A Beginner's Psychology

by Edward Bradford Titchener

So the objection is answered. Coming back to the subject, we note that some of our more complex perceptions have a twofold basis: thus the perception of melody is at once qualitative and temporal, and the perception of movement is at once temporal and spatial. Nay more, the perception of a scene, a situation, an event, is threefold: qualitative, temporal and spatial; think of a scene of grand opera, or of an accident on the street. In general, the analysis of these complex perceptions follows from that of the simpler modes, though every one of them has its own psychological problem.

It may seem strange that we have not distinguished a group of perceptions based upon sensory intensity. The fact is, however, that while intensity enters into all sorts of perceptions (lemonade must not be too sour, the members of a rhythm must be variously accented, a distant sound is faint), it only rarely characterises a perception; and when it does, the perception thus characterised belongs to one or other of the groups already mentioned. We say ‘What a heavy child!’—but the perception of weight, like that of resistance, is itself qualitative. Or we say of a certