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T.D. Bornhorst, N. Fogt; The effect of retinal image motion on discrimination of single and surrounded letters . Invest. Ophthalmol. Vis. Sci. 2004;45(13):2536.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Discrimination of an image moving across the retina is reduced when velocity exceeds about 3°/s. As image velocity increases, retinal image smear increases, decreasing the contrast available for image discrimination. The purpose of this experiment was to determine the relationship between image velocity and acuity for single letter targets and for letter targets surrounded by other letters. It was expected that retinal smearing of the images of the surrounding letters would result in a more rapid decline in visual acuity as velocity increased. Methods:10 subjects participated. They were seated 4m from a black target plane with their chin in a rest. The target was either a single white Landolt C or a white Landolt C in the center of 8 surrounding Landolt Cs. The 9 letters in the latter case were arranged in 3 vertical columns. The letters were equivalent to 20/34 Snellen letters. The target was projected onto the target plane by a mirror galvanometer. In each trial subjects fixated a dim laser spot straight ahead, which was extinguished just prior to target presentation. In a forced–choice manner, subjects then identified the orientation of the (central) Landolt C target as it moved across fixation. The target orientation (up, down, left, or right), target velocity (1–5°/s in 0.5°/s steps), target direction (left, right), and target appearance (single or SI condition, surrounded or SU condition) were all randomized. Targets were presented for about 113ms to prevent eye movements. Each subject completed 144 trials. Results:At all target velocities, the mean number of targets whose orientation was correctly identified was greater in the SI than in the SU condition. These differences were statistically significant at most target velocities. For the SI condition, the threshold level of 62.5% correct was reached or nearly reached at velocities less than 3.5°/s, while this level was never reached in the SU condition. As the target velocity increased, the mean number of targets correctly identified decreased at exactly the same rate for the SI and SU conditions. Conclusions: The surrounding letters reduced the ability to determine the orientation of the central letter. However, the difference in performance for the SI and SU conditions was the same at each velocity. This suggests that the reduction in letter discrimination with the surrounding letters occurred because these letters shifted the focus of attention away from the central letter. The poorer discrimination with the surrounding letters could not be attributed to a reduction in contrast of the central letter from smearing of the retinal images of the surrounding letters.
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