Combined effects of spatial frequency and retinal eccentricity upon fixation disparity

The variations in fixation disparity (FD) with spatial frequency and retinal locus of fusion locks were examined in two groups of subjects having either steep or flat forced-duction FD curves. FD for subjects with flat curves was unaffected by either spatial frequency or diameter of fusion locks. FD of observers with steep forced-duction curves increased with both the coarseness and eccentricity of fusion locks. Panum's horizontal fusion range increased markedly with target spatial period but only modestly with retinal eccentricity. These results indicate that factors other than the dimensions of Panum's fusional area (PFA) result in the increase of FD found in some observers with peripheral fusion locks.