Download Sleep, Dreaming and Memory

PERIODICALS OF IMPLICIT COGNITION (2006) 7, 1-7
Implicit Laboratory Association, Budapest, Hungary
Sleep, Dreaming and Memory
Péter Simor
ImpLab Association, Budapest &
Institute of Psychology, Eötvös Loránd University,
46, Izabella utca, Budapest, 1076, Hungary
[email protected]
http://www.implab.org
Abstract: In this target article I briefly summarize some of the experimental results and
theoretical assumptions about the information processing mechanisms during the different
stages of sleep. The main differences between the phenomenological aspects of NREM and
REM sleep are discussed along with their different contributions for informational
processes. Neuropsychological evidences of the dream state will also be considered.
Keywords: dreaming, episodic memory, emotion, explicit memory, implicit memory,
memory consolidation, NREM sleep, REM sleep, semantic memory.
Introduction
Mr. Z. has no formal training in psychology.
For him it might be a really strange idea that
during the ostensibly passive state of
sleeping, a lot of important things might
happen in his mind. However, if Mr. Z.
could recall his last night dream where he
was riding a red horse and saving the world
from the evil conspiracy of his own boss he
may realize something. Although he thought
he was „switched off” his mind was
obstinately working. Imagine that Mr. Z. is
extremely curious and he wants to
understand the above mentioned paradox,
so he starts to look for all the scientific
publications concerning the role of sleep and
dreaming. It can be taken granted that after
reading a few articles Mr. Z. will become
even more confused then ever as a result of
the numerous contradictory results, opinions
and theories.
In the last few decades sleep has been
associated with mood-regulation, coping and
with stress (e.g., Berger, Riemann, & Lauer,
1986
in
Bódizs,
2000),
memory
consolidation (e.g., Payne & Nadel, 2004),
semantic rewriting of episodic memories
(Stickgold, 2002), emotional information
processing (Hartmann, 1996), facilitation of
social bonding (McNamara, 1996), or even
that sleep has nothing to do with
information processing, but only serves pure
biological functions (e.g., Vertes & Eastman,
2000).
1. The memory consolidation hypothesis
1.1 The first steps
The first (mainly animal) experiments
focusing on the effect of sleep in learning
and memory used the different methods of
sleep deprivation paradigm. According to
the main hyphothesis (Fishbein 1970 in
Ellman, Spielman, Luck, Steiner, &
Halperin, 1991), the newly aquired
informations are stored in a fragile state, and
REM sleep is the necessary process to
consolidate the new information. In one
popular experimental design for example the
animal’s sleep was selectively blocked after a
learning task, and then the investigators
analized if the REM deprived, NREM
deprived and the control group showed
different performance in the previously
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Simor: Sleep, dreaming and memory
learnt task. In some studies the REMdeprived group showed some performance
deficits compared with the other groups
(Bódizs, 2000), but in other studies the
investigators failed to demonstrate such
relationship (Siegel, 2001). Among the great
number of the righteous methodological
critics, maybe the most important is, that the
sleep deprivation technique apart from
depriving the subject from a certain stage of
sleep, introduces a lot of uncontrolled
variables such as stress, hipersexuality,
irritability, hiperphagia, etc., which can
interfere with the required learning process.
1.2 Implicit and explicit memory
After the first REM deprivation techniques,
with the appearance of more sophisticated
experimental paradigms (including human
experiments), interesting results enriched
our knowledge about the relationship
between sleep and memory consolidation.
One important finding is that different types
of memory (implicit vs. explicit) are best
consolidated during specific stages of sleep
(Payne & Nadel, 2004). The experimental
data suggests the following conclusions:
1. REM sleep may facilitate the
consolidation of implicit, procedural
memories (Plihal & Born, 1997).
2. REM sleep is an opened state for
information processing: the classical
conditioning can be generated in
REM, and the conditioned response
appears in waking too. Equally in
REM sleep the subject responds
(with the activity in the specific brain
areas) to the conditioned stimulus
learnt while awake (Bódizs, 2000).
3. NREM sleep, especially slow wave
sleep is critical for the consolidation
of explicit, episodic memories.
