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Dirk Kerzel, Blandine Ulmann; Suppression of steady state smooth pursuit by irrelevant flashes. Journal of Vision 2007;7(9):143. doi: 10.1167/7.9.143.
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We examined the effects of briefly flashed stimuli on steady state pursuit. Subjects pursued a small cross moving at a velocity of 11 deg/s. After about 1 sec of pursuit, two bars subtending 45 deg horizontally were presented above and below the pursuit target. Presentation time was 30 ms. Because of the elongated shape and short presentation duration, very little perceivable motion along the direction of pursuit was produced by the stimulus. After flash presentation, smooth pursuit continued for another second. We observed a decrease of pursuit gain of about 10% starting 200 ms after flash onset. At the same time, the probability of catch-up saccades decreased from 4–5% to 1–2%. Pursuit gain recovered after another 200 ms. Increasing the eccentricity of the bar or decreasing its contrast reduced the effect of the distractor on smooth pursuit gain, but did not affect its latency. Compared to a visual flash, the presentation of a brief click sound produced only a very small decrease of pursuit gain. However, the click inhibited catch-up saccades at the same latency as the visual flash. Making the occurrence of the flash highly predictable or highly unpredictable did not change the pattern of results. Overall, our findings resemble previous studies showing that task-irrelevant flashes suppress the occurrence of saccades during reading and increase the latency of goal directed saccades. However, the latency of the suppression of smooth pursuit is much longer: Saccadic suppression occurs after ∼80 ms, while smooth pursuit was affected after ∼200 ms. The short latencies of saccadic suppression suggest a subcortical origin, maybe in the superior colliculus. The much longer latencies for the suppression of smooth pursuit suggest that cortical circuits are involved. This seems to be true for smooth pursuit and catch-up saccades.
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