Anatomical Mapping and Optogenetic Projection-Targeting in the Rat Decision-Making Circuit

Abstract

Our place the world is inextricably linked to the decisions that we make everyday as a result of events that occur. The process of decision-making has long been a focus of study using well-controlled behavioral tasks in primates and rodents, which have revealed several candidate brain areas that are involved. The rodent model organism has proved particularly useful recently in its allowance of studies with large sample size, which have focused attention on the frontal orienting fields (FOF) within premotor cortex, the posterior parietal cortex (PPC), and the superior colliculus (SC) as potential centers of decision-making. These areas send and receive signals from each other and between other neural areas, but which signals are sent and their specific functions are not well understood. While inactivations of whole brain areas involved in decision-making have allowed basic description of the functions of these areas, few perturbations have been attempted to specific connections between brain areas during these tasks. Optogenetics has allowed the extensive targeting of projections in studies of other behavioral functions, and here we applied these techniques to the study decision-making. To facilitate the study of projections involved in this behavior, we first used retrograde and anterograde tracing experiments to determine a precise connectivity map between the FOF, PPC, SC, and possibly involved thalamic areas. The anatomical experiments showed the existence of several neuronal populations within each brain area that are distinct in the projections they send to other areas. Before this map was applied to study behavior, we showed and modeled the functionality in the brain of an optogenetic projection-targeted inhibitory technique using channelrhodopsin-2, a technique that has only been used in the peripheral nervous system. In this technique, Channelrhodopsin-expressing axon terminals are tonically optically stimulated, which we showed reduces post-synaptic action potentials that result from incoming pre-synaptic potentials entering the axon terminals. This technique together with the excitation of projections using pulsed optical stimulation was used to perturb the connection between bilateral FOFs in a small number of rats performing a memory-guided threshold discrimination task and an accumulation of evidence task. The early but not late excitation of these projections caused behavioral biases in task performance, but no significant effect was observed as a result of inhibition of these projections. While the latter absence of an effect must still be validated using more established projection-inhibition methods, these data suggest that the anatomical projection between the two FOFs can alter early activity in the decision-making neural circuit, but that this connection is not normally active in the tasks we studied. Overall, the circuit map constructed will guide future experimentation on the decision-making functions of the connections between brain areas. Indeed we used the map to guide the exploration of the functions of one described connection using an established and a newly modified optogenetic projection-targeting technique, which will continue to be used to determine how it is that we decide.