Prediction and a stationary, structured visual background influence the dynamics of the smooth-pursuit offset in humans

 It is still not clear whether the transition from pursuit eye movements to fixation is mediated by the same system that initiates pursuit, or whether another system, a specialized fixation system, is responsible. To investigate this question we measured smooth-pursuit eye movements and smooth-pursuit termination in five normal subjects using both predictable and unpredictable step-ramp stimuli (velocities 10° and 20°/s) in front of a homogeneous and a structured visual background in order to compare the profile of eye velocity under these different conditions. With the predictable and/or structured visual background there was a gradual transition of eye velocity toward zero. In contrast, with the unpredictable stimulus in front of a homogeneous background, eye velocity during the offset was characterized by an overshoot (on the average, 2.2±1.0°/s for 10°/s ramps) before eye velocity settled at zero. Under this condition, steady-state velocity gain and the deceleration of the offset were significantly higher than during the other paradigm with the same target velocity. The latency of the pursuit offset was significantly shorter when a predictable stimulus was used. The duration of the offset did not depend on the experimental condition used. These findings imply that the pursuit onset and offset have some similarities and may be mediated by the same oculomotor system. Received: 6 February 1998 / Accepted: 8 July 1998