The Role of Dreams in the Evolution of the Human Mind

Evolutionary Psychology
human-nature.com/ep – 2005. 3: 59-78
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Original Article


The Role of Dreams in the Evolution of the Human Mind

Michael S. Franklin, Department of Psychology, University of Michigan, 525 East University Ann
Arbor, MI, 48109, USA. Email: msfrankl@umich.edu. (Corresponding author)

Michael J. Zyphur, Tulane University, Department of Psychology, 2007 Percival Stern Hall, New
Orleans, LA 70118, USA. Email: mzyphur@tulane.edu.

Abstract: This paper presents an evolutionary argument for the role of dreams in the
development of human cognitive processes. While a theory by Revonsuo (2000)
proposes that dreams allow for threat rehearsal and therefore provide an evolutionary
advantage, the goal of this paper is to extend this argument by commenting on other
fitness-enhancing aspects of dreams. Rather than a simple threat rehearsal
mechanism, it is argued that dreams reflect a more general virtual rehearsal
mechanism that is likely to play an important role in the development of human
cognitive capacities. This paper draws on current work in cognitive neuroscience and
philosophy of mind in developing the argument.

Keywords: Dreams, sleep, REM sleep, evolution, philosophy of mind, cognitive
neuroscience.

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Introduction


Although Freud (1900) proposed that dreaming and, specifically, the
meaningful content of dreams are related to mental functioning, the tenuous and
misunderstood nature of dreams has made the proposition of empirically providing
support for, or falsifying, this claim very problematic. The inability to study the
effects of dreams on mental functioning has forced many researchers to view dreams
as the result of random neural activity (e.g., the activation-synthesis hypothesis;
Hobson and McCarley, 1977). If postulations regarding the random nature of dreams
are indeed true, then it becomes challenging to construct a theory of how the
phenomenology of the dream state could serve a functional role and be better
understood through an evolutionary analysis. However, recent research, to be
discussed in this paper, which takes into account the physiological mechanisms
underlying sleep and dreams, the content of dreams, and the environmental conditions

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The Role of Dreams in the Evolution of the Human Mind
of selection, points toward the natural selection of dreaming as a state of
consciousness which has persisted across the development of the human species.
This tends to suggest that the dream state was selected for as an adaptation which
increases overall fitness. The leading theory addressing the adaptive qualities of
dreaming uses the concept of virtual threat, defined as a dream-state wherein a
threatening situation is constructed virtually, and explains that through the rehearsal
of various threatening scenarios we may be better equipped to handle real-world
threats (Revonsuo, 2000). While this theory offers a plausible evolutionary account
of dreaming, the goal of the current paper is to extend the theoretical underpinnings
of this hypothesis by commenting on other fitness-enhancing aspects of dreams and
the broader influence of dreaming in the evolution of higher mental functioning.

The Subjective Nature of Dreams


The nature of the dream-state is highly subjective and a truly personal
experience making the scientific analysis of dreaming somewhat prohibitive. Dreams
often contain material that is nonsensical and challenging to interpret rationally,
making the characterization of dreams from an objective point of view a perplexing
task. While we all dream (though see Solms, 1997, for an example of
neuropsychological patients who do not dream), there is incredible variability in the
subjective dream experience (Hall and Van de Castle, 1966; Spadafora and Hunt,
1990). Some people rarely remember their dreams and erroneously conclude that
they do not dream at all (a condition discussed by Freud, 1900), while others
experience vivid dreams with rich visual imagery and emotional content. Sometimes,
the story-lines that make up people’s dreams follow a tight narrative and have a
relatively smooth transition from scene to scene, while at other times dreams appear
as illogical and haphazard associations lacking a coherent sense of flow. Some
people have full control of their dreams, exerting conscious control over the
supposedly random events which typify dreaming (Laberge, Levitan, Dement, 1986),
while others are mere bystanders watching the events unfold without any sense of
agency approximating waking volition. With the multiplicity of dream dynamics, it
is no surprise that there are differing views on the nature of dreams, as a researcher’s
views on dreaming may directly relate to their own subjective experience of dreaming
(Potter, 1996).

