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J. B. Jacobs, L. F. Dell'Osso; Smooth Pursuit Errors in Normals First Discovered in Infantile Nystagmus Syndrome. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1998.
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On occasion, patients with infantile nystagmus syndrome (INS) have reported a transient inability to accurately fixate/track a moving target. In previous work, our behavioral ocular motor system (OMS) model predicted, and patient data confirmed, this effect to be due to the relative timing between the attempt to initiate pursuit and the occurrence of a foveating or braking saccade in their INS cycle. To examine whether this was an intrinsic property of saccade-pursuit interaction or limited to INS, we attempted to replicate this finding in normal subjects by presenting them with a novel stimulus whose timing was adjusted to mimic this critical condition seen in the INS patients.
We constructed a new ocular motor stimulus, the "step-pause-ramp." It was presented to five normal subjects as combinations of horizontal saccades with randomized delays (0 [i.e.,Rashbass condition], 150, 200 or 250 ms) before the onset of pursuit, and randomized direction of pursuit relative to the initial refixation saccade (either "with" or "against"). Their eye movements were analyzed using custom-written software in the MATLAB environment. Positive results were determined to be instances where the eye position lagged target position by more than one foveal radius (0.5 deg) for more than the expected interval (~150ms) needed to generate a corrective saccade to eliminate the error. We also performed simulations of these paradigms using our OMS model, which is capable of simulating normal ocular motor behavior as well as that of INS.
Model simulations predicted that when target-velocity onset occurred just before, during, or just after the initial refixation saccade, pursuit behavior in normals mimicked that of individuals with INS by making various errors in the initial saccade, pursuit velocity and initiation, target acquisition time, and steady-state position error (eyes lagging target) errors. Model predictions were confirmed by analysis of subject data. Subjects showed combinations of these errors including sustained position errors.
Saccades interfere with accurate and timely smooth pursuit. The implications for accurate ocular motor behavior in response to real-world stimuli (target onsets not limited to intervals when the eyes are still) are profound, especially in any activity where response speed and accuracy are required. Failure to maintain steady fixation until the stimulus occurs may drastically impair an individual’s ability in many sports and cause errors in other dynamic situations.
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