Exploring behavioral circuits with holographic optogenetics and network imaging.

Understanding how individual neurons within a circuit process sensory information and influence broader network dynamics to drive behavior is a longstanding goal of neuroscience. The larval zebrafish, Danio rerio, is an excellent model organism for behavioral circuit investigation, providing genetic and optical access and a variety of visually evoked behaviors. Psychophysics can investigate features of visual stimuli crucial for behavior. Together with two-photon calcium imaging this approach can locate brain regions involved in processing these stimuli, for example retinorecipient areas such as the optic tectum. These sensory processing regions send information downstream towards motor related areas, including the nucleus of the medial longitudinal fasciculus (nMLF). The nMLF is a midbrain nucleus, shown to be active during visually evoked behavior. Optogenetics is a powerful technique for exploring circuit function, especially in motor areas. Light from an optic fiber can be used to activate a population of neurons in the nMLF, inducing behavior, and together with ablations shows a crucial role for this area in postural control. These approaches can find the role of a population of neurons, however understanding the functional impact of individual neurons within a circuit is a greater challenge. To increase the resolution of circuit investigation, we have designed a holographic light-shaping system that enables activation with single neuron precision. A protocol was developed that combines this approach with simultaneous imaging and behavior, to relate behavioral output to functional properties of the circuit. This system creates a combined approach that can extract functional information from individual neurons in behavioral circuits.