Blind people who partially recover their sight can end up processing both visual and auditory information in the same region of the visual cortex, according to a recent study published in the Journal of Neuroscience. This and other recent findings in the field of “cortical plasticity” have encouraged researchers to believe that the sensory cortices are much more flexible in their organization than was once thought.
The Journal of Neuroscience study, led by Melissa Saenz at the California Institute of Technology, was inspired by observations that the brains of people who go blind can in some cases compensate for the deficit by using the spare processing power of the visual cortex to enhance or expand other sensory functions.
People who are blind have been shown to, for example, process Braille—a touch-based perception—using parts of their visual cortex. Prosthetic devices that translate visual images to auditory signals for people who are blind also appear to be handled partly by the visual cortex. The younger the age of onset of blindness, the more plasticity the visual cortex seems to have.
One of the mysteries that has lingered over this field is why non-visual functions go where they do in the visual cortices of blind people. “It’s been a problem to determine that in blind subjects,” Saenz says. “The subregions of the visual cortex can’t be reliably defined just based on anatomical coordinates or landmarks, in other words in the absence of functioning visual inputs.”
Recently, however, Saenz and her colleagues were able to locate a man and a woman, each of whom had gone blind early in life and then had partly recovered the sense of sight in adulthood, after surgery. Because they now had some restored sight capabilities, regions of their visual cortices could be mapped with visual stimuli and then tested to see which, if any, of these same regions also reacted to auditory stimuli.
Saenz and her colleagues were particularly interested to see whether a visual cortex area known as MT+ would show signs of this “cross modal” response. “We wanted to test the general hypothesis that brain reorganization would map onto pre-existing specialized regions, and MT+ was a good candidate to target because of its well-studied role in motion processing,” she says.
An array of tests of the two subjects, while their brains were being scanned with functional magnetic resonance imaging, confirmed that their MT+ regions responded not only to visual motion but also to sounds that simulated a moving source.
“In fact we found that these responses are about equally strong,” says Saenz. “The two functions seem to be coexisting in the same region.”
To Franco Lepore, whose laboratory at the University of Montreal has been one of the leaders in the field of sensory plasticity, such a sensory coexistence was somewhat unexpected. “What’s nice about this study, the really original part, is that the [auditory-motion] function remains and they show that it remains in the same way even after vision functions are re-established in the same region,” he says. “There might have been a negative interaction between the two.”
Studies in Lepore’s lab and others have shown, for example, that when deaf individuals receive cochlear implants, their auditory cortices often have only a limited ability to process the new inputs of sound data, having been reorganized during deafness to handle other functions. Those who perform better with the implants tend to have received them earlier in life, when the brain is still quite flexible, and in such cases the auditory cortex appears largely to revert to its usual sound based functions.
Another prominent researcher in the field, Amir Amedi of the Hebrew University of Jerusalem, points out that for both of Saenz’s subjects, “their vision is not fully restored. So maybe the competition from visual inputs is not as strong.”
Amedi notes that one of Saenz’s subjects was Mike May, a California businessman who has been the subject of a documentary and a high-profile biography because of his partial vision recovery at age 46, following an experimental corneal stem cell transplant. “May writes that when he goes skiing, for example, he still closes his eyes and uses his other senses, years after his vision was restored,” Amedi says.
“So he and the other subject might still depend a lot on their audition and their touch senses. They might have learned, better than we have, to count on those other senses and to get information from them.”
Not a Random Takeover
Perhaps most important, Saenz’s study confirmed that in blindness, the takeover of the visual cortex by auditory functions can proceed according to a clear logic.
“It wasn’t a random takeover that happened just because that region [MT+] wasn’t doing anything,” says Lepore. “It was a takeover by another motion-related function.”
There have been indications of this phenomenon from other recent studies. Last year Lepore’s laboratory published a study in which blind subjects, wired with devices that convert image data to sounds, were shown to use a region of their visual cortex broadly associated with spatial processing to handle location-related sound stimuli.
