When a subject monocularly views a bar grating that is oriented counterclockwise from vertical, and then immediately looks at a vertically oriented grating, he/she perceives it rotated clockwise. This tilt aftereffect occurs when the adapting and test targets are viewed by the same eye (direct effect) or opposite eye (interocular transferral effect). An apparatus has been developed to measure the tilt aftereffect in a clinical setting and to use such measures in assessing the prognosis for functional correction of strabismus on the assumption that the transferred tilt aftereffect is an indicator of the percentage of binocularly driven cortical neurons. Results from six normal and two strabismic subjects are presented.