convergence

Meaning: When you look at a nearby object, the extraocular muscles rotate both eyes inward so the object's image lands on the fovea of each eye. The brain uses feedback from the muscle tension as a cue to distance: more inward turning signals a closer object, and eyes that are nearly parallel signal a distant one. Convergence needs both eyes, so it is a binocular cue, and it is most useful for objects within a few meters.

On the MCAT: It appears in sensation and perception questions on depth perception. The usual task is to sort cues. Binocular cues are convergence and retinal (binocular) disparity. Monocular cues include relative size, interposition, linear perspective, texture gradient and motion parallax. Other items ask what happens as an object approaches (convergence increases) or which cues remain for a person with vision in one eye (the monocular ones).

Common confusion: Convergence is not retinal disparity. Disparity is the difference between the left and right retinal images, which the brain compares. Convergence is the angle of the eyes' inward turn. Both are binocular cues. In biology, convergence also names many neurons synapsing onto one neuron, as with rods onto bipolar cells, which is a different idea.