Binocular vision - A different angle

The human binocular system has a reflex desire for heterophoria and when using a 'normal' viewing environment the binocular lock will hide small vertical phorias. The effort required in maintaining compensation may cause significant visual symptoms, argues Tony Nixon. It may also be a factor in reduced reading abilities

It is known that the human visual system is highly sensitive to small misalignments evidenced by the accuracy of vernier acuity. However, this same sensitivity may also be a source of visual symptoms, particularly if the binocular fields are misaligned vertically by only a small amount.

Some researchers suggest reduced visual functions when the perceived images come from misaligned eyes. Hubel and Weisel (reported by Solomons)1 showed a reduced cortical binocular receptor cell response with induced ocular phoria in cats. Tunnicliffe and Williams2 reported reduced binocular contrast sensitivity and Jenkins et al3 reported reduced binocular acuity associated with small phorias.