ASA 124th Meeting New Orleans 1992 October

2pPP8. Neutral coding of virtual acoustic space.

J. F. Brugge

J. C. K. Chan

J. E. Hind

A. D. Musicant

P. W. F. Poon

R. A. Reale

Dept. Neurophysiol. and Waisman Ctr., Univ. of Wisconsin---Madison, Madison, WI 53706

The spectrum of a free-field sound source is transformed by the head, pinna, and ear canal before reaching the eardrum. Prominent spectral features in the free-field to eardrum transfer function (FETF) vary systematically with changes in sound-source direction. Inner ear transduction mechanisms subsequently filter the transformed sound before the information is encoded in the discharge rates of auditory nerve fibers. This information may then be modified by neural interactions at all levels if the central auditory system. An earphone delivery system has been implemented that incorporates FETFs to reproduce the spectral properties of a free-field click. This simulation of sound sources in different directions creates an acoustic virtual space (AVS). Responses of auditory nerve fibers and primary auditory cortical neutrons of anesthetized cats were obtained to a set of clicks in AVS. The stimulus set was derived from a family of 3600 FETFs obtained as part of the related study [Musicant et al., J. Acoust. Soc. Am. 87, 757 (1990)]. For both the auditory nerve and cortex, vigorous neutronal responses were obtained to clicks from a restricted number of virtual space locations. An azimuth-versus-elevation map of evoked spike activity derived from responses to clicks in AVS is termed a ``virtual space receptive field'' (VSRF). The VSRF of a single auditory nerve fiber is well matched by the spatial pattern of sound pressure for frequencies near a fiber's CF, reflecting the combined filtering action on free-field sound by both the cochlea and the external ear. Thus, the auditory nerve array transmits faithfully information about the spatial distribution of sound pressure over the full range of frequencies to which the animal is sensitive. This information is preserved by certain neurons at the level of the cortex. For these cells, monaural VSRFs are similar in shape to those of auditory nerve fibers having the same CF. The VSRF of most cortical neurons is modified by binaural interactions.