutral), the
magnitude of encoding impairment differed (figure). In those that had slept, both positive and negative
stimuli were associated with superior retention levels relative to the neutral condition, consistent with
the notion that emotion facilitates memory encoding. However, there was severe disruption of
encoding and hence later retention for neutral and especially positive emotional memory in the
deprivation group. In contrast, a relative resistance of negative emotional memory was observed in the
deprived group.
These data suggest that, while the effects of sleep deprivation are directionally consistent across
memory subcategories, the most profound impact is on the encoding of positive emotional stimuli,
and to a lesser degree, emotionally neutral stimuli. In contrast, the encoding of negative memory
appears to be more resistant to the effects of deprivation.
These data may offer insight into affective mood disorders that express co-occurring sleep
abnormalities. If one compares the memory encoding in the figure, it is clear that those who slept
encoded and retained a balanced mix of both positive and negative memories. In contrast, those who
did not sleep displayed a skewed relative distribution of encoding, resulting in a dominance of
negative memories, combined with a deficit of positive and neutral memories.
The impact of sleep deprivation on the neural dynamics associated with declarative memory has been
examined using fMRI. In addition to performance impairment after sleep deprivation, a highly
significant and selective deficit was identified in bilateral regions of the hippocampus; a structure
known to be critical for learning episodic information. The success of encoding was further associated
with activity in different regions of the prefrontal lobe. In those that slept prior to learning, the right
dorsal/middle lateral PFC showed a strong positive relationship with the proficiency of encoding.
3
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