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International Journal of Dream Research Volume 7, No. 1 (2014) 85
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Commentary
1. Introduction
Previous research has found evidence of relationships be-
tween memory consolidation and sleep, as well as dreams
(Smith, 2008; DeKoninck, Christ, Hebert, & Rinfret, 1990).
However, dominant neurological models of dream genera-
tion are inconsistent with experimental ndings that have
arisen in recent years. In addition, no dominant models
of dream generation account for the relationship between
dreams and memory function (Wamsley, Tucker, Payne,
Benavides, & Stickgold, 2010), nor do they account for the
fact that dream content is often found to hold dreamer-spe-
cic meaning (DeCicco, 2007). Despite signicant research
contributions in the area, the way in which dreams are gen-
erated (as well as their purpose) has remained almost entire-
ly elusive. Herein a model of dream generation is proposed
that concurs with empirical observations of dream charac-
teristics, and which suggests that dreams may actually be
a by-product (or readout) of the processes of memory con-
solidation occurring during sleep – processes that are nec-
essary for normal memory function within the human brain
(Peigneux & Smith, 2011; Smith, 2008).
Major unanswered questions concerning sleep state
mentation (dreaming) involve the concepts of the purpose
of dreams and their production by the brain – what are the
physiological functions served by dreams (if there is a func-
tion), how are they generated by the brain, and do they
contain information which is meaningful for the dreamer? A
number of recent studies have found evidence that supports
a variety of perspectives on dream meaningfulness and the
reection of individuals’ waking day lives in their dreams
(Dale, DeCicco, & Miller, 2013; DeCicco, 2007). However,
there is a lack of denitive support for any particular per-
spective in terms of explaining the origin of dreams; even
the dominant neurological models of sleep mentation, while
revolutionary in their time, are unable to explain more recent
experimental observations (Hobson, 1988; Solms, 1997).
2. Neurological Theories of Dream Generation
Research suggests that dreams may contain meaningful
information which is dreamer-specic (i.e. dreams contain
information meaningful to the dreamer rather than random
information that could produce meaningful insight when an-
alyzed by another person; DeCicco, 2007). However, neu-
rological theories of dreams have proposed models that do
not allow specic meaning for imagery in dreams – instead,
it is often suggested that the brain makes sense out of im-
ages generated by random cortical activations (Hobson,
1988). In terms of explaining dream generation in neuro-
logical terms, two major perspectives emerge as dominant
theories in the eld.
The rst (and more prevalent) example is J. Allan Hob-
son’s Activation-Synthesis hypothesis (Hobson, 1988),
which explains dream imagery as the brain’s interpreta-
tion of random activations of the cortex caused by brain-
stem activity. In The Dreaming Brain, Hobson (1988) out-
lines this process and explains that REM-Sleep (Rapid Eye
Movement Sleep; a sleep state characterized by 7-10 Hz
brainwave activity, as well as repetitive and rapid conjugate
movement of the eyes) activation is capable of activating
almost every neuron in the brain in a stereotyped fashion.
The activation-synthesis theory suggests that this provides
stimulation of neuronal pathways that would otherwise not
be activated on a day-to-day basis. Without this activation,
Hobson suggests that we would lose those memories and
functions (because the underlying neural pathways would
atrophy). Structures in the brainstem/pons stimulate cells
A neuro-cognitive model of sleep mentation and memory
consolidation
Anthony Murkar1, Carlyle Smith2, Allyson Dale1, and Nicolle Miller2
1University of Ottawa, Ontario, Canada
2Trent University, Ontario, Canada
Corresponding address:
Anthony Murkar, MA/PhD Candidate, University of Ottawa,
Vanier Hall Rm. 2090, 136 Jean-Jacques Lussier. Ottawa,
Ontario (Canada). K1N 6N5.
Email: amurk054@uottawa.ca
Summary. Previous research has found evidence of relationships among sleep states and memory consolidation. How-
ever, no dominant models of sleep mentation (dreaming) account for the relationship between sleep and memory, despite
the fact that the underlying neural processes of ofine memory consolidation and sleep mentation happen simultane-
ously (and are therefore produced by the same brain state). In addition, experimental brain research contradicts what
dominant neurological models of sleep mentation predict. Here a model of dream generation that agrees with empirical
ndings on dreams, and which is based on the neural process of memory consolidation during sleep, is proposed. The
implications of the model are such that is suggests sleep state mentation may actually be a by-product (or readout) of the
consolidation of memories produced by non-random cortical/sub-cortical reactivation during REM and Non-REM sleep
(processes which are known, and are necessary for normal human memory function).
