NU Student SpeakerKiran BhattacharyyaGraduate StudentProfessor Malcolm MacIver LabNorthwestern UniversityHost: Professor Malcolm MacIver

Title:"Larval Zebrafish Perform Graded Evaluations of Threat to Modulate Escape Responses and Mauthner Cell Activity"Abstract:To avoid becoming a meal, mobile prey must decide if, where, and when to escape. A common strategy for escaping predation is the recruitment of `giant fiber' pathways, which minimize the number of synaptic connections between sensation and action. While these pathways expedite signal propagation and ensure quick reflexes, the powerful and relatively stereotyped reactions they produce can inflict energetic and behavioral costs. Consequently, a fundamental decision facing many animals is whether a threat warrants activation of giant fiber pathways. We used naturalistic looming stimuli combined with high-speed kinematic analysis and in vivo imaging of Mauthner cells in larval zebrafish, a giant fiber neuron in an aquatic vertebrate, to explore two possibilities for how risk may be calculated to decide if the giant fiber should be activated. 1) Risk assessment is simply a threshold function. In this case an object approaching at any speed that gets close enough is perceived as an urgent threat and triggers a Mauthner-associated response; or 2) risk assessment is a more graded process that accounts for approach rate. In this case, more slowly approaching objects are perceived as less threatening and result in alternate, less costly strategies, like no response or a less urgent one. More broadly, the story of the role of Mauthner versus non-Mauthner mediated behavior is a microcosm of a larger logic at play which offers a trade-off between deciding to execute fast andrelatively inflexible responses mediated by few synapses or slower and more flexible responses. Thus, our work investigates a more general relationship between sensing, movement, time, and behavioral control in animals with implications for decision-making and flexible behavior.