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1 CESAME, Universite chatholique de Louvain, Louvain-la-Neuve, Belgium; Laboratory of Neurophysiology, Universite catholique de Louvain, Brussels, Belgium; Centre for Vision Research, York University, Toronto, Ontario, Canada
2 Laboratory of Neurophysiology, Universite catholique de Louvain, Brussels, Belgium
3 CESAME, Universite chatholique de Louvain, Louvain-la-Neuve, Belgium; Laboratory of Neurophysiology, Universite catholique de Louvain, Brussels, Belgium; National Eye Institute, Laboratory of Sensorimotor Research, National Institutes of Health, Bethesda, Maryland, USA
* To whom correspondence should be addressed. E-mail: lefevre{at}csam.ucl.ac.be.
When objects move in our environment, the orientation of the visual axis in space requires the coordination of two types of eye movements: saccades and smooth pursuit. The principal input to the saccadic system is position error, whereas it is velocity error for the smooth pursuit system. Recently, it has been shown that catch-up saccades to moving targets are triggered and programmed by using velocity error in addition to position error. Here, we demonstrate that when a visual target is flashed during ongoing smooth pursuit, it evokes a smooth eye movement towards the flash. The velocity of this evoked smooth movement is proportional to the position error of the flash; it is neither influenced by the velocity of the ongoing smooth pursuit eye movement nor by the occurrence of a saccade but the effect is absent if the flash is ignored by the subject. Furthermore, the response started around 85 ms after the flash presentation and decayed with an average time constant of 276 ms. Thus, this is the first direct evidence of a position input to the smooth pursuit system. The present study brings further evidence for a coupling between saccadic and smooth pursuit systems. It also suggests that there is an interaction between position and velocity error signals in the control of more complex movements.
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