Brains of the previously blind retain 'wiring' even after sight is recovered.
Two people have been found to retain a mark of blindness years after their sight was partially restored. A part of their visual cortex that normally responds to visual motion now also responds to auditory motion. Researchers who have studied the pair suspect that the subjects have an enhanced ability to track moving sounds, although they have yet to test this.
The brains of people who lose their sight at a young age have long been known to turn parts of the visual cortex to non-visual tasks, making use of its spare capacity to handle the processing of auditory and tactile inputs. But it is hard to study exactly how this happens or which subregions are involved, says Melissa Saenz, a neuroscientist at the California Institute of Technology in Pasadena who led the team that conducted the study. “There’s a lot of variability in the brain from one person to another, and in this large region [the visual cortex] particularly. And obviously if a subject is blind, you can’t use visual stimuli to make a functional map of his or her visual cortex.”
Saenz found two very rare patients who had lost their sight at an early age but partly recovered it in their 40s: one following a cataract operation, the other after an experimental stem-cell therapy and a corneal transplant. The latter individual, Mike May, now 54, has had his sight-recovery experience portrayed in a book and a television documentary.
Using a technique called functional magnetic resonance imaging (fMRI) to monitor their brain activity, Saenz and her colleagues presented May and the other, female patient, as well as a normally-sighted control group, with an array of visual and auditory stimuli. The researchers found that a region of the visual cortex called MT+/V5, which specializes in the processing of motion, was activated about equally in the two patients who recovered their sight when either visual or auditory stimuli with motion-related content were presented. In normal-sighted subjects, only visual-motion stimuli activated this area, the researchers report in the Journal of Neuroscience1.
The fact that the rewiring of the visual cortex to process audio signals was so targeted — that the area specifically associated with visual motion was rewired for audio motion — suggests that the brain rewiring is very “efficient”, says Saenz. “It was able to take advantage of the pre-existing specialization of this brain region for processing motion,” she notes.
“It wasn’t just a random takeover,” agrees Franco Lepore, a brain plasticity expert at the University of Montreal in Canada. “It was a functionally relevant takeover.”
Previous research has suggested that the young brain’s sensory cortices are at least partially wired to take inputs from more than one sensory channel, and that some regions retain more of that multi-sensory potential than others. A team including Lepore reported last year that blind people wearing prostheses to convert image data to sounds seemed to map location-related stimuli broadly to an area of the visual cortex known for spatial processing2.
But Lepore finds it surprising that sound-derived and vision-derived processing of motion co-existed amicably in Saenz’s patients for so long after they recovered their sight. “You would expect that either vision takes over and the other one disappears, or that they interfere with each other,” he says.
The effect this rewiring has on the patients' perception remains unclear. The subjects don’t report any strange melding of sight and sound. “It could be that this sense of motion is really not so dependent on the sensory channel that feeds it,” says Saenz.
Even though their visual acuity is lower than normal, the patients seem to have regained a relatively good sense of visual motion. As for their hearing, blind people tend to have better auditory capabilities than sighted ones, and Saenz now plans to test her patients to see whether such a performance advantage has persisted despite partial sight recovery. “One of our major predictions is that they will have a better ability to detect whether or not a sound source is moving, or in which direction it’s moving,” she says. “We hope to have the data from that by the end of the summer.”
Saenz, M., Lewis, L. B., Huth, A. G., Fine, I. & Koch, C. J. Neurosci. doi:10.1523/jneurosci.0803-08.2008 (2008).
Collignon, O., Lassonde, M., Lepore, F., Bastien, D. & Veraart, C. Cereb. Cortex 17, 457-465 (2007).