October 30, 2003

Brain fakes it

Nerve-cell activity when eyes are shut reveals internal views of the world.

by Tanguy Chouard
Nature News

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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 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

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