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.
A Simple Experiment
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.
