Are the differences between intra-line and return-sweep fixation durations driven by linguistic, oculomotor, or visual processing? A comparison of eye movements during reading and z-string scanning.

Return-sweeps, which move the reader's gaze from the end of one line to the beginning of the next, typically result in shorter line-final fixations and longer accurate line-initial fixations compared to intra-line fixations. The mechanisms underlying these differences have been the subject of debate. To assess the linguistic and oculomotor contributions to these return-sweep fixation differences, we compared the eye movements of 41 participants during reading and z-string scanning, an oculomotor control condition that is devoid of useful linguistic content. Our results indicate that line-final fixations are shorter than intra-line fixations, while accurate line-initial fixations are longer than intra-line fixations, under both tasks, underscoring the significant role of the oculomotor system in determining fixation durations across tasks. Notably, the reduction in line-final fixation durations compared to intra-line fixations did not differ between tasks. This suggests that oculomotor coordination or visual processing, rather than linguistic processing, drives shorter line-final fixations. In contrast, the difference in the increase in duration for accurate line-initial fixations between reading and z-string scanning implies that longer accurate line-initial fixations are likely a result of lexical processing, oculomotor coordination, and visual processing. These findings advance our understanding of eye movement control by highlighting the combined influence of linguistic and oculomotor processes on return-sweep fixation durations.