2020 年 38 巻 2 号 p. 214-226
Despite numerous Vertical–Horizontal Illusion (VHI) studies conducted since that of Adolf Fick in 1851, VHI has yet to attain a uniformly accepted consensus due to conflicting conclusions. As a result, a re-examination of VHI was undertaken focusing on contact position of a vertical (mast) line on a horizontal (base) line, orientation of the VHI stimulus, and viewing conditions, i.e., monocular vs. binocular-vision. In pseudorandom fashion on a computer, 35 adults adjusted mast lines of varying length and contact position in reference to a 50-mm base line to the same perceived length as the base line. The Point of Subjective Equality (PSE) was measured over 168 trials (two trials of 7 contact positions×4 orientations×3 viewing conditions). Perceived lengths were significantly affected by contact position in an M-shaped manner (Marma et al., 2015) rather than V-shaped (Künnapas, 1955a), and PSE was shorter when the baseline was horizontal rather than vertical, confirming the anisotropy in vertical–horizontal axis (Künnapas, 1955a).