Visual feedback in the control of reaching movements David Knill and Jeff Saunders

Two types of motor control Ballistic Feedback control

Motor planning Physical plant Target state Initial system state Motor commands New System states Ballistic control

Feedback control Motor planning Physical plant Target state Initial System state Motor commands New System states Sensory system

Baseball examples Ballistic control –Hitting –Throwing Feedback control –Running to catch a ball

Target

EOG signal Finger position Eye-hand coordination

EOG signal Finger position Movement start

Speed profile for pointing movement

Motor planning Physical plant Target state Initial system state Motor commands New System states Ballistic control

Questions Does the visuo-motor system use visual information about target on-line to update motor program?

Feedback control Motor planning Physical plant Target state Initial System state Motor commands New System states Sensory system

Questions Does the visuo-motor system use visual information about target on-line to update motor program? Does the visuo-motor system use continuous feedback from the hand during a movement to control the movement

Questions Does the visuo-motor system use visual information about target on-line to update motor program? Does the visuo-motor system use continuous feedback from the hand during a movement to control the movement –What visual information is used?

Question Does the visuo-motor system use visual information about target on-line to update motor program?

Question Does the visuo-motor system use visual information about target on-line to update motor program? Yes - for detectable target motion (e.g. catching a moving object)

Question Does the visuo-motor system use visual information about target on-line to update motor program? Yes - for detectable target motion (e.g. catching a moving object) ?? - for imperceptible changes in target position

Experiment Perturb position of target during a saccade (imperceptible change) Does motor system correct for change in target position?

Perturbed trials Unperturbed trials

Results Automatically correct for imperceptible target perturbations. Correct for perturbations –Perpendicular to movement –In direction of movement Reaction time = 150 ms Smooth corrections

Question Does the visuo-motor system use continuous feedback from the hand during a movement to control the movement?

Hypotheses Classic model –Ballistic control during fast phase of motion –Feedback control during end, slow phase of motion Continuous model –Feedback control throughout movement

Arguments against continuous feedback Visuo-motor delay (~100 ms) is too large for effective control during fast phase. Removing vision of hand early in motion does not affect end-point error. Corrections to target perturbations are just as strong with or w/o vision of hand.

Experiment Imperceptibly perturb the position of the hand during a movement and measure motor response. Add perturbations early and late in pointing movement. Measure reaction time to perturbations.

Reaction time predictions End-phase feedback Continuous feedback Late perturbation Early perturbation

Sample finger paths

Autoregressive model Baseline (unperturbed) trajectories Perturbed trials

Subject 1: Trajectories for early perturbed trials Positive perturbations Negative perturbations

Subject 1: Trajectories for early perturbed trials Positive perturbations Negative perturbations

Subject 1: Trajectories for late perturbed trials Positive perturbations Negative perturbations

Subject 1: Trajectories for late perturbed trials Positive perturbations Negative perturbations

Subject 2: Trajectories for early perturbed trials Positive perturbations Negative perturbations

Subject 2: Trajectories for late perturbed trials Positive perturbations Negative perturbations

Perturbation weight function for in-line perturbations

Perturbation weight function for perpendicular perturbations

Conclusions Visuomotor system uses directional error signal for feedback control? Position / speed error in direction of movement is not effective feedback signal? Why? –Position along path blurred by motion –Insensitivity to acceleration along direction of motion

Question What visual information about hand does visuomotor system use –Position error? –Motion error? –Position and motion?

Conclusions Visuomotor system uses continuous visual feedback to control reaching movements. Feedback signals include positional error. Feedback signals include motion error. System is approximately linear.