A high dimensional quantification of mouse defensive behaviours reveals enhanced diversity and stimulus specificity

Instinctive defensive behaviours, consisting of stereotyped sequences of movements and postures, are an essential component of the mouse behavioural repertoire. Since defensive behaviours can be reliably triggered by threatening sensory stimuli, the selection of the most appropriate action depends on the stimulus property. However, since the mouse has a wide repertoire of motor actions, it is not clear which set of movements and postures represent the relevant action. So far this has been empirically identified as a change in locomotion state. However, the extent to which locomotion alone captures the diversity of defensive behaviours and their sensory specificity is unknown. To tackle this problem we developed a method to obtain a faithful 3D reconstruction of the mouse body that enabled to quantify a wide variety of motor actions. This higher dimensional description revealed that defensive behaviours are more stimulus-specific than indicated by locomotion data. Thus, responses to distinct stimuli that were equivalent in terms of locomotion (e.g. freezing induced by looming and sound) could be discriminated along other dimensions. The enhanced stimulus-specificity was explained by a surprising diversity. A clustering analysis revealed that distinct combinations of movements and postures, giving rise to at least 7 different behaviours, were required to account for stimulus-specificity. Moreover, each stimulus evoked more than one behaviour revealing a robust one-to-many mapping between sensations and behaviours that was not apparent from locomotion data. Our results indicate that diversity and sensory specificity of mouse defensive behaviours unfold in a higher dimensional space spanning multiple motor actions.