Binocular summation was studied with two different hyperacuity tests, a bisection test and a vernier-type test. The summation factor differed between the two tests and depended on the test configuration. In the bisection test, binocular summation increased with decreasing line separation, and for the largest separation binocular inhibition was found. In the vernier test summation was essentially more constant for different separations. It is argued that, for a bisection test with a small intertarget distance, the hyperacuity thresholds involve a component of intensity discrimination that is due to overlapping retinal line-spread functions. An additional, unexplained finding was that in a vertically oriented three-dot alignment test the monocularly perceived alignment always occurred while the center dot was displaced toward the nasal side.
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