THE TACTUAL HORIZONTAL-VERTICAL ILLUSION DEPENDS ON RADIAL MOTION OF THE ENTIRE ARM

We sought to clarify the causes of the tactual horizontal-vertical ill usion, where vertical lines are overestimated as compared with horizon tals in L and inverted-T figures. Experiment 1 did not use L or invert ed-T figures, but examined continuous or bisected horizontal and verti cal Lines. It was expected that bisected lines would be perceived as s horter than continuous lines, as in the inverted-T figure in the horiz ontal-vertical illusion. Experiment 1 showed that the illusion could n ot be explained solely by bisection, since illusory effects were simil ar for continuous and bisected vertical and horizontal lines. Experime nts 2 and 3 showed that the illusory effects were dependent upon stimu lus size and scanning strategy. Overestimation of the vertical was min imal or absent for the smallest patterns, where it was proposed that s timuli were explored by finger movement, with flexion at the wrist. La rger stimuli induce whole-arm motions, and illusory effects were found in conditions requiring radial arm motion. The illusion was weakened or eliminated in Experiment 4 when subjects were forced to examine sti muli with finger-and-hand motion alone, that is, their elbows were kep t down on the table surface, and they were prevented from making radia l arm motions. Whole-arm motion damaged performance and induced percep tual error. The experiments support the hypothesis that overestimation of the vertical in the tactual horizontal-vertical illusion derives f rom radial scanning by the entire arm.