Sensorimotor structure of Drosophila larva phototaxis

Significance Small animals such as Drosophila provide an opportunity to understand the neural circuitry for complex behaviors from sensory input to motor output without gaps. Here, we define the algorithms for Drosophila larva phototaxis (i.e., the maps between sensory input and motor output) by quantifying the movements of individual animals responding to a battery of illumination conditions. Surprisingly, the distinct rules that define different components of the overall photosensory response begin to segregate at the first synapses after the photoreceptor cells. These results lay the foundation for mapping the circuits for phototaxis in the compact nervous system of the larva by first elucidating the algorithms that define behavior and then mapping these algorithms to specific circuit pathways. The avoidance of light by fly larvae is a classic paradigm for sensorimotor behavior. Here, we use behavioral assays and video microscopy to quantify the sensorimotor structure of phototaxis using the Drosophila larva. Larval locomotion is composed of sequences of runs (periods of forward movement) that are interrupted by abrupt turns, during which the larva pauses and sweeps its head back and forth, probing local light information to determine the direction of the successive run. All phototactic responses are mediated by the same set of sensorimotor transformations that require temporal processing of sensory inputs. Through functional imaging and genetic inactivation of specific neurons downstream of the sensory periphery, we have begun to map these sensorimotor circuits into the larval central brain. We find that specific sensorimotor pathways that govern distinct light-evoked responses begin to segregate at the first relay after the photosensory neurons.