The Function/Content Word Distinction and Eye Movements in Reading

A substantial quantity of research has explored whether readers' eye movements are sensitive to the distinction between function and content words. No clear answer has emerged, in part due to the difficulty of accounting for differences in length, frequency, and predictability between the words in the two classes. Based on evidence that readers differentially overlook function word errors, we hypothesized that function words may be more frequently skipped or may receive shorter fixations. We present two very large-scale eyetracking experiments using selected sentences from a corpus of natural text, with each sentence containing a target function or content word. The target words in the two classes were carefully matched on length, frequency, and predictability, with the latter variable operationalized in terms of next-word probability obtained from the large language model GPT-2. While the experiments replicated a range of expected effects, word class did not have any clear influence on target word skipping probability, and there was some evidence for a "content" word advantage in fixation duration measures. These results indicate that readers' tendency to overlook function word errors is not due to reduced time spent encoding these words. The results also broadly support the implicit assumption in prominent models of eye movement control in reading that a word's syntactic category does not play an important role in decisions about when and where to move the eyes.