Limit of single vision in stereopsis depends on contour sharpness.

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OUR two eyes see slightly different images of the world, and this enables us to estimate the position of objects in depth. When we look at a fixed point in space we can correctly judge the distance of an object that is in front of, or behind this point by the amount of disparity of its retinal images. Our brain combines these disparate images in such a way that we can see the object as single within a certain limit of disparity. This limit is called Panum's fusional area; for fine-line targets the disparity limits are found to be ±7 min arc for central (foveal) vision, and larger for peripheral vision (for review see ref. 1). However, Fender and Julesz2 demonstrated that fusion may be extended beyond Panum's area when random-dot stereograms are used. Experiments reported here demonstrate that Panum's area also increases for stimuli with blurred contours, that is, for contours containing lower spatial frequencies.