Absence of the horizontal-vertical illusion in haptic space.

An earlier experiment showed that the so-called horizontal-vertical (HV) illusion with an L is a function of the retinal meridians with which the two lines correspond. This finding indicated that the illusion is specifically visual and, therefore, unlike other geometrical illusions, would not be expected to occur haptically. The experiment showed that, as found in earlier studies, a large haptic illusion occurred with an inverted-T figure. However , as predicted, no illusion occurred with the L figure. The vertical line in an invcrled-T figure is judged visually to he about 7-14% longer than the horizontal line o[ equal length. This effect is called the horizontal-vertical (HV) illusion. Avery and Day (1969), in an examination of the visual-frame explanation of the HV illusion (Kunnapas, 1957), using an L figure, showed that the greater apparent length of one line was not due to its alignment with the gravitational vertical, but to its correspondence with the retinal meridian, which is vertical when S is upright. With S recumbent, this meridian is nearly horizontal and, accordingly, the horizontal line is judged the longer. This finding suggests that visual structures or processes associated with different retinal meridians form the basis of the illusion. There is evidence showing that some geometrical illusions, including the HV with an inverted T, occur in haptic space when the fingers are moved around a raised figure visual illusion with an L is a function of the visual meridians stimulated, a haptic illusion would not be expected with this figure. On the other hand, the effect occurring haptically with the inverted T could be due to the bisection factor. In the experiment reported here, it was predicted that a haptic overestimation of the vertical length would not occur with an L figure. An invertcd-T condition was included for comparison. Method.—The visual illusion with an L figure is about 3-5%, so that a haptic effect, if any, would be expected to be similarly slight. The apparatus, shown in Fig. 1, consisted of a 30 X 30 cm. vertical board, supported fronto-parallcl to 6" by being clamped to a bench top. In the surface of the board were two vertical slots and one horizontal slot, each 6 mm. deep and 2 mm. wide. The vertical slots were arranged so that one formed an L and the other an inverted T. Polished mild steel bars, 12 mm. wide and 1.5 mm. thick, could be …