The Influence of Declarative Processes upon Human Motor Cortex Physiology

Abstract

Skilled movements require the ability to efficiently extract and manipulate incoming sensory information relating to our body and environment to inform motor output. To facilitate efficient sensory to motor transformations humans have developed highly tuned cognitive abilities featuring constructs such as attention and working memory. Such cognitive constructs support the development of declarative knowledge pertaining to skilled actions. Yet, our understanding of how declarative knowledge shapes the function and reorganization of subconscious procedural knowledge about a skill is limited. Importantly, understanding how declarative strategies may influence motor cortical physiology is an essential step towards understanding why some skills benefit from explicit knowledge while others do not. The purpose of this dissertation was to determine how declarative functions, specifically verbal working memory, shape procedural motor control through modulation of sensory afference. Chapter 1 reviews the role of the motor and somatosensory cortices in motor behavior. The role of attention in the activation of the sensorimotor cortex is then described. Finally, the role of verbal working memory in motor performance is discussed. Previous research looked at the role of working memory from a behavioral perspective, but the studies in this thesis investigated the neural substrates, and notably the sensory afference of the interaction of working memory and control of movement. Chapters 2 through 4 detail a series of studies investigating how working memory load and verbal instructions alter motor cortex physiology and plasticity. Specifically, Chapter 2 demonstrates that engaging verbal working memory processes can change the potential for plasticity in the motor cortex, a substrate of the procedural motor system. Chapter 3 demonstrates that working memory acts upon the motor cortex through intracortical circuits that are distinct from other cognitive functions such as attention. Finally, Chapter 4 extends these results from a model where working memory is a distractor to working memory as a task-relevant construct. Overall, the findings from the studies described in this dissertation demonstrate that working memory has the ability to influence motor cortex physiology through circuits distinct from the circuits affected by attention. Further, the way in which working memory is employed can have important modulatory effects in the motor cortex, which could then impact the acquisition and execution of motor skills. These results lay the groundwork for future studies investigating whether declarative strategies may control and limit procedural learning such that the procedural system serves to perfect the optimal kinematics and dynamics for the imposed strategy even if the imposed strategy results in sub-optimal performance.