Keywords: REM dreams; NREM dreams; dream recall frequency; neurological substrates
Commentary
International Journal of Dream Research Volume 7, No. 1 (2014)86
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in the visual cortex (as well as other cortical areas). These
stimulations are random, and the dream is considered to be
the result of the brain interpreting and “making sense” of
randomly produced visual events. In this view, sleep menta-
tion images are not implicitly meaningful – it is the rational
component of the brain that applies meaning to the random
imagery that is generated (Hobson, 1988).
In addition to activation-synthesis, another similar model
(often considered as the successor to activation-synthesis)
was proposed by Hobson (2009). This model suggests that
REM sleep may constitute a proto-conscious state (a virtu-
al-reality world produced within the brain during sleep that
is of use to the development and maintenance of waking
consciousness). This model, termed the AIM model, relies
on three processes: A (Activation; the level of energy of the
brain and its constituent circuits), I (Input-Output Gating;
the facilitation or inhibition of sensory information from the
outside world), and M (Modulation of the chemical micro-
climate of the brain by neurons in the brainstem). The AIM
model describes REM sleep as being similar to wakeful-
ness – however, during wakefulness, input-output gating is
“open,” allowing the perception of external stimuli. During
REM sleep, however, high levels of A but low levels of I and
M result in brain activity (and thus mentation) in the absence
of access to sensory information from the external world.
Dreaming is therefore proposed to be the subjective experi-
ence of a brain state similar to waking consciousness.
While the activation-synthesis and AIM models have pow-
erful implications for explaining dreams, ndings suggest
the models may not accurately represent empirical observa-
tions of sleep mentation characteristics. For example, truly
random cortical activations (as dened in the activation-
synthesis model) do not allow for the possibility of recur-
rent nightmares (a phenomenon frequently reported, often
following experiences of trauma). Neither activation-synthe-
sis nor the AIM model account for the existence of NREM
dreams; in addition, both activation-synthesis and the AIM
model rely on the brainstem as a major component neces-
sary for dream generation. The second dominant neurologi-
cal viewpoint in terms of dream generation is incompatible
with Activation-Synthesis and the AIM model – however, it
has been supported by experimental ndings.
This second perspective, introduced by Solms (1997), uti-
lized the brain scans of lesioned patients to examine the
brain areas which are critical for dream generation. The re-
sults of Solms’ (1997) investigations have shown that when
individuals had damage to the brainstem, 81% of partici-
pants still reported a “preservation of the subjective experi-
ence of dreaming.” These ndings are in contradiction to
Hobson’s (1988; 2009) perspective, which would predict
that the mechanism responsible for generating sleep state
mentation must involve the functioning brainstem. Howev-
er, Solms (1997) has not provided an extensive alternative
theory of dream generation other than to suggest that intact
temporo-parietal-occipital and medial prefrontal areas are
important.
With regards to neural activation during sleep, recent re-
search has demonstrated that one major pillar of the ac-
tivation-synthesis theory (random cortical activations) has
further inconsistencies with experimental ndings that are
not addressed by the more recent AIM model. One imag-
ing study conducted using positron emission tomography
(PET) to look at cerebral blood ow revealed that following
the learning of a serial reaction time task, brain activity dur-
ing REM sleep was signicantly higher for those who had
learned the task than those who had not – however, activa-
tion during REM sleep was of the same brain areas that had
increased activation during the learning of the task (Maquet
et al., 2000). This suggests that cortical activation during
REM sleep is in fact not random, but rather is inuenced
by experience (i.e. memories acquired during the day are
strengthened in the same areas of the brain at night during
REM sleep, so that the specic brain areas activated during
REM are inuenced by pre-sleep memory acquisition). This
pattern of neural reactivation during sleep has also been
found in Slow Wave Sleep (SWS – a Non-REM sleep state
dened by 1-3 Hz/Delta brainwave activity); also using cere-
bral blood ow measurements, similar patterns of reactiva-
tion have been shown in the hippocampus (the amount of
hippocampal reactivation expressed during SWS was found
to be associated with improvement on the learned task the
next day; Peigneux et al., 2004). Furthermore, this pattern
has been demonstrated repeatedly using multiple research
techniques in both humans and animals (Euston, Tatsuno,
& McNaughton, 2007; Hoffman & McNaughton, 2003; Ma-
quet et al., 2000; Peigneux et al., 2004; Wilson, 1994). These
ndings collectively suggest that brain activity during REM
and Non-REM (NREM) sleep is inuenced by pre-sleep
memory acquisition. In addition, dreams also appear to re-
ect memory processes (Maquet et al., 2000; Peigneux et
al., 2004; Wamsley et al., 2010), and may therefore be linked
with this process of reactivation.
With regards to previous theories of dreaming, it must be
noted that no current model of dreams accounts for the ap-
parent relationship between dreams and memory. While it
may be the case that a proto-conscious state arises during
REM sleep, the actual purpose of this state has not been
addressed sufciently. In support of the idea that REM sleep
may provide the construction of a proto-conscious state,
one historical research study conducted using the lesion
method in cats demonstrated that when REM sleep paraly-
sis is inhibited, animals appear to act out their dreams (Jou-
vet & Delorme, 1965). This suggests that the existence of
such an internal world during REM sleep is very likely - how-
ever, the purpose of such a system and the way in which
these brain processes relate to the memory processes of
the brain that exist during sleep has not been explained.
