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How quickly do they learn to walk?
How many years until they reach ado
lescence? The more rapid an animal’s
development, the more pre programmed
(less flexible) the brain.
As predicted, we found that species
with more flexible brains spend more
time in REM sleep each night. Although
these two measures—brain flexibility
and REM sleep—would seem at first
to be unrelated, they are in fact linked.
Dream circuitry is so fundamen
tally important that it is found even in
people who are born blind. However,
those who are born blind (or who be
come blind early in life) don’t experi
ence visual imagery in their dreams;
instead they have other sensory experi
ences, such as feeling their way around
a rearranged living room or hearing
strange dogs barking. This is because
other senses have taken over their vi
sual cortex. In other words, blind and
sighted people alike experience activity
in the same region of their brain during
dreams; they differ only in the senses
that are processed there.
We developed our defensive acti
vation theory to explain visual hallu
cinations during extended periods of
darkness, but it may represent a more
general principle: the brain has evolved
specific circuitry to generate activity
that compensates for periods of dep
rivation. This might occur in several
scenarios: when deprivation is regu
lar and predictable (e.g., dreams dur
ing sleep), when there is damage to the
sensory input pathway (e.g., tinnitus
or phantom limb syndrome) and when
deprivation is unpredictable (e.g., hal
lucinations induced by sensory depriva
tion). Hallucinations during deprivation
may in fact be a feature of the system.
Dreams have long perplexed philoso
phers, priests and poets. We suggest
that dream sleep exists, at least in part,
to prevent the other senses from tak
ing over the brain’s visual cortex when
it goes unused. Dreams are the counter
balance against too much flexibility.
Dreams may be better understood as the
strange love child of brain plasticity and
the rotation of the planet.Eagleman, a neuroscientist at Stanford,
is the author of Livewired. Vaughn is
a neuroscientist at UCLA.exactly what happens: in humans, REM accounts for half of
an infant’s sleep time, but the percentage decreases steadily
to about 18% in the elderly. REM sleep appears to become
less necessary as the brain becomes less flexible.
Of course, this relationship is not sufficient to prove the
defensive activation theory. To test it on a deeper level, we
broadened our investigation to animals other than humans.
The defensive activation theory makes a specific prediction:
the more flexible an animal’s brain, the more REM sleep it
should have to defend its visual system during sleep. To this
end, we examined the extent to which the brains of 25 spe
cies of primates are “preprogrammed” vs. flexible at birth.
We looked at the time it takes animals of each species to de
velop. How long do they take to wean from their mothers?
Dreams are
the strange
love child of
the brain’s
plasticity
and the
rotation of
the planet