From Nicholas Carr, The Shallows:
In 1879, a French ophthalmologist named Louis Emile Javal discovered that when people read, their eyes don’t sweep across the words in a perfectly fluid way. Their visual focus advances in little jumps, called saccades, pausing briefly at different point along each line. One of Javal’s colleagues at the University of Paris soon made another discovery: that the pattern of pauses, or “eye fixations,” can vary greatly depending on what’s being read and who’s doing the reading. In the wake of these discoveries, brain researchers began to use eye-tracking experiments to learn more about how we read and how our minds work. Such studies have also proven valuable in providing further insights into the Net’s effects on attention and cognition.
In 2006, Jako Nielsen, a longtime consultant on the design of Web pages who has been studying online reading since the 1990s, conducted an eye-tracking study of Web users. He had 232 people wear a small camera that tracked their eye movements as they read pages of text and browsed other content. Nielsen found that hardly any of the participants read online text in a methodical, line-by-line way, as they’d typically read a page of text in a book. The vast majority skimmed the text quickly, their eyes skipping down the page in a pattern that resembled, roughly, the letter F. They’d start by glancing all the way across the first two or three lines of text. Then their eyes would drop down a bit, and they’d scan about halfway across a few more lines. Finally, they’d let their eyes cursorily drift a little farther down the left-hand side of the page. This pattern of online reading was confirmed by a subsequent eye-tracking study carried out at the Software Usability Research Laboratory at Wichita State University.
“F,” wrote Nielsen, in summing up the findings for his clients, is “for fast.”