Neuroplasticity of the Auditory System

Plasticity occurs at various time scales. In addition to neuronal spike-firing adaptation at a relatively short time scale, central neural responses can be decreased over much longer time periods, e.g., as in habituation. In particular, spectrotemporal receptive fields might have slower timescales of adaptation on the order of minutes to hours. A common hypothesis is that tinnitus results from a maladaptive imbalance between excitation and inhibition as a result of downregulation of inhibitory amino acid neurotransmission in the central auditory pathway. However, the effects of neural plasticity underlying noise exposure do not need to be maladaptive because the chronic tinnitus percept may be the result of experience-dependent plasticity with a percept engrained in memory as a result of continuous attention to it. Plastic changes in the adult auditory system do occur following noise exposure, from traumatic to environmental, causing permanent hearing loss, temporary loss, or no audiometric hearing loss. Tonotopic map changes are not required for tinnitus to occur; however, tonotopic maps in people with hearing loss without tinnitus were significantly different from those with normal hearing, whereas the maps of people with hearing loss and tinnitus were not. These results corroborate that map reorganization is a characteristic of hearing loss, not of tinnitus. It further raises the possibility that the spontaneous activity underlying tinnitus may prevent cortical map changes.