Action-specific Decision-related Signals in Monkey Parietal and Human Motor Cortex

Decision-making is a hallmark of higher-order cognitive processing. The neural substrates of decision-making have been studied extensively in the monkey oculomotor system. It has been found that neurons in the oculomotor system modulate their activity by the variable on which animals base their decision, and also appear to encode the dynamics of the decision process. An outstanding question raised by this work is whether the decision-related findings in the oculomotor system generalize to other systems beyond the oculomotor system. This question is of critical importance to decision sciences as decision-related neural signals serve to construct and constraint models of choice behavior. However, the work on the neural basis of decision-making in the monkey oculomotor system cannot address this question because in this literature animals make choices using eye movements. In these tasks, the decision-related neuronal modulation could therefore specifically reflect the decision to make an eye movement, and not a generic, movement-independent decision. We addressed this question by engaging monkeys in decision tasks in which animals choose a target using one of two movement kinds---an eye movement, or an arm movement. While animals performed the task, we recorded neuronal activity in the parietal oculomotor system (lateral intraparietal area) and in the parietal somatomotor system (parietal reach region). We found that decision-related signals in the oculomotor system were observed particularly during choices made using an eye movement. In contrast, the decision signals in the somatomotor system were observed only for choices made using an arm movement. In parietal cortex, decision-related signals thus appear to be encoded for each movement in specific circuits. In addition to these findings in monkey parietal cortex, we found that human motor cortex also shows decision-related signals, and that these signals are observed specifically for choices made using a hand movement, and not for choices made using an eye movement. Thus, neural circuits that give rise to specific movements in monkeys and humans also encode higher-order cognitive, decision-related variables. These findings are in line with the view of embodied cognition in which cognitive processes do not function separately from but are instead closely tied to movement-related processes