SOUND DIRECTION MODIFIES THE INHIBITORY AS WELL AS THE EXCITATORY FREQUENCY TUNING CHARACTERISTICS OF SINGLE NEURONS IN THE FROG TORUS SEMICIRCULARIS (INFERIOR COLLICULUS)

Single-unit recordings were made from the frog inferior colliculus to determine whether or not the direction-dependent sharpening of a unit' s free-held excitatory frequency-threshold curve (FTCe) was accompanie d by a broadening of its inhibitory frequency-threshold curve (FTCi). To determine the FTCi, a two-tone-suppression paradigm was employed. T he unit's FTC(i)s and FTC(e)s were collected at three azimuths: contra lateral to the recording site, ipsilateral to the recording site, and frontal midline. The result showed that: (1) most inferior colliculus neurons (95%) displayed two-tone suppression, (2) the majority (54%) o f neurons displayed stronger two-tone-suppression leading to broader F TC(i)s when the sound was presented from the ipsilateral side than fro m the contralateral side, (3) for some neurons, the borders of the FTC (e)s and FTC(i)s were closely aligned, and this juxtaposition persiste d at all sound azimuths (namely, when a change in sound direction prod uced a narrowing of a unit's FTCe, its FTCi was broadened concomitantl y). For the remaining neurons, however, direction-dependent sharpening of the FTCe was not accompanied by an increase in two-tone-suppressio n. The neural mechanisms that underlie the direction-dependent changes in the FTC(e)s and FTC(i)s are discussed.