Despite this subjective nature of dreams, an evolutionary analysis of dreams
should not be disregarded and considered outside the realm of scientific inquiry
(although for a competing view see Thompson, 2000). Since the cognitive
revolution, psychology and other disciplines have made significant progress in
developing and implementing methodologies meant to reveal truths about the mental
processes underlying our subjective experiences (Miller, 2003). For example, the
tools of cognitive neuroscience have allowed neuroimaging data to inform our
theories of cognition (Kandell and Squire, 2000). It is not unreasonable to think that
these methods will one day allow for a correlation to be established between certain
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patterns of brain activity and corresponding dream content, not unlike how current
technology now allows accurate prediction of information from subjective
experiences.

As an example, neuroimaging evidence can provide information to distinguish
between lower-level sensory experiences (e.g., the experience of visual vs. auditory
stimuli) as well as higher-level perceptual experiences (e.g., visual processing of a
face stimulus vs. a house stimulus; O'Craven and Kanwisher, 2000). In this vein, it is
important to approach the study of dreams in a scientific fashion, not biased by our
own subjective dream experiences, but rather by letting our theories rest on
scientifically collected data. Towards this aim of objective and scrutinizing scientific
inquiry, below we present data concerning the function of dreaming.

REM Sleep and Dreaming


One of the first and most important findings in the history of research on
dreams and dreaming is that which relates the phenomenon of dreaming and the
physiological occurrence of rapid eye movement (REM) sleep (Dement and
Kleitman, 1957). While dreaming refers to “the subjective conscious experiences that
we have during sleep” (Revonsuo, 2000, p.878), REM sleep is a physiologically-
defined stage of sleep. It has been established that dreaming does occur during REM
sleep through the collection of dream reports from subjects awoken from REM sleep,
though the same is true for non-REM sleep (NREM; Hobson, 1988). Rather than
being a static process, sleep contains a number of discrete states defined by various
physiological measures (Rechtschaffen and Kales, 1968).

The use of electroencephalography (EEG), electro-oculography (EOG), and
electromyography (EMG) has proven useful in distinguishing between arousal states
during sleep, by measuring brain activity, eye movements, and muscle activity,
respectively. As we sleep, our brain passes through various stages in a cyclical
manner. Some of these stages are characterized by slow brain activity and other
stages occur in which the electrical activity of the brain mimics the waking brain, and
can even be considered hyperactivated. This specific, hyperactive stage of sleep is
known as REM sleep and has three characteristics that define it: 1) The brain is more
active than while in other stages and the EEG consists of alpha and beta activity,
similar to waking, 2) Muscle activity is actively inhibited within the central nervous
system in order to promote paralysis, and 3) Eye-movements occur during REM sleep
because the muscle paralysis does not extend to the eye muscles.

A link between REM sleep and dreaming has been established through
various experimental studies (Hobson, 1988). First, it is known that people awakened
from REM sleep as opposed to NREM sleep are significantly more likely to produce
dream reports and these reports are likely to be more detailed and vivid than NREM
dream reports. Also, evidence implicating REM sleep with dreams appears when
REM sleep mechanisms malfunction. Normally during REM sleep, signals that elicit
all motor output (except for eye movements) are actively inhibited. Disorders that
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naturally occur in humans and lesions in other species that damage the inhibitory
response can result in physically acting out dreams while asleep (Sforza, Krieger,
Petiau, 1997). Further, humans can give a verbal report to substantiate the
correspondence of dream actions to waking actions (Ferini-Strambi and Zucconi,
2000).

Other species cannot provide information about mental processes during
sleep, so controversy surrounds the question of whether or not animals are dreaming
during REM sleep. One perspective is that animals such as cats, which display
threat-induced posturing and appear startled by invisible objects while in REM sleep,
have a reason to produce such behavior. The reason is linked to their perception of
information relevant to these displays without actual corresponding sensory
information. In fact, studies using electrophysiological measures to record activity in
hippocampal place cells indicate that rats which have spent a considerable amount of
time during the day running through a maze show activation of the same place cells
during REM sleep which were active during maze running (Louie and Wilson, 2001;
Wilson and McNaughton, 1994). These data point towards the possibility that
dreaming serves some type of rehearsal function, allowing animals to practice the
activities performed while awake, namely running through the maze.

