CORTICAL REPRESENTATION OF AUDITORY SPACE

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Clinical and animal studies indicate that lesions of the auditory cortex can result in deficits in the ability to localize a sound. Efforts to account for these observations in physiological terms have failed to identify a cortical representation of sound location, and the current view of the tonotopically organized primary auditory pathway to the cortex is inconsistent with a simple one- to-one map of auditory space. A midbrain structure, the superior colliculus, does contain such map in which the location of a sound source is represented by the position of a single focus of maximally activated neurons, and it is puzzling that a similar map has not been described in the cortex. One possibility is that the auditory map in the superior colliculus is a product of the sensorimotor integration which takes place in the colliculus and, thus, is unique to that structure. In the cortex, the location of a sound might be represented by characteristic patterns of activation comprising multiple active areas rather than single foci of activity. An alternative possibility is that auditory space is mapped in the cortex, but previous efforts to find it have focused on the wrong areas of the cortex or have used inappropriate stimuli. The proposed experiments will use broad band stimuli presented in a free sound field to characterize the patterns of cortical activity associated with specific locations. Particular attention will be given to the largely-unexplored non-tonotopic areas, including the second auditory area (A2), which appear a priori to be the most likely areas to contain a spatial representation. Results of these experiments will indicate whether auditory space is represented in the cortex by a one-to- one map of space or by some other, more distributed, pattern of activity. The auditory cortex is a key element of the temporal lobe, which is itself the cortical substrate for many communicative processes. An understanding of functional organization in the auditory cortex could lead to the development of diagnostic procedures for evaluation of temporal lobe pathology. Auditory spatial representation is particularly relevant in that sense, since source location is one of the few basic features of a sound which apparently is not represented bilaterally. On a broader scale, the proposed research will contribute to the basic understanding of cortical function which is essential for the development of appropriate therapeutic responses to brain injury and disease.