Objects, raised lines, and the haptic horizontal-vertical illusion

Experiments were conducted to determine whether the haptic horizontal - vertical illusion occurs with solid, three-dimensional objects as well as with tangible lines. The objects consisted of round or square bases, with dowel rods projecting above them at heights equal to the widths of the horizontal bases. A negative illusion, with overestimation of horizontals, was found with free haptic exploration, but not with tracing with the fingertip. The negative illusion occurred when subjects felt wooden Ls and inverted Ts with a grasping, pincers motion of the index finger and thumb. The presence or absence of illusory misperception was dependent upon exploration strategy, since the negative illusion vanished with finger tracing. A negative illusion was also found when subjects adjusted a vertical dowel so that it was judged to be equal in extent to a round or square base. A general overestimation of judged size derived from the pincers response measure, but was not found with the use of a tangible ruler. Comparable illusory results are most likely when drawings and objects promote similar haptic scanning methods. The results were consistent with the idea that the orientation of an edge or line is more important than whether one explores a tangible line or a three-dimensional object.