However, we will never know if the subjective experience of dreaming is the
same for these animals as it is for humans, as we will also never truly know if another
person’s subjective dream experience is similar to our own. Just as behaviorists
concluded the human mind was a ‘black box’ incapable of scientific study (Watson,
1913), there is a tendency to assume that we will never be able to gain an
understanding of animals’ mental states and that any attempt is simply
anthropomorphism. However, the neurophysiological evidence mentioned above
makes plausible the claim that during REM sleep these animals are experiencing
something similar to what people call dreaming, with the caveat that the dream
experience will be specific to the perceptual and cognitive abilities of the animal.

While there is a strong correlation between REM sleep and dreaming, it is
also clear that dreaming can occur outside of REM sleep, and similarly, instances of
REM sleep without dreaming are also feasible (Hobson, 1988; Solms, 1997). An
analysis of dream content suggests that there are systematic differences between
REM and NREM dream reports (Hobson, Pace-Schott, Stickgold, 2000). This data
indicates that just as sleep is not a static unitary process, but rather made of discrete
stages, the cognitive processes that take place throughout the sleep cycle, and that are
normally uniformly called dreams, differ and can result in different classes of dreams
(Fosse, Stickgold, Hobson, 2004). Dreams that occur during NREM sleep lack vivid
imagery and, while they may contain themes similar to REM dreams, they often
consist of a simple recurring theme.

For the purpose of this paper we will concentrate on the types of dreams that
are normally reported when subjects are awakened from REM sleep. From this
perspective, it is possible to make a stronger inference that certain physiological
mechanisms of REM sleep influence dreaming. Specifically, activation can be
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examined in forebrain areas that are more likely to be informative for a cognitive
theory of dreaming, and are claimed to selectively influence dreaming without
affecting REM sleep (Solms, 2000). This is not to say that we are unconscious
outside of REM while sleeping and that NREM dreams are not also of potential
interest, rather, it is argued that the type of consciousness that mostly occurs during
REM sleep is of special interest and represents a prototypical dream. Since we
currently lack the technology to achieve a highly detailed understanding of the
physiological correlates of dreaming, a logical starting point is to use existing
technologies to acquire data during REM sleep, in order to see how they can inform a
theory of dreaming.

Theories of Dreaming


The theory of dreaming most generally accepted, which offers an explanation
of dreaming based on the physiology of REM sleep, is Hobson and McCarly’s (1977)
activation-synthesis hypothesis. According to this hypothesis, dreams are the result
of the forebrain responding to random activity initiated at the brainstem. This is
demonstrated by the PGO waves that occur during REM sleep. Specifically, PGO
refers to the pons, where the activity originates; the lateral geniculate nucleus of the
thalamus, which is the area through which sensory information passes; and occipital
areas, where visual information is processed. According to Hobson and McCarly
(1977), this random activity, or noise, emanating from the pons, passes through
similar sensory-relay stations as information from the environment, and is interpreted
in a way that leads to the phenomenology of dreaming. Overall, this theory has
received general support for some time because it fits well with physiological data
and its explanation of dreaming appeals to a majority of peoples’ dream experiences,
again, being somewhat haphazard and random. This theory posits that the bizarre
nature of dreams is attributed to certain parts of the brain attempting to piece together
a story out of what is essentially random information.

The activation-synthesis theory does make intuitive sense, based not only on
how we generally remember and report information from dreams, but also on how
difficult it is to piece together memories of a dream upon waking.

Neuropsychological evidence points towards our tendency to confabulate stories that
we believe to be true in order to fit together disparate pieces of information
(Gazzaniga, 1985). If true, however, the supposedly random information that leads to
dreaming would weaken the evolutionary analysis presented here. If there is no bias
towards a particular type of information processed during REM sleep, then it
becomes hard to imagine how dreaming could be selected for in an evolutionary
context. Specifically if there is no rhyme or reason with regards to the content that
makes up dreams, it becomes difficult to understand the advantage of experiencing
such a haphazardly concocted virtual dream environment.

A more detailed analysis of dream content and the relation between REM
sleep and dreaming, however, demonstrates that the activation-synthesis theory is
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incomplete (Domhoff, 2000b). Although dreams tend to be rather bizarre, they are
certainly not as disjointed as would be the case if this hypothesis were unilaterally
true. In fact, large samples of dream reports from numerous studies point toward the
fact that individuals see the majority of dreams as realistic and containing a connected
storyline (Foulkes, 1985; Snyder, 1970; Domhoff, 2000a). This is something which
should not occur if the information processed in dreams is truly random. Likewise, to
be discussed below, certain information is differentially represented in dreams (Hall
and Van de Castle, 1966).

