Link Between Deafness and Speech Deterioration
The way the senses play on our output modalities is fascinating. How as one sense increases it opens up new ways to explore the world, and triggers new and different modes of behaviour. This is particularly important to the growing trend towards virtual embodiment, and taking on new virtual bodies with different sense characteristics, because as your senses change, so too does the way you interact with the world.
However, it seems the differences are not just psychological. There are tangible changes in the wiring of the brain when sensory input changes, that affects the output quite significantly.
In the case being looked at here, when the sense of hearing fades, related abilities in the brain also fade with it. When hearing declines, speech declines with it its based at least partially on hearing your own voice after all. Yet, in the brain itself, the neurons responsible for speech are undergoing physical changes that occur within hours of total deafness setting in. We have nowhere near enough information to do much more than speculate at the moment, but it is quite plausible, based on what we do have, that there is a danger with long-term immersion, that if a simulated sensory channel is removed, it may well cause actual changes in the brain of the individual that go on to affect them long after they have exited the virtual world.
The work being focussed on here concerns songbirds, and focusses on how the portions of a songbird's brain that control how it sings have been shown to decay within 24 hours of the animal losing its hearing.
The findings, by researchers at Duke University Medical Center, show that deafness penetrates much more rapidly and deeply into the brain than previously thought. As the size and strength of nerve cell connections visibly changed under a microscope, researchers could even predict which songbirds would have worse songs in coming days. The study was published in Neuron journal online on March 7, 2012.
"When hearing was lost, we saw rapid changes in motor areas in that control song, the bird's equivalent of speech," said senior author Richard Mooney, PhD, professor of neurobiology at Duke. "This study provided a laser-like focus on what happens in the living songbird brain, narrowed down to the particular cell type involved."
"I will go out on a limb and say that I think similar changes also occur in human brains after hearing loss, specifically in Broca's area, a part of the human brain that plays an important role in generating speech and that also receives inputs from the auditory system."
"Our vocal system depends on the auditory system to create intelligible speech. When people suffer profound hearing loss, their speech often becomes hoarse, garbled, and harder to understand, so not only do they have trouble hearing, they often can't speak fluently any more."
The nerve cells that showed changes after deafening send signals to the basal ganglia, a part of the brain that plays a role in learning and initiating motor sequences, including the complex vocal sequences that make up birdsong and speech.
Although other studies had looked at the effects of deafening on neurons in auditory brain areas, this is the first time that scientists have been able to watch how deafening affects connections between nerve cells in a vocal motor area of the brain in a living animal, said Katie Tschida, PhD, a post-doctoral research associate in the Mooney laboratory who led the study.
Using a protein isolated from jellyfish that can make songbird nerve cells glow bright green when viewed under a laser-powered microscope, they were able to determine that deafening triggered rapid changes to the tiny connections between nerve cells, called synapses, which are only one thousandth of a millimetre across.
"I was very surprised that the weakening of connections between nerve cells was visible and emerged so rapidly -- over the course of days these changes allowed us to predict which birds' songs would fall apart most dramatically," Tschida said. "Considering that we were only tracking a handful of neurons in each bird, I never thought we'd get information specific enough to predict such a thing."
The implications are important for other aspects of embodiment as well, of course. We know the brain is plastic enough to accept a third arm when it is grafted into the nervous system we have done that many times, with monkeys to observe the results. Likewise, when a limb is amputated, the brain tends to pull neurons away from areas specialised in dealing with it, and assign them to other tasks.
This data, taken alongside what we already know about brain plasticity, strongly suggests that exposure to an embodied form other than the one a person was born with and extended sole use for some period of time, is going to have a profound effect on their natural embodiment should they choose to return to it.