3. Sleep & Memory
Turning to a non-neurological theory of dreams (that is
rmly based on experimental observation), the continuity
hypothesis suggests that information from waking day life
is reected in dream imagery (Schredl & Hoffman, 2003).
This well-documented dream phenomenon indicates that
dream imagery is not random, but instead is a reection of
waking day experiences had by the dreamer. As an extreme
example of this, one recent study suggests that among indi-
viduals commonly experiencing nightmares, those who had
experienced trauma generally developed more depressive
symptoms and experienced more traumatic events in their
dreams (David, Pruiksma, Rhudy, & Byrd, 2011). Previous
research has indicated that sleep may play a signicant role
in the consolidation of newly acquired memories. The pur-
pose of dreaming in relation to empirical ndings associating
memory consolidation and the processes of the sleep-state
brain, however, has remained largely unexplored – despite
the fact that these two processes (memory consolidation
and dreaming) occur simultaneously.
International Journal of Dream Research Volume 7, No. 1 (2014) 87
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Commentary
Recent research suggests that REM preferentially sup-
ports the consolidation of emotionally signicant - as well as
procedural - memories (Diekelmann, Wilhelm, & Born, 2009;
Nishida, Pearsall, Buckner, & Parker, 2009; Payne, Cham-
bers, & Kensinger, 2012; Vandekerckhove & Cluydts, 2010;).
Reviews of experimental ndings also suggest the idea that
SWS is more specically implicated in the consolidation of
declarative memories (Diekelmann, Wilhelm, & Born, 2009;
Smith, 2008).
Similarly to the memory functions of the brain, mentation
also differs between REM and NREM brain states. REM
dreams have been characterized by one study as having
a “dream-like” quality - bizarre imagery, movement, and
emotions to name some (Monroe, Rechtschaffen, Foulkes,
& Jensen, 1965). The same study also found that NREM
dreams can be characterized as more “thought-like.” More
recent research has also noted differences between REM
and NREM mentation, especially in terms of recall frequen-
cy, with a higher frequency of dream recall reported follow-
ing REM sleep (Foulkes, 1966; Nielsen, 2000). Mentation fol-
lowing REM sleep seems reective of emotional experience
and procedural (implicit) learning, while NREM mentation is
more reective of factual (declarative) memories (Cavallero,
1993; Cicogna, Cavallero, & Bosinelli, 1986; Monroe et al.,
1965; Nielsen, 2000).
While research has strongly suggested a role for sleep
in memory consolidation (Maquet, 2001; Rasch, Büchel,
Gais, & Born, 2007; Stickgold, 2005; Wilson & McNaughton,
1994), fewer experimental investigations have noted asso-
ciations between dreams and memory. One investigation
showed that French immersion students who experienced
incorporation of the French language into their dreams earli-
er and who had more verbal communication in their dreams
overall showed more signicant progress in their learning of
the French language (DeKoninck et al., 1990). In addition,
one more recent investigation found that when participants
were presented a memory task, improved performance on
the task at re-test was associated with task-related dream
imagery (Wamsley et al., 2010). These ndings further sug-
gest that dreams reect memory processes.
All of the aforementioned investigation results suggest
that dreams are not just a replay of experiences, but are in-
stead a reection of those waking-day experiences that the
brain is actively consolidating during sleep into a more sta-
ble form. A comprehensive theory of dreams must account
for the possibility of recurring dreams, the question of why
dreams seem to contain information which is meaningful for
the dreamer, continuity, and the relationships that have been
discovered between sleeping brain states and the neuro-
logical processes of memory consolidation. In addition, it
should also agree with imaging studies demonstrating how
neural activity changes during sleep (in comparison to the
normal/waking day), and must therefore account for the ap-
parent non-random brain activation during REM and NREM
sleep (Maquet et al., 2000; Peigneux et al., 2004).
4. Neuro-Cognitive Model of Dream Generation
According to the culmination of evidence from previous re-
search, it is proposed that the dream-experience is simply a
reection of the neural processes associated with memory
consolidation, and exists as a natural by-product (or read-
out) of those processes during the brain’s transition from
an unconscious/sleeping state to a conscious waking state.
