Eye movement affords a good picture of the mobility of attention. Ordinarily the eye shifts frequently from one part of the field of view to another. When simply exploring a scene, it shifts about in what seems an indiscriminate way, though really following the principle of deserting each object as soon as it has been examined, and jumping to that other object which next has the advantage on account of movement, brightness, color, definite form, or habit of attention. In reading, however, the eye is governed by a definite interest, and moves consecutively along the series of words, instead of shifting irregularly about the page.

A moving object, or an object that is doing something, or even a complex object that presents a number of parts to be examined in turn, can hold the eye for some time. But it is almost impossible to hold the eye fixed for any length of time on a simple, motionless, unchanging object.{252}

Attention is mobile because it is exploratory; it continually seeks something fresh for examination. In the presence of a complex of sights and sounds and touch stimuli, it tends to shift every second or two from one part of the situation to another. Even if you are lying in bed with your eyes closed, the movement of attention still appears in the rapid succession of thoughts and images, and some shift usually occurs as often as once a second.

A few simple experiments will serve to throw the shifting of attention into clearer relief. Look fixedly at a single letter written on a blank sheet of paper, and notice how one part after another of the letter stands out; notice also that attention does not stick absolutely to the letter, since thoughts obtrude themselves at intervals.

Fig. 42.--A dot figure, from Sanford. Look steadily at it.]

Or, make a "dot figure", composed of six or eight or more dots arranged either regularly or irregularly, and look steadily at the collection. Probably you will find that the dots seem to fall into figures and groups, and that the grouping changes frequently. Objectively, of course, the dots are grouped in one way as much as another, so that any particular grouping is your own doing. The objective stimulus, in other words, is capable of arousing several grouping reactions on your part, and does arouse different reactions one after another

Shifting also appears in looking at an {253} "ambiguous figure", drawn so as to represent equally well a solid object in either of two different positions. The transparent cube, showing near and far edges alike, is a good example. Look steadily at such a drawing, and the cube will appear to shift its position from time to time. Numerous such figures can be constructed; the most celebrated is the ambiguous staircase. Look steadily at it, and suddenly you see the under side of a flight of stairs, instead of the upper; and if you keep on looking steadily, it shifts back and forth between these two positions.

Fig. 43.--The ambiguous cube figure.

Fig. 44.--The ambiguous staircase figure.

A still more striking case of shifting goes by the name of "binocular rivalry", and occurs when colors or figures that we cannot combine into a single picture are presented, {254} one to one eye, and the other to the corresponding part of the other retina. Hold red glass close in front of one eye and blue before the other, and look through both at once towards a bright background, and you will see red part of the time and blue part of the time, the two alternating as in the case of ambiguous figures.

Fig. 45.--Another ambiguous figure, which can be seen in three ways.

The stereoscope is a great convenience in applying inconsistent stimuli to the two eyes, and by aid of this instrument a great variety of experiments can be made. It is thus found that, if the field before one eye is a plain color, while the other, of a different color, has any little figure on it, this figure has a great advantage over the rival plain color and stays in sight most of the time. Anything moving in one field has a similar advantage, and a bright field has the advantage over a darker one. Thus the same factors of advantage hold good in binocular rivalry as in native attention generally.

A different kind of shifting appears in what is called "fluctuation of attention". Make a light gray smudge on a white sheet of paper, and place this at such a distance that the gray will be barely distinguishable from the white {255} background. Looking steadily at the smudge, you will find it to disappear and reappear periodically. Or, place your watch at such a distance that its ticking is barely audible, and you will find the sound to go out and come back at intervals. The fluctuation probably represents periodic fatigue and recovery at the brain synapses concerned in observing the faint stimulus.

Shiftings of the fluctuation type, or of the rivalry type either, are not to be regarded as quite the same sort of thing as the ordinary shiftings of attention. The more typical movement of attention is illustrated by the eye movements in examining a scene, or by the sequence of ideas and images in thinking or dreaming. Rivalry and fluctuation differ from this typical shifting of attention in several ways:

(1) The typical movement of attention is quicker than the oscillation in rivalry or fluctuation. In rivalry, each appearance may last for many seconds before giving way to the other, whereas the more typical shift of attention occurs every second or so. In fact, during a rivalry or fluctuation experiment, you may observe thoughts coming and going at the same time, and at a more rapid rate than the changes in the object looked at. Attention does not really hold steady during the whole time that a single appearance of an ambiguous figure persists.

(2) Rivalry shifts are influenced very little, if at all, by the factor of momentary desire or interest, and are very little subject to control.

(3) In rivalry, the color that disappears goes out entirely, and in looking at a dot figure or ambiguous figure you get the same effect, since the grouping or appearance that gives way to another vanishes itself for the time being. But when, in exploring a scene with the eyes, you turn from one object to another, the object left behind simply retires to the background, without disappearing altogether; and, {256} in the same way, when attention shifts from one noise to another, the first noise does not lapse altogether but remains vaguely heard. Or when, in thinking of a number of people, one after another comes to mind, the first one does not go out of mind altogether when attention moves to the next, but remains still vaguely present for a few moments.