Effects of Observer Orientation on Perception of Ego- and Exocentric Spatial Locations

Perceived eye position and/or the perceived location of visual targets are altered when the orientation of the surrounding visual environment (Cohen et al, 1995 Perception & Psychophysics 571 433) or that of the observer (Cohen and Guzy, 1995 Aviation, Space, and Environmental Medicine 66 505) is changed.

Fourteen subjects used biteboards as they lay on a rotary bed that was oriented head-down −15°, −7.5°, supine, head-up +7.5°, and +15°. In the dark, subjects directed their gaze and set a target to the apparent zenith (exocentric location); they also gazed at a subjective ‘straight ahead’ position with respect to their head (egocentric location). Angular deviations of target settings and changes in vertical eye position were recorded with an ISCAN infrared tracking system.

Results indicated that, for exocentric locations, the eyes deviate systematically from the true zenith. The gain for compensating changes in head orientation was 0.69 and 0.73 for gaze direction and target settings, respectively. In contrast, ‘straight ahead’ eye positions were not significantly affected by changes in the subject's orientation. We conclude that subjects make systematic errors when directing their gaze to an exocentric location in near-supine positions. This suggests a systematic bias in the integration of extra-ocular signals with information regarding head orientation. The bias may result from underestimating changes in the orientation of the head in space. In contrast, for egocentric locations, where head orientation information can potentially be discarded, gaze directions were unaffected by head orientation near supine.