Additional neuropsychological evidence reveals that the brainstem
mechanism, which is a key ingredient in activation-synthesis theory, is not necessary
for dreams to occur. Rather, work by Solms (1997, 2000) points towards the
forebrain region as being crucial in the generation of dreams. If there is reason to
believe that dreaming is not just the random processing of information, but instead
there is some pattern to the types of themes present in dreams and the possibility that
dreams can consist of cohesive story-lines, then it seems logical to investigate why
these patterns exists and what purpose they serve. Before delving into these details
on the functional aspects of dreaming, it is necessary to briefly describe more about
the phenomenology of dreaming and how this could be reflected in the brain.

Mental Rehearsal


It can be assumed that the brain is optimally designed for the processing of
“real- world” sensory information, so that we can react in appropriate manner when
confronted with environmental stimuli. Despite this fact, a large portion of mental
life consists not of the processing of actual information, but rather the rehearsal of
what to do when we encounter stimuli from the environment (Klinger, 1978). This
rehearsal and the cognitive skills involved are likely to have a strong adaptive value.

Present neuroimaging data suggests that this “non-real” information, or
information not tied to any current environmental stimuli, is treated in a similar
fashion as information processed in a real physical environment. Data from a
neuroimaging study, specifically using positron emission topography (PET), supports
the notion that when we imagine something of a visual nature and manipulate that
image, our visual cortex is activated (Kastner et al., 1999). Likewise, in studies that
control for actual movement, it has been shown that by simply imagining the actions
involved in a repetitive motor task, the physical representation of the associated
pattern of activity in the motor cortex increases (Pascual-Leone et al., 1995).

A question, then, is why would mental imagery of a physical activity activate
the same brain regions as the activity itself? This double-activation would make
sense if mental imagery reflects exercise/practice for the brain (or if imagining a thing
and “really” doing a thing are not as distinct as many assume they are). By being
able to practice a response, or exercise a part of the brain without having to physically
experience a behavior-eliciting stimulus (especially one that is potentially dangerous),
we can optimize mental functioning and, ultimately, our response to an actual
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situation (Cumming and Hall, 2003). It is well known that mental imagery
techniques greatly facilitate multiple aspects of performance from sports to music
(e.g., Feltz and Landers, 1983). Further, the most successful individuals at creative
endeavors are usually those that have the best imagery skills (Intons-Peterson, 1993).
Thus, it appears advantageous to be able to create vivid representations in the mind’s
eye of various scenarios, which in fact, is what dreaming entails.

Threat Rehearsal


When awoken abruptly from a terrifying nightmare, it is easy to understand
the strength dream imagery has in generating both physiological and cognitive
responses. In the case of a nightmare, heart rate is accelerated, sweating occurs, and a
general feeling of fear and anxiety can extend for some time after the dream has
finished (Mellman et al., 1985). Even though dreams are a form of mental
representation, in the sense that perception is not tied to stimuli in the environment,
they are generally experienced as real and the content is perceptually
indistinguishable from waking perception (Freud, 1900).

If merely imagining an event has the power to better prepare us for an actual
event by physically activating comparable brain regions, then it should follow that the
more realistic the simulation of events, the more the brain treats the information as
real. Also, if this capacity to simulate an environment allows us to be optimally
prepared to deal with challenges in a real environment, it should affect fitness and be
naturally selected for across generations (Darwin, 1995). The threat-simulation
hypothesis of dreaming argues that this is the purpose of dreams and the reason why
dreaming has evolved (Revonsuo, 2000). It is suggested by this theory that dreams
serve the purpose of allowing for the rehearsal of threatening scenarios in order to
better prepare an individual for real-life threats. This is supported by evidence from
dream reports to be discussed below.
An Evolutionary Perspective


In order to evaluate the threat simulation theory of dreaming (of the kind
found in REM sleep), it is useful to discuss it in an evolutionary context, and consider
whether dreaming meets the necessary requirements of evolution by natural selection;
namely, genetic variation, inheritance, and differential fitness. As for the first
condition, there is evidence that REM sleep is genetically varied between and within
species. REM sleep seems to be exclusive to placental and marsupial mammals
(Winson, 1993). This suggests a particular phylogeny of dreaming, and that there
was some point in time in which this characteristic was acquired and further spread to
evolving species. Also, the amount of REM sleep placental and marsupial animals
tend to require varies in a shared manner throughout their life cycle (Siegel, 1995),
pointing towards an underlying genetic control over dreaming.