Also last year, a group led by Amedi, then working in the laboratory of Alvaro Pascual-Leone at Harvard Medical School, reported that a cortex region known as the lateral occipital tactile-visual area, normally activated when shapes are recognized via touch or sight, was also activated when blind subjects—and even sighted volunteers wearing blindfolds—recognized shapes using sounds generated by devices that convert visual images to auditory signals.
Amedi points out that it is not just these higher sensory areas that seem flexible enough to accept multiple inputs. Primary regions of the visual cortex, including the V1 region (which normally handles relatively raw data from the retinas in sighted individuals), also appear to have this flexibility. Indeed, he says, there is evidence that the normal hierarchy of the visual cortex can become “inverted” in people with congenital blindness.
In a paper in 2003, Amedi, Ehud Zohary and their colleagues showed that V1 became relatively active during verbal memory tasks—normally processed in a higher region—in congenitally blind subjects, and that this activation correlated with performance on these tasks. At the same time, the group found evidence that Braille reading by such subjects engaged visual areas normally associated with higher level processing.
“The story in the blind might be that V1, when it is not getting the input from the eye and from the thalamus, actually moves farther from the sensory world, as far as the prefrontal cortex normally would be,” says Amedi.
In their experiments with sighted volunteers who are blindfolded, Amedi and Pascual-Leone and their colleagues have found that the brain can show signs of this flexibility very rapidly. A recent series of experiments, described in conference presentations, involved subjects blindfolded and kept in a dark environment for five days.
“The results were quite amazing,” Amedi says. “For touch and even for verbal memory, where we have shown massive recruitment of the visual cortex in the blind, there was the beginning of this recruitment even in these sighted individuals. It’s not as dramatic as what we see in congenitally blind individuals, but it’s definitely a very strong recruitment of small parts that might be the anchors for the later plasticity.”
“It’s possible that everybody might have some weak auditory connections in some parts of their visual cortex,” says Saenz. “And if so, that would provide the opportunity for the strengthening of those connections under the right circumstances.”
One might think that when a person processes inputs from two very different sensory organs in one brain region, he or she would experience a certain blurriness about the origin of sensation. “Maybe when they hear something it feels like they see something, or vice versa?” says Saenz, whose interest in cortical plasticity extends to the study of the sensory cross-talk phenomenon known as synesthesia. (Individuals with this wide-ranging and relatively common condition somehow get two or more sensory experiences from a single stimulus, perceiving colors when they hear certain sounds, for example, or tastes when they read certain words.)
But somehow the processes of consciousness in her two formerly blind subjects maintained the sight-sound barrier. “They told me they knew distinctly what it was to hear something vs. what it was to see something,” Saenz says. “They seemed to have no obvious sense of synesthesia.”
Saenz plans further tests on the two subjects this summer, among other things to determine whether their expanded auditory-motion processing power has made it easier for them to track moving sound sources. And as stem-cell and gene-transfer therapies for restoring sight are developed successfully, she expects that more people with mixed-mode sensory capabilities will become available for such testing.
As Amedi points out, tests of such patients could also lead to the resolution of an old conundrum, best known in its formulation by a 17th century Irish scientist, William Molyneux: If someone is born blind and learns to distinguish, for example, a cube and a sphere by touch, will he be able to distinguish and to name them solely by sight if his vision is ever restored?
“The philosopher John Locke’s answer was that vision and touch are completely different entities, so this hypothetical blind man would have no possibility to distinguish these objects by sight after having been blind,” says Amedi. “And modern neuroscience for a long time agreed with Locke.”
Thanks to sensory plasticity studies by Amedi, Lepore, Saenz and others, neuroscience has started to move in the other direction. Amedi suspects that with extensive retraining it might be possible to restore most or all visual functions, even for people already in adulthood when they are given their sight.
But clearly some sensory regions are more plastic than others. Years after their sight-recovery surgeries, for example, Saenz’s two recovered-sight subjects still find it hard to recognize complex visual information, including the faces of family members and friends.
“Motion, color and simple form perception are consistently regained in the few case reports, but more complex object recognition remains highly impaired,” Saenz says.