Patterns of neuronal activity underlying behavioral decisions

National Institute Of Mental Health

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Abstract

Cortical neurons fire in complex patterns of activity during behavior and cognition. In sensory regions of the cortex, animals' interactions with the sensory world evoke neural activity in the cortex. But not all patterns of cortical activity can be elicited by sensory stimuli. We study how non-sensory, artificially induced activity patterns can be used by animals to make behavioral responses. These studies shed light on the limits of cortical function, and how circuit properties like neural connections constrain the set of activity patterns the cerebral cortex can process. Trained animals (Emx1-Cre) report non-sensory optogenetic stimuli delivered to excitatory cells (Flex-ChrimsonR) in primary visual cortex. We find that animals improve their detection performance for such stimuli over the course of days, exhibiting learning in both sensitivity (more accurate responses to stimuli) and speed (faster responses to stimuli). Animals show a mean decrease in reaction time per session for constant stimulus intensity (p<0.01, Wilcoxon rank sum test against median of 0). Over many days, subjects improve their detection sensitivity by several orders of magnitude in stimulus intensity. Recent findings show that cortical responses to the optogenetic input also increase, suggesting a remodeling of recurrent connectivity that can increase amplification of the input as animals learn to use that input. The implications are broad: that cortical networks use recurrent connectivity to amplify certain patterns of input, and that this amplification can developed, via learning, in the adult. We have also examined how decisions are controlled at the level of interacting brain areas, by examining the roles of multiple brain areas in a sensory task. Primary visual cortex (V1) in the mouse projects to numerous brain areas, including several secondary visual areas, frontal cortex, and basal ganglia. While it has been demonstrated that optogenetic silencing of V1 strongly impairs visually guided behavior, it is not known which downstream areas are required for visual behaviors. We trained mice to perform a contrast-increment change detection task, for which substantial stimulus information is present in V1. Optogenetic silencing of visual responses in secondary visual areas revealed that their activity is required for even this simple visual task. In vivo electrophysiology showed that, although inhibiting secondary visual areas could produce some feedback effects in V1, the principal effect was profound suppression at the location of the optogenetic light. The results show that pathways through secondary visual areas are necessary for even simple visual behaviors. Together with other work in the lab, these findings contribute to our understanding of how cortical networks compute that is, how the connectivity within and between brain areas transforms inputs to process information, a function that is at the core of how brains work.