The brain has an amazing capacity for change, and one of the iconic examples is the enhancement of other senses when a sense is lost. For instance, if a person loses hearing, a heightened capacity for certain aspects of vision can be gained.
And while super vision sounds great, it involves restructuring of pathways in the brain. The exact way that this compensation happens has important implications for how to treat hearing loss, because it’s important to know where the pathways of auditory information get lost, blocked, or otherwise taken over by new pathways.
If a cochlear implant could help restore hearing, reversing the changes in the deaf brain to make use of native neural pathways could be the simplest way to go.
Stephen Lomber, professor of physiology and psychology at Western University, studies the anatomy behind sensory enhancements after hearing loss and how the deaf brain reorganizes.
Studying young cats with hearing loss, Lomber showed that deafness changes how the brain’s sensory cortex connects to the superior colliculus, a region of the midbrain that takes sensory information and helps a creature move to more favourable environments.
The brain regions that are normally involved in processing sound are affected, but not in the way that is generally assumed. The afferent neurons, ones that carry sensory information towards the auditory cortex, remain stable. That means that inputs from other senses, like vision, aren’t remapping to shuttle visual information to the auditory cortex for increased brain capacity.
Instead, it’s the patterns of efferent neurons, the ones that carry information out from the auditory cortex to the superior colliculus, that change.
“What we found was the normal projection from the auditory cortex to the superior colliculus is all but gone in the deaf model, and is instead replaced by a more dispersed pattern,” said PhD student and lead author Blake Butler. “It is a much different pattern of outputs than what we see in the case of normal hearing.”
So if this is where the neural pathways that accept and process sound get pruned back, then delivering sound information to the auditory cortex won’t restore the full pathway to hearing.
This important insight will help inform the best ways to re-engage the brain to restore the structure and function of the full auditory pathway to treat hearing loss.