Binaural inhibition is important in shaping the free-field frequency selectivity of single neurons in the inferior colliculus. Gooler, David M., Jinghua Xu, and Albert S. Feng. Department of Molecular and Integrative Physiology, and the Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, IL 61801.
APStracts 3:0138N, 1996.
SUMMARY AND CONCLUSIONS
1. We have previously shown that under free-field stimulation in the frontal field, frequency selectivity of the majority of inferior colliculus (IC) neurons became sharper when the loudspeaker was shifted to ipsilateral azimuths. These results indicated that binaural inhibition may be responsible for the direction-dependent sharpening of frequency selectivity. To test the above hypothesis directly, we investigated the frequency selectivity of IC neurons under several conditions: i) monaural stimulation using a semi-closed acoustical stimulation system, ii) binaural stimulation dichotically also using a semi-closed system, iii) free-field stimulation from different azimuths, iv) free-field stimulation when the ipsilateral ear was monaurally occluded (coated with a thick layer of petroleum jelly which effectively attenuated acoustic input to this ear). 2. The binaural interaction pattern of 98 IC neurons of northern leopard frogs (Rana pipiens pipiens) were evaluated of which there were 34 EE and 64 EO neurons. The majority of IC neurons (92 of 98) showed some degree of binaural inhibition (i.e., showing diminished response when the ipsilateral and contralateral ears were stimulated simultaneously) whether they were designated as EE or EO; these IC neurons were thus classified as EE-I or EO-I. Neurons were classified as exhibiting strong inhibition if the ILD function showed a pronounced response decrement, i.e., a decrease of ³ 50% of the response to monaural stimulation of the contralateral ear. Those neurons that showed smaller response decrements (decrease was ³25% but < 50%) were designated as showing weak inhibition. Most of these EE-I and EO-I neurons (n=68) showed strong binaural inhibition. 3. In agreement with results from our earlier studies, frequency threshold curves (FTCs) of IC neurons were altered by sound azimuth. Independent of binaural interaction pattern, most IC neurons (59 of 98) showed a narrowing of the FTC as sound direction was changed from contralateral 90[acute]i (c90[acute]i) to ipsilateral 90[acute]i (i90[acute]i). IC neurons that exhibited the largest direction-dependent changes in frequency selectivity were typically those that displayed stronger binaural inhibition. Occlusion of the ipsilateral ear, which reduced the strength of binaural inhibition by this ear, abolished direction-dependent frequency selectivity. 4. FTCs of IC neurons that exhibited little to moderate direction-dependent effects on frequency selectivity were typically associated with neurons that displayed weak binaural inhibition. Associated with this weak binaural inhibition, central neural responses under monaural occlusion also displayed only small effects; the FTCs were only slightly broader than those derived in the intact condition, and as before, the experimental manipulation resulted in abolishment of direction-dependent frequency selectivity. 5. In contrast to most IC neurons which showed direction-dependent narrowing of the FTC, about one-third (34 of 98) of IC neurons studied showed a broadening of the FTC when sound direction was shifted to ipsilateral azimuths. Interestingly, for 90% of these 34 neurons, monaural occlusion resulted in narrowing of the bandwidth at each azimuth instead of broadening of the FTC bandwidth. We have evidence to suggest that this direction-dependent broadening is actually a consequence of a truncation or loss of the tip of the FTC derived at c90[acute]i which results from strong binaural inhibition. 6. To compare the frequency threshold tuning in response to monaural stimulation of each ear with free-field FTCs we measured FTCs for each of the 34 EE neurons to independent contralateral and ipsilateral stimulation. FTCs derived from ipsilateral monaural stimulation were significantly narrower than those resulting from contralateral monaural stimulation, independent of a neuron's direction-dependent changes in frequency selectivity. These results demonstrated that the narrower excitatory FTCs to ipsilateral monaural stimulation cannot be the primary contributor to the sharpening of FTCs in some EE cells, and broadening in other EE cells, in response to stimulation from ipsilateral azimuths in the free field. 7. These results support our hypothesis that binaural interactions, particularly binaural inhibition, play a role in the sharpening of frequency selectivity in the frog IC. 

Received 7 August 1995; accepted in final form 18 June 1996.
APS Manuscript Number J511-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 4 July 96