The perception of the vertical: I. Visual and non-labyrinthine cues.

Abstract

Modification of receptor cues may be expected to influence the accuracy of visual and postural perception. Subjects placed at angles of lateral tilt were required to return themselves to the postural vertical in the absence of visual cues. Somesthetic cues were modified by the introduction of a thickly padded soft seat. Constant errors did not differ significantly under hard and soft seat; there was a significant difference in variability in favor of soft seat. The arc of uncertainty (U[degree]) was defined as the middle 75% of judgments. Under hard seat, the mean value of U varied from 3.4[degree] to 7.4[degree]; under soft seat U varied from 6.4[degree] to 10.3[degree], depending upon the method. The mean judgments of the visual vertical and the visual horizontal made by subjects seated in the upright position were 0.3 and 0.4, respectively; when placed in positions of tilt there was a significant tendency towards increase in the errors of judgment of the visual horizontal.