The curious case of spillover: Does it tell us much about saccade timing in reading?

In completing daily activities, the eyes make a series of saccades by gazing at stimuli in succession. The duration of gaze on each stimulus has been used to infer how the initiation of a saccade is timed relative to the underlying mental processing. In reading, gaze dwells longer on a word that occurs infrequently in English text (low frequency) than on a more frequent word (high frequency), but also on the following word, which is referred to as spillover. Accounts of spillover attribute it to mechanisms of lexical access. A low-frequency word n is assumed to delay the onset of cognitive processing of word n+1 more than it delays the saccade to n+1, leaving more processing to be done on n+1 once it is fixated. We tested this assumption by having participants perform a series of speeded lexical decisions on a linear array of letter strings spaced 5° apart, using low- and high-frequency words to vary the lexical difficulty. Lexical decision adds a response selection stage that is absent in reading, which should eliminate differential effects on saccades and cognitive processing. Nonetheless, we found the typical pattern of lengthened gaze duration and spillover for low-frequency words, with effects that were consistent in magnitude with those seen in studies of reading. These data challenge existing accounts of spillover and argue against the idea that reading has a unique interaction with oculomotor control. Instead, the similarity of our gaze patterns to those of reading suggests a common pattern of saccade initiation across tasks.