Does line bisection contribute to the Horizontal-Vertical Illusion?

To determine whether line bisection in inverted-T figures contributes to the Horizontal Vertical Illusion (HVI), subjects adjusted and selected lines to match 50-mm long horizontal standards. In general, the lengths of bisected horizontal lines were slightly underestimated. In a forced choice psychophysical task, the points of subjective equality for inverted-T and L figures were 48.9 mm and 50.9 mm, respectively. The mean HVI adjustment score was 2.6 mm greater with L than with inverted-T figures, a value only slightly greater than the combined constant errors made in the forced choice task (2.0 mm). Thus, the difference in the HVI obtained with these figures is due, in part, to the combined perceptual shortening and lengthening of the horizontal lines in inverted-T and L figures, respectively.