Single neurons in the frog inferior colliculus exhibit direction-dependent frequency selectivity to isointensity tone bursts.

The effects of sound direction on frequency selectivity of inferior colliculus (IC) neurons were investigated by measuring the neuron's isointensity frequency responses (FRs) to tone bursts emanating from a free-field loudspeaker at several sound levels. The loudspeaker was rotated across the frontal field at 0 degrees elevation through 180 degrees of azimuth (from contralateral 90 degrees or C90 degrees to ipsilateral 90 degrees or I90 degrees). At each frequency, to assess the magnitude of response change with sound direction, the mean spike count obtained at an azimuth was compared to that at C90 degrees. The FR of most IC neurons (75/83 or 90%) was direction dependent. For most of these neurons, bandwidths of FRs were narrower when sounds originated from ipsilateral azimuths. Remarkably, with a change in sound azimuth, some segments of these FRs showed very distinct changes in shape, while other portions of the same FRs remained essentially unchanged. These narrow-band changes associated with restricted portions of the FR, were also exhibited by neurons with direction-dependent frequency-threshold characteristics (Gooler et al., 1993). Additionally, the most frequent direction-dependent change in the FRs occurred in a narrow frequency band around the units' best excitatory frequency.