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 The Johns Hopkins University 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 appears online today in an advance publication of The Journal of Neuroscience.
The authors believe this work not only may provide insight into how people see, but also aid in the design of future medical interventions, says senior author Michael McCloskey of Johns Hopkins.
BL's Drawings of Visual ObjectsWhen a stimulus appears just below BL's blind area, the shape elongates upwards and into the blind area. He perceived circles as cigar shaped, squares as rectangles, and triangles as pencil shaped. Graphic courtesy Daniel Dilks, McGovern Institute for Brain Research at MIT, formerly of Johns Hopkins University
“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 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. Dilks led the study while he was 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 upwards into the blind area. Likewise, he saw triangles as “pencil-like” and circles as “cigar-like.”
Subsequent functional magnetic resonance imaging (fMRI) 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 said.
McCloskey said: “Before this, 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 include Steven Yantis and Benjamin Rosenau of Johns Hopkins and John Serences of the University of California at Irvine. An Integrative Graduate Education and Research Traineeship and a Graduate Research Fellowship, both from the National Science Foundation, and the NIH funded the Johns Hopkins work.
Credit: Johns Hopkins Medical Institute