The influence of a stationary single line in darkness on the visual perception of eye level.

The angle of pitch of a visual field consisting of only a single vertical, 64 degrees-long, eccentrically-located line in otherwise total darkness influences the elevation of a target set to appear at eye level (VPEL). The influence changes linearly with the magnitude of pitch over the range from -30 degrees to +20 degrees. The average slope of the VPEL-vs-pitch function is +0.53. The influence on VPEL of a pitched visual field consisting of two parallel vertical lines is slightly greater (slope = +0.56), and the influence of the pitch of a complexly-structured well-illuminated pitched room is slightly greater yet (slope = +0.63). The pitch of a frontoparallel plane containing one horizontal line has a small influence on VPEL (slope = +0.08); the influence with two horizontal lines is slightly greater (slope = +0.18). The slope of the VPEL-vs-pitch function differs among individual subjects but is linear for each of the eight subjects. A great deal of consistency is manifested by individual subjects across all of the visual fields: an individual with a steep slope with one visual field tends to have a steep slope with all visual fields. The individual's characteristic response in total darkness is strongly correlated with the response to an erect well-illuminated visual field. The significant aspect of the pitched-from-vertical line stimulus is the change in orientation of its retinal image. An additional experiment with a small pupil (pilocarpine) indicates that cues related to other retinal gradients or to accommodation play no role in the influence of the visual field on VPEL. The experiments provide support for treating the visual influence on VPEL by means of the Great Circle Model.