ABSTRACT
We will investigate the problem of how the brain generates reaching movements. We propose that the brain uses a geometric planner (GP) as an intermediate stage between perception and action. The role of GP is to provide spatial paths independently from the movement dynamics. This implies that earlier sensory inputs to areas in the perceptual system are sufficient to formulate a temporal estimate of movement based on distances rather than having to rely on later feedback from the actual execution of the action. We characterize the behavior of GP with a simple differential equation that links the task space with an abstract representation of the biomechanics to encode postures. Our objective function is the notion of distance defined by the goals of a task, so it changes as a function of the action’s purpose. This scalar function that operates in the posture and the task spaces gives the error from task completion. Its gradient provides the direction that changes the arm posture so that the system gets closer to the goals. The paths thus generated are length minimizers (time-invariant, geodesics) with respect to a task specific distance measure. We have shown that if the equation runs recursively it provides the spatial path useful to estimate movement duration. If the equation unfolds iteratively the guiding geometric signal at each step can be paired with the execution dynamics for on-line error correction. We recorded from neurons in the Parietal Reach Region (PRR) concurrently with arm behavior. The results suggest involvement of the PRR cells in the distance-based (geometric) planning of spatio-temporal aspects of a pending trajectory.

SPEAKER BIO:

Elizabeth Torres is a postdoctoral fellow in computational and neural systems at the California Institute of Technology (Caltech). She received her Ph.D. in cognitive science from UCSD in 2001, and her thesis explored a "theoretical framework for the study of sensory-motor integration." Torres is interested in the study of goal-directed movement, in particular, natural arm movements in the context of reach for and grasping an object, and her work integrates both behavioral and neurophysiological perspectives. Born in Havana City, Cuba, Torres began university there but later transferred to San Jose State University, where she earned her B.S. in mathematics and computer science.