Brain fakes it
Nerve-cell activity when eyes are shut reveals internal views
of the world.
30 October 2003
TANGUY CHOUARD
 |
| The brain may produce visual maps even when the eyes are
closed. |
| © Brand X |
|
|
People's tendency to see what they expect to see may be caused by their
brain constantly generating virtual sensations. So suggests a new study of
sleeping cats1.
When the animals' eyes were closed, researchers recorded spontaneous
patterns of neuronal activity similar to those evoked by real scenes.
Strikingly, this happened in the primary visual cortex - a region thought to
record visual stimuli passively.
Like a detuned television screen flashing up occasional pictures, the
resting cortex spontaneously produces clear maps of the outside world. It is as
if the eyes were actually looking at objects.
This is not dreaming. Because it happens so low in the normal
information processing chain, it is almost as if the mental images had sprung
right in front of the eyes.
The meaning of these internal states remains unclear. They might reflect
memories, expectations or items most worthy of attention.
If the same is true for humans, these virtual scenes might even
represent our brain's best guess of what our surroundings should look like at
any given time. Sensory stimulation might then update such preconceptions -
provided that we open our eyes.
"It is necessary to verify that the results apply to awake animals,"
comments visual processing specialist Dario Ringach of the University of
California in Los Angeles, California2. Then, he
explains, neuroscientists could find out how these internal pictures "interact
with external stimuli to influence visual perception and
performance".
Cat scan
To make the surprising discovery, Tal Kenet and colleagues at the
Weizmann Institute of Science in Rehovot, Israel, applied voltage-sensitive
dyes to the surface of anaesthetized cats' brains. These dyes make nerve cells
change colour under the microscope, depending on their state of electrical
excitation.
The researchers adapted mapping algorithms to analyse the resulting
deluge of imaging data. "This approach is truly cutting-edge," says Larry Katz
of Duke University in Durham, North Carolina.
It may turn vision science on its head, Ringach adds. It challenges "the
traditional view of the cortex as a pure stimulus-encoder machine," he
says.
Normally, when the eyes detect a tiny speck -a fly on the wall, say - a
patch a few millimetres wide on the surface of the brain becomes excited. The
activity of hundreds of thousands of nerve cells therein further refines what
the speck looks like - dark, green, furry and vertical, for instance.
Some nerve cells get extremely excited when a speck is vertical; others
react more to horizontal or diagonal objects2. So a vertical speck produces
islands of highly active nerve cells in a sea of quieter ones, whereas an
oblique speck results in a different pattern altogether. Neuroscientists call
such cortical footprints 'orientation maps'.
Eyes wide shut
Until now the brain was thought not to produce these maps when the eyes
are closed. Ongoing activity in the cortex was assumed to be random, like
static on a television.
Intriguingly, the brain seems to scroll through its internal images
methodically. It scans related orientation maps, one after the other, the
Israeli researchers observed.
|
It is necessary to verify that the
results apply
to
awake animals
|
|
Dario Ringach
University of
California
|
|
|
It also shows strong biases. For example, the visual cortex dwells on
maps that correspond to vertical or horizontal objects. This might explain why
people are much better at spotting real vertical and horizontal specks.
The findings strongly support theories of a 'top-down' mechanism of
perception, says Ringach. The prevailing wisdom favours a 'bottom-up' concept -
in which information flows only from the eyes to higher processing centres in
the brain.
Tanguy Chouard is a senior biological sciences editor at the
journal
Nature
|
