The oblique effect is a decreased sensitivity for oblique orientations compared with horizontal/vertical orientations. We investigated whether orientation-matching acuity for binocularly presented stimuli is dependent on orientations of retinal projections or on the perceived orientation of the stimulus. Using a stereoscope, we constructed a stimulus, such that retinal orientations were oblique, while the perceived orientation was vertical. We then used this stimulus to test observers in an orientation-matching task. If orientation matching depends on the orientations of retinal projections, low matching performance would be expected, since retinal projections are oblique. However, if orientation matching depends on perceived orientation (vertical), high matching performance would be expected. We found that observers' performance can only be predicted from the perceived orientation, not the orientations of retinal projections, implying that in binoc-ular viewing the oblique effect occurs after binocular fusion.
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