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Tuesday, 29 May, 2001, 14:49 GMT 15:49 UK
How the brain 'sees'
The brain appears to filter out certain visual cues
By BBC News Online science editor Dr David Whitehouse

Your brain does not tell you everything it sees, according to new research.

Scientists at the University of Minnesota, Minneapolis, US, have shown that neurons in the human visual cortex - the region that processes visual information - can detect patterns that are too fine for subjects to "see".

Your visual cortex isn't telling you everything

Sheng He, University of Minnesota
The work shows that some types of visual information, while not consciously perceived, are still registered by the brain.

Researchers say that this discovery contributes to the understanding of vision and the puzzling question of consciousness.

"This is probably the first demonstration that visual cortical neurons are capable of resolving fine lines past the limits previously thought to exist," said Sheng He, assistant professor of psychology at the University of Minnesota.

Blurred lines

Everyone with normal vision can perceive patterns of lines up to a certain point. But when the spacing of lines becomes too fine a uniform blur is seen.

Previously, researchers thought this was due to the eye itself - that is, a failure of the retina to resolve the lines. Now, it seems that at least some of the blurring takes place in the brain.

The researchers studied the responses of two subjects to patterns of lines projected directly onto their retinas by lasers. The lines were either horizontal or vertical.

MRI brain scan
Some types of visual information are processed unconsciously
It is already known that certain neurons in the visual cortex respond preferentially to either vertical lines, horizontal lines or in-between orientations.

It is also known that when subjects are shown a pattern of vertical or horizontal lines for several seconds and then shown a second grid of lines, they are better able to perceive the orientation of the second grid if it is perpendicular to the original one.

The explanation for this is that the neurons responding to a vertical grid become fatigued and have trouble perceiving a second vertical grid. The neurons that respond to horizontal lines, however, are fresh.

But when the researchers projected lines so fine that they were just beyond the subjects' ability to resolve, their subjects exhibited the same difficulty perceiving a second grid of clearly visible lines with the same orientation.

Conscious knowledge

This, said Dr He, is evidence that the cortical neurons geared to that orientation perceived the lines the first time, when the lines were invisible to the subjects.

Therefore, the subjects' inability to see the too-fine lines must be due to a blurring that occurs after the visual cortex receives input.

The blurring of lines appears to be due to processes inside the visual cortex that holds back some information from the eyes.

"This suggests that not everything in the cortex can become conscious knowledge," said Dr He. "Your visual cortex isn't telling you everything."

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