Tracker effector-specific and motor planning signals in human frontal and parietal cortices : relevance for goal-directed action and neural prosthetics

Abstract

Delayed response tasks and functional magnetic resonance imaging were employed to map the neural architecture underlying goal-directed action planning in the human brain, examine interactions between motor planning and effector-specification (arm vs. eye), and explore other related processes and variables. Studies in healthy human subjects revealed a frontoparietal network of brain areas selectively involved in motor planning compared to control processes. Nodes within this network were characterized based on their functional properties, including effector-specificity. In frontal cortex, the dorsal premotor and supplementary motor areas preferentially encoded motor plans for arm reaches compared to saccadic eye movements, while the inferior frontal eye field was identified based on its selective involvement in eye movements. In parietal cortex, a similar dissociation of arm- and eye-specific brain areas was observed in the superior lobule. A medial branch of the intraparietal sulcus preferentially encoded eye movements, in contrast to more anterior medial, and posterior medial, portions of the intraparietal sulcus that preferentially encoded arm movements. Additionally, motor planning areas were engaged during voluntary shifts of spatial attention and during working memory for visual cues when these cues were relevant for upcoming movements. Many of these brain areas also encoded the type of arm movement (reach vs. point), arm posture, and limb contralaterality, a property that co-varied with increasing ties to motor execution. Also, a comparison of real vs. imagined arm movements revealed that the imagined arm could be used as a proxy for the real arm to drive activity in motor planning areas. Another study completed in healthy control and spinal cord-injured subjects demonstrated the preservation of a relatively normal pattern of brain activity after the brain is functionally disconnected from the limbs. The degree of preservation of healthy/normal BOLD activity levels, particularly in the medial parietal cortex, strongly correlated with clinical and behavioral variables and could predict functional motor improvements in spinal cord-injured subjects six months later. These studies contribute to our understanding of the representation of goal-directed action planning in the human brain, elucidate human-monkey interspecies functional homologies, and have implications for the design and implantation of cortical neural prosthetic devices.