The previous lecture dealt with one of the hidden facets of deafness, namely the synaptopathies triggered by overexposure to noise. We saw that such synaptopathy, which involves the synapse of inner hair cells (IHCs), often goes undetected on the tonal audiogram. A normal or transiently elevated auditory threshold masks a degeneration of auditory neurons (mainly low-threshold neurons with a high rate of spontaneous discharge), which will progress inexorably.
This fourth lecture focused on damage to the auditory cortex. First, we discussed the plasticity of the auditory system, especially studied at the level of the auditory cortex, which plays a major role in deriving benefit from any mode of restoration of peripheral hearing, including via the cochlear implant. We then discussed a study on another unsuspected hidden facet of hereditary deafness, which the study of animal models has enabled us to discover only recently. This study highlights for the first time the existence of molecules expressed in both the cochlea and the auditory cortex, the loss of which causes not only profound deafness linked to peripheral damage, but also intrinsic damage to the auditory cortex, masked by the peripheral damage, and which we have characterized.
Following this example, we wondered about the scope of these observations and their possible generalization, through the following questions:
- Is there a neurogenetic basis for the development and physiology of the auditory cortex?
- Are the audiogenic seizures (epileptic seizures triggered by exposure to intense sound) observed in hereditary hearing impairment in animals a way of understanding how the balance between inhibitory and activating neurons in the auditory cortex is established and maintained?
- Are peripheral and cortical disorders caused by the same gene damage related in terms of cellular mechanisms?
- How does a sensory system evolve, with certain molecules playing essential but undoubtedly different roles at peripheral and central levels? What advantages might this bring (coordinated evolution, release of compensatory brain plasticity)?
- Is this sharing of key proteins at peripheral and central levels a general feature of sensory systems?
Finally, the medical implications of these data, the evaluation of the same anomalies in humans and their consequences for the auditory rehabilitation that must accompany any peripheral restoration of hearing were discussed.