In keeping with this way of conceptualizing the genera-
tion of dreams, meaningfulness in dreams should naturally
emerge as a common characteristic of dream imagery since
most dreams recalled are from REM sleep - the sleeping
brain state state associated with emotional memory con-
solidation (Foulkes, 1966; Nielsen, 2000; Smith, 2008). The
functional aspect of the proto-conscious state described by
the AIM model may also be explained by memory consoli-
dation processes, since REM sleep is also implicated in the
consolidation of procedural memories (Peigneux & Smith,
2011; Smith, 2008). One additional aspect of dreams not
considered by previous models is the fact that dreams ap-
pear to only be recalled from the transition between sleep
and wakefulness (i.e. dreams do not seem to be recalled
from throughout the entire night in the absence of an arous-
al).
Figure 1. Model of REM/NREM Dream Production based on memory consolidation during sleep.
Commentary
International Journal of Dream Research Volume 7, No. 1 (2014)88
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The proposed model suggests that dreams may be de-
ned as the experience of perceiving the processes of mem-
ory consolidation during the brain’s transition from sleep
to wakefulness. The model consists of ve components
(see Figure 1): 1) The brain during sleep is consolidating a
stage-dependent memory type, and non-random cortical/
sub-cortical reactivation within the sleeping brain is inu-
enced by pre-sleep memory acquisition; 2) during arousal,
perception begins as the brain transitions from a sleeping
state to a waking/perceptive state – it is during arousal (be-
tween the sleeping and waking states, while cortical/sub-
cortical reactivation and memory consolidation are shutting
down in some parts of the brain and the waking/percep-
tive brain state is simultaneously beginning in others) that
both memory consolidation and waking perception briey
coexist within the brain; 3) stage-dependent memory con-
solidation is experienced as dream recall; 4) dream recall
is submitted as a written report of subjective experience;
5) the written dream report reects the consolidation of a
stage-dependent memory type.
Traditionally, the processes of sleep mentation and mem-
ory consolidation have been studied independently, and
therefore considered as two separate and independent pro-
cesses. However the fact that these two processes occur
simultaneously suggests that the underlying neurological
bases for these two phenomena must also be occurring at
the same time, and may therefore be linked together (and
noted in research ndings as two different observable ac-
tivities of the same single underlying brain process).
This way of conceptualizing neurological dream genera-
tion allows for the possibility of recurring dreams, and ex-
plains dreams as the by-product of the already-known pro-
cesses of neural reactivation during different sleep states
that facilitate the consolidation of newly acquired memories
into a more stable, long term form. In addition, this approach
further accounts for both the qualitative difference between
REM and NREM mentation noted by previous research
(Monroe, Rechtschaffen, Foulkes, & Jensen, 1965; Caval-
lero, 1993; Cicogna, Cavallero, & Bosinelli, 1986; Monroe
et al., 1965; Nielsen, 2000) and the continuity hypothesis of
dreams (since stage-dependent cortical/sub-cortical reacti-
vation facilitates the consolidation of newly acquired mate-
rial from an individual’s prior waking experience). Research
has also demonstrated that the frequency of incorporation
of memory sources into dreams is inuenced by the emo-
tional signicance of the experience (Schredl, 2006); this
bias toward the incorporation of emotionally signicant
memories can also be explained by the proposed model,
since most dreams are recalled from REM - the sleep stage
associated with emotional memory consolidation (Peigneux
& Smith, 2011; Smith, 2008).
While the AIM model and activation-synthesis (Hobson,
1988; Hobson, 2009) do not account for the occurrence of
NREM dreams, the processes of neural reactivation during
both REM and NREM have already been conrmed in dif-
ferent (stage-dependent) areas of the brain using functional
brain imaging and other techniques in both humans and
animals (Maquet et al., 2000; Peigneux et al., 2004; Wilson,
1994; Hoffman & McNaughton, 2003; Euston, Tatsuno, &
McNaughton, 2007).
The existence of a proto-conscious state within REM
sleep may also possibly be accounted for by the fact that
procedural memory consolidation is associated with REM
rather than NREM sleep (Peigneux & Smith, 2011; Smith,
2008) – however, the stage dependent processes of reac-
tivation allow for both REM and NREM dreams while also
explaining the qualitative difference between the two that
have been noted by previous research ndings (Monroe,
Rechtschaffen, Foulkes, & Jensen, 1965; Cavallero, 1993;
Cicogna, Cavallero, & Bosinelli, 1986; Monroe et al., 1965).
While this model offers an approach to explaining many of
the observable characteristics of both sleep mentation and
brain activity during REM and NREM sleep, challenges arise
in terms of designing practical experiments that can link
brain reactivation during sleep to sleep mentation. However,
one recent study demonstrated that, by using functional
brain imaging, it is possible to decode visual imagery during
sleep using computer learning models (Horikawa, Tamaki,
Miyawaki, & Kamitani, 2013). This experimental approach
has already demonstrated that brain activation during sleep
relates to the production of visual imagery in a very specic
way. However, in order to conclusively determine whether
this brain activation both facilitates memory consolidation
and produces visual imagery simultaneously, a more exten-
sive experimental protocol is necessary.
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