Neurons in the inferior colliculus use multiplexing to encode features of frequency-modulated sweeps

Within the central auditory pathway, the inferior colliculus (IC) is a critical integration center for ascending sound information. Previous studies have shown that many IC neurons exhibit receptive fields for individual features of auditory stimuli, such as sound frequency, intensity, and location, but growing evidence suggests that some IC neurons may multiplex features of sound. Here, we used in vivo juxtacellular recordings in awake, head-fixed mice to examine how IC neurons responded to frequency-modulated sweeps that varied in speed, direction, intensity, and frequency range. We then applied machine learning methods to determine how individual IC neurons encode features of FM sweeps. We found that individual IC neurons multiplex FM sweep features using various strategies including spike timing, distribution of inter-spike intervals, and first spike latency. In addition, we found that decoding accuracy for sweep direction can vary with sweep speed and frequency range, suggesting the presence of mixed selectivity in single neurons. Accordingly, using static receptive fields for direction alone yielded poor predictions of neuron responses to vocalizations that contain simple frequency changes. Lastly, we showed that encoding strategies varied across individual neurons, resulting in a highly informative population response for FM sweep features. Together, our results suggest that multiplexing sound features is a common mechanism used by IC neurons to represent complex sounds.