Effects of sound direction on spectral and temporal processing in the frog midbrain

The effects of sound direction on two aspects of signal processing, spectral and temporal processing, were investigated by single-unit extracellular recording from the frog inferior colliculus (IC). The neural activities were evaluated from 3 to 5 sound directions within 180$\sp\circ$ of frontal field in the free-field stimulus system. The results from this study provide physiological evidence that sound direction can bolster the spectral and temporal selectivities of central auditory neurons.(1) The frequency selectivity of most IC neurons (90%) was direction dependent. For most of these neurons, the response bandwidths was narrower when sounds originated from ipsilateral azimuths. The directional influence on frequency selectivity was stimulus intensity dependent.(2) In response to amplitude-modulated stimuli, IC neurons displayed five classes of modulation transfer functions (MTFs): low-pass, high-pass, band-pass, multi-pass and all-pass. For 64% of IC neurons, the MTF class remained unchanged when sound direction was shifted from contralateral 90$\sp\circ$ to ipsilateral 90$\sp\circ$. However, the MTFs of more than half of these neurons exhibited narrower bandwidths when the loudspeaker was shifted to ipsilateral azimuths. Direction-dependent changes in the shapes of the tone and noise derived MTFs were different for the majority of IC neurons tested. It indicates that a spectrally-based and a temporally-based mechanism may be responsible for the observed results.(3) The majority of IC neurons (94%) showed some degree of binaural inhibition independent of the unit's binaural interaction pattern, i.e., whether they were designated as EE or EO. Neurons that exhibited the largest direction-dependent changes in frequency selectivity were typically those that displayed stronger binaural inhibition. Neurons that exhibited little or moderate direction-dependent effects on frequency selectivity were typically associated with those that displayed weak binaural inhibition.(4) Occlusion of the ipsilateral ear, which reduced the strength of binaural inhibition by this ear, abolished direction-dependent frequency selectivity.(5) The application of bicuculline, an antagonist of GABAa receptor, also abolished (completely or partially) the direction-dependent changes in frog tuning through presumably the removal of the binaural inhibition. In contrast, strychnine, an antagonist of glycine receptor, had no effect on the FTCs of IC units and on the firing activity of IC neurons.U of I OnlyETDs are only available to UIUC Users without author permissio