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Cues for Sound Localization Are Encoded in Multiple Aspects of Spike Trains in the Inferior Colliculus

¹Department of Neurobiology, University of Pittsburgh and Department of Statistics, Carnegie Mellon University, Pittsburgh, Pennsylvania; and ²Center for Hearing and Balance and Department of Biomedical Engineering, Johns Hopkins University; Baltimore, Maryland

Submitted 11 June 2007; accepted in final form 27 January 2008

To localize sound, information from three cues—interaural timing differences (ITDs), interaural level differences (ILDs), and spectral notch cues (SNs)—must be properly integrated. The inferior colliculus (IC) receives convergent input from neurons encoding all three cues. Using virtual space stimuli and information theoretic techniques, we investigated the coding of the various localization cues in single neurons of the IC under different encoding schemes. Here we focus on the analysis of information encoded by first-spike latency, in comparison to previous results on discharge rate and ongoing spike timing. The results show that the localization cues converge to different degrees in particular neurons. ITD information is conveyed most strongly by spike rate, with small amounts of independent information in latency and ongoing spike timing. ILD information shows a similar pattern, with larger mutual information values for all three cues. For these cues, ongoing spike timing does not typically contribute independent information over that captured by a joint rate/first-spike latency code. SNs are coded by both rate and first-spike latency, but ongoing spike timing significantly enhances their representation in a best frequency–dependent manner, as long as the temporal envelope of the stimulus can be used in the decoder. The differential coding of the localization cues suggests that information about multiple cues could be multiplexed onto the responses of single neurons.