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Eye Movements during Reading

This entry briefly details what is known about eye movements during reading. It includes an overview of properties of the eye, types of eye movements, commonly used eye movement measures, research methodologies, a discussion of visual and linguistic factors of words that affect eye movements during reading, and a brief mention of a computational model that attempts to simulate these influences.

Eye Movements and Properties of the Eye

When we read, our eyes don't glide smoothly across the page. Rather, we make a series of eye movements (called saccades) separated by fixations. Eye movements are tightly linked to cognitive processing and offer the opportunity to rigorously test hypotheses about how the mind operates during reading. The reason we move our eyes is so that we can place the fovea—that part of our retina with the highest acuity (resolution)—over the text we wish to process. The fovea extends 2 degrees of visual angle around fixation and outside of this area acuity drops off rapidly in the parafovea (2–5 degrees) and periphery (5 + degrees). Saccades, which typically last 25 to 60 milliseconds (ms) (depending on their size), are considered to be ballistic—so that it is difficult to change trajectory once initiated—and take approximately 175 ms to plan and execute if no other cognitive processing must be completed. Between the saccades, our eyes are in relatively stable states—fixations—which last approximately 225 to 250 ms. Information is obtained during fixations as we are essentially “blind” during saccades because of saccadic suppression. For the most part, overt attention (where the eyes fixate) and covert attention (where the mind attends) are tightly linked in reading.

In reading, the average saccade extends 7 to 9 letter spaces. Letter spaces are the appropriate indicator of how far the eyes move since regardless of the reading distance of the text (which modulates text size on the retina), eye movements extend the same number of letter spaces, not the same degree of visual angle. Not all saccades move forward in the text; 10% to 15% of saccades are regressions, which means they return to previously read or skipped text. More difficult text leads to more regressions as well as shorter saccades and longer fixations.

Eye Movement Measures

Although the abovementioned metrics—global measures of processing—are important, researchers more often discuss local measures of processing, such as first fixation duration (duration of the first fixation on a word), single fixation duration (duration of the fixation on a word when the word was only fixated once, excluding regressive fixations), and gaze duration (the sum of all fixations on a word before the eyes move to another word). These measures are more useful because they are more robust to skipping and regressions, whereas global measures have the implicit assumption that all words are fixated once. Approximately a third (31%) of words in text are skipped—not directly fixated. In general, these words are short, high-frequency words with little semantic content (e.g., the, in, and) or words that are highly predictable from prior text. Conversely, words likely to be refixated (i.e., fixated more than once before the eyes continue) tend to be longer and less frequent. The abovementioned local measures are useful to estimate how long it takes to process a word.

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