Plasticity in the neural coding of auditory space in the mammalian brain

Sound localization relies on the neural processing of monaural and binaural spatial cues that arise from the way sounds interact with the head and ext ernal ears. Neurophysiological studies of animals raised with abnormal sens ory inputs show that the map of auditory spare in the superior colliculus i s shaped during development by both auditory and visual experience. An exam ple of this plasticity is provided by monaural occlusion during infancy, wh ich leads to compensatory changes in auditory spatial tuning that tend to p reserve the alignment between the neural representations of visual and audi tory space. Adaptive changes also take place in sound localization behavior , as demonstrated by the fact that ferrets raised and tested with one ear p lugged learn to localize as accurately as control animals. In both cases, t hese adjustments may involve greater use of monaural spectral cues provided by the other ear. Although plasticity in the auditory space map seems to b e restricted to development, adult ferrets show some recovery of sound loca lization behavior after long-term monaural occlusion, The capacity for beha vioral adaptation is, however, task dependent, because auditory spatial acu ity and binaural unmasking (a measure of the spatial contribution to the "c ocktail party effect") are permanently impaired by chronically plugging one ear, both in infancy but especially in adulthood. Experience-induced plast icity allows the neural circuitry underlying sound localization to be custo mized to individual characteristics, such as the size and shape of the head and ears, and to compensate for natural conductive hearing losses, includi ng those associated with middle ear disease in infancy.