The "vertical effect" in human squint amblyopia.

Grating acuity was tested in seven squint amblyopes as a function of orientation. In the squinting eyes of six unilateral amblyopes, the resolution for vertical gratings was much lower (by about 1/2 octave) than that of horizontal gratings. The non-amblyopic eyes of these subjects showed a normal "oblique effect". In one bilateral amblyope the selective loss of resolution for vertical contours was found in both eyes. This effect is well correlated with the reduced incidence of cortical cells encoding vertical contours in squinting cats. Both findings can be interpreted as an adaptive modification of the central visual system to alleviate the selective doubling of the vertical contours caused by strabismus.