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 co lliculus (IC) neurons were investigated by measuring the neuron's isoi ntensity frequency responses (FRs) to tone bursts emanating from a fre e-field loudspeaker at several sound levels. The loudspeaker was rotat ed across the frontal field at 0 degrees elevation through 180 degrees of azimuth (from contralateral 90 degrees or C90 degrees( )to ipsilat eral 90 degrees or I90 degrees). At each frequency, to assess the magn itude of response change with sound direction, the mean spike count ob tained at an azimuth was compared to that at C90 degrees. The FR of mo st IC neurons (75/83 or 90%) was direction dependent. For most of thes e neurons, bandwidths of FRs were narrower when sounds originated from ipsilateral azimuths. Remarkably, with a change in sound azimuth, som e segments of these FRs showed very distinct changes in shape, while o ther portions of the same FRs remained essentially unchanged. These na rrow-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 frequen t direction-dependent change in the FRs occurred in a narrow frequency band around the units' best excitatory frequency.