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Casper J. Erkelens; Properties of saccade generation revealed by smooth pursuit. Journal of Vision 2004;4(8):113. doi: 10.1167/4.8.113.
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Voluntary saccades are made to change fixation to another target. Such saccades require engagement of the saccadic system to the new target and disengagement from the old one. Here we study the occurrence of these changes by investigating smooth pursuit preceding and following voluntary saccades. The rationale is that smooth pursuit may reveal the changes because it is an ongoing oculomotor response. Subjects viewed random dot patterns (h × v: 80 × 60 deg) on a computer screen. The patterns were divided in 2 to 10 horizontal bands of which all dots moved either in the leftward or rightward direction at constant speed. The direction of motion was opposite in adjacent bands. Two-dimensional eye orientation was measured of the left eye. Saccades were detected online, providing us the opportunity to change stimulus motion within saccades. Subjects were asked to pursue the fixated band and to make voluntary vertical saccades between different bands. Eye movement recordings showed that subjects pursued the fixated band at constant speed until the onset of voluntary saccades. After these saccades, the novel band was pursued immediately. As a consequence, pursuit reversals occurred within periods as short as 50 ms. If band motion was reversed within saccades, pursuit reversals started after about 150 ms and took about 150 ms to complete. As a result, saccades shortened pursuit reversals by about 100 ms. The shortening is explained by assuming that, while a common mechanism prepares a saccade to and pursuit of the novel target, the eyes remain engaged to the old target for another 100 ms. We speculate that the frontal eye fields are involved in the preparations whereas the superior colliculi keep the eyes fixated on the current target.
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