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Archana Bora, Scott B. Stevenson; Catch-up Saccades Do Not Influence Pursuit Direction For Mixed First Order / High Order Plaids. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4866.
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© ARVO (1962-2015); The Authors (2016-present)
Previous work suggests that initial catch-up saccades during pursuit mark a transition from first order to high order motion processing mechanisms. We examined pursuit responses to a stimulus with both types of information in which the direction of pursuit reveals the level of motion processing responsible.
Four participants viewed a mixed plaid pattern though the apertures of a dual Purkinge image eye tracker. The mixed plaid comprised an oblique luminance-defined square wave grating (first order) and an orthogonal, dynamic random line orientation-defined square wave grating (higher order) target, 0.15cycles/deg. First order and high order gratings were also tested separately. On each trial the entire plaid moved either to the right or left at 5degrees/sec.Observers were instructed to pursue the pattern motion. Horizontal and vertical eye positions were recorded at 120 Hz in synchrony with the CRT display. Traces were converted to velocity and the initial saccade was identified with a velocity threshold of 20 deg/sec. Pursuit latency was identified with a velocity threshold of 0.5 deg/sec after smoothing and interpolation across saccades. Pursuit latency and pre- and post-saccadic tracking eye directions were compared.
Pursuit responses to individual first order and higher order grating components were orthogonal to the bar orientations, consistent with the aperture effect. Higher order gratings showed longer and more variable latencies (range 272 to 423 ms) than luminance gratings (range 93 to 261 ms). Responses to the combined plaid stimulus showed a brief initial bias towards the luminance grating component motion. Trials were divided into equal groups based on saccade latency relative to the median latency for three of the four subjects (260- 470ms). Pursuit directions at this median saccade latency were compared and there was no significant difference between pre- and post-saccade trials.
The initial bias of pursuit toward the first order component is consistent with the latency difference for luminance and texture. However, the bias from luminance to texture did not change with the initial saccade. In most cases, pursuit was in the plaid direction by the time of the first catch-up saccade.
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