Illusory concomitant motion in ambiguous stereograms: evidence for nonstimulus contributions to perceptual organization.

Three experiments were performed to test whether perceptual organization is cognitively or motivationally penetrable. In Experiments 1 and 2, subjects viewed a reversible stereogram while instructed to hold one depth organization. Responses about depth were recorded indirectly by recording responses about direction of the illusory concomitant motion that is perceptually coupled to depth in a stereogram. Inasmuch as perceptually coupled variables covary without necessary stimulus covariation, a post-perceptual locus for any intention effects they exhibit is unlikely. Experiments 2 and 3 examined the possibility that instructed intention might influence perception indirectly by influencing eye movements: Viewers' vergence position was measured directly through responses about alignment of a vernier nonius fixation. In all three experiments, a residual effect of instructed intention was found. Therefore, instructed intention may influence perceptual organization by influencing internal nonstimulus components integral to the perceptual process.