The senses of touch and hearing often get enhanced when people lose their vision. Now US scientists have gained insights into why this happens.
Experts at the Berenson-Allen Center for Noninvasive Brain Stimulation at Beth Israel Deaconess Medical Center (BIDMC) have found that sudden and complete loss of vision leads to profound, but rapidly reversible, changes in the visual cortex in the brain.
Writing about their findings in a research paper, published in the journal PLOS One, the researchers said that their study provided new insights into how the brain compensates for the loss of sight, and suggest that the brain is more adaptable than originally thought.
"The brain's ability to reorganize itself is much greater than previously believed. In our studies [in which a group of sighted study subjects were blindfolded for five days], we have shown that even in an adult, the normally developed visual system quickly becomes engaged to process touch in response to complete loss of sight. The speed and dynamic nature of the changes we observed suggest that rather than establishing new nerve connections - which would take a long time - the visual cortex is unveiling abilities that are normally concealed when sight is intact," says senior author Dr. Alvaro Pascual-Leone, Director of the Berenson-Allen Center and Professor of Neurology at Harvard Medical School (HMS).
In a previous study, the researchers had seen that subjects with normal vision who were blindfolded for a five-day period performed better than non-blindfolded control subjects on Braille tests.
The researchers said that subsequent brain scans during the same study had found that blindfolded subjects also experienced dramatic changes in the brain's visual cortex.
In their latest study, they set out to determine whether those outcomes were the result of new nerve connections being developed, or they resulted from the unmasking of the latent capabilities in the brain's visual cortex in response to the loss of sight.
"We recruited 47 subjects to participate in the study. Half of the study participants remained completely blindfolded, 24 hours a day, for a total of five days under the careful watch of the staff of BIDMC's General Clinical Research Center. The other half were only blindfolded for testing, but spent the rest of the day seeing normally. During their stays, both sets of study participants underwent intensive Braille instruction for four to six hours a day from a professional instructor from the Carroll Center for the Blind," says Merabet, Assistant Professor of Ophthalmology and Neurology at HMS.
The study participants also underwent serial brain scans - functional magnetic resonance imaging (fMRI) - at both the beginning and end of the five-day study period.
The researchers observed that the subjects who were blindfolded were superior at learning Braille than their non-blindfolded counterparts, and that the brain scans of the blindfolded subjects showed that the brain's visual cortex had become extremely active in response to touch.
When the researchers removed the blindfolds 24 hours later and rescanned the subjects, they discovered that their visual cortices were no longer responsive to tactile stimulation: reading Braille no longer activated "sight" among the study subjects.
According to the researchers, disruption of the visual cortex impaired tactile function and Braille reading after five days of blindfolding, but not a day after the blindfold was removed and never in the control subjects.
"This extremely rapid adaptation indicates that functions that are normally inhibited in the brain's visual cortex will come to the surface when they are needed. We believe that over time, if these adaptive functions are sustained and reinforced, they will eventually lead to permanent structural changes," says Merabet.
"Our brain captures different types of information from the world -- sounds, sights, smells or tactile sensations. The impressions we form require us to merge these various different elements, but science's traditional view of brain function is that it is organized in separate and highly specialized systems," adds Pascual-Leone.
He further states: "Our study shows that these views are incorrect and illustrate the potential for the human brain to rapidly and dynamically reorganize itself. We have shown that even in an adult, the normally developed visual system quickly becomes engaged to process touch in response to complete loss of sight. And we believe that these principles may also apply to other sensory loss, such as deafness or loss of function following brain injury."