DEVELOPMENTAL AND ADULT PLASTICITY IN THE AUDITORY PATHWAY

Nodal FR, Bajo VM, King AJ

Department of Physiology, Anatomy and Genetics. Parks Road OX1 3PT Oxford.

To resolve the spatial location of a sound source the mammalian brain computes different cues that depend ultimately on the shape of the pinna (monaural cues) and size of the head (binaural cues). Changes in the values of those localization cues, because of morphological changes of the structures responsible, pinna or head, or because alterations on the input at each ear, would lead neural plastic changes to attain an accurate sound localization performance.

During early postnatal development, due to pinna and head growth, changes in spatial sensitivity neurons in the superior colliculus can be observed. Progressively individual neurons show a sharper sensitivity and are organized to form a map of the acoustic space. However, auditory projections to the SC that will determine its spatial sensitivity are topographically organized before the onset of the hearing, suggesting that electrophysiological changes can be observed even with a relative anatomical stability.

During adulthood changes in the acoustic transmission, e.g. otitis media, might alter the normal input balance between both ears causing a degradation of the localization performance. We have shown behaviourally that in conditions of monaurally reduced acoustic transmission, training and therefore feedback is essential for plastic changes to occur. Using this model of adult plasticity we have established the role of the cerebral cortex in the adult plasticity. Lesions or reversible inactivation of different cortical areas reduced or even prevented the expression of adult plasticity. Work supported by the Wellcome trust.