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.

Two experiments on the difference in haptically judged extent of two raised edges forming an L are described. The data from Exp. I showed that when movements of the outstretched arm and finger along both components of the L are tangential, there is no difference in their apparent lengths, but that when movement along one is tangential and the other radial, the latter is apparently longer. In Exp. II a different type of movement involving different joints was required, but the results were essentially the same. The data from the two experiments confirm earlier findings on the relative apparent extents of tangential and radial movement and are interpreted as demonstrating the absence of a relationship between visual and haptic illusions of length in an L figure.

In four experiments involving 156 5s, stimulus templates were traced onto the volar surface of 5's forearm. Experiment I showed that rotation of a T figure over 180° resulted in an illusion function, with a reversal of the effect at 90°. A similar function was obtained in Experiment II with an L figure, indicating the absence of a bisection effect. The component line along the shorter lateral axis was judged longer than that aligned with the longitudinal axis of the forearm. Experiments III and IV indicated that the effect was independent of the degree of stimulation on the skin and that an inscribed circle was similarly affected and perceived as a lateral ellipse. It was concluded that the horizontal-vertical illusion in passive touch is a function of the orientation of the figure with reference to the elongated frame of the receptor organ and is independent of the effect in vision and active touch.

Two groups of 24 subjects each were tested with either haptic or visual Müller-Lyer figures. The size of the obtained haptic and visual illusion was found to be unaffected by the degree of excursive movement of the limb and the eye, respectively. It was concluded that the illusion is the cause rather than the result of peripheral scanning behaviour of the limb and eye, and that the similar illusion across modalities is probably determined by common central mechanisms.