Binocularly Perceived Direction and Outline Shape

Abstract

Suppose there is a square Julesz random-dot pattern located in front of a background pattern and suppose there is a binocularly visible slider in the background. If subjects align the slider with the rims of the square, the outline of the occlusion in the background pattern has a vertically elongated rectangular shape (Erkelens, Muijs, and van Ee Vision Research in press). The cyclopean-eye concept predicts the outline shape of occluder and occlusion to be the same. However, subjects had the impression that the occluder had the shape of a square. We investigated this impression psychophysically. We measured perceived outline shape of stereoscopically defined rectangles presented in front of a random-dot pattern. In a series of 2AFC tasks subjects decided whether they perceived a horizontally or a vertically elongated rectangle. The horizontal side subtended 8 deg and the vertical side varied between 86% and 114% of 8 deg. The disparity between foreground and background was varied and all trials were presented either in the presence or the absence of monocularly visible contours. We repeated the experiment also during monocular viewing with the left and the right eye. We used exposure durations of 0.5 and 3.0 s. In all circumstances we found that the outline shape of the occluder was veridically perceived; it was not vertically elongated in accordance with the elongation found in the alignment tasks. Apparently information from the binocular alignment of objects in different depth planes is not used for extracting the outline shape of an object.