Sound elicits stereotyped facial movements that provide a sensitive index of hearing abilities

Abstract

Sound elicits rapid movements of muscles in the face, ears, and eyes that protect the body from injury and trigger brain-wide internal state changes. Here, we performed quantitative facial videography from mice resting atop a piezoelectric force plate and observed that broadband sounds elicit rapid, small, and highly stereotyped movements of a facial region near the vibrissae array. Facial motion energy (FME) analysis revealed sensitivity to far lower sound levels than the acoustic startle reflex and greater reliability across trials and mice than sound-evoked pupil dilations or movement of other facial and body regions. FME tracked the low-frequency envelope of sounds and could even decode speech phonemes in varying levels of background noise with high accuracy. FME growth slopes were disproportionately steep in mice with autism risk gene mutations and noise-induced sensorineural hearing loss, providing an objective behavioral measure of sensory hyper-responsivity. Increased FME after noise-induced cochlear injury was closely associated with the emergence of excess gain in later waves of the auditory brainstem response, suggesting a midbrain contribution. Deep layer auditory cortex units were entrained to spontaneous facial movements but optogenetic suppression of cortical activity facilitated - not suppressed - sound-evoked FME, suggesting the auditory cortex is a modulator rather than a mediator of sound-evoked facial movements. These findings highlight a simple involuntary behavior that is more sensitive and integrative than other auditory reflex pathways and captures higher-order changes in sound processing from mice with inherited and acquired hearing disorders.