The role of target position in smooth pursuit deceleration and termination

Subjects smoothly pursued a target moving horizontally at 15 deg/s. After p ursuit for 1 s, the target jumped 3 deg ahead of the fovea. At the moment o f the jump, target velocity became 0 and 'effective visual feedback' assume d a value of either 0 (target retinally stabilized), - 0,2, - 0.4, or - 1.0 (target fixed in space). With 0 visual feedback the eye continued to move smoothly at a moderate velocity, an apparent response to target position re lative to the fovea. When negative visual feedback was present eye velocity decreased. With - 0.2 and - 0.4 feedback, this decrease was not a simple e xponential, but often consisted of an initial fast decrease followed by slo wer decrease. With - 1.0 feedback, eye velocity quickly decreased in an app roximately exponential manner, and stopped. We were able to simulate these pursuit responses using a simple model of the pursuit system. key features of the model are: (a) a target-velocity channel whose output decreases with target offset from the fovea, and whose gain switches from high to low as pursuit velocity approaches zero; (b) a target-position channel with a satu ration non-linearity at 1-3 deg; and (c) a positive feedback loop with gain of less than 1.0. All of these features are essential to simulate the purs uit responses, especially with visual feedback values of - 0.2 and - 0.4. O ur results and model suggest that target position serves as an important st imulus in guiding smooth pursuit as pursuit velocity decreases, and especia lly during pursuit termination. (C) 2001 Published by Elsevier Science Ltd. All rights reserved.