A. Lindner et Uj. Ilg, Initiation of smooth-pursuit eye movements to first-order and second-ordermotion stimuli, EXP BRAIN R, 133(4), 2000, pp. 450-456
Since normal human subjects can perform smooth-pursuit eye movements only i
n the presence of a moving target, the occurrence of these eye movements re
presents an ideal behavioural probe to monitor the successful processing of
visual motion. It has been shown previously that subjects can execute smoo
th-pursuit eye movements to targets defined by luminance and colour, the fi
rst-order stimulus attributes, as well as to targets defined by derived, se
cond-order stimulus attributes such as contrast, flicker or motion. In cont
rast to these earlier experiments focusing on steady-state pursuit, the pre
sent study addressed the course of pre-saccadic pursuit initiation (less th
an 100 ms), as this early time period is thought to represent open-loop pur
suit, i.e. the eye movements are exclusively driven by visual inputs procee
ding the onset of the eye movement itself. Eye movements of five human subj
ects tracking first- and second-order motion stimuli had been measured. The
analysis of the obtained eye traces revealed that smooth-pursuit eye movem
ents could be initiated to first-order as well as second-order motion stimu
li, even before the execution of the first initial saccade. In contrast to
steady-state pursuit, the initiation of pursuit was not exclusively determi
ned by the movement of the target, but rather due to an interaction between
dominant first-order and less-weighted second-order motion components. Bas
ed on our results, two conclusions may be drawn: first and specific for ini
tiation of smooth-pursuit eye movements, we present evidence supporting the
notion that initiation of pursuit reflects integration of all available vi
sual motion information. Second and more general, our results further suppo
rt the hypothesis that the visual system consists of more than one mechanis
m for the extraction of first-order and second-order motion.