It's well known that the child's brain has a
remarkable capacity for change, but controversy
rages about the extent to which such plasticity exists in
the adult human brain, particularly in the
part responsible for vision. Now, scientists from Johns
Hopkins and MIT offer evidence — derived
from both brain imaging and behavioral studies — that
the adult visual cortex (the area of the brain
that receives images from the eyes) does, indeed, have the
ability to reorganize. Moreover, that
reorganization affects visual perception. The study
appeared online Sept. 5 in an advance publication
of The Journal of Neuroscience.
According to senior author Michael McCloskey of Johns
Hopkins, the authors believe this work
not only may provide insight into how people see but also
may aid in the design of future medical
interventions.
"The results show that the visual areas of our brains
are capable of reorganization, and that
this reorganization affects how we see the world," said
McCloskey, a professor in the Krieger School's
Department of Cognitive Science. "Our findings will not
lead immediately to treatments but may
eventually contribute to developing therapies for people
who have suffered strokes or brain injuries,
or people with conditions affecting their eyes, such as
macular degeneration."
In an effort to learn more about cortical plasticity,
the research team studied "BL," an
individual who had suffered a stroke six months previously.
BL's stroke damaged the fibers that
transmit information from the eyes to the visual cortex,
though the cortex itself remained
undamaged. The damage cut off input from the upper left
visual field to the corresponding region of
visual cortex. Because his brain was not receiving
information from the upper left visual field, BL was
blind in that area; he could not see anything above and to
the left of where his eyes were focused.
What, though, happened to the piece of the visual cortex
that was no longer being fed information?
"We discovered that it took on new functional
properties, and BL sees differently as a
consequence of that cortical reorganization," said first
author Daniel Dilks of the McGovern Institute
for Brain Research at MIT, who led the study as a graduate
student in McCloskey's laboratory at
Johns Hopkins.
The researchers came to this conclusion because BL
reported that things "looked distorted" in
the lower part of his left visual field, directly below the
blind area. The researchers hypothesized
that the distortions resulted from reorganization of the
visual cortex that was now deprived of
sensory input. In order to isolate that distortion, they
asked BL to focus his gaze on a center dot
while objects such as squares appeared in various parts of
his visual field. BL's reaction was just what
they had anticipated: Though he saw nothing in his blind
area, whenever he focused on a square shape
just below his blind area, the patient perceived it as a
rectangle extending upward into the blind area.
Likewise, he saw triangles as "pencillike" and circles as
"cigarlike."
Subsequent functional magnetic resonance imaging
studies confirmed that the visually deprived
cortex in BL's upper left visual field was indeed
responding to information coming from the lower left
visual field. The deprived area was assuming new properties
— a hallmark of plasticity — and resulting in
visual distortions.
"In a nutshell, the cells in BL's blind area which
were no longer getting any information were
borrowing or grabbing it from the surrounding cortex which
was getting visual information," Dilks
explained.
"Before this," McCloskey said, "we had no idea whether
a cell deprived of information would just
wither away and die, or do something else. We found that
what it does is adapt. The adaptation in this
case turns out to be unfortunate for BL, who is suffering
from added vision problems. But what this
reveals about the brain is great news because eventually we
should be able to find ways to use this
adaptive ability to help people who find themselves in a
position like BL's."
Dilks is continuing this work in postdoctoral studies
at MIT. Other authors are Steven Yantis
and Benjamin Rosenau, both of Johns Hopkins; and John
Serences, of the University of California,
Irvine. The Johns Hopkins work was funded by an Integrative
Graduate Education and Research
Traineeship and a Graduate Research Fellowship, both from
the National Science Foundation, and the
National Institutes of Health.
GO TO