The horizontal vertical illusion: Transfer of illusion decrement

The present study examined illusion decrement and the transfer of illusion decrement in different tests for the Horizontal-Vertical Illusion. Subjects initially adjusted the vertical line of an inverted T figure and were then either tested on a different sized figure or were asked to draw a one inch line in the horizontal or vertical plane. On the adjustment task, subjects initially adjusted the vertical line of the inverted T figure to be significantly shorter than the horizontal line, but, over trials, their accuracy gradually improved. At the end of the adjustment trials, their vertical line estimations did not differ significantly from the horizontal line. This improvement transferred to the second adjustment test but not to the production test. These results demonstrate that the magnitude of the Horizontal-Vertical Illusion decreases with practice and that this practice effect readily transfers to other size figures but not to an alternate form of the illusion. The potential factors responsible for the improvement in transfer effects are discussed.