A Sensorimotor Model for Computing Intended Reach Trajectories

The presumed role of the primate sensorimotor system is to transform reach targets from retinotopic to joint coordinates for producing motor output. However, the interpretation of neurophysiological data within this framework is ambiguous, and has led to the view that the underlying neural computation may lack a well-defined structure. Here, I consider a model of sensorimotor computation in which temporal as well as spatial transformations generate representations of desired limb trajectories, in visual coordinates. This computation is suggested by behavioral experiments, and its modular implementation makes predictions that are consistent with those observed in monkey posterior parietal cortex (PPC). In particular, the model provides a simple explanation for why PPC encodes reach targets in reference frames intermediate between the eye and hand, and further explains why these reference frames shift during movement. Representations in PPC are thus consistent with the orderly processing of information, provided we adopt the view that sensorimotor computation manipulates desired movement trajectories, and not desired movement endpoints.Author Summary: Does the brain explicitly plan entire movement trajectories or are these emergent properties of motor control? Although behavioral studies support the notion of trajectory planning for visually guided reaches, a neurobiologically plausible mechanism for this observation has been lacking. I discuss a model that generates representations of desired reach trajectories (i.e., paths and speed profiles) for point-to-point reaches. I show that the predictions of this model closely resemble the population responses of neurons in posterior parietal cortex, a visuomotor planning area of the monkey brain. Several aspects of population responses that are puzzling from the point of view of traditional sensorimotor models are coherently explained by this mechanism.