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-field excitatory frequency-threshold curve (FTCe) was accompanied by a broadening of its inhibitory frequency-threshold curve (FTCi). To determine the FTCi, a two-tone-suppression paradigm was employed. The unit's FTCis and FTCes were collected at three azimuths: contralateral 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%) of neurons displayed stronger two-tone-suppression leading to broader FTCis when the sound was presented from the ipsilateral side than from the contralateral side, (3) for some neurons, the borders of the FTCes and FTCis were closely aligned, and this juxtaposition persisted at all sound azimuths (namely, when a change in sound direction produced a narrowing of a unit's FTCe, its FTCi was broadened concomitantly). For the remaining neurons, however, direction-dependent sharpening of the FTCe was not accompanied by an increase in two-tone-suppression. The neural mechanisms that underlie the direction-dependent changes in the FTCes and FTCis are discussed.