Every night, in a state the brain enters without being asked, we become the
authors of experiences we did not choose and cannot fully control. We fly. We
sit exams for subjects we never studied. We speak with people who have been
dead for years. Dreaming is so ordinary that it can feel like it needs no
explanation. But for neuroscientists, it remains one of the most genuinely open
questions in the field.
The problem with obvious answers
The most commonly repeated explanations for why we dream fall into two
categories: memory consolidation and emotional processing. Sleep, and REM sleep
in particular, does appear to help the brain sort and store the experiences of
the day. Studies going back decades show that people who sleep after learning
something remember it better than people who stay awake. And there is good
evidence that REM sleep plays a specific role in processing emotionally loaded
experiences, which may be part of why disrupted sleep and mood disorders so
often travel together.
But neither explanation fully accounts for the specific phenomenology of
dreaming: the narrative quality, the bizarre juxtapositions, the visual
vividness, the near-complete loss of self-awareness that allows us to mistake a
dream for waking life. Memory consolidation happens during non-REM sleep too.
Emotional processing happens during waking life. Neither tells us why any of
this needs to feel like something.
Defending the visual cortex
One of the
more striking recent theories comes from Stanford neuroscientist David Eagleman
and his colleague Don Vaughn. Published in Frontiers in
Neuroscience and developed in subsequent years, their
Defensive Activation Theory proposes that dreams exist primarily to protect the
visual cortex from being taken over by neighboring sensory systems during the
hours of darkness.
The brain is radically adaptive. When a brain region goes unused,
neighboring regions begin to colonize it through synaptic competition. Research
on people wearing blindfolds shows that the visual cortex begins generating
touch-related activity within roughly 60 to 90 minutes of sensory deprivation.
Blind people who lost their sight in adulthood show cross-modal rewiring within
months. The brain does not hold territory it is not using.
Eagleman and Vaughn’s argument is that the visual cortex is especially
vulnerable during the long stretch of darkness that constitutes the ancestral
night. With no visual input for eight hours, the cortex risks being colonized
by adjacent auditory or somatosensory regions. REM sleep, with its bursts of
intense visual activity, keeps the territory occupied. Dreams, on this view,
are not the point. They are the side effect of the brain running a maintenance
program.
“The brain is in a
use-it-or-lose-it competition with itself every night. Dreaming may be how the
visual system fights back.”
Evidence from
across species
The theory makes several testable predictions. One is that animals that are
more visually dependent should spend more time in REM sleep. Predators, who
rely on vision for hunting, do tend to have more REM than prey animals, which
often have eyes positioned for wide peripheral awareness rather than acute
forward vision. Newborn humans, whose visual systems are the most immature and
therefore the most vulnerable to competitive takeover, spend roughly half their
sleep time in REM, far more than adults. As visual plasticity declines with
age, REM percentage drops in parallel.
A 2023 study on lucid dreamers added another angle. Researchers found that
the frontal regions associated with self-awareness show unusually strong
connectivity to visual areas during lucid REM sleep, suggesting that the
balance between self-monitoring and visual generation is actively managed
during the dream state. When self-awareness comes back online, as in lucid
dreaming, something in the visual experience changes too.
What remains unknown
The Defensive Activation Theory is compelling and falsifiable, which is
more than can be said for many dream theories. But it is still contested. It
does not explain non-visual dream content, the emotional intensity of many
dreams, or the fact that blind people who have never had visual experience
still enter REM sleep and report dream experiences involving the senses they do
use.
What the current generation of research agrees on is that dreaming is not
random noise. It is correlated with specific brain states, specific
neurochemical environments, and specific life circumstances in ways that point
toward function. Whether that function is visual maintenance, memory
processing, emotional regulation, threat simulation, or something yet unnamed,
the brain goes to considerable metabolic expense to make dreams happen every
night. That alone suggests they are not nothing.
Sources
·
Eagleman, D.M., & Vaughn, D.A. (2021). The Defensive
Activation Theory: REM sleep as a mechanism to prevent takeover of the visual
cortex. Frontiers in Neuroscience.
·
Baird, B., et al. (2023).
Prefrontal-visual connectivity in lucid dreaming. Nature Neuroscience.
·
Walker, M. (2017). Why We Sleep. Scribner.
(contextual reference)
Source: Why Do We Dream? –
Scents of Science
