Physiological studies of binaural tone-in-noise detection in the inferior colliculus of awake rabbit

Abstract

With identical noise maskers presented to both ears, human listeners have lower tone detection thresholds when the tone is presented out-of-phase between the ears (N0Sπ) rather than in-phase (N0S0). The threshold difference is called the binaural masking level difference (BMLD). Human listeners have BMLDs up to at least 4 kHz, but previous studies to understand neural mechanisms have only used low-frequency tones. Here, we recorded single-neuron responses from the inferior colliculus in awake rabbit to a wide range of overall noise levels and tone frequencies near the neuron’s characteristic frequency. Maskers were 1/3-octave gaussian noise centered at the tone frequency. Neural thresholds at each noise level were estimated from average-rate responses at signal-to-noise ratios of -12 to +8 dB based on receiver-operating-characteristic analysis. Neural thresholds were similar across noise levels and were pooled together for further analysis. BMLDs of all recorded neurons with measurable thresholds for both N0S0 and N0Sπ ranged up to +17 dB (i.e., a substantial masking release for the N0Sπ condition). Across the population of neurons tuned to different frequencies, the largest BMLDs at each frequency decreased with increasing frequency, consistent with human psychophysical studies, but with a more gradual slope.