Likewise, different physiological processes occurring during REM must have
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undergone processes of natural selection. Consider disorders in which people
physically act out their dreams, and the potential dire consequences that could result
from such disorders. Those who acted out their dreams may have put themselves at
great risk. As the trait of physical inhibition during dreaming varies in humans, those
individuals with the trait which inhibits paralysis during REM sleep seem to have
been predominately removed from the current dreaming population, indicating also
that the second condition of inheritance is satisfied.

When considering the third proposition of the differential fitness of dreaming
in modern humans, it is important to understand the environment in which selection
was occurring. Our human ancestors faced a number of challenges posed by
interactions with conspecifics within and between groups (Foley, 1989), as well as in
procuring food and protecting themselves from predators (Kaplan and Hill, 1985). In
this environment, the ability to most efficiently react when a real threat is apparent
would obviously confer a survival advantage. Evidence from mental imagery and
dream studies suggest that rehearsal in the dream is treated as a real threat and,
therefore, those individuals with these imagery skills to rehearse threatening scenarios
should have an improved ability to deal with threat, making them more likely to be
the progenitors of offspring. Through the survival and procreation of their offspring,
this ability of, and propensity towards, imagery would be differentially passed on to
future generations.

If dreaming was selected for because of its adaptive function, the general
content of dreams should certainly reflect this, and consist of situations that allow the
rehearsal of scenarios that ultimately lead toward increased fitness. This is exactly
what is seen, with studies indicating that dream content is biased toward negative
elements reflecting threat, as opposed to positive elements. Data collected from over
500 dream reports by Hall and Van de Castle (1966) indicate that about 80%
contained negative emotions, while only about 20% contained positive emotions.
These negative dreams are also disproportionably likely to contain threatening
elements such as animals and male strangers in threatening encounters. The evidence
points towards the overrepresentation of threatening events in dreams, which should
not occur if dream content is random. Through appropriating and learning to deal
with these threats in dreams, it is proposed here that an animal could increase its
overall evolutionary fitness.

Beyond Threat Simulation


While Revonsuo (2000) limits his argument to the effectiveness of dreams in
preparing for real-world threats, it is our goal now to extend this argument. We
propose that the fitness-enhancing benefits of dreaming is not restricted to threat
rehearsal, and the evolution of other higher-order cognitive faculties has been
strongly influenced by a dreaming mechanism. By commenting on other fitness-
enhancing aspects of the phenomenology of dreaming, besides threat, it also becomes
possible to integrate our theory with portions of Hobson and McCarley’s (1977)
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activation-synthesis hypothesis, with particular regards to their view on the random
information that leads to dreaming.

While dream content is not completely random, as demonstrated by the fact
that there tends to be an over-representation of negative affect (Hall and Van de
Castle, 1966; Merrit et al., 1994)) and social interactions (Kahn et al., 2002), there
still is a great deal of variability and randomness observed in dream content. We
argue that this variability is likely due to activation propagated from the brainstem,
and that this noise in the system is beneficial. The advantages of having noise as a
crucial factor in a dream-generation mechanism could be likened to the benefits of
genotypic variability in the evolution of species (cf., Darwin, 1995). Given an
unpredictable and variable environment, variability in traits increases the possibility
that a certain trait will randomly confer an advantage under certain circumstances,
this being the crux of Darwin’s theory of natural selection. In dreams, the potential
advantage of noise and variability in the system allows for a broad range of scenarios
to be simulated and new scenarios to be created rather than having the same type of
dream occur repeatedly. This concept relates to ideas discussed by Kahn, Combs,
and Krippner (2002), in terms of stochastic resonance which they contend prevents
mental activity from perseverating, which allows for novel situations to be developed
through the presence of noise in the system.

Aside from our theory being in a state of consonance with theories of both
activation-synthesis and threat-simulation, we also contend that increased fitness is
not limited to situations of threat rehearsal and that the information processing
occurring in dreams should be similarly represented in the brain as is waking
cognition. This is the case because if sleeping and waking cognition are quite
different, then rehearsing threatening situations in a dream may not transfer into the
ability to better handle similar situations in waking life. However, evidence from
lucid dream studies (described below) indicate that tasks such as counting and singing
during a dream, which should activate the left and right hemispheres, respectively, do
just that. When a person is singing in a dream, their right hemisphere is more active,
and conversely when a person counts, the left hemisphere becomes more activated
(LaBerge and Dement, 1982). A more recent PET study demonstrated that subjects
trained on a serial reaction time task showed task-related increases in brain activity
during REM sleep which was correlated with improved performance on the task after
sleep (Maquet et al., 2000).

