Subjective vertical and postural activity

Three experiments were conducted to test the effect of postural information, resulting from the active control of balance, on the perception of the vertical. Subjects were required to adjust a luminous rod in two different visual contexts: in the dark or within a tilted visual frame. In these experiments, postural activity was manipulated by placing observers either in a situation of slight postural imbalance (Experiment 1) or in a situation of postural immobilization (Experiment 2). In both situations performance was compared with a control condition in which subjects were standing upright freely (Experiment 1) or sitting unconstrained (Experiment 2). Results showed no main effect of active posture or of immobilization on the visual perception of the vertical. In the third experiment, subjects were supine with their Z body axis perpendicular to the plane of the luminous rod. Thus, body orientation relative to gravity was modified and motor activity reduced. In this position, the physical vertical was perceived quite accurately in a dark environment. Moreover, in the tilted frame condition, the supine body position clearly improved vertical judgements. These results are discussed in relation to the ecological theory of orientation (Stoffregen and Riccio, 1988).