Different parts of the brain handle fantasy and reality
By David F. Salisbury
March 28, 2002
The ability to recognize objects in the real world is handled by different parts of the brain
than those that allow us to imagine what the world is like. That is the result of a brain mapping
experiment published in the March 28 issue of the journal Neuron.
The study focused on two cognitive tasks widely used by experimental psychologists.
One is mental rotation – mentally a rotating a complex object into a different position to compare
it with a second similar shape – and object recognition – determining whether two complex
objects are the same or different.
“Mental rotation and object recognition are indistinguishable from a behavioral viewpoint:
You can’t tell them apart,” says the paper’s first author, Isabel Gauthier, assistant professor of
psychology at Vanderbilt. “As a result, the field has been deadlocked over the question of
whether the brain uses the same mechanism and different mechanisms for the two tasks.”
Michael J. Tarr, one of the paper’s co-authors and professor of cognitive and linguistic
sciences at Brown University, had proposed in several papers with Steven Pinker at the
Massachusetts Institute of Technology that the same mechanism must be involved in the two
tasks. “There are parts of our brain that are involved in our ability to imagine the world,” he says.
“The question is, ‘Are those the same as the parts of the brain that we use to know what things
are?’ And the answer appears to be, ‘No, they are not.’”
Also collaborating on the study were William G. Hayward of Chinese University of Hong
Kong and an fMRI team from the Yale School of Medicine headed by John C. Gore.
To find out what parts of the brain are involved in these two mental tasks, the researchers
began with six unfamiliar geometric shapes that look something like the pieces from an unfolded
Rubic’s cube. They used three of these objects – two of which were mirror images – for the
mental rotation tasks and three which were slightly different but similar in appearance for the
object recognition tasks. Next, they assembled a group of 15 subjects and used functional
Magnetic Resonance Imaging machine to measure activity levels in different parts of their brains.
During the fMRI sessions, the researchers had the subjects perform two types of tasks
using the objects. They displayed the objects in pairs on a screen and, in one case, they asked
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Different parts of the brain handle fantasy and reality
the subjects whether the two are identical or mirror images – a basic mental rotation task. In the
other case, they asked them whether the two objects were the same or different – a basic object
recognition task. The objects were shown at different angles from each other on the horizontal,
vertical and out-of-plane axes and the researchers measured the time it took the people to
answer.
When they examined the brain scans, the scientists found that the areas of activated
during the two tasks tended to lie on two different pathways in the visual system. These two
pathways, called the ventral and dorsal, are sometimes called the “what” and “where” pathways.
When asked questions about the identity of an object – for example, is it the same shape as a
second object? – then the ventral pathway, which includes the temporal lobe, is activated. But
when a person is asked where an object is located the dorsal pathway, which lies in the parietal
lobe, becomes active.
The first place the researchers looked was the parietal lobe because previous studies
had shown that it is involved in mental rotation tasks. They confirmed these observations and
found that when the difference in orientation in the mental rotation tasks was large, the amount of
parietal lobe activity was greater than when the difference was small.
In the object recognition tasks, however, the researchers saw a much different pattern.
They did see some activity in the parietal region. Surprisingly, however, the amount of activity in
the parietal lobe decreased at larger orientation differences. In addition, they found that the brain
area that did show an increase in activity with larger differences in orientation was in the ventral
pathway.
“This is the first indication we have that the brain doesn’t rely on the same processes to
accomplish these two tasks, despite the fact that they appear to be so similar,” says Gauthier.
During the course of evolution, it seems as if the same solutions have arisen more than
once for similar problems in the way our brains work, adds Tarr. “They look very similar
behaviorally, but it turns out they use completely different neural circuits and the brain doesn’t
know how to put them together.”
-VUADDITIONAL INFO:
Isabel Gauthier’s home page
http://www.psy.vanderbilt.edu/faculty/gauthier/
Michael J. Tarr’s home page
http://www.brown.edu/Research/IGERT/prof_cmm_tarr.html
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Different parts of the brain handle fantasy and reality
John C. Gore’s home page
http://mri.med.yale.edu/individual/gore.html
William G. Hayward’s home page
http://www.psy.cuhk.edu.hk/people/whayward/
Article on mental rotation by Michael Tarr, MIT Encyclopedia of the Cognitive Sciences
http://cognet.mit.edu/MITECS/Entry/tarr.html
Article on object recognition by Martha J. Farah; MIT Encyclopedia of the Cognitive
Sciences
http://cognet.mit.edu/MITECS/Entry/farah2.html
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