Also, from a neuropsychological perspective, evidence comparing bizarre
dream cognition with certain psychopathology indicates another link between brain
activity in dreams and waking. For example, people who suffer from damage to
frontal and temporal brain areas typically report the misidentification of faces during
waking life, a condition known as Fregoli syndrome. Some research has indicated
that a decrease of activity in these regions, reported from neuroimaging studies in
sleep, correspond to similar reports of misidentification during dreaming (Schwartz
and Maquet, 2002). So, the functional architecture of our brains similarly influences
both sleep and waking cognition and perception, supporting the idea that
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neurophysiological correlates of cognition appear stable across the two forms of
consciousness.

Situated Cognition in Dreams


While the above argument points towards the similarity between thoughts
while dreaming and in waking life, clearly there is a difference in how the two states
are experienced and the type of cognition occurring in each. As discussed earlier, for
the majority of time spent dreaming, we accept as real even the most bizarre
scenarios, and are able to make rationalizations allowing us to treat the dream as real.
Generally speaking, we are fooled into accepting a dream experience as a real
experience, until we awake and reflect on the content of the dream. This indicates a
general deficit in certain aspects of executive functioning (e.g., deficits in planning,
monitoring, attention switching, etc.), including skills relating to critical-thinking and
our ability to access specific types of memories.

While dreaming, an effect of the general deficit in executive functioning is
that our cognitive machinery becomes fully engrossed in perceptions and goal-states
directly relevant to perceptions of the dream. This has a considerable resemblance to
the idea of situated cognition, in which cognition is tied to the moment and restricted
to satisfying goals pertaining to current concerns (also, perceptual narrowing has been
shown in alternate contexts within the rubric of the threat-rigidity effect, proposed by
Staw, Sandelands, and Dutton, 1981). It can be argued that all non-human cognition
is situated, and that it is the ability to extend thinking beyond the here-and-now of
perception and motivation that makes human cognition unique (Bogdan, 1997). It
has even been hypothesized that what humans currently experience during REM sleep
shares a similarity to waking consciousness in early hominid brain evolution
(Panksepp, 1998). Jaynes (1976) takes this idea even further by arguing that there
was a time, roughly 3000 years ago, when humans lacked consciousness and acted in
a way that parallels the situated nature of dream consciousness.

This situated aspect of dreaming also makes sense from an evolutionary
perspective and further supports aspects of the threat-simulation theory. While it is
advantageous to rehearse situations that are subjectively deemed as threatening, it is
equally disadvantageous to come across a threatening scenario in real life and invest
the time required to wonder whether or not that situation is real. Therefore, in order
for this dream mechanism to be selected for, an important aspect of its initial
selection is that the perceived threats encountered during a dream must be
experienced as a real. This means that certain higher-order mental processes, which
would function to appraise the situation in an intellectual fashion (mostly frontal
areas), would likely have to be deactivated, which research indicates is the case
(Mazur, Pace-Schott, Hobson, 2002).

In most dreams there are deficits in the ability to solve complex problems.
Evidence from fMRI studies during REM sleep, show that there is a decrease in
activity of the prefrontal cortex, which would normally be associated with a decrease
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Document Outline

• Evolutionary Psychology
  • • • • • • The Subjective Nature of Dreams
            • REM Sleep and Dreaming
            • Theories of Dreaming
            • Mental Rehearsal
            • Threat Rehearsal
              • An Evolutionary Perspective
            • Beyond Threat Simulation
• Situated Cognition in Dreams
• Social Cognition in Dreams
  • • • • • • Dream Ontogeny
            • An Important Exception
            • Conclusion
        • References
  • Pascual-Leone, A., Nguyet, D., Cohen, L. G., Brasil-Neto, J. P., Cammarota, A. and Hallett, M. (1995). Modulation of muscle responses evoked by transcranial magnetic stimulation during the acquisition of new fine motor skills. Journal of Neurophysiolo

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