The effect of Kanizsa's compression illusion on reflexive saccades.

When a horizontal line is occluded by a surface, subjects misperceive the line as being compressed, that is, they perceive it to be shorter than it really is (Kanizsa's compression illusion). The size of the compression effect usually ranges from approximately 5 to 10%. In this experiment, subjects executed reflexive saccades from one end of a line, presented at fixation, to the other end, with and without an occluding square. Saccade amplitude was reduced in all subjects in the presence of an occluding square; the effect averaged about 5%. These results demonstrate that saccade amplitude is modified by the Kanizsa compression illusion. They provide further evidence that reflexive saccade amplitude can be altered by illusion inducing stimuli, to the same degree as perceptual effects, even in circumstances in which other motor behaviours resist the illusion. They are difficult to reconcile with any strong version of the "two visual systems hypothesis".