A brain in doubt leaves it out
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 dots 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 back 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." http://www.nature.com/nsu/010614/images/motion_tn.gif
Fix your eyes on the
blue dots and the yellow ones may
disappear ?
A.P.Opposing effects on perceptual rivalry
caused by right vs. left TMS. Society for Neuroscience Abstracts
-- Cherri (jessam6@home.com), August 02, 2001