Vertical disparity detection: is there an induced size effect?

Horizontal, vertical, and over-all size differences were introduced in the retinal images of the two eyes of normal subjects during brief presentations of simple foveal targets. Horizontal disparities, whether accompanied by vertical disparities or not, induced the appearance of a rotation of the target around a vertical axis out of the frontal plane, according to the expectation from geometry, but vertical disparities had no effect. Over-all size changes in one eye induced the effect of the horizontal component. Threshold experiments showed that even with practice and error feedback, vertical disparity detection has at most only one-tenth the sensitivity of horizontal disparity detection. Although at variance with findings on the induced size effect obtained under more complex observation conditions, these results confirm that the processing of horizontal disparity plays a special role in the integration of the signals coming from the two eyes.