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M. B. Kaufman, M. Zhu, R. W. Hertle, K. Zawrotny, D. Yang; Threshold of Dynamic Vernier Stimuli Detected With Oculomotor Responses. Invest. Ophthalmol. Vis. Sci. 2007;48(13):896.
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Hyperacuity is usually assessed subjectively using static stimuli. In this study ocular motor responses were used to detect moving vernier offsets. We will report the threshold of the dynamic vernier stimuli and properties of the ocular motor responses.
The vernier stimulus is a vertical black offset on a horizontal black bar spanning the width of a 19" monitor, situated 2.5m from the subject. In control conditions, the vernier offset was displayed at the midline of the horizontal bar prior to motion. In experimental conditions, the vernier offset appeared at the midline of the horizontal bar only after the onset of motion. Guided by a brief presentation of a fixation dot in the center of the horizontal bar, subjects triggered a horizontal motion of the vertical offset by pressing a button. VEX software controlled the offset, moving it from the midline of the horizontal bar in the randomly selected direction of right or left at speeds of 0.5°, 1.0°, 2.0°, 3.0°, or 4.0°/sec. As the offset moved, its vertical height would increase from a minimum of 26 seconds of arc at appearance to a maximum of 247.5 seconds of arc as it moved off the screen. Subjects were instructed to follow the movements of the vernier offset as soon as it could be detected. The subjects’ eye movements were recorded with an infrared eye movement recording system. Initial measurable smooth pursuit responses and catch-up saccades were analyzed to determine the timing of visual detection of the moving offset. Three subjects with normal oculomotor function and acuity of 20/20 or better with correction participated in this study.
All subjects could detect the static vernier offsets and responded to the motion of it with reaction time of 350-450 ms in control conditions. The reaction time of the ocular motor responses to stimuli that appeared in motion in the experimental conditions was longer by 80-125 ms at slow speeds (0.5°, 1.0°, and 2.0°/sec). The detection thresholds were 50-150 second of arc and increased as the speed of offset increased gradually for both control and experimental conditions. At speeds of 0.5°, 1.0°, and 2.0°/sec, thresholds for detecting the offset were higher in the experimental condition than in the control condition. At higher speeds the difference was not significant.
Dynamic threshold of vernier acuity under current conditions was determined in normal subjects. The approach used in this study can be a simple and useful method for clinical application of evaluating ocular motor function in patients with potential ocular motor deficits.
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