Sometimes the brain ignores what the eyes tell it

Fix your eyes on the blue dots and the yellow ones may disappear

We are the prisoners of our brains. We see only what they decide to let us see. Researchers now illustrate this with an illusion in which the brain erases some aspects of the visual field.

Yoram Bonneh, of the Smith-Kettlewell Eye Research Institute in San Francisco, and colleagues have been showing people a swirling pattern of blue dots superimposed on some stationary yellow dots1.

The yellow dots seem to wink in and out. But the erasing happens in the mind, not the computer. Nearly everyone tested saw the effect.

The brain seems to have internal theories about what the world is like. It then uses sensory input - which tends to be patchy and disorganized - to choose between these. In some sensory situations, different theories come into conflict, sending our perceptions awry.

The illusion, which Bonneh's team calls motion-induced blindness, catches the brain ignoring or discarding information. This may be one of the brain's useful tricks, a deficiency - or perhaps both, says Bonneh.

The researchers speculate that this phenomenon could happen in everyday life without us noticing it. A highway at night, with drivers staring dully at a mass of moving lights, might recreate the kind of conditions used in the experiments, says Bonneh, causing objects - the tail lamp of the car in the next lane, for example - to temporarily vanish.

Jack Pettigrew, a neuroscientist at the University of Queensland in Brisbane, believes that the illusion results from a tussle for supremacy between the left and right halves of the brain.

He has found that applying a pulse of magnetism to the brain to temporarily disrupt its function affects the occurrence of motion-induced blindness. When the pulse is applied to the right hemisphere (leaving the left dominant) the dots disappear; zapping the left brings them back2.

The left hemisphere seems to suppress sensory information that conflicts with its idea of what the world should be like; the right sees the world how it really is. Some people with paralysis caused by injuries to their right hemisphere will deny that they are disabled.

"The right hemisphere is the cautious devil's advocate and the left hemisphere is the confident general with a plan of action," says Pettigrew.

The brain's theories about what the world should be like seem to emanate from a region called the parietal lobe - where Pettigrew's team aimed their magnetic. Some patients with injuries to this area see objects - their legs, for instance - vanish before their eyes. Others cannot perceive more than one object at a time.

An understanding of motion-induced blindness may bring insights into these conditions. "I don't have a strong theory," Bonneh says, "but the similarities are compelling."