Task dependent coarticulation of movement sequences

Abstract

Classical theories propose that multiple goals of a sequence are specified simultaneously in the motor system, enabling each elements’ execution to be influenced by others in the sequence, a process known ascoarticulation. However, recent neural recordings suggest independent execution within circuits generating motor commands, with each individual goal separately specified to the controller. Consequently, the manner in which movement sequences are produced remains unclear. In a two-reaches sequence task, we tested the idea that both coarticulation and separation are related to flexible feedback control. More precisely, simulations revealed that an optimal feedback controller planning for multiple reaches at once can produce both separated and coarticulated sequences based on constraints imposed at intermediate goals. Human experiments confirmed that the coarticulation of sequential reaches was flexibly modulated by task instructions, matching simulation results closely. Moreover, the location of the second goal impacted long-latency muscle stretch responses to mechanical perturbations applied prior to the first goal, indicating that sequence combination was expressed in the sensorimotor network mediating fast feedback control. Overall, our results uncover the role of long-latency feedback pathway as a flexible control policy, supporting efficient sequence production without the need to individually specify each target one-by-one to the controller.SignificanceFrom preparing coffee to executing a tennis serve, our nervous system combines multiple elementary actions into longer sequences. Traditional research has explored whether these elements are combined in low-level control circuits for rapid execution or independently recalled from higher-level circuits. Surprisingly, the role of feedback control in the production of movement sequences has remained unexplored. Here, using simulations of optimal feedback control alongside human experiments, we shed light on the crucial role of task-dependent feedback control in shaping how sequences manifest within the motor system. Our findings reveal that the combination of sequence elements can result from flexible processing of multiple goals within the motor system involved in fast feedback control.