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.
Schnurmacher Institute for Vision Research, State University of New York, State College of Optometry, 33 West 42nd Street, New York, NY 10036, USA.
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D005246:Feedback / Q000502:physiology; D005260:Female; D005403:Fixation, Ocular / Q000502:physiology; D006801:Humans; D008297:Male; D008954:Models, Biological; D009039:Motion Perception / Q000502:physiology*; D011698:Pursuit, Smooth / Q000502:physiology*