CODING FOR AUDITORY SPACE IN THE NUCLEUS OF THE BRACHIUM OF THE INFERIOR COLLICULUS IN THE FERRETT. Jan W.H. Schnupp and Andrew J. King. University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK.
APStracts 4:159N, 1997.
ABSTRACT
The nucleus of the brachium of the inferior colliculus (BIN) projects topographically to the deeper layers of the superior colliculus (SC), which contain a two-dimensional map of auditory space. In this study, we have used broad-band stimuli presented in the free field to investigate how auditory space is represented in the BIN of the ferret. Response latencies and temporal firing patterns were comparable to those in the SC and both properties showed some variation with stimulus location. We obtained spatial response profiles at two sound levels (5-15 dB and 25-35 dB above unit threshold). A large proportion of azimuth profiles (41% in the supra-threshold condition, 80% in the near threshold condition) presented a single peak, indicating that they were tuned to single regions in space. For some of these units, the preferred speaker position varied considerably with sound level. The remaining units showed predominantly either broad "hemifield", or spatially ambiguous "bilobed" response profiles. At supra-threshold sound levels the preferred azimuths of the tuned cells were topographically ordered along the rostro- caudal axis of the BIN, although this representation is considerably more scattered than that in the SC. In contrast to the SC, we observed no systematic variation in the distribution of near-threshold best azimuths, which were instead concentrated around the inter-aural axis in the contralateral hemifield. The azimuth tuning of individual units in the BIN was generally broader at both sound levels than that in the SC. Many units were also tuned for the elevation of the sound source (48% for supra-, 77% for near threshold stimulation), but there was no evidence for topographic order in the distribution of preferred elevations within the BIN. These results suggest that the BIN sends inputs to the SC which are already selective for sound azimuth and elevation, and which show some degree of topographic order for sound azimuth. These inputs are then presumably sharpened and their topography refined by a mechanism that is likely to involve convergence of other inputs and activity-dependent fine tuning of terminal connections, to result in a precise two-dimensional map of auditory space in the SC. 

Received 27 January 1997; accepted in final form 16 July 1997.
APS Manuscript Number J074-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997