Reading is a behavior that is commonly investigated with eye tracking technology. Believe it or not, eyes do not move in a smooth motion across lines of text as you might imagine. In fact, reading behavior is actually a series of fixations and saccades as your eyes dart from one point to the next. Fixations last about 200-250 milliseconds and do not occur on every word. Saccades are typically 7-9 letters in length as a reader skims the text and moves from one fixation point to the next.
Fixation points are affected by several different factors, including the type and length of the word, the syntactic or contextual difficulty, and whether the reading is aloud or silent. Content words are generally fixated on 85% of the time, whereas function words, like prepositions, are only 35%. Words that are 2-3 letters long get skipped over in a saccade 75% of the time, and 8 letter words are almost always fixated on. The fixations usually fall at the beginning or middle of a word, and information is gathered around the fixation point. This is called the “perceptual span,” and it is asymmetric; it extends 3-4 letters to the left and 14-15 to the right, indicating that the reader is taking in the upcoming text. The reverse is true for languages that run right to left, and it is rotated for languages that are read vertically.
Saccades are mostly forward movements; however, about 10-15% are regressive, meaning the eyes skip backward to fixate on previously read words. This indicates that there were difficulties in processing the previous word or meaning of the sentence, so it must be revisited for clarity. The human brain is able to process written language without having to fixate on each individual letter or even every word. The brain fills in the gaps of letters and words skipped over by saccades to assemble the meaningful context of the sentence.
Eye tracking research on eye movements during reading has significant applications for education psychologists. It can be used to study differences in eye movement patterns in children of different grade levels to analyze language processing abilities. It can also be used to compare the reading behaviors of dyslexics and non-dyslexics. Check out this previous Eye-Com Research article about how eye tracking research on reading behavior is being used to adapt e-learning programs.