Brain areas that are usually devoted solely to vision take on new duties following severe or total sight loss, two new brain-imaging studies suggest. In some cases, that switch occurred even when blindness arose after early childhood
For some children who go blind, parts of their brains that would otherwise have handled visual tasks end up pinpointing the origins of nearby sounds, report neuroscientist Franco Lepore of the University of Montreal and his colleagues.
“It’s clear that the visual cortex participates in enhanced sound localization for certain people with early-onset blindness,” Lepore says. “We don’t yet know how this type of reorganization occurs in the brain.”
The researchers theorize that sound localization gets a boost (SN: 10/16/04, p. 245: Hearing Better in the Dark: Blindness fuels ability to place distant sounds) when children receive lots of encouragement from parents and others to pay special attention to sounds as a way to compensate for their lost sight.
In the study, the scientists administered positron-emission tomography scans to 7 sighted adults and 12 adults who had lost their vision during childhood. Each participant listened to sounds randomly emitted by any of 16 loudspeakers in an echofree room and was asked to point in the direction of each sound.
Five of the blind volunteers displayed a keen ear for sound sources. These participants showed prominent blood flow—a sign of neural activity—in two areas deep within the visual cortex at the back of the right brain. This response appeared most strongly when a participant wore an earplug and could use only the right or left ear during tests.
The remaining seven blind volunteers showed neither enhanced sound localization nor pronounced reactions in the visual cortex to noises, Lepore and his coworkers report in the February PLoS Biology. In an ongoing study, the researchers are investigating whether these people instead have developed a heightened sense of touch that partly depends on brain areas formerly associated with vision.
The results of Lepore’s group “underscore the plasticity of the visual cortex in the face of visual loss,” remarks neuroscientist Chris I. Baker of the Massachusetts Institute of Technology. So do the results of a preliminary study, directed by Baker, of two men who had macular degeneration, a condition in which the central area of the retina degrades and destroys a person’s capacity to see anything directly in front of him or her. The pair of men had been diagnosed with macular degeneration at ages 11 and 35.
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Baker’s team obtained functional magnetic resonance imaging scans of the men’s brains during visual tests. Parts of the visual cortex that typically respond only when a person gazes directly at an item showed pronounced activity when the volunteers used their peripheral vision, the scientists report in the Jan. 19 Journal of Neuroscience.
Baker and his coworkers are now testing whether people with macular degeneration possess better peripheral vision on everyday tasks, such as face recognition, than people with normal vision do. The team will also track individuals with macular degeneration to determine how long it takes for changes to occur in the visual cortex.