The tale of reading begins when the retina receives photons reflected off the written page. But the retina is not a homogeneous sensor. Only its central part, called the fovea, is dense in high-resolution cells sensitive to incoming light, while the rest of the retina has a coarser resolution. The fovea, which occupies about 15 degrees of the visual field, is the only part of the retina that is genuinely useful for reading. When fovea] information is lacking, whether due to a retinal lesion, to a stroke having destroyed the central part of the visual cortex, or to an experimental trick that selectively blocks visual inputs to the fovea, reading becomes impossible.”

The need to bring words into the fovea explains why our eyes are in con­stant motion when we read. By orienting our gaze, we “scan” text with the most sensitive part of our vision, the only one that has the resolution needed to determine letters. However, our eyes do not travel continuously across the page.’ Quite the opposite: they move in small steps called saccades. At this very moment, you are making four or five of these jerky movements every second, in order to bring new information to your fovea.

Even within the fovea, visual information is not represented with the same precision at all points. In the retina as well as in the subsequent visual relays of the thalamus and of the cortex, the number of cells allocated to a given por­tion of the visual scene decreases progressively as one moves away from the center of gaze. This causes a gradual loss of visual precision. Visual accuracy is optimal at the center and smoothly decreases toward the periphery. We have the illusion of seeing the whole scene in front of us with the same fixed accu­racy, as if it were filmed by a digital camera with a homogeneous array of pixels. However, unlike the camera, our eye sensor accurately perceives only the precise point where our gaze happens to land. The surroundings are lost in an increasingly hazy blurriness.

One might think that, under these conditions, it is the absolute size of printed characters that determines the ease with which we can read: small letters should be harder to read than larger ones. Oddly enough, however, this is not the case. The reason is that the larger the characters, the more room they use on the retina. When a whole word is printed in larger letters, it moves into the periphery of the retina, where even large letters are hard to discern. The two factors compensate for each other almost exactly, so that an enormous word and a minuscule one are essentially equivalent from the point of view of retinal precision. Of course, this is only true provided that the size of the characters remains larger than an absolute minimum, which corresponds to the maximal precision attained at the center of our fovea. When visual acuity is diminished, for instance in aging patients, it is quite logical to recommend books in large print.

Because our eyes are organized in this way, our perceptual abilities depend exclusively on the number of letters in words, not on the space these words occupy on our retina.9 Indeed; our saccades when we read vary in absolute size, but are constant when measured in numbers of letters. When the brain prepares to move our eyes, it adapts the distance to be covered to the size of the characters, in order to ensure that our gaze always advances by about seven to nine letters. This value, which is amazingly small, thus corresponds approximately to the information that we can process in the course of a single eye fixation.

To prove that we see only a very small part of each page at a time, George W. McConkie and Keith Rayner developed an experimental method that I like to call the “Cartesian devil.” In his Metaphysical Meditations, Rene Descartes imagined that an evil genius was playing with our senses:

I shall then suppose, not that God who is supremely good and the fountain of truth, but some evil genius not less powerful than deceitful, has employed all his energy to deceive me; I shall consider that the heavens, the earth, colors, figures, sound, and all other external things are naught but the illusions and dreams of which this genius has availed himself in order to lay traps for my credulity. I shall consider myself as having no hands, no eyes, no flesh, no blood, nor any senses, yet falsely believing myself to possess all these things.

Much like the supercomputer in the Matrix movies, Descartes’ evil genius produces a pseudo-reality by bombarding our senses with signals carefully crafted to create an illusion of real life, virtual scene whose true side remains forever hidden. More modestly, McConkie and Rayner designed a “moving window” that creates an illusion of text on a computer screen.”‘ The method consists in equipping a human volunteer with a special de, ice that tracks eye movements and can change the visual display in real time. The device can be programmed to display only a few characters left and right of the center of gaze, while all of the remaining letters on the page are replaced with strings Xs: As soon as the eyes move, the computer discreetly refreshes the display. Its goal is to show the appropriate letters at the place where the person is looking, and strings of x’s everywhere else:

Xx xxx people of the xxxxxxxx xxxxxx, xx xxxxx xx

Xx xxx xxxxxxxhe United xxxxxx, xx xxxxx xx

Xx xxx xxxxxx xx xxxXxxxed States, in xxxxxx xx

Xx xxx xxxxxx xx xxx Xxxxxx Xxxxxx,in order to

Using this device, McConkie and Rayner made a remarkable and para­doxical discovery. They found that the participants did not notice the manip­ulation. As long as enough letters are presented left and right of fixation, a reader fails to detect the trick and believes that he is looking at a perfectly normal page of text.

