The multifunctional nature of motor cortex

The cerebral cortex is responsible for neural functions ranging from basic sensory processing to complex decision making. However, the underlying neural processes and the precise function of some cortical areas are not well understood. For humans the cortex is invaluable, providing us with our most powerful cognitive traits. For some species, the cortex can be removed leaving only minor deficits. Stark contrasts like these blur the overall function and importance of cortex. Recent advancements in recording and analysis technology present us with a unique opportunity to search for neural processes previously impossible to find. Recent work has found cortical regions in mice that do more than their functional name implies, in contrast to the specialized cortical regions in higher order species. In this study we explore the hidden functions of mouse motor cortex and elaborate on its role in the whisker system. The mouse whisker system is traditionally divided into sensory regions and motor regions, with sensory cortex (vS1) and motor cortex (vM1) sitting on the border. The functional divisions between sensory and motor cortex have recently blurred, both regions able to drive movements and encode sensory information. Whisker motor cortex, specifically, exhibits disparate functions, making it an ideal place to study multirole cortex. Here we take advantage of advanced neural recording and analysis techniques and uncover a motor cortex that acts as a sensory and possibly a higher order cortical region as well.In chapter 1, I provide an overview of how cortical regions were defined and then summarize how modern research is blurring the lines between functionally defined cortical areas. I then introduce sensory processing in the mouse whisker system with a focus on motor cortex, a cortical region where its function is increasingly blurring, and then describe how my research explored non-motor functionality in a motor region. In chapter 2, I present my first first-author publication where we determined if somatosensory cortex integrates sensory information over short or long timescales in order to estimate “mean” variables. In this work I first use neural decoding to quantify how well each neuron represents pieces of sensory information and find that some neurons correlate with choice. Chapter 3 is work that I collaborated on, we determined how multiple sensors contribute to the receptive field of individual neurons and the broader population. We discovered a map of sensory space distributed across somatosensory cortex and determined the map was dependent on neurons integrating information from multiple sensors in parallel. Chapter 4, we explore whether somatosensory cortex is necessary for a whisker dependent discrimination task and further determine what arrangement of sensors are required for task completion. Finally, in Chapter 5 I present my main project investigating vibrissae motor cortex. Here I study the sensory responses present in motor cortex, quantifying vM1 sensory tuning for the first time, and ultimately determining that vS1 does not drive the activity as previously thought. Incredibly, vM1 is not required for whisker movements in general but is required during demanding whisker dependent contexts, such as a whisker discrimination task, affecting both choice, whisker movements, and onset of lick response. Finally, in Chapter 6, I summarize what this tells us about sensory processing, propose some ideas for future research, and discuss how modern tools can enable us to find hidden functionality.

Advisor: Adesnik, Hillel