Interaural delay sensitivity to tones and broad band signals in the guinea-pig inferior colliculus

We have measured the sensitivity of 243 low-frequency cells in the central nucleus of the guinea pig to the interaural time delay of best frequency (BF) tones, wideband noise and synthetic vowels. The highest rate of firing for the majority of cells occurred when the stimulus to the contralateral ear arrived 100-400 microseconds before that to the ipsilateral ear. The best delays for tones and noise measured in the same cell were highly correlated. In contrast to the tone delay functions, the majority of the delay functions obtained in response to wideband signals did not cycle, but were characterized by a single dominant peak or trough. The response frequency calculated from the delay functions to the vowel often did not correspond to the unit's BF, suggesting that the unit was responding to a component close to the first formant frequency (730 Hz) of the vowel. Phase-locked responses, on the other hand, only occurred to the fundamental frequency of the vowel (100 Hz) and not to higher frequency components. The responses to delayed tone and noise signals in the guinea pig are very like those obtained in the cat and other mammals. The similarity of the range of best delays for the guinea-pig with those reported for the cat, despite the difference in head size in these two species, suggests that the sensitivity to interaural delays reflects the properties of the binaural pathways rather than an adaptation to the delays normally experienced by the animal.