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Elisabeth M. Fine, Sergey Yurgenson, Cathleen M. Moore; Path length and number of saccades affect saccade accuracy. Journal of Vision 2006;6(6):494. doi: 10.1167/6.6.494.
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Background: The last saccade in a series was less accurate when the series was longer, especially when the spacing between targets was wider (Fine et al., 2005; ECEM). Number of saccades and distance to the farthest target were confounded. It is unclear whether error accumulated with each saccade or because the distance to the farthest target was greater as series length and spacing increased. Methods: We recorded the eye movements of 18 observers using a dual-Purkinje-image eye tracker. Observers were told at the start of each trial to make either one saccade to the farthest target (3.75 or 7.5 deg from fixation) or a series of saccades to each target in a linear display. In the latter case, the number of saccades matched the number of targets (1, 3, or 5, positioned semi-randomly). The average space between targets was greater with the farther target. Results: Overall, observers were less accurate with the farther target. There was a significant interaction between distance to farthest target and number of saccades. For the 3.75 deg target, number of saccades did not matter; for the 7.5 deg target observers were more accurate when they made a series of saccades than when they made one saccade. Conclusions: Saccades are less accurate the farther the endpoint is from fixation, regardless of the number of saccades made to reach it. This suggests that when planning a series of saccades, the total distance from fixation plays an important role in the calculation of saccade metrics.
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