4. The effect of NREM sleep applies
both for verbal tasks and spatial
tasks (Plihal & Born, 1997).
1.3 Emotional and neutral information
Another interesting result is that REM sleep
can also facilitate the consolidation of
explicit memories, if the information is
emotionally relevant and even more if it is
emotionally threatening for the subject
(Karni, Tanne, Rubenstein, Askenasy, &
Sagi, 1994). While the emotionally neutral
information may benefit preferentially from
NREM sleep. The emotional information
processing in REM sleep is probably related
to the mood-regulating effect of REM sleep
and the emotional aspect of dreams, but the
mechanism underlying this emotional
functioning is still unclear, and open for
debate.
1.4 The sceptics
However some researchers (e.g., Siegel,
2001; Vertes & Eastman, 2000) are still
sceptical about the information processing
in sleep; they say that sleep serves “cruder”,
purely biological functions. For example,
according to Vertes the general purpose of
sleep is the recuperation of the CNS, and
the function of REM sleep is to “mobilize”,
prepare the brain for the recovery of sleep.
Perhaps the most persuasive argument of
these “sceptics” is that a lot of SSRI
antidepressant reduces REM sleep, but
people taking these antidepressants doesn’t
show any cognitive or memory deficits. As
an alternative hypothesis we can suggest,
that in the cases of reduced REM phase
maybe other compensatory mechanisms (in
NREM sleep or waking) assume the “job”
of the “dismissed” REM sleep.
2. Dreams: The movie of the sleeping
consciousness
2.1 The explicit reflection of unconscious processes
If we speak about dreams we have to be
aware of that we jumped to the
phenomenological, psychological level of
analysis. Are the aforementioned implicit
processes reflected in dreams? The
experimental results regarding this question
PERIODICALS OF IMPLICIT COGNITION (2006) 7
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Simor: Sleep, dreaming and memory
are really scarce, but there is some evidence
that the consolidating memory traces can be
incorporated to our dreams (Koulack, 1975
in Bódizs, 2000; Schwartz, 2003), but in a
transformed way, influenced by the “logic”
of the dream narrative. For example if I
dream that I fight against a terrible orc,
using a newly aquired aikido techique, the
dream may reflect a procedural memory
consolidating process (the movement that I
learnt the day before). And the face of the
orc could be the phenomenological correlate
of the activation of the face processing
modules of my brain, working with an
emotionally relevant dvd experience. Of
course these assumptions are speculative,
but maybe with the merger of the Brain
Imaging methods and sleep mentation
techniques this kind of young research trend
will also gain strength.
2.2 NREM dreams vs. REM dreams –
phenomenological aspects
The nature of the dream mentations of the
awakened subjects, is different if awakened
of NREM or REM sleep. Generally we can
say that REM dreams are more bizarre,
story-like,
visually
dominated
and
emotionally more intense, than NREM
dreams, which are more thought-like, more
logical, less visual, and emotionally less
absorbing, less vivid mental experiences
(Bódizs, 2000)
2.3 The memory sources of dreaming
Although our dreams are influenced by our
memories, they are not a replay of complete
episodic memories (Fosse, Fosse, Hobson,
& Stickgold, 2003), only isolated fragments,
or metamorphosed elements appear in the
healthy1 subjects dream. But once again
REM and NREM dreams seem to be
different according to the memory sources
1
In PTSD the traumatic event realistically (with
coherent perceptual and spatio-temporal details)
“returns” in the nightmares of the traumatized
person (Stickgold, 2002).
of the dream formation (Baylor & Cavallero,
2001). While REM dreams are mainly
associated with the semantic, and abstract
self-referential knowledge system, NREM
dreams may be built mainly by elements of
episodic memory (Cavallero & Cicogna,
1993).
3. The neuropsychological perspective
3.1 The neocortical-hippocampal dialog
Apart from the slight differences among the
theorists, there is a consensus, that altered
hippocampal functioning during sleep
accounts for the fragmented episodic
memories during dreaming (Nielsen &
Stenstrom, 2005), for the different memory
consolidating effects of the different stages,
and for the different memory sources in the
two main stages of the sleeping brain. In
waking the hippocampal-neocortical dialog
is bi-directional, information flowing in both
directions. In NREM sleep only the
hippocampus sends information to the
cortex, while in REM sleep only the
neocortex send information to the
hippocampus, and the hippocampalneocortical outflow is blocked (Buzsáki,
1996, in Stickgold, 2002).
