Project Summary In the mammalian brain, action, perception, and cognition arise from the coordination of many neurons working in concert. For decades, neuroscientists have observed these multi-neuron patterns of neural activity, and developed complex ‘population codes’ to describe how a pattern correlates with a behavior. These population codes make up our core understanding of how the brain works. However, simply observing activity cannot distinguish between activity that drives a behavior (i.e., causality) vs that which only reports that a behavior happened (i.e., correlation). Even modern optogenetic or chemogenetic approaches struggle to assess causality in complex interconnected systems, like the mammalian cortex, and are incapable of testing the importance of a pattern of activity, where information is encoded in the timing and relative firing rates of neurons. This has led to calls for revised methodologies to study causality in neural systems. Here we leverage a unique and novel approach, that we developed, to test the causal link between neural activity and specific actions. This new approach enables the previously impossible task of recreating population activity de novo, writing different numbers of action potentials into adjacent cells with millisecond precise timing. We will use this technique to ask a fundamentally different class of questions than ever previously possible and create a framework determining the causal role of population codes on behaviors. Vector Optogenetics is the most advanced form of optogenetic stimulation yet, and the only approach able to reproduce population activity. Evolving from multiphoton optogenetics, vector optogenetics allows a user to specify not just which cells are activated but how many action potentials they will fire and when – anywhere within the field of view of a conventional two-photon microscope and in a behaving mouse. In this proposal, we use this technique to address several critical questions in motor cortex. Exploring, quantitatively, the nuance of what patterns of activity drive specific behaviors, and other cells. We explore possible important features of a population code (e.g., identity of activated cells, relative firing rates, synchrony, or sparsity of a pattern), as well as popular theories of the importance of ‘low dimensional’ activity spaces. By combining vector optogenetics with electrophysiology in the striatum, we further explore how patterns of activity drive downstream nuclei – addressing questions about inter-area communication and the initiation of actions. My experience as a systems neuroscientist, combined with being the inventor of these optical approaches, makes me ideally suited to execute this plan, and overcome any obstacle. This proposal promises to revolutionize how we study and validate models of cortical interaction and neural coding. Far from being limited to motor control alone, these findings will have far-reaching implications – helping to create next-generation neural prosthetics, treat complex neurological diseases, and change the way we study complex systems.

Project Narrative: Complex, time varying, patterns of neural activity are thought to give rise to all of the behavioral complexity that animals engage in. However, technical limitations have made it impossible to recreate these patterns de novo in vivo, and thus we poorly understand how these patterns causally drive different movements. In this proposal, we use a unique and novel technique to drive precise, multi-neuronal, patterns of activity in awake animals to understand what features of neural activity drive action and ultimately to understand neural population coding.