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Girish Kumar, Scott B. Stevenson, Austin Roorda; Localized distortions within saccadic mapping contribute to the variance in saccadic landing points. Journal of Vision 2008;8(17):19. doi: 10.1167/8.17.19.
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Aim: Previous work1 showed that saccades placed stimuli within a zone of approximately 5–10 arc minutes in diameter within the fovea. Our aim is to determine the contribution of localized targeting distortions towards this variability.
Methods: An Adaptive Optics Scanning Laser Ophthalmoscope was used to collect videos from 3 normal subjects who fixated a 7 arc minute square that jittered randomly among nine positions within a 2 degree field. Eye movements were extracted and saccades identified using a velocity criterion of 15o/sec. The retinal locations of the stimulus in the frame prior to a saccade was taken as its “starting location”, while the location following the saccade was the “ending location”. Saccades were grouped based on the proximity of their starting locations, and the spread of the ending locations between these groups and randomly chosen saccades were compared.
Results: Spread for all ending locations ranged from 10 to 17 arc minutes. For two subjects, a sign test showed a statistically significant proportion (α - 0.05) of saccade groups had smaller spread when compared to randomly chosen saccades. Subjects‘ saccades undershot by an average of 1.5 arc minutes, however saccadic errors correlated poorly with starting location (r2 - 0.16 to 0.18).
Conclusions: Our findings show saccades place stimuli within a “preferred retinal zone” that tends to undershoot a central foveal location. However, local retinal regions have unique and separate targets within this zone, leading us to infer the presence of localized distortions within saccadic mapping.
CornweetT. N. (1956), Determination of the Stimuli for Involuntary Drifts and Saccadic Eye Movements, Journal of the Optical Society of America, 46(11), 987–993.
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