Binocular coordination and return-sweep saccades among skilled adult readers

During reading, binocular coordination ensures that a unified perceptual representation of the text is maintained across eye movements. However, slight vergence errors exist. The magnitude of disparity at fixation onset is related to the length of the preceding saccade. Return-sweeps are saccadic eye movements that span a line of text and direct gaze from the end of one line to the start of the next. As these eye movements travel much farther than intraline saccades, increased binocular disparity following a return-sweep is likely. Indeed, increased disparity has been a proposed explanation for longer line-initial fixations. Thus, we sought to address the following questions: Is binocular disparity larger following a return-sweep saccade than it is following an intraline saccade, and is the duration of a line-initial fixation related to binocular disparity and coordination processes? To examine these questions, we recorded binocular eye movements as participants read multiline texts. We report that, following return-sweeps, the magnitude of disparity at fixation onset is increased. However, this increased magnitude of disparity is unrelated to the duration of line-initial fixations. We argue that longer line-initial fixations result from a lack of parafoveal preview for words at the start of the line.