It's well known that when people lose one of their senses, the others become more acute to compensate. It's much rarer for people to regain a long lost sense, so science is only just coming to grips with what happens. New research suggests that, contrary to expectations, the heightened senses assist, rather than prevent, recovery.
The famously sharp hearing of the blind, or sight of the deaf, are reflected in the brain. In a process known as cross-modal plasticity, parts of the brain normally devoted to the missing sense get re-purposed. In particular, deaf people become better than hearing individuals at motion detection, including lip reading. Yet if new technology brings the lost sense back, these reassigned brain regions need to do their traditional job. This raises the question: does building the capacity of the brain to process input from one source interfere with the recovery of another?
Dr Carly Anderson of Nottingham Biomedical Research Center has investigated how the brains of 15 profoundly deaf individuals regained the ability to process speech once the physical capacity to hear was restored.
Anderson used functional near-infrared spectroscopy (fNIRS) to study activation of parts of the superior temporal cortex normally associated with processing auditory speech. Unlike some other methods of studying brain activation, fNIRS is compatible with cochlear implants, and Anderson looked at the brains of recipients both before and six months after they were given a cochlear implant. Her observations were compared with recipients' capacity to understand auditory speech as people became used to the implant.
Anderson found that cross-modal activation not only didn't harm implant recipients capacity to understand what they heard, it actually helped. The stronger the response to lip reading cues in the superior temporal cortex, the greater the response to hearing words post-implant, and the better the understanding of what was being said. Despite the small size of Anderson's sample, the correlation was unambiguous. Nevertheless, the report in the Proceedings of the National Academy of Sciences notes that the large-scale averaging involved in fNIRS prevents us characterizing the precise neurons involved, let alone understanding the relationship.
The work was done in response to recommendations against people with a cochlear implant continuing lip reading, as this might prevent the brain regaining its capacity to respond to inputs from the ears rather than eyes. However, this has been based more on guesswork than solid evidence, and Anderson's results suggest the exact opposite. With hundreds of thousands of cochlear implant recipients worldwide, the findings are important for maximizing their rehabilitation. On a broader level, they could alter thinking about neuroplasticity and suggest the brain adapts to sensory deprivation in more complex ways than we have assumed.
