Time - USA (2021-02-15)

26 Time February 15/February 22, 2021

TheView Science

How dreams

defend our brains

By David Eagleman and Don Vaughn

What do brain flexibility and rapid

cortical takeover have to do with dream­

ing? Perhaps more than previously

thought. Ben clearly benefited from

the redistribution of his visual cortex

to other senses because he had perma­

nently lost his eyes, but what about the

participants in the blindfold experi­

ments? If our loss of a sense is only tem­

porary, then the rapid conquest of brain

territory may not be so helpful. And

this, we propose, is why we dream.

In the ceaseless competition for

brain territory, the visual system has a

unique problem: because of the planet’s

rotation, all animals are cast into dark­

ness for an average of 12 out of every

24 hours. Our ancestors effectively were

unwitting participants in the blindfold

experiment, every night of their lives.

So how did the visual cortex of our

ancestors’ brains defend its territory,

in the absence of input from the eyes?

We suggest that the brain preserves

the territory of the visual cortex by

keeping it active at night. In our “defen­

sive activation theory,” dream sleep ex­

ists to keep neurons in the visual cortex

active, thereby combatting a takeover

by the neighboring senses. In this view,

dreams are primarily visual precisely

because this is the only sense that is dis­

advantaged by darkness. Thus, only the

visual cortex is vulnerable in a way that

warrants internally generated activity

to preserve its territory.

In humans, sleep is punctuated by

rapid eye movement (REM) sleep every

90 minutes. This is when most dream­

ing occurs. REM sleep is triggered by

a specialized set of neurons that pump

activity straight into the brain’s visual

cortex, causing us to experience vision

even though our eyes are closed. This ac­

tivity in the visual cortex is presumably

why dreams are pictorial and filmic. The

anatomical precision of these circuits

suggests that dream sleep is biologically

important—such precise and universal

circuitry rarely evolves without an im­

portant function behind it.

The defensive activation theory

makes some clear predictions about

dreaming. For example, because brain

flexibility diminishes with age, the frac­

tion of sleep spent in REM should also

decrease across the life span. And that’s

When he Was 2 years old, Ben sTopped seeing ouT of
his left eye. His mother took him to the doctor and soon dis­
covered he had retinal cancer in both eyes. After chemo­
therapy and radiation failed, surgeons removed both his eyes.
For Ben, vision was gone forever.
But by the time he was 7 years old, he had devised a tech­
nique for decoding the world around him: he clicked with his
mouth and listened for the returning echoes. This method
enabled Ben to determine the locations of open doorways,
people, parked cars, garbage cans and so on. He was echo­
locating: bouncing his sound waves off objects in the environ­
ment and catching the reflections to build a mental model of
his surroundings.
Echolocation may sound like an improbable feat for a
human, but thousands of blind people have perfected this
skill, just as Ben did. How could blindness give rise to the
stunning ability to understand the surroundings with one’s
ears? The answer lies in a gift bestowed on the brain by
evolution: tremendous adaptability.
Whenever we learn something, pick up a new skill or mod­
ify our habits, the physical structure of our brain changes.
Neurons, the cells responsible for rapidly processing informa­
tion in the brain, are interconnected by the thousands—but
like friendships in a community, the connections between
them constantly change: strengthening, weakening and
finding new partners. Neuroscience calls this phenomenon
brain plasticity, referring to the ability of the brain, like plas­
tic, to assume new shapes and hold them. More recent discov­
eries in neuroscience, though, suggest that the brain’s brand
of flexibility is far more nuanced than holding onto a shape.
To capture this, we refer to the brain’s plasticity as “live­
wiring” to spotlight how this vast system of 86 billion neu­
rons and 0.2 quadrillion connections rewires itself every mo­
ment of your life. The brain’s livewiring allows for learning,
memory and the ability to develop new skills.
Recent decades have yielded several revelations about live­
wiring, but perhaps the biggest surprise is its rapidity. Brain
circuits reorganize not only in the newly blind but also in the
sighted who have temporary blindness. In one study, sighted
participants intensively learned how to read braille. Half the
participants were blindfolded throughout the experience.
At the end of five days, the participants who wore blindfolds
could distinguish subtle differences between braille charac­
ters much better than the participants who didn’t wear blind­
folds. Even more remarkably, the blindfolded participants
showed activation in visual brain regions in response to touch
and sound. In other words, the blindfolded participants per­
formed better on the touch­ related task because their visual
cortex had been recruited to help.
But such changes don’t have to take five days; that just hap­
pened to be when the measurement took place. When blind­
folded participants are continuously measured, touch­related
activity shows up in the visual cortex in about an hour.

26

YEARS

Amount of time

you will spend

asleep in an

average lifetime

21%

Proportion of

time sleeping

that we spend

dreaming

4 TO 6

CYCLES

Number of times

most people

experience a

dream sequence

every night

KANGHEE KIM