Locating sound sources such as prey or predators is critical for survival in many vertebrates. Terrestrial vertebrates achieve this by measuring the time delay and amplitude difference of sound waves arriving on each ear. For fish however, the faster speed of sound in water and the proximity of the two ears make such cues useless. Yet, directional hearing has been confirmed behaviorally, and the mechanisms have puzzled researchers for decades. Theoretical studies attempted to explain this remarkable ability, proposing that acoustic pressure and particle velocity signals must be measured separately and then be compared. However, the locus of this computation is unknown and its neuronal and biophysical mechanisms remain obscure. This is because most vertebrate brains and inner ears are highly opaque, rendering them inaccessible to systemic optical investigation.
Addressing this challenge, we recently identified the tiny glass fish Danionella as a unique vertebrate model for neuroscience. Danionella are among the smallest living vertebrates and are transparent throughout their lifespan. Despite having the smallest known vertebrate brain, they display a rich set of complex behaviors, including acoustic communication, illustrating the ethological relevance of hearing for this species. Building on our experience with acoustics and brain-wide imaging, we will exploit this model to (1) image the vibrational response of the inner ear, (2) study the neuronal activity of the sensory organs, and (3) follow the neuronal integration of sensory signals by circuits across the brain with functional imaging. These measurements will, for the first time, allow us to study the entire acoustic processing chain from acoustic stimulus, via mechanical transmission, to brain-wide neuronal integration at single cell resolution. If successful, they will constitute a major step for our understanding of hearing mechanisms in fish and illuminate the evolutionary origin of vertebrate audition.
