The brain has a remarkable capacity to adapt to changes in sensory inputs and to learn from experience. However, the neural circuits responsible for this flexible processing remain poorly understood. A new study from the King Group in collaboration with Associate Professor Ed Mann and Associate Professor Ed Boyden from MIT identifies the key part of the brain that drives auditory perceptual learning and enables us to adapt to hearing loss.
In essence, their study shows that the auditory cortex plays a critical role in learning to adapt to the abnormal auditory spatial cues that result from hearing loss in one ear. Training leaves a memory trace, which facilitates adaptation to a second period of hearing loss. However, evidence suggests that after the initial learning, the recovery in sound localisation accuracy is independent of cortical activity. "This type of rapid plasticity is likely to be extremely important when people experience hearing loss or other sensory impairments, and particularly when their hearing is partially restored via a hearing aid or cochlear implant." (Professor Andrew King).
The methodology behind these conclusions is particularly innovative. The team, which includes Associate Professor Victoria Bajo Lorenzana and Dr Fernando Nodal, used optogenetic tools to silence the activity of the auditory cortex. They silenced the auditory cortex in ferrets with a laser green light at the same time as the animals heard a sound. The light activated an proton pump called ArchT in the neural membrane that has been expressed after viral transfection of the ArchT gene. As a consequence, when the light is on, the neurons in the auditory cortex were shown to be hyperpolarized and therefore silenced.
Crucially, the results demonstrate the essential role of primary sensory cortices in perceptual learning, and could inform how hearing aids or cochlear implants are developed in future. Furthermore, according to first author Prof Bajo Lorenzana, the significance of these results does not only relate to hearing loss:
The wider significance is not only related to the auditory field, but potentially to other senses such as vision or touch. There are implications for the design and generation of new prosthesis that can change the activity of the cerebral cortex and therefore promote sensory learning. - A/Prof Bajo Lorenzana
The full publication Silencing cortical activity during sound-localization training impairs auditory perceptual learning is available to read here.