Losing your sight early in life is often said to heighten other senses, particularly hearing. A new study now shows that even a short, weeklong stint in complete darkness can result in superhuman hearing, at least temporarily. This suggests that a kind of darkness therapy may help restore hearing to adults suffering from hearing loss.
Musicians Stevie Wonder and Ray Charles, who were both blinded at a young age, are often cited as examples of how the lack of sight can enhance hearing. And over the years, research has suggested that people who lost their sight early in life do, in fact, hear better than their non-blind counterparts.
"They have better frequency discrimination and spatial localization," explained Patrick Kanold, a biologist at the University of Maryland, who studies how the brain processes sound. "They have superhuman hearing."
Similarly, research has also shown that people born deaf use their auditory cortex — the region of the brain that deals with sound — to process tactile and visual information, and even have enhanced sight.
These effects, however, only appear to occur in people who lost one of their senses at an early age, or from birth. Reason being: There is a so-called critical period, during which the brain is most adaptable to sensory stimuli, including new sights and sounds. "Your brain is very malleable during early development, and then after a certain point, it's not," Kanold told io9.
Studies have shown that the connections, or synapses, between cortical neurons — nerve cells in the cerebral cortex of the brain, which processes sensory information and is vital for thinking and reasoning, among other things — remain malleable long into adulthood. On the other hand, the synapses between neurons in the thalamus (a kind of sensory information relay center) and the cortex, called thalamocortical synapses, were thought to be pretty much set after the critical period.
But recent research has suggested otherwise, leading Kanold and his colleagues to wonder if thalamocortical synapses in the visual cortex and auditory cortex change when the senses are disrupted after the critical period. In effect, this would mean that vision or hearing loss in adulthood could actually alter and even improve other senses. The team decided to find out if this was the case.
The researchers conducted a simple experiment, in which they placed mice that were "way beyond their critical period" in complete darkness for 6-8 days. These mice, Kanold explained, had normal hearing and vision. They then brought the mice back into the light to test their hearing.
The researchers put the mice into a soundproof chamber and played one-note tones of varying frequencies and loudness, while they recorded the mice's brain activity. The auditory neurons of the temporarily blinded mice were better able to discriminate pitch and respond to softer sounds than the neurons of mice that weren't subjected to weeklong darkness. What's more, the visual deprivation strengthened the thalamocortical synapses in the mice's primary auditory cortex, but not those in the mice's primary visual cortex.
"The results were quite surprising," Kanold said. "Given what we know about blind people, we expected to see some change in the mice's brains, but these results were a lot larger than we expected."
The researchers suspect that a similar effect would occur in people placed in prolonged darkness. However, they don't know how long the darkness would need to last — it may take many more weeks before any hearing improvements develop. Before anyone can conducts such tests, the researchers will be investigating how to make the hearing improvement permanent. In their mouse experiments, the rodents reverted back to normal hearing a few weeks after returning to a normal dark-light cycle. Simply keeping the mice in pure darkness for a longer period of time may prolong the hearing enhancements.
"If it works in humans, we could use this technique in a variety of clinical situations to improve recovery of functions in the brain," Kanold said. Specifically, he foresees darkness therapy being useful for patients with cochlear implants, which are much more effective in children than adults. These electronic devices work by electrically simulating the auditory nerves in the inner ear, ultimately providing a sense of sound to people who are hard-of-hearing or completely deaf. Putting adults in darkness while they begin to use the implants may help them adjust to the devices and help their brains better process the new sounds.
The technique could also, presumably, be used to help people better process visual information. In another experiment, Kanold and his colleagues experimentally deafened mice and looked at their thalamocortical synapses. In this case, the synapses became stronger in the visual cortex, but not the auditory cortex.
Read the study in the journal Neuron.