Contributions of Sound Localization Cues to Azimuth Tuning in the Auditory Midbrain

Neurons in the central nucleus of the inferior colliculus exhibit spatial receptive fields due to underlying neural sensitivity to acoustic cues that covary with sound source location, including interaural time difference (ITD), interaural level difference (ILD), and spectral shape and average acoustic gain within each ear. While neural sensitivity to individual cues is generally known, what remains unknown is how individual cues contribute to a neuron's receptive field when all cues are combined and how these contributions vary with the neuron's characteristic frequency (CF). In the present study, broadband noise stimuli were presented to awake rabbits of both sexes in virtual acoustic space using the rabbit's own head-related transfer functions. Contributions of each cue to the azimuth tuning curve (i.e., the receptive field within the front horizontal plane) were assessed by manipulating transfer functions to fix some cues while allowing others to vary naturally with azimuth. On average, firing rates of low-CF neurons (<2.8 kHz) were determined by the combination of ITD and one or more of ILD and individual-ear acoustic gains, whereas rates of high-CF neurons (>2.8 kHz) were largely determined by either ILD; contralateral-ear spectrum and ILD; or a combination of ITD and non-ITD cues, depending on whether source location was ipsilateral to the recording site, contralateral, or straight ahead, respectively. The CF transition coincided with the acoustic frequency above which the range of ILDs rapidly expands. Despite CF-dependent differences in the contributions of localization cues, rate sensitivity to azimuth was the same, on average, across the tonotopic axis.