This surprising blindness occurs because the eye attains its maximum speed at the point when the letter change occurs. This trick makes the letter changes hard to detect, because at this very moment the whole retinal image is blurred by motion. Once gaze lands, everything looks normal: within the fovea, the expected letters are in place, and the rest of the visual field, on the periphery, cannot be read anyway. McConkie and Rayner’s experiment thus proves that we consciously process only a very small subset of our visual inputs. If the computer leaves four letters on the left of’ fixation, and fifteen letters on the right, reading speed remains normal.” In brief, we extract very little information at a time from the written page. Descartes’ evil genius would only have to display twenty letters per fixation to make us believe that we were reading the Bible or the U.S. Constitution!

Twenty letters is, in fact, an overestimate. We identify only ten or twelve letters per saccade: three or four to the left of fixation, and seven or eight to the right. Beyond this point, we are largely insensitive to letter identity and merely encode the presence of the spaces between words. By providing cues about word length, the spaces allow us to prepare our eye movements and ensure that our gaze lands close to the center of the next word. Experts con­tinue to debate about the extent to which we extract information from an upcoming word—perhaps only the first few letters. Everyone agrees, however, that the direction of reading imposes asymmetry on our span of vision. In the West, visual span is much greater toward the right side, but in readers of Arabic or Hebrew, where gaze scans the page from right to left, this asymmetry is reversed.” In other writing systems such as Chinese, where character, density is greater, saccades are shorter and visual span is reduced accordingly. Each reader thus adapts his visual exploration strategy to his language and script.

Using the same method, we can also estimate how much time is needed to encode the identity of words. A computer can be programmed so that, after a given duration, all of the letters are replaced by a string of x’s, even in the fovea. This experiment reveals that fifty milliseconds of presentation are enough for reading to proceed at an essentially normal pace. This does not -can that all of the mental operations involved in reading are completed in one twentieth of a second. As we shall see, a whole pipeline of mental processes continues to operate for at least one-half second after the word has been presented. However, the initial intake of visual information can be very brief.

In summary, our eyes impose a lot of constraints on the act of reading. The structure of our visual sensors forces us to scan the page by jerking our eyes around every two or three tenths of a second Reading is nothing but the word-by-word mental restitution of a text through a series of snapshots. ‘file some small grammatical words like “the,” “it or “is” can sometimes be skipped, almost all content words such as nouns and verbs have to be fixated at least once.

These constraints are an integral part of our visual apparatus and cannot be lifted by training. One can certainly teach people to optimize their eye movement’s patterns, but most good readers, who read from four hundred to hundred words per minute, are already close to optimal. Given the reti­nal sensor at our disposal, it is probably not possible to do much better. A simple demonstration proves that eye movements are the rate-limiting step in reading. If a full sentence is presented, word by word, at the precise point where gaze is focalized, thus avoiding the need for eye movements, a good reader can read at staggering speed—a mean of eleven hundred words per minute, and up to sixteen hundred words per minute for the best readers, is about one word every forty milliseconds and three to four times faster than normal reading! With this method, called rapid sequential visual presentation, or RSVP, identification and comprehension remain satisfactory, thus suggesting that the duration of those central steps does not impose a strong constraint on normal reading. Perhaps this computerized presentation mode represents the future of reading in a world where screens progressively replace paper.

At any rate, as long as text is presented in pages and lines, acquisition through gaze will slow reading and impose an unavoidable limitation. Thus, fast reading methods that advertise gains in reading speed of up to one thou­sand words per minute or more must be viewed with skepticism.  One can no doubt broaden one’s visual span somewhat, in order to reduce the number of saccades per line, and it is also possible to learn to avoid moments of regres­sion, where gaze backtracks to the words it has just read. However, the physi­cal limits of the eyes cannot be overcome, unless one is willing to skip words and thus run the risk of a misunderstanding. Woody Allen described this situ­ation perfectly: “I took a speed-reading course and was able to read War and Peace in twenty minutes. It involves Russia.”

Excerpted from ‘Reading in the Brain’ by Stanislas Dehaene Page 13-18