The Effect of Stimulus Size on Stereoscopic Fusion Limits and Response Criteria

The stereoscopic fusion limit denotes the largest binocular disparity for which a single fused image is perceived. With few exceptions, this limit has been measured with thin lines or bars employing a criterion of single or double images. We wondered if the fusion limit and criterion for fusion (for example, diplopia or lustre) changed systematically as a function of bar width. We first measured horizontal and vertical disparity fusion limits for thin bars and for the edge of an extended surface. Seven observers increased or decreased the disparity of the target and responded when fusion broke or reappeared, respectively, using either the criterion of diplopia or lustre. Fusion limits were equal for thin bars and extended surfaces in both horizontal and vertical disparity conditions. We next measured fusion limits for a range of bar widths and instructed four observers from experiment 1 to indicate which criterion they employed on each trial. Fusion limits were constant across all stimulus widths. However, there was a sharp change in criterion from diplopia to luster when the bar width was approximately twice the fusion limit. Stereoscopic fusion limits do not depend on stimulus size, but the criterion for fusion does.