APPARENT BODY TILT AND POSTURAL AFTEREFFECT

Apparent orientation of the body tilted laterally in the frontal plane was studied with the methods of absolute judgments in four experiment s. In Experiment 1, 17 subjects, who maintained the normal adaptation of body to gravity estimated their body tilts under the condition of s eeing the gravitational vertical and under the condition of eliminatin g it. The results showed that (1) there was not a significant differen ce between the two conditions and (2) the small tilts of less than 45 degrees were exactly estimated, whereas the large tilts of 45 degrees- 108 degrees were overestimated. In Experiment 2, 10 subjects estimated their body tilts under three velocities of a rotating chair on which each subject was placed. Although both body tilt and chair velocity we re found to influence tilt estimation, the effect of body tilt was ove rwhelmingly greater than that of chair velocity. In Experiment 3, 11 s ubjects adapted their bodies to a 72 degrees left tilt for 10 min and then estimated various body tilts around the adapting tilt. The estima tions obtained under the 72 degrees adaptation were lower than those o btained under the 0 degrees adaptation, and this reduction was greater for the test tilt that was farther away from the adapting tilt. In Ex periment 4, 11 subjects adjusted their own body tilts to designated an gles. The results confirmed the outcomes of absolute estimation in Exp eriments 1-3. From these findings and past literature, the judgments o f body tilt were considered to be subserved by a single sensory proces s that was based on the cutaneous and muscular proprioceptors, rather than the vestibular and joint proprioceptors.