Receptive fields and binaural interactions for virtual-space stimuli in the cat inferior colliculus

Receptive fields and binaural interactions for virtual-space stimuli in the cat inferior colliculus. J. Neurophysiol. 81: 2833-2851, 1999. Sound local ization depends on multiple acoustic cues such as interaural differences in time (ITD) and level (ILD) and spectral features introduced by the pinnae. Although many neurons in the inferior colliculus (IC) are sensitive to the direction of sound sources in free field, the acoustic cues underlying thi s sensitivity are unknown. To approach this question, we recorded the respo nses of IC cells in anesthetized cats to virtual space CVS stimuli synthesi zed by filtering noise through head-related transfer functions measured in one cat. These stimuli not only possess natural combinations of ITD, ILD, a nd spectral cues as in free field but also allow precise control over each cue. VS receptive fields were measured in the horizontal and median vertica l planes. The vast majority of cells were sensitive to the azimuth of VS st imuli in the horizontal plane for low to moderate stimulus levels. Two-thir ds showed a "contra-preference'' receptive field, with a vigorous response on the contralateral side of an edge azimuth. The other third of receptive fields were tuned around a best azimuth. Although edge azimuths of contra-p reference cells had a broad distribution, best azimuths of tuned cells were near the midline. About half the cells tested were sensitive to the elevat ion of VS stimuli along the median sagittal plane by showing either a peak or a trough at a particular elevation. In general receptive fields for VS s timuli were similar to those found in free-field studies of IC neurons, sug gesting that VS stimulation provided the essential cues for sound localizat ion. Binaural interactions for VS stimuli were studied by comparing respons es to binaural stimulation with responses to monaural stimulation of the co ntralateral ear. A majority of cells showed either purely inhibitory (Br) o r mixed facilitatory/inhibitory (BF&I) interactions. Others showed purely f acilitatory (BF) or no interactions (monaural). Binaural interactions were correlated with azimuth sensitivity: most contra-preference cells had eithe r BI or BF&I interactions, whereas tuned cells were usually BF. These corre lations demonstrate the importance of binaural interactions for azimuth sen sitivity. Nevertheless most monaural cells were azimuth-sensitive, suggesti ng that monaural cues also play a role. These results suggest that the azim uth of a high-frequency sound source is coded primarily by edges in azimuth receptive fields of a population of no-sensitive cells.