EFFECTS OF SOUND DIRECTION ON THE PROCESSING OF AMPLITUDE-MODULATED SIGNALS IN THE FROG INFERIOR COLLICULUS

Single-unit recordings were made from 143 neurons in the frog (Rana p. pipiens) inferior colliculus (IC) to investigate how free-field sound direction influenced neural responses to sinusoidal-amplitude-modulat ed (SAM) tone and/or noise. Modulation transfer functions (MTFs) were derived from 3 to 5 sound directions within 180 degrees of frontal fie ld. Five classes of MTF were observed: low-pass, high-pass, band-pass, multi-pass, and all-pass. For 64% of IC neurons, the MTF class remain ed unchanged when sound direction was shifted from contralateral 90 de grees to ipsilateral 90 degrees. However, the MTFs of more than half o f these neurons exhibited narrower bandwidths when the loudspeaker was shifted to ipsilateral azimuths. There was a decrease in the cut-off frequency for neurons possessing low-pass MTFs, an increase in cut-off frequency for neurons showing high-pass MTFs, or a reduction in the p ass-band for neurons displaying bandpass MTFs. These results suggest t hat sound direction can influence amplitude modulation (AM) frequency tuning of single IC neurons. Since changes in periodicity of SAM tones alter both the temporal parameters of sounds as well as the sound spe ctrum, we examined whether directional effects on spectral selectivity play a role in shaping the observed direction-dependent AM selectivit y. The directional influence on AM selectivity to both SAM tone and SA M noise was measured in 62 neurons in an attempt to gain some insight into the mechanisms that underlie directionally-induced changes in AM selectivity. Direction-dependent changes in the shapes of the tone and noise derived MTFs were different for the majority of IC neurons (55/ 62) tested. These data indicate that a spectrally-based and a temporal ly-based mechanism may be responsible for the observed results.