Effects of body axis on estimation of horizontal linear translation

Extending the work of Israel et al [1993], this study investigated human path integration of linear horizontal motion and, in particular, the presence of different responses between body axes of motion. Six blindfolded subjects were passively accelerated along a horizontal track and instructed to signal when they perceived themselves passing a remembered target. We tested three orientations: upright Y-axis (lateral), and two supine Z-axes (starting acceleration headward or footward). The cart trajectory was specified by a square wave in acceleration. Two acceleration levels (1.0 and 0.2 m/s2) and two target distances (0.8 and 2.4 m) were used. The subjects each performed four repetitions of each set of parameters. We consistently found (501/554) anticipatory responses, with the only late responses occurring on the return path; this agrees with the findings of Israel et al. Footward acceleration produced the greatest anticipation, followed by Y-axis and headward accelerations. However, a striking asymmetry appeared when responses to headward and footward accelerations in the supine orientation were compared to one another. ANOVA on the latencies for both pooled and individual subjects showed orientation as significant (P<0.001). This asymmetry may reflect nonlinearities in the generation or processing of otolith information. Moreover, from a behavioral standpoint, the heightened anticipation during footward motions may be an expression of the human need to be particularly sensitive to downward accelerations, such as falls.