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PremNandhini Satgunam, Monica Chitkara, Nick Fogt; Ocular tracking of transiently occluded targets. Journal of Vision 2005;5(8):444. doi: 10.1167/5.8.444.
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Purpose: When individuals pursue an object moving horizontally that disappears for a short time and then reappears, they continue to move the eyes during target occlusion but at a lower velocity than the target. This study had 2 purposes. (1) To compare horizontal and vertical tracking of a transiently occluded two-dimensional (2D) target. (2) To determine whether subjects could learn to more accurately track this 2D target after repeated exposures.
Methods: 3 subjects tracked a letter “E” (0.3°) monocularly (distance=1.6m). The target started 11° above the eyes and then moved down (14°/s). At random angles of 3° above to 3° below the eyes, a horizontal ramp (14°/s, random direction) was added to the vertical motion (2D target). At random intervals (0.7–0.8s) after the 2D motion began a shutter occluded the target for 0.3–0.4s. The target continued to move at the same velocity. The shutter then reopened and the target continued moving for <0.14s. Subjects were to always pursue the target as if it were visible. 15 consecutive trials were recorded. Eye movements were recorded with a search coil.
Results: After occlusion subjects continued to track the target with pursuit and saccades. However, mean retinal position errors (PE) increased throughout the occlusion period in most trials. The PE prior to occlusion (after 2D motion onset) were 0.8°±0.5° horizontal (H) and 1°±0.5° vertical (V). The PE during occlusion were 2°±1° H and 4°±1° V. The PE after occlusion were 3°±2° H and 7°±1° V.
Discussion: The increase in tracking errors during occlusion and the lack of improvement in tracking during occlusion over 15 trials shows that the memory store that allows for tracking during occlusion is short lived. Thus an efferent copy of the eye movements in a trial cannot be modified for use in subsequent trials by the visual feedback provided when the target reappears. Finally, the short-term memory store for tracking is more effective for horizontal than for vertical tracking.
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