Saccades and fixations: When we read text it feels as if the eyes traverse the page smoothly and continuously, left to right and line by line, without interruption. Examination of the eye movements of skilled readers, however, reveals a rather more complicated process. Our eyes, when we read, are actually stationary for 90 per cent of the time, the remaining ten per cent being taken up with controlled but jerky movement on to the next pause. The period of sudden movement onward is called a saccade, the stationary period between is called a fixation. Each fixation lasts about 250 msecs. (about 1/4 sec.). Each saccade takes about 25 msecs. (about 1/40 sec.). We are, therefore, making just under 4 fixations a second and the eyes are moving for about 1/10 of that time. (As a matter of interest it is not only during reading that this pattern is observed; when we look around at a scene, to ‘take it all in’, our eyes go into much the same saccade/fixation routine.) A skilled reader clocks in at about 90 fixations for every 100 words read, depending, of course, on the difficulty or familiarity of the text itself. Each fixation, therefore, is covering a single word except that very familiar words may be skipped or fixated together with a neighbour and longer, less familiar or composite words (like saccade or fixation) may be fixated at two or more points or for longer than simpler words. Easier, or more familiar, text will require, and receive, fewer fixations. Harder, or less familiar, text will require, and receive, more fixations. More demanding reading will also produce a greater proportion of backward saccades and re-reading of words.

During a fixation the eye sees acutely over a five degree scan; in other words it sees more than simply the word it is fixated on. Interestingly, this scan is biased in a forward direction, that is we see about 2/3 to the right and 1/3 to the left of the word fixated. (An Arabic language reader, who reads, of course, from right to left, sees the opposite, about 2/3 to the left and 1/3 to the right, when reading Arabic script.) Our eyes are obviously getting a sneak preview of the upcoming text. This will slightly reduce uncertainty and improve speed of decision-making. When reading simple text aloud, if it is suddenly obliterated (e.g. if the lights go out) the reader is able to continue for a time, usually to the end of the current grammatical unit (clause, or even sentence). There is evidence that we fixate for longer, in fact, at grammatical boundaries (where there will be helpful punctuation marks) perhaps to allow the mind to digest and organise material before continuing. The eyes also, especially with easier reading, march ahead of the mind which is decoding and recoding to meaning.

So, we have eyes fixating on words yet able to see ahead by another couple of words or thereabouts, and perhaps running ahead of the mind's comprehension when reading simple text. This may allow the mind the chance of prediction, of marrying what has gone with what may be coming in order to eliminate unlikely hypotheses and suggest likely ones in advance of decision-making time, thereby reducing uncertainty and thereby increasing both accuracy and speed of the decision-making process. For example, hippopotami are said to kill more people than crocodiles do. Putting this completely irrelevant remark into this discussion of reading surprised you, I guess. If you did not suspect I was purposefully fooling around you would be jolted to a halt and have to look about in the text a bit before deciding it was a misprint, or I had gone off my head, and (perhaps) continuing. Read in the context of a discussion of the relative dangers of various forms of African wild life, of course, that sentence about hippopotami excites no such shocking dislocation. It would, in that context, fit the hypothesis you had formed as to what was going on and inform and support your onward reading.