Cite as:
Scocchia L, Baud-Bovy G, 2005, "Visuo-motor interactions in the flash-lag effect" Perception 34 ECVP Abstract Supplement

Visuo-motor interactions in the flash-lag effect

L Scocchia, G Baud-Bovy

The flash-lag effect (FLE) is a visual illusion that consists in perceiving a flashed stimulus behind an aligned moving target. In this study, we investigated whether the motor system interacts with the visual system in this illusory perception. Subjects (N = 24) performed a visual judgment task in two visuo-motor conditions and in a control visual condition: they had to state whether a flash appeared behind or ahead of a small disk (diameter 0.27 deg) moving on a computer screen along a circular trajectory (diameter 5.12 deg). Fixation was held during the task. In the control condition, the computer controlled the movement of the disk along the trajectory. The moment at which the flash was presented and the position of the moving stimulus at the time of the flash were random. A double staircase algorithm controlled the position of the flash relative to the moving disk. The first visuo-motor condition was similar to the control condition except for the fact that subjects controlled the movement of the disk by moving a robotic manipulandum (Phantom 1.5, Sensable Technology). The robot produced a force field that maintained the manipulandum on the circular trajectory. The instantaneous velocity was monitored to ensure that subjects moved the disk at a similar speed in all conditions. The second visuo-motor condition was the same as the first one, but subjects had to produce a constant force (203.9 g force) against the robot to move the manipulandum along the trajectory. The flash-lag effect was observed in all conditions. A repeated-measures ANOVA and a Duncan a posteriori test revealed that it was significantly greater in the visuo-motor conditions than in the control condition. This finding demonstrates that the motor system interacts with the visual system while perceiving moving objects.

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