Tactile-kinesthetic space estimation: The influence of gravity

Summary The capacity to estimate the orientation of an object with respect to gravity was investigated by having Ss rotate a tactile-kinesthetically (tk) perceived bar (under exclusion of all visual cues) to the subjective horizontal. The 1st experiment was conducted to compare tk judgements with the visual judgements obtained by Udo de Haes (1970a,b) about an object's orientation in order to study the underlying mechanism, particularly to show possible vestibular influence, if any. For this purpose Ss were required to orient the tk-perceived bar while subjected to changes in both the magnitude (by centrifugation) and the direction (by tilting the body) of the gravitational force. Great variability of S's adjustments in the nearly inverted body positions (120°, 150°), indicating a low stability of spatial orientation in this range, was common to both tk and visual estimations. The relationship between body tilt and the corresponding judgement about the object's orientation, was however different for the two systems: No linear proportionality to the shear force in the utricles, as was established earlier in the visual experiments of Udo de Haes, was found in the tk settings. Vestibular influence was further studied in the 2nd experiment by comparing bar adjustments of the right hand with those of the left hand. Increasing angles of body tilt produced similar results in the responses of both hands although shifted differently from the true values. Standard deviations of left-hand settings were lower than those of right-hand ones. Reactions to increased gravity differed between hands. In the 3rd experiment the hand-weight alone, not the whole gravitational force, was increased. A direct effect of gravity on the adjusting hand was found. It was concluded that 1) the underlying mechanisms for tk and visual space estimations with respect to gravity are different; 2) with the given tk stimulation vestibular influence is not recognizable; 3) muscle receptors are probably significant, in addition to joint receptors, for this mode of space estimation, and 4) both arms may represent a system to perceive the direction of gravity independent of each other and of the labyrinth.