Similar effects of a motion-in-depth illusion on manual tracking and perceptual judgements

We previously demonstrated that changing the apparent extent of a target's apparent motion-in-depth, by manipulating pictorial depth cues in the surrounding, affected perceptual judgements and manual pursuit to the same extent. Here, we investigated whether a different manipulation of the extent of motion (expanding and contracting the object itself) also has the same effect on both tasks. Objects were presented that changed in size as they moved on an elliptical path. The size was related to the object's position in the sagittal plane, suggesting additional motion in depth; therefore the illusion was expected to affect sagittal measures for both perception and action. We measured manual tracking and perceptual judgements of the lateral and sagittal extents of the object's elliptical trajectories. A significant correlation was found across subjects between the effect of the illusion on the perceptual and the motor task. As expected, the illusion only had a significant influence on the sagittal dimension. The size of this illusory effect was equal for perception and action.