A new study from Monday indicates that early eye loss can lead to subtle changes in the brain can circuit that is mainly responsible for hearing
It is generally believed that a blind birth or early loss of vision can make the hearing more sensitive. Blindness, especially if a person is born with it or if it occurs early in life, can make the hearing more sensitive. Although studies have consistently shown that blind people in some ways seem to have a closer ear, we do not know exactly how or where this increased ability actually appears in the brain.
The authors behind this latest study, published in the Journal of Neuroscience, say theirs is one of the first to examine what's happening in the auditory cortex of people living with blindness.
"Previous studies have really dealt with behavioral issues, and we're one of the first to try to approach it with a more modeled approach," said lead author Kelly Chang, a vision and knowledge researcher at the University of Washington, opposite Gizmodo.
They examined the auditory cortices of people who were born or were blind at an early stage in life (including some people with anophthalmia whose eyes are completely absent) using MRI. They were also scanned when they were subjected to hearing tests where they heard pure tones. These sounds were played on different frequencies and their brain activity in the auditory cortex was measured when they heard these sounds. Their results were then compared to a control group with average view. Everyone involved had average hearing.
They found that the auditory cortex was similar in both groups, including its size. There was, however, a marked difference in how blind and seeing people processed the sound. For those who were blind, the auditory cortex seemed to be better tuned to certain sound frequencies played in the test, based on the type of brain activity the researchers saw in the scans.
"Let's say you wanted between a low frequency and high frequency. For people with eyesight, this is actually pretty easy, because the notes are very far apart. But in the early blind, probably because they only rely on their hearing system, it's actually much better to be able to distinguish between very tight frequencies, "said Chang.
Other research suggests that neuronal connections of the brain can reorganize this When a person becomes blind, especially in those areas where vision is normally processed, this is an example of the well-known brain plasticity and the results, the authors say, are "some of the first evidence in humans," that this compensation can occur in areas of the brain that are not directly affected by blindness.
"Seeing this reorganization in such a basic location is fundamentally remarkable," she said.
The small sample size of the study (total nine subjects with early blindness, including five with anophthalmia), means that the results of the team are by no means unique. But even if that were the case, there are still some important questions to be found out, say the authors.
The first is what makes this compensation possible in the auditory cortex of the brain. Previous research, including Chang's team, has suggested that such changes are only possible when someone becomes blind as a child and does not appear in those who lose their sight as adults. And perhaps it is not the loss of sight itself that drives this reorganization in the auditory cortex, but the need for a blind person to focus on specific sounds to navigate the world. It's also worth studying how people who become blind at an early stage move sounds and speech – complex sounds that require more collaboration between different areas of the brain.
Future studies that solve these questions, on which Chang and others work, might investigate what happens to the auditory cortex of people who are temporarily blinded. They could also look at people who were blind in adulthood, including those who were able to see again later.