In recent decades, advances in eye tracking technology have made eye movement studies practical, non-invasive, and precise. Computers outfitted with sophisticated eye tracking equipment can record and analyze the movement of a subject’s eyes with great precision, giving us a tremendous amount of data about eye movement and vision. The traditional qualifier is that, while we can know with great accuracy where a subject’s eyes are looking, and for how long, we can’t know the subject’s cognitive focus. (In other words, we can tell where the subject’s eyes are focused, but we can’t be sure he or she is thinking about what the eyes are focused on.) Researchers have reason to believe, for example, that a reader’s cognition “trails” the movement of the eye. In recent years there have been attempts to better understand the relationship between a reader’s visual focus and his or her cognitive process, but speaking cautiously we are still limited in the claims we can make about a reader’s mental activity as it relates to eye movement.
To understand eye movement during reading, it’s necessary to understand the saccade. The saccade is one of the most important aspects of human vision and eye movement. Pick a spot on the far wall, and try to keep your gaze held as steadily as possible. You’ll likely find that it’s quite hard not to move your eye after a short period. (In fact, it’s harder than you know– you’re moving your eye even without realizing it, as I’ll discuss.) Looking at something for more than a very brief without eye movement is in fact quite unnatural; our eyes are constantly moving in short, fast movements. These movements are saccades.
Why do our eyes move so quickly and ceaselessly? Resolution, for one. The retina doesn’t produce equally sharp images all along its tissue. For example, peripheral vision, while better for movement detection, is significantly less sharp than forward vision. The highest resolution area of the retina, and the part involved in central vision, is called the fovea. The fovea “records” the sharpest images, but is in fact quite small– maybe 2 degrees wide. (The analogy of the retina to film, while problematic in many ways, is a useful way of thinking about the relationship between retinal area and image producing.) By constantly moving around in short, quick movements, the eye resolves a sharper, higher-quality image. Additionally, saccadic movement is understood to help depth perception and rendering objects in three dimensions.
How short and quick are these movements? Try 20 milliseconds– that’s 20 thousandths of a second– during a close-focus visual activity like reading. As I said above, we’re often entirely unaware that our eyes are making these short visual adjustments. In such a situation, we perceive neither the movement of our eyes themselves, nor the blurring that should occur when we move our central vision (when our foveal focus shifts). This is saccadic masking, a neurological process through which the brain limits our perception of our own eye movement. As an experiment, you can have a friend stand in front of you and look from one of your eyes to another. As fast as the eye movement is, you’ll clearly see his or her eyes moving from left to right. Then try standing in front of a mirror and looking from one eye to the other. You won’t perceive any of the same movement; your eyes will simply “blink” from one to the other. Given the speed and frequency of saccades, you can imagine why saccadic masking is to our advantage. It would likely be maddening to move our eyes so quickly and so often and be completely aware that it was happening.
Consideration of saccadic masking is important when understanding reading, as it speaks to an important insight into literacy: reading is a substantially nonlinear activity. The traditional concept of reading as a steady progression from one word to the next, in the order proscribed by a given language (left to right from word to word and up to down from line to line, in English), is not an accurate picture of reading in practice. (I say word to word, because adult, non-disordered readers read in whole word chunks, not phonically.) While a general movement from left to right and from top to bottom of a given page is present in the reading of English, within smaller units of observation there are constant deviations from this practice. Readers revert to previously focused-on words and phrases constantly, and again often without being conscious of doing so. This isn’t an artifact of poor reading ability or disorder, either; even adults capable of reading at high speeds constantly violate word order while reading.
There are many consequences to this understanding of the visual habits of readers, and lots of generative possibilities for research. My mentor and friend Robert Schwegler, for example, has performed research utilizing eye tracking technology to better understand reader perception of error. Some of his findings include the fact that many grammatical errors (it’s/its inversion, for example) don’t produce saccadic events from readers, but syntactic errors (subject deletion, for example) produce major saccadic reactions, speaking to the preeminence of syntactic cohesion and universal grammar. Or consider well-meaning parents and instructors who, faced with children whose dyslexia presents as an inability to parse word order appropriately, counsel them to read in a rigid linear order, unaware that such behavior is not in keeping with typical adult reading practices.
As you’d imagine, the scientific exploration of the neurocognitive interface between visual stimuli, eye movement, and reader perception is well above my level of expertise. I do believe, however, that with assistance I can read and understand new scholarship on these issues, and that such scholarship has significant consequences for the empirical study of literacy and writing. It’s a very exciting time.