Using traditional horizontal-vertical illusion figures and single lines to directly compare haptics and vision

The horizontal-vertical illusion (HVI) is the tendency for a vertical line to be perceived as longer than a horizontal line of the same length. The HVI is commonly reported and investigated as a visual phenomenon. It has, however been found to occur haptically. The comparatively small number of haptic HVI papers paired with varied stimuli and measures make it problematic to directly compare visual and haptic forms of the illusion. The current paper reports a study in which the visual and haptic HVIs were directly compared. Three sets of stimuli were used in an attempt to resolve previous limitations: L-figures, inverted T-figures, and separated horizontal and vertical lines. These stimuli were presented in two lengths: 3 and 9 cm. The dependent variable was percentage error between the horizontal and vertical - no error represents an absence of illusion. As expected, inverted T-figures produced an illusion significantly stronger than both the L-figures and single lines, which did not differ from each other. Stimuli of 9 cm produced stronger illusions than those that measured 3 cm, and stimulus size interacted with modality. The consequences of these findings for earlier research are discussed and suggestions are offered as to what causes this and other illusions.