Gesture planning integrating knowledge of the motor plant's dynamics: A literature review from motor control and speech motor control

In order to study how dynamical properties of the motor plant are accounted for in movement planning, an optimal motor planning hypothesis was proposed in the late eighties. It suggested that forward and inverse internal models of the dynamics of the motor plant are learned in the brain, and that these models are used to compute and minimize a cost along the movement sequence. In this paper, the original form of this optimal motor planning hypothesis is described in detail in order to introduce the main issues that have since been investigated in relation to this hypothesis. A selected number of experimental studies carried out in the context of arm and speech movements are presented and analyzed. It is shown that while models of the motor plant’s dynamics seem to be used in arm movement planning, no evidence currently exists in support of this hypothesis for speech movement. In general for both arm and speech movements, recent findings suggest that representations of the motor plant’s dynamics in the brain could be spatially local or dedicated to a specific task. This raises the question of whether these representations are based on models or on motor memory. These findings are discussed more specifically in the context of speech motor control. Updates of the original hypothesis are given and some alternative hypotheses are briefly introduced.