The perception of angle size is more accurate in vision than haptics (Appelle 1971). Lakatos and Marks (1998) asked blindfolded adults to explore (according to different procedures defined by the experimenter) the angles of wooden plates or of raised lines forming isoscele triangles, and to verbally estimate their size in degrees. The authors proposed different angles (from 20° to 150°) and, furthermore, modified the extent of continuity of the intersection between the two angle branches by inserting portions of "circles" with different curvature radii. The results showed that the angles of both types of stimulus (3-D plates or raised lines) were systematically underestimated in all the exploration procedures proposed. The amplitude of this underestimation varied inversely to that of angle size. This underestimation was also present in vision but to a lesser degree. In addition, the estimation of an angle size increased as the curvature radius increased. This effect, present in the haptic modality, was absent in vision.
Thus, haptic performance in shape perception is always inferior to visual performance, and, although the general processing modes are analogous in both modalities, the particular characteristics of manual exploration (spontaneous or imposed) often have specific effects on haptic perceptual estimations.
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