Effect of gravitational cues on visual search for orientation.

To determine whether nonvisual (vestibular and somatosensory) information participates in low-level orientation processing, subjects in different postural conditions (upright, supine, and sitting immobilized) searched for a target distinguishable from distractors by difference in orientation (A. Treisman's, 1985, "pop-out" paradigm). Searches for vertical and horizontal targets were dramatically modified as a function of the postural position, indicating that the processing of orientation in early vision is not only retinal but integrates information from the sensory graviceptors. This visuovestibular phenomenon is interpreted in the conceptual framework of D. H. Foster and P. A. Ward's (1991a) model based on local orthogonal orientation filters and T. A. Stoffregen and G. E. Riccio's (1988) dynamics of balance theory.