Dynamic properties of radial and tangential movements as determinants of the Haptic Horizontal-Vertical Illusion with an 'L' figure.

In four experiments involving blindfolded subjects, constant errors in the haptic judgment of extent in the horizontal plane were found to relate consistently to the time and velocity of limb movement.Radial movements, executed at a slower speed and for a longer time, are judged longer than tangential movements of equal extent.The data were considered in relation to certain physiological and kinematic properties of the actively moving limb.Taken together with additional information on judgments of movement duration, the results suggest that the illusion of extent is modulated by the perception of differential time cues.In these terms, it was noted that the haptic horizontal-vertical illusion with the L figure is another instance of the interaction of apparent space and time commonly found in studies of psychological relativity.Following Reid's (1954) documentation of the haptic horizontal-vertical illusion with an L figure, recent studies have shown that the effect is functionally related to the components of radial and tangential exploratory arm movements as described earlier by Davidon and Cheng (1964).Specifically, radial arm movements toward and away from the body, whether along the sagittal or coronal planes, are judged longer than equal tangential movements along the front or side of the body.The effect operates regardless of the spatial separation of the two extents to be judged (Cheng, 1968) and independently of two different types of movement involving different joints and muscle groups (Day & Wong, 1971).As radial movements are specific to the horizontal plane, no relative overestimation of haptic extent is found when the L figure is presented in the vertical (fronto-parallel) Experiments 1, 2, and