The Coding of Sound Localization Cues in the Inferior Colliculus
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The auditory system uses three cues to decode sound location: interaural time differences (ITDs), interaural level differences (ILDs), and spectral notches (SNs). Initial processing of these cues is performed in several auditory brainstem nuclei that send projections to neurons of the inferior colliculus (IC). This work addresses how information about these different sound localization cues is integrated into the responses of single neurons of the IC. Virtual space techniques were used to create stimulus sets varying in two soundlocalization parameters each. By manipulating pairs of cues within a stimulus set, the relative coding of each cue could be compared. Using a variety of information theoretic methods, the mutual information between the localization cues and the neural response was quantified under the assumption of several different encoding schemes. The results show that the three localization cues are best represented by different codes. ITD information is conveyed by spike rate alone, and is contained only in low frequency neurons. ILD information is best represented by a joint rate/first spike latency code. The coding of SNs changes with the best frequency (BF) of the neuron. Low BF neurons represent SNs by the timing of spikes distributed throughout the response, where the spike times are locked to particular stimulus features. High BF neurons, on the other hand, represent SNs by spike rate and, to a lesser extent, first spike latency. The differential coding of the localization cues suggests that information about multiple cues could be multiplexed onto the responses of single neurons. iii These results have implications for how the localization cues might be integrated into a percept of sound location. The accuracy with which each cue can contribute to the overall location percept changes depending on sound conditions, such as the frequency content of the stimulus, temporal characteristics of the stimulus, or the reverberant qualities of the environment. That the cues are differentially coded in the IC implies that the brain may have access to the individual cues at cortical levels, and the weight with which each cue contributes to the location percept could be tailored to the stimulus, environment, and perceptual task.