Opposite haptic and visual induction effects observed with Titchener’s ⊥

Abstract

Nineteen psychology undergraduates were requested to "grasp" the lines of a computer image of Titchener's perpendicular with a thumb and index finger pincer grip immediately after the line to be grasped had been indicated and the whole figure extinguished. The hand remained visible. Lines were of three different lengths (6.5, 7, and 7.5 cm), and perpendiculars were presented at eight different orientations (rotated in steps of 45 degrees ). Lines also had to be judged longer, shorter, or equal, relative to one another. Haptically, subjects responded correctly to the perpendiculars' undivided lines, but scaled their responses to the divided ones according to the undivided ones. Categorical judgments yielded comparable psychometric functions for both kinds of lines, with a significant shift of points of subjective equality from a difference in length between lines of 9.8% to 13.5% when the divided or the undivided line was target. Haptic sensitivity, therefore, appears to be constrained by context, whereas the classical "visual illusion," associated with Titchener's perpendicular, may be regarded a mere decisional criterion shift.