UC San Diego

Producing movements is a fundamental output of the brain. Over many millennia the brain has undergone significant evolutionary changes to accommodate the increasingly complex ways that we can move and interact with our environment. The mammalian cortex is one such structure that has evolved to provide animals with the capacity to perform highly skilled movements. Among the many cortical areas, the primary motor cortex (M1) has been recognized as a necessary structure for learning to produce such skilled movements. Alternatively, it is also known that M1 is not necessary for executing all movements. The consensus of the field is that whether or not a movement is dependent on M1 is determined entirely by the relative complexity of the movements. Therefore movements are divided into two categories: complex skilled movements that are M1 dependent, and simple movements that can be executed independent of M1. However there has been no work attempting to understand how the state of learning affects a movement’s dependency on M1 for execution. For example, is it possible for movements that are initially M1 dependent to become M1 independent after learning? The second chapter of this dissertation investigates this variable role of M1 in movement execution by combining a novel two-direction forelimb movement task with in vivo two-photon calcium imaging and optogenetic perturbation. Briefly, we found that once movements become sufficiently learned and reproducible, they are indeed no longer dependent on M1 for execution. In conjunction with this result, we found that neurons in M1 are more active and their activity was more consistent for the less-learned, less-stereotyped movements.

Following the acquisition of skills and movements, how do we choose which movements to make given the environmental conditions and understanding of possible outcomes? The third chapter of this thesis investigates this question using a novel memory guided sensorimotor discrimination task, in vivo two-photon calcium imaging and optogenetic perturbation to identify the posterior parietal cortex as a possible locus assembling choice-outcome history prior to decision output.