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 widely debated. To assess linguistic and oculomotor contributions to these return-sweep fixation differences, we compared the eye movements of 41 participants during normal reading and z-string scanning, an oculomotor control condition to reading, which is devoid of useful linguistic content. Our results indicate that line-final fixations are shorter and accurate line-initial fixations are longer 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 accurate line-initial fixation durations between reading and z-string scanning implies that longer accurate line-initial fixations are likely a result of lexical processing and oculomotor coordination or 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.