Perceptual judgements and manual tracking are equally affected by an expansion/contraction illusion

Abstract

We previously showed (Lopez-Moliner et al., to appear in Psych. Sci.) that a pictorial depth illusion affected both manual pursuit and perceptual judgements to the same extent. In that study both the perceived extent of the target's changing position and the amplitude of the trajectory of the manual tracking were affected by manipulating the target's apparent depth with perspective cues in the surrounding. In the present study we explore whether manipulating the apparent depth by expanding and contracting the object itself will also influence both tasks to the same extent. We measured perceptual judgements of lateral and sagittal extents of a target moving along an elliptical path, and manual tracking paths of a target moving along the same trajectory. Since the expansion/contraction was only modulated along the sagittal plane, we expect the illusion to affect both sagittal measures and not the lateral ones. The magnitude of the effect was the same for both tasks. Moreover, a significant correlation across subjects between the effect of the illusion on the perceptual and the motor task was found and no significant effect on the measures for the lateral dimension. We conclude that the equal effect of illusions on perception and action is not limited to contextual illusions.