Sound direction modifies the inhibitory as well as the excitatory frequency tuning characteristics of single neurons in the frog torus semicircularis (inferior colliculus)

Abstract

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 (FTC_e) was accompanied by a broadening of its inhibitory frequency-threshold curve (FTC_i). To determine the FTC_i, a two-tone-suppression paradigm was employed. The unit's FTC_is and FTC_es 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 FTC_is when the sound was presented from the ipsilateral side than from the contralateral side, (3) for some neurons, the borders of the FTC_es and FTC_is 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 FTC_e, its FTC_i was broadened concomitantly). For the remaining neurons, however, direction-dependent sharpening of the FTC_e was not accompanied by an increase in two-tone-suppression. The neural mechanisms that underlie the direction-dependent changes in the FTC_es and FTC_is are discussed.