Simultaneous hand tracking does not affect human vergence pursuit

In order to find out whether human vergence eye movements are influenced by simultaneous hand tracking movements, vergence was studied when sinusoidal (expressed in vergence angles) target movements were tracked. The target motion was externally generated and the target actually moved in depth. Tracking was done by the eyes alone or by the eyes and hand together, in both light and dark viewing conditions. Our data show that the target motion was tracked by the eyes with a short delay (on average 48 ms), independent of the tracking condition. This suggests that vergence modeling should include some predictive mechanism similar to that proposed for the smooth pursuit subsystem. Furthermore, in contrast to effects on smooth pursuit, simultaneous hand tracking movements did not influence vergence eye movements. From this, we argue that the balance between smooth pursuit and saccadic eye movements is adjustable and can be adapted to the requirements of different tasks.