Neural representations of movement planning within the human prehension system

Object manipulation is central to our daily interactions with the environment. Failing to select, prepare or perform correct prehension movements results in dramatic limitations for the affected individual. Whereas we begin to have a better understanding of the neural mechanisms underlying the execution of object-directed movements, less is known about how exactly our brain makes the plan for action. Previous studies examining movement planning suggested that neuronal populations in parieto-frontal areas contain information about upcoming movements moments before they actually take place. However, such studies typically used experiments in which the participant was instructed about the movement to plan with visual or auditory cues, making it difficult to disentangle movement planning from the processing of cues and stimulus- response (S-R) mapping. In our first functional magnetic resonance imaging (fMRI) study (Study I), we compared an instructed condition with a free-choice condition that allowed participants to select which prehension movement to perform: a condition in which the task was not tied to specific external cues (i.e., no direct S-R mapping). Using multi-variate pattern analysis (MVPA), we found contralateral parietal and frontal regions containing abstract representations of planned movements that generalize across the way these movements were generated (internally vs externally). The majority of previous studies were based on delayed-movement tasks, which introduce brain responses unrelated to movement preparation. Consequently, whether these findings would generalize to immediate movements remained unclear. In our second fMRI study (Study II), we directly compared delayed and immediate reaching and grasping movements. Using time-resolved MVPA allowed us to reveal shared representations for delayed and non-delayed movement planning in human primary motor cortex and examine how movement representations unfolded throughout the different stages of planning and execution. Overall, our findings expand previous understanding of the regions implicated in movement planning and offer new insights into the dynamics of the human prehension system.