Interaction between Perspective Visual Cues and Monocular versus Binocular Vision in the Perception of Pitch Subjective Vertical

Abstract

In a pitch vertical adjustment, a rectangular and plane surface (rod) could be assessed as being upright when its lateral sides looks parallel. According to this hypothesis, geometrical analysis underscores that adjustment errors relative to the vertical are a function of the width of the rod, its gradient of perspective, and its distance from the observer's eyes. In order to verify this relationship, thirty-two subjects assessed pitch subjective vertical (PSV) in two visual conditions (monocular versus binocular) by rotating in darkness different luminous rods in the median plane. The trapezoid rods varied both in their width and in their angles of convergence measured from each lateral side. The rod-to-eyes distance was fixed at 60 cm and the length of the rod was 25 cm. The results showed that the data fitted in with the geometrical analysis in monocular and binocular conditions. However, in the binocular condition the weight of the shape visual cues decreased considerably. Moreover, a significant constant error remained even with the rectangular surface. It is concluded that rod adjustment to PSV is essentially a visual process similar to the perception of surface slant as Perrone's model develops it (1982 Perception 11 641 – 654). Indeed, the visual cues involved in the vertical adjustment task are in accordance with this model. Also, the constant error observed in PSV could be explained by the deviation of the subjective eye level which Perrone supposes to account for the slant underestimation in his model.