LIGHT AND DARK-ADAPTATION OF VISUALLY PERCEIVED EYE LEVEL CONTROLLED BY VISUAL PITCH

The pitch of a visual field systematically influences the elevation at which a monocularly viewing subject sets a target so as to appear at visually perceived eye level (VPEL). The deviation of the setting from true eye level averages approximately 0.6 times the angle of pitch wh ile viewing a fully illuminated complexly structured visual field and is only slightly less with one or two pitched-from-vertical lines in a dark field (Matin and Li, 1994a). The deviation of VPEL from baseline following 20 min of dark adaptation reaches its full value less than 1 min after the onset of illumination of the pitched visual field and decays exponentially in darkness following 5 min of exposure to visual pitch, either 30 degrees topbackward or 20 degrees topforward. The ma gnitude of the VPEL deviation measured with the dark-adapted right eye following left-eye exposure to pitch was 85% of the deviation that fo llowed pitch exposure of the right eye itself. Time constants for VPEL decay to the dark baseline were the same for same-eye and cross-adapt ation conditions and averaged about 4 min. The time constants for deca y during dark adaptation were somewhat smaller, and the change during dark adaptation extended over a 16% smaller range following the viewin g of the dim two-line pitched-from-vertical stimulus than following th e viewing of the complex field. The temporal course of Light and dark adaptation of VPEL is virtually identical to the course of Light and d ark adaptation of the scotopic luminance threshold following exposure to the same luminance. We suggest that, following rod stimulation alon g particular retinal orientations by portions of the pitched visual fi eld, the storage of the adaptation process resides in the retinogenicu late system and is manifested in the focal system as a change in lumin ance threshold and in the ambient system as a change in VPEL. The Line ar model previously developed to account for VPEL, which was based on the interaction of influences from the pitched visual field and extrar etinal influences from the body-referenced mechanism, was employed to incorporate the effects of adaptation. Connections between VPEL adapta tion and other cases of perceptual adaptation of visual direction are described.