3.2 The Emotional Brain and the Dreaming Brain
After borrowing the Emotional Brain and
Dreaming Brain denomination from
LeDoux (1996) and Hobson (1988)
respectively, now let’s see some of the
neuroanatomical correlates of dreaming, and
the aforementioned (1.2; 1.3; 2.3) memory
processes.
According
to
the
neuropsychological perspective (Muzur,
Pace-Schott, & Hobson, 2002; Hobson,
2000) the lack of self-awareness, and the
impairment of logic planning, and other
executive functions is due to the selective
deactivation of the prefrontal cortex (namely
the deactivation of the dorsolateral
prefrontal cortex) during REM sleep. At the
same time the elevated activation of the
limbic structures, such as the amigdala can
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Simor: Sleep, dreaming and memory
contribute to the emotional aspect of
dreams, and the enhancement of emotional
information processing during REM sleep,
because the amigdala can modulate the
consolidation of emotional memories
(McGaugh, 2000).
3.3 Emotions as the deep structure of dream
narratives
If the higher-order cognitive functions can’t
organize the dream scenario, what are the
main principles that structure these often
surrealistic stories? One proposal is that
emotional patterns, or the individuals main
emotional concerns interlaces the dream
content (Nielsen & Stenstrom, 2005). So
maybe at the background of the bizarre
associations in dreams there are ego-relevant
emotional patterns.
Conclusion
Sleep and dreaming may serve several
functions simultaneously, according to the
different needs of the organism, so taking
Hunt’s (1989) opinion it’s worth to be really
“polytheist” if we want help MR.Z to
resolve his confusion.
Invited Peer Commentary
Slow wave sleep, brain oscillations
and the reptilian wake state
Andras Czurko
Institute of Behavioral Sciences, University of Pecs,
12, Szigeti Street, Pecs, H-7624, Hungary
[email protected]
3.1 In connection to the hippocamponeocortical dialogue (Buzsaki, 1996), I think
it is important to note that the (declarative)
memory trace formation is supposed to
happen in two stages (Buzsaki, 1989). The
acquisition of information may happen
during the theta/gamma oscillations (awake
state and REM sleep). The consolidation of
the memory, the hippocampal-neocortical
transfer, on the other hand, is carried by
discrete quanta of cooperative neuronal
bursts during slow wave sleep (SWS or
NREM) and associated with sharp
wave/ripple (SWR) network patterns (140200Hz
frequency
oscillations)
in
hippocampus and enthorhinal cortex
(Buzsaki, 1998) (Chrobak & Buzsaki, 1996;
Csicsvari, Hirase, Mamiya, & Buzsaki, 2000;
Ylinen et al., 1995).
There is also direct experimental evidence
that certain firing sequences are replayed in a
time-compressed form during SWS and their
occurrence is correlated with SWRs (Lee &
Wilson, 2002; Nadasdy, Hirase, Czurko,
Csicsvari, & Buzsaki, 1999).
Recently on humans explicit memory
formation during SWS can be boosted by
transcranial application of slow oscillating
potentials (Marshall, Helgadottir, Molle, &
Born, 2006).
3.2 In case of implicit memory formation, it
is well known that not the hippocampus and
the mediotemporal memory system is the
key player. From the neuropsychological
perspective, not only the prefrontal cortex
and the amygdala but equally the caudate
nucleus, the cerebellum and sensory cortical
areas are involved in the different domains
of implicit memory (Squire & Zola, 1996).
From this perspective I think it is easy for
Mr. Z. to understand that here the REM
sleep is likely to play the primary role (1.2).
Finally, I think an evolutionary perspective is
missing from the target article. I’m sure that
first it can be surprising for Mr. Z that our
mammalian SWS can be a homologous
states to the reptilian waking state, but I’m
sure that from this perspective several of the
confusing issues of our sleep and dreams
can be resolved (Nicolau, Akaarir, Gamundi,
Gonzalez, & Rial, 2000; Rial, Nicolau,
Lopez-Garcia, & Almirall, 1993).
