Binaural cues occurring in natural environments are frequently time varying, either from the motion of a sound source, or through interactions between the cues produced by multiple sources. Yet a broad understanding of how the auditory system processes dynamic binaural cues is still lacking. Here, we directly compared neural responses in the inferior colliculus (IC) of unanesthetized rabbits to broadband noise with time-varying interaural time differences (ITD) with responses to noise with sinusoidal amplitude modulation (SAM) over a wide range of modulation frequencies. Based on prior research, we hypothesized that the IC, one of the first stages to exhibit tuning of firing rate to modulation frequency, might use a common mechanism to encode time-varying information in general. Instead, we found weaker temporal coding for dynamic ITD compared to amplitude modulation and stronger effects of adaptation for amplitude modulation. The differences in temporal coding of dynamic ITD compared to SAM at the single-neuron level could be a neural correlate of "binaural sluggishness", the inability to perceive fluctuations in time-varying binaural cues at high modulation frequencies, for which a physiological explanation has so far remained elusive. At ITD-variation frequencies of 64 Hz and above, where a temporal code was less effective, noise with a dynamic ITD could still be distinguished from noise with a constant ITD through differences in average firing rate in many neurons, suggesting a frequency-dependent tradeoff between rate and temporal coding of time-varying binaural information.
- inferior colliculus
- auditory motion
- binaural sluggishness
- amplitude modulation
- Copyright © 2016, Journal of Neurophysiology