A light inside the eye

Blink and you miss it
Jim Giles

Why the world doesn't go black when your eyelids flicker.

A light shone inside the mouth helped to prove that the brain switches
off when we blink.

Why doesn't the world go dark when we blink? Because a critical part of
the brain switches off and fails to detect the blackness behind closed
eyes, says a team of neuroscientists.

We blink about ten times a minute without noticing any change in what
we see. Researchers had suspected that this is because the visual
system is inactivated during blinking, but were not able to prove this.

A team at University College London have cracked the problem by
inserting an optical fibre into the mouths of people wearing black-out
goggles. The fibre illuminated the back of the subjects' retinas, so
that they saw a light at all times, even when they blinked. This
allowed the researchers, who publish their results in Current Biology1,
to distinguish between the effects of the act of blinking and the
darkness that it causes.

It's not that the visual gap is filled in. It's that you're not aware
of it.

Using magnetic resonance imaging (fMRI) brains scans, Davina Bristow
and colleagues revealed that activity in a part of the visual system
known as V3 was suppressed in subjects when they blinked. V3 is one of
a series of brain areas that handle signals sent from the eyes. With it
out of action, the blink goes unnoticed.

"It's not that the visual gap is filled in," says Bristow. "It's that
you're not aware of it."

Blinded by the light

Similar inactivation is known to allow us to see a smooth image even
when our eyes jump between two parts of a scene. But working out what
happens in the brain during blinking has proven very difficult.

Blinks normally cause an abrupt change in the amount of light reaching
the retina, which in turn causes a massive change in brain activity in
the visual region. Seeing any effect in the V3 area has been obscured
by this in the past.

"Blinks are hard to study," says Tim Gawne, a vision researcher at the
University of Alabama in Birmingham.

Blinking is not the only process that causes areas of the brain to be
suppressed. Bristow points out that fMRI scans have shown that tactile
areas of the brain are suppressed when we tickle ourselves, but not
when someone else does it2.

Bristow says studies of tickling and blinking add to our understanding
of how our brains deal with different types of events.

"How do we distinguish between what is caused by you and what is caused
by the outside world?" she asks. "It's more important, for all animals,
to pay attention to outside causes. Blinking is a way of studying how
they do so."

Bristow D., Haynes J. D., Sylvester R., Frith C. G. & Rees G. Current
Biology, 15. 1296 - 1300 (2005).
Blakemore S. J., Wolpert D. M. & Frith C. D. Nat. Neurosci., 1. 635 -
640 (1998).



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