The role of target position in smooth pursuit deceleration and termination.

Subjects smoothly pursued a target moving horizontally at 15 deg/s. After pursuit for 1 s, the target jumped 3 deg ahead of the fovea. At the moment of the jump, target velocity became 0 and 'effective visual feedback' assumed 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 relative 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 exponential, but often consisted of an initial fast decrease followed by slower decrease. With -1.0 feedback, eye velocity quickly decreased in an approximately 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 saturation 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 pursuit responses, especially with visual feedback values of -0.2 and -0.4. Our results and model suggest that target position serves as an important stimulus in guiding smooth pursuit as pursuit velocity decreases, and especially during pursuit termination.