Provided by the National Institute of Mental Health
Watching the Brain at Work
Is the brain hard-wired for houses?
When NIMH neuroscientists Dr. James Haxby and colleagues flashed pictures of
faces, houses and chairs in front of subjects while their mental activity was
being visualized by an MRI scanner*, each category of object seemed to activate
its own distinct circuitry in the brain's visual system. 1 But the researchers
were skeptical. Except for certain biologically relevant objects, such as faces,
which emerge through evolution, how could the brain have evolved specific
circuits for such relatively modern objects? So they analyzed their data in a
different way; they looked at the pattern of response to each stimulus category
across the three regions that were ostensibly specialized—paying particular
attention to areas of secondary activation. In each case, they found that even
though a particular region might respond maximally to one category, it also
responds to a lesser degree—but still significantly—to objects in the other
categories as well.
Since this pattern of activation is widely distributed, the brain's visual
circuitry appears to process what it sees based on features of objects, suggest
the researchers. Information most characteristic of objects within a single
category clusters together, thus giving the appearance of object-specific
circuitry.
Still, faces appear to be an exception to the rule. Brain areas activated by
faces were not as widely distributed as those for houses and chairs, adding to
evidence that there may indeed be face-specific circuitry. Also, a task in the
study that required subjects to pay greater attention to the stimulus had more
effect on responses to houses and chairs than to faces, hinting that face
perception is more automatic.
Keeping those blinders on
We simply can't become conscious of and remember everything that we see.
Multiple representations of objects in our visual field are constantly competing
with each other for our brain's limited visual processing capacity. What's more,
they mutually cancel each other out; visual clutter actually suppresses the
brain's ability to respond; it reduces its activity. So how does the brain cope?
An NIMH research team led by Dr. Robert Desimone had a hunch: by focusing its
attention on just one stimulus, the brain cancels out this suppressive influence
of nearby stimuli—enhancing information processing of the desired stimulus.2
They first measured the suppression of brain activity caused by a cluttered
scene, using fMRI to visualize activity in the cerebral cortex. Subjects were
scanned while distracted away from the scene. The researchers compared the
cortical response to four complex pictures presented at the same time, i.e., the
"cluttered" condition, to the cortical response to the same four pictures
presented one at a time, i.e., when the cortex could fully process each picture
individually. This comparison confirmed that the cluttered scene caused a
suppression of activity in a circuit critical for the recognition of objects,
which explains why it is not possible to recognize objects in a complex scene
without focusing your attention.
The clincher came when subjects were asked to fix their eyes on a corner of the
screen, while shifting their attention to one of the four pictures. It is well
known that we can attend to things we're not looking at. The researchers found
the effect of attention was to counteract the suppression caused by the other
distracting pictures on the screen. In other words, with attention, the cortex
could devote all of its processing to the single most important picture,
filtering out distraction. The results point to the modulation of suppression in
the cortex as a critical mechanism of attention.
"I just saw it. Now where is it?"
Where does the human brain hold information momentarily about where things are
located? This specialized circuitry for spatial working memory keeps track of,
for example, the ever-changing locations of other cars while you're driving.
Working memory—what we're aware of from one moment to the next—bridges time and
is the content of our consciousness. After a decade of following blind
alleys—led astray by its location in the monkey brain—neuroscientists recently
discovered that the elusive circuitry has been displaced rearward and upward
through evolution, as areas serving more distinctly human functions emerged.
These newer areas mediate cognitive abilities, such as abstract reasoning,
complex problem solving and planning for the future—functions impaired in some
severe mental disorders, such as schizophrenia.
While other researchers focused on the human anatomical counterpart to the area
in the monkey, a region in the middle of the frontal lobe, NIMH's Dr. Leslie
Ungerleider and colleagues took their clue from a functional landmark.3 They
hypothesized that, as in the monkey, they would find the human spatial working
memory circuits just in front of an area specialized for controlling eye
movements. Brain imaging studies hinted that this circuitry had evolved into a
higher and more rearward location in the human frontal cortex.
Using fMRI, they saw high activity, during an eye movement task, in the middle
upper part of the frontal cortex, confirming location of the eye movement
circuits. Just in front of this area they discovered an area that showed
sustained activity during a pause in a spatial working memory task, confirming
that it harbors the circuits for that function.
--------------------------------------------------------------------------------
For More Information
Please visit the following link for more information about organizations that
focus on the human brain.
--------------------------------------------------------------------------------
All material in this fact sheet is in the public domain and may be copied or
reproduced without permission from the Institute. Citation of the source is
appreciated.
NIH Publication No. 01-4609
--------------------------------------------------------------------------------
References
1 Ishai A, Ungerleider LG, Martin, A, et al. Distributed representation of
objects in the human ventral visual pathway. Proceedings of the National Academy
of Sciences USA, 1999; 96(16): 9379-84.
2 Kastner S, De Weerd P, Desimone R, et al. Mechanisms of directed attention in
the human extrastriate cortex as revealed by functional MRI. Science, 1998;
282(5386): 108-11.
3 Courtney SM, Petit L, Maisog JM, et al. An area specialized for spatial
working memory in human frontal cortex. Science, 1998; 279(5355): 1347-51.
Back to Mental Health Articles