Dependence of the interaural phase difference sensitivities of inferior collicular neurons on a preceding tone and its implications in neural population coding

This study examined the sensitivities of the neuronal responses in the inferior colliculus (IC) to the interaural phase difference (IPD) of a preceding tone, and explored its implications in the neural-population representation of the IPD. Single-unit responses were recorded from the IC of anesthetized gerbils. The stimulus was a dichotic tone sequence with a common frequency (typically the unit’s best frequency) and level (10-20 dB relative to the threshold), consisting of a conditioner (200 ms) followed by a probe (50 ms) with a silent gap (5-100 ms) between them. The IPDs of the two tones were varied independently. The presence of a conditioner generally suppressed the probe-driven responses; the effect size increasing as the conditioner IPD approached the unit’s most responsive IPD. The units’ preferred IPDs were relatively invariant with the conditioner IPD. Two types of models were used to evaluate the effects of a conditioner on the IPD representation at the level of IC neural population. One was a version of the population vector model. The other was the hemispheric channel model, which assumed that the stimulus IPD is represented by the activities of two broadly tuned hemispheric channels. Both models predicted that, in the presence of a conditioner, the IPD representation would shift in a direction away from the conditioner IPD. This appears to emphasize the difference between the conditioner and the probe IPDs. The results indicate that in the IC, neural processes for IPD adapt to the stimulus history to enhance the representational contrast between successive sounds.