Binaural Properties

Many auditory cortical neurons are sensitive to differences in the way a sound reaches the two ears. These interaural differences are primarily of two types: intensity and time of arrival. Both these properties arise as a consequence of the distribution of sounds in space and the acoustics of how such sounds arrive at the two ears. Consequently, these differences are the cues used by the auditory nervous system to compute the spatial position of sounds. Physiological responses that correspond to these two features have been described in several different locations in the auditory core, belt, and parabelt regions. Many cortical neurons are very sensitive to slight differences in interaural intensity, whereas others respond differentially to interaural timing differences. These neurons tend to respond preferentially to sound sources located in the contralateral spatial hemifield, although there are also many units that show broad spatial tuning, extending into both hemifields.

Although there is evidence that neurons sensitive to similar spatial positions are located within cortical columns, there appears to be no topographic representation of space in the auditory cortex. Thus, unlike the visual system in which retinal position, and hence space, is coded by retinotopic mechanisms, the representation of auditory space is not topographically organized. However, recent evidence suggests that auditory space may be represented by population codes based on neural activity rates. Cortical neurons are also sensitive to auditory motion and sometimes respond preferentially to one direction of motion in the azimuthal plane.



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