Mixed representations of sound and action in the auditory midbrain

Abstract

Linking sensory input and its consequences is a fundamental brain operation. Accordingly, neural activity of neo-cortical and limbic systems often reflects dynamic combinations of sensory and behaviorally relevant variables, and these "mixed representations" are suggested to be important for perception, learning, and plasticity. However, the extent to which such integrative computations might occur in brain regions upstream of the forebrain is less clear. Here, we conduct cellular-resolution 2-photon Ca2+ imaging in the superficial "shell" layers of the inferior colliculus (IC), as head-fixed mice of either sex perform a reward-based psychometric auditory task. We find that the activity of individual shell IC neurons jointly reflects auditory cues and mices actions, such that trajectories of neural population activity diverge depending on mices behavioral choice. Consequently, simple classifier models trained on shell IC neuron activity can predict trial-by-trial outcomes, even when training data are restricted to neural activity occurring prior to mices instrumental actions. Thus in behaving animals, auditory midbrain neurons transmit a population code that reflects a joint representation of sound and action. Significance StatementNeurons in ICs superficial "shell" layers preferentially project to higher-order thalamic nuclei that are strongly activated by sounds and their behavioral consequences. This integrative computation is thought critical for a variety of behaviorally relevant functions, such as establishing learned sound valence. However, whether such "mixed representations" reflect unique properties of thalamocortical networks, or rather are inherited from afferent inputs, is unclear. We show that in behaving mice, many shell IC neurons are modulated by sounds and mices actions. Consequently, shell IC population activity suffices to predict behavioral outcomes even prior to the goal-directed action. Our data thus establish shell IC nuclei as a novel, ascending source of mixed representations for the thalamocortical system.