Double-target experiments with a sequential neuro-controller

This paper describes some double-target experiments simulated with a neural system that drives a three-joint redundant limb. The system is composed of a controller (a sequential network) and a limb emulator. The system was trained to generate reaching movements of the limb towards targets specified as sensory stimuli; it was not trained to perform the double-target task itself. The experiments demonstrate that the system possesses the ability to carry out the double-target task, which requires generalization with respect to both initial limb posture and sensory stimulation. I performed the experiments under two different perceptual conditions: 1. /off switching of the two stimuli mporal overlap of the two stimuli. econd case refers to a hypothesis proposed by some experimental investigators about two different systems being involved in the programming of movements: a “where-system” that would build an internal representation of the target that shifts gradually to the new value, and a “when-system” that would start the motor program generator. The “where-system” would be able to account for the observed differences in path, while the “when-system” would be able to account for the reaction-time phenomenon. The case of temporal overlap of the two stimuli is a simulation of the “where-system.” I present a qualitative comparison of data generated by the neural system under conditions (1) and (2), namely 1. e endpoint paths and velocity profiles he patterns of muscular activation. t of the comparison show that in the case of temporal overlap of the stimuli, the controller can account for the variability in paths and the basic two-peak structure of the velocity profiles commonly observed in psychophysical experiments. In the case of on/off target-change, the behavior of the controller is, on the contrary, highly stereotyped.