“Trait” and “state” aspects of fixation disparity during reading

Eye movement research in reading has traditionally been associated with the investigation of visual processing and language comprehension (see, for example: (Kliegl, Nuthmann & Engbert, 2006, Liversedge, White, Findlay & Rayner, 2006b, Rayner, 1998)). Central to the description (and prediction) of eye movement behaviour during reading are saccades and fixations, which are traditionally extracted from the recorded movements of only one eye. But we read with both eyes (binocularly), and besides saccadic eye movements (both eyes move in the same direction) our eyes perform vergence eye movements (the eyes move in opposite directions). In other words, binocular vision of the text requires that the vergence angle between the two visual axes is adjusted for proper fusion of the two retinal images for each fixation. In (theoretically) optimal binocular vision, the principal visual directions of both eyes intersect at the fixation point; slight deviations fixation disparities (FD) or vergence errors from this optimal state typically amount to a few minutes of arc and are thus smaller than the Panum’s area (i.e. the range of disparity where sensory fusion of the two retinal images is performed), not leading to double vision. These fixation disparities are called exo or eso when the visual axes of the eyes converge slightly behind or in front of the fixation point, respectively.