Spending more time in the dark could boost hearing in old age

In the Marvel Comic Daredevil our eponymous hero is the victim of a radioactive spill, leaving him blind but also with an extraordinary heightening of his other senses, particularly hearing.

For Julie, her superhuman hearing isn't the result of radioactivity, but is instead due to the reorganisation of neurons in her brain, enhancing her sense of hearing after the loss of her sight at the age of 16.

The cells in our brain are plastic, not static, meaning they can adapt and grow according to our experiences. So if one area of your brain is damaged, disrupting a certain process, other neurons in neighbouring regions will often take over, growing new projections or strengthening existing ones to pick up the slack. This has been most dramatically shown in children who have an entire hemisphere of their brain removed, usually to treat extreme cases of epilepsy. Miraculously, these children grow up to function almost entirely normally – walking, running, talking and thinking – going through school and even on to university as if nothing were out of the ordinary.

Co-ordinated reorganisation between our auditory and visual areas is particularly efficient, and it is this type of cross-sensory plasticity that has led to tales of enhanced hearing and even extraordinary musical ability in individuals who are blind (Stevie Wonder, for example), the auditory pathway compensating for disability in the visual circuit.

Now, new research has shown that this cellular sensory adaptation can occur with temporary disability in old age, suggesting our cells are even more malleable than previously thought, and opening the door for potential non-invasive treatments for hearing loss.

Published in Neuron this month, US scientists from Johns Hopkins University in Baltimore and the University of Maryland found evidence of enhanced auditory ability and cell firing in mice that were subjected to visual sensory deprivation for a week. First, the researchers locked the mice in a darkened chamber for six to eight days. After re-exposing them to light, the researchers played the mice a series of tones, comparing their responses to those of a group of control mice that had simply been hanging out in their cages during this time.

Though probably unpleasant, the time in the visual sensory deprivation chamber apparently did the mice some good, at least as far as their hearing was concerned. Those that had been "blinded" for a week showed greater activity in response to the tones in neurons in the auditory cortex that originated in the thalamus, a type of way-station in the brain that processes sensory information and thus may help integrate responses from both our eyes and our ears.

The cells fired more strongly and faster, demonstrating that the auditory cells in the blinded group were more sensitive to the noise than the same cells in the normal group. The visually deprived mice were also better at detecting quieter tones. Moreover, these super-hearing mice were better at discriminating between different types of tones, meaning their perception of pitch was also improved.

Importantly, this research was carried out in adult mice, showing that this sensory reorganisation can still occur in older animals. Previously, researchers thought that neural plasticity in these thalamo-cortical cells was only available while our brains were still developing. Emily Petrus, a graduate student at Johns Hopkins University who was first author on the paper, told me: "Most people thought these synapses stopped changing after a critical age, but we show that cross-modally you do still see a change. These findings go against the dogma that this part of the brain is aplastic [unchanging]."

In other words, there is hope for translating these findings into new treatments for ageing adults experiencing hearing loss. While locking people in a dark room for a week probably isn't a feasible treatment now, shorter spells of sensory deprivation may achieve the same results. In fact, past research suggests that being blindfolded for only 90 minutes can lead to enhanced hearing.

For now, the benefits are only temporary – the mice returned to their normal abilities after a week in the light. But according to Petrus the potential is there for more long-term changes, particularly for individuals who are struggling to learn how to use cochlear implants after going deaf.

"You have to force the brain to do something it doesn't usually do. And if you do it long enough – if you were put in the dark long enough and forced to use the cochlear implant – you could learn how to use it. Once the brain realises it is important, it will start to adapt."