Microstrabismus: Fixation point scotoma, a perimetric artefact

The contribution of the strabismic eye to binocular vision has frequently been studied with stimuli presented solely to the strabismic eye, on a binocular background. These studies revealed a central suppression scotoma in the strabismic eye, the so-called fixation point scotoma. Considering that this scotoma might be an artefact due to the unnatural viewing condition, we employed stereoperimetry that allowed examining the contribution of the strabismic eye under natural viewing, and compared the stereoresolution with the Vernier resolution of the strabismic eye. On the retina of the strabismic eye, the stereotarget was imaged at one of seven locations, between 2 degrees nasally and 2 degrees temporally, whereas on the retina of the non-strabismic eye, the stereotarget was imaged always in the centre. The mean stereoresolution of three micro-esotropic observers was 96'', averaged over all seven locations. A reduction of the stereoresolution in relation to the monocular Vernier resolution and to the performance of three non-strabismic observers indicated a slight diffuse suppression, rather than a circumscribed scotoma. We conclude that the strabismic eye contributes more to binocular vision than has been assumed on the basis of tests with targets presented solely to the strabismic eye (on a binocular background).