Binocularity during reading fixations: Properties of the minimum fixation disparity

The present study was based on the physiologically reasonable assumption that the binocular system aims for a reduction of fixation disparity during fixation and that the minimum amount of fixation disparity reflects the optimal binocular status. We measured eye movements (EyeLink II) of 18 participants, while they read 60 sentences from the Potsdam-Sentence-Corpus (PSC) at a viewing distance of 60 cm. The minimum fixation disparity was frequently reached directly after the post-saccadic drift, sometimes at the end of fixation and sometimes somewhere in between. Minimum fixation disparity was strongly influenced only by fixation position (within the sentence) while the amplitude of incoming saccade had a negligible effect. Moreover, the effect of fixation position on minimum fixation disparity was correlated with the individual ability to compensate for binocular disconjugacy (due to saccades) while fixating during reading. Generally, we found fixation disparity to be correlated between conditions of reading and fixating single targets, while the reading fixation disparity tended to be more crossed (eso).