Thursday, June 11, 2009
It's rare when real-world events perfectly mirror experiments that
scientists are conducting.
That's why neuroscientists at the University of Washington were
delighted at the reactions of former President George W. Bush and
Iraq's Prime Minister Nouri al-Maliki when an Iraqi reporter flung
his shoes toward the two men during a Baghdad news
conference.
When Bush ducked and Maliki didn't flinch as the first shoe
sailed toward them, it was a real-world example supporting the
theory that there are two independent pathways in the human visual
system.
"The original idea proposed is that one system guides your actions
and the other guides your perception. The interesting part is the
'action' system allows the brain to 'see' things your eyes do not
perceive," said Jeffrey Lin, a UW psychology doctoral student and
lead author of a paper appearing June 11 in the journal Current
Biology. Co-authors are Scott Murray and Geoffrey Boynton, UW
psychology assistant and associate professors, respectively.
"When we throw two balls at you with very similar trajectories,
they may look the same to your perceptual system, but your brain
can automatically calculate which one is more threatening and
trigger a dodging motion before you've even realized what has
happened," said Lin.
"If you look at the shoe-throwing video you will see that
the prime minister doesn't flinch at all. His brain has already
categorized the shoe as non-threatening which does not require
evasive action. But Bush's brain has categorized the shoe as
threatening and triggers an evasive dodge, all within a fraction of
a second."
To explore how this dual visual system works, the researchers set
up several experiments that were similar to what baseball players
experience when they step into the batter's box and get ready for
the pitcher to throw the ball. In a split second their action
system determines if the ball is going to hit their body and
whether to initiate a defensive bail out of the batter's box.
Instead of baseballs, college students participating in three
experiments looked at a computer monitor and were instructed to
quickly locate a target oval among a field of circular discs,
determine its path and press a key when they had found the oval.
The key manipulation in the study was that some of the trials began
with a looming stimulus at the position of the target oval. When
this looming motion was on a collision path with a student's head,
the participant responded faster to the target than when the
looming motion just missed the head. The experiment showed that a
stimulus on a collision path with a student captured attention but
one on a near-miss path did not. Critically, subjects could not
differentiate between the subtly different collision and near-miss
looming stimuli in a separate experiment.
The authors strongly believe the research supports the idea that
the human visual system is composed of two independent
systems.
"A major focus of neuroscience is understanding how we deal with
sensory information," said Boynton. "There is no way the brain can
possibly process and analyze everything we are exposed to. We have
to select what is important. In the real world you are on your own
and what you pay attention to is part of survival. This experiment
shows that threatening stimuli grab your attention, even those we
can't consciously identify. That this is more accurate than our
conscious perception is pretty amazing."
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University of Washington
Thanks to University of Washington for this article.