Early stages in a sensorimotor transformation

TLDR

Later stages, such as the transformation from kinematic (position) to dynamic (force) parameters or to levels of muscle activation, are beyond the scope of the present model. The study presents a model of several early stages of the sensorimotor transformations involved in targeted arm movement. The authors conducted psychophysical experiments where subjects pointed to remembered target locations in 3‑D space, and the model attributes distance errors to a linear approximation in transforming shoulder‑centered target parameters into arm orientations, placing visual information into a reference frame that can be combined with kinesthetic data about initial arm orientations. Subjects consistently undershot distal targets, indicating that errors arise in the sensorimotor transformation rather than distance perception; the error was most consistent when evaluated in a spherical coordinate system based at the right shoulder, suggesting a transformation from retinocentric to shoulder‑centered representation, and the combination of initial and final arm orientation representations may underlie movement direction signals recorded in motor cortex.

Abstract

Abstract We present a model for several early stages of the sensorimotor transformations involved in targeted arm movement. In psychophysical experiments, human subjects pointed to the remembered locations of randomly placed targets in three-dimensional space. They made consistent errors in distance, and from these errors stages in the sensorimotor transformation were deduced. When subjects attempted to move the right index finger to a virtual target they consistently undershot the distance of the more distal targets. Other experiments indicated that the error was in the sensorimotor transformation rather than in the perception of distance. The error was most consistent when evaluated using a spherical coordinate system based at the right shoulder, indicating that the neural representation of target parameters is transformed from a retinocentric representation to a shoulder-centered representation. According to the model, the error in distance results from the neural implementation of a linear approximation in the algorithm to transform shoulder-centered target parameters into a set of arm orientations appropriate for placing the finger on the target. The transformation to final arm orientations places visually derived information into a frame of reference where it can readily be combined with kinesthetically derived information about initial arm orientations. The combination of these representations of initial and final arm orientations could give rise to the representation of movement direction recorded in the motor cortex by Georgopoulos and his colleagues. Later stages, such as the transformation from kinematic (position) to dynamic (force) parameters, or to levels of muscle activation, are beyond the scope of the present model.