VISUAL PERCEPTIONS OF VERTICAL AND INTRINSIC LONGITUDINAL AXES

The purpose of these experiments was to investigate whether visual per ceptions of the earth-fixed vertical axis are more accurate than those of intrinsic body-fixed axes. In one experiment, nine neurologically normal young adult subjects' abilities to position a luminescent rod v ertically or parallel to the longitudinal axis of the head or trunk we re studied in four conditions: (1) earth-fixed - subjects stood erect with the head aligned to the trunk and visually aligned a hand-held ro d to vertical; (2) earth - subjects aligned the rod to vertical as in 1, but the orientations of the head and trunk were varied in the sagit tal and frontal planes on each trial; (3) head - frontal and/or sagitt al plane orientation of the subject's head was varied on each trial an d the rod was aligned parallel to the longitudinal axis of the head; ( 4) trunk - frontal and/or sagittal plane orientation of the subject's trunk was varied on each trial and the rod was aligned parallel to the longitudinal axis of the trunk. Note that in conditions 2, 3, and 4 t he head and trunk were never aligned with each other. Also, each condi tion was carried out in normal light and in complete darkness. Percept ual errors were measured in both the frontal and the sagittal planes. The results showed that the variable errors were significantly lower w hen subjects aligned the rod to vertical rather than to the longitudin al axis of the head or trunk. Also, errors were similar in size in the two planes and were unaffected by vision of the surrounding environme nt. In a second experiment, subjects were seated and controlled the po sition of a luminescent rod held by a robot. They aligned the rod eith er to the longitudinal axis of their head or to the vertical in comple te darkness, under three conditions similar to those described above: (1) earth-fixed, (2) earth, and (3) head. There was no possibility of use of kinesthetic information for controlling rod position in this ex periment as in the first experiment. The results were similar to those of the first experiment, as subjects aligned the rod more accurately to vertical than to the longitudinal axis of the head. These results s how convincingly that visual perceptions of earth-fixed vertical are m ore accurate than perceptions of intrinsic axes fixed to the head or t runk.