PERIODICALS OF IMPLICIT COGNITION (2006) 7
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Simor: Sleep, dreaming and memory
Dream and memory - in waking
Szinovszki Bence
Faculty of Pedagogy, Eötvös Loránd University,
23/27, Kazinczy utca, Budapest, 1075, Hungary
[email protected]
1.
Adding thoughts to the target article from
the perspective of waking consciousness I
sketch some connections between the
previous article about unintended implicit
recall (Marno, 2006) and dreams.
In the cultural tradition of dream
interpretation (included some scientific
researches nowadays as well) the main
question of the waking consciousness is if
the dream movies contain any useful
information for the waking mind or not
(Halász, 2000 in Bódizs, 2000). In scientific
approaches could transform this point of
view to the question of adaptation: an
important part of that could be the analysis
of dream's recall mechanism. How can we
access to the movie seen in sleep? What are
the conditions of recall? And what could be
the role of the access? This approach is not
interested in the differences of explicit and
implicit structures but take notes of their
connections,
relationships
and
communications.
2.
In waking it is easy to forget and difficult to
recall the dream. Memory or oblivion
depends on awakening. Hobson's (1988, in
Atkinson, 1996) fixed that the distraction of
awakening fails the dream consolidation and
dream contents disappear from memory. On
the other hand we may presume these
contents only get unaccessible for the
intentional search in waking consciousness.
In many cases after in a natural situation
(not in laboratory) the dream comes back
"by accident". For example when Mr. Z
going home from his work suddenly set to
remember his dream of last night. Possibly
he can't tell us why he recalls or what kind
of stimulus led him to call it back. While he
insist that he dreamt today and he didn't
remember it in the morning.
About process of non-strategic recall Marno
cites Moskovitch's theory (2000, in Marno,
2006) which appointed that during
spontaneous recall the limbic structures are
activated without prefrontal cortex and this
allows to associate directly the abstract
ambient stimulus with particular memory
experiences. In addition, Kónya (2006)
makes a distinction between the two
principle part of spontaneous memory
contents: actual pre-semantic core deriving
from amygdala (unintended or automatic
memory) and the associated semantic part
of that (memory without intention).
Comparing this mechanism with the
neuropsychological aspects of dream
mentioned in the target article (3.2) we could
explore analogue processes between the
dream arising in REM phase and
spontaneous recall in waking. Using the
previous distinction in the field of dream it
seems that during REM pre-semantic
memories, namely amygdala-memories are
associated with semantic reflections in
consciousness. Otherwise it would offer a
good explanation that the deep structures of
returning dreams and their emotional
connotations and reflections given by the
individual in waking are consistent in spite
of the contrasts in their content.
3.
I would like to bring back once again the
example about Mr Z, who suddenly recalls
the dream of his last night. We could see it
as a special case within the spontaneous
memory when effected by the actual
perception,
an
unintended memory
interlaces with the semantic element coming
from representation of dream and not from
the representation of autobiographical
memory. Marno agrees with Mace (2004, in
Marno, 2006) that for supporting to realize
the intentions during unconscious recall
process the individual's aim is to recall a
PERIODICALS OF IMPLICIT COGNITION (2006) 7
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Simor: Sleep, dreaming and memory
particular memory from the past that is
accordant with his actual situation.
The memory of a dream could be more
informative to interpret a particular situation
than memories coming from waking stage?
Probably, it could. If we presume the
primary role of pre-semantical memories in
this interpretation process. Anyway this
example can illuminates the working
mechanism of implicit memory showing the
connections between the sleeping and
waking consciousness and emphasizing the
important role of emotions in the
adaptation. In addition, referring a previous
issue about the evolution of consciousness I
would like to cite those theories within the
authors presumed that the function of
implicit structures is the adaptation to the
environment, contrary to explicit structures
which support the adaptation of the
environment to ourselves (Holyoak and
Spellman, 1993 in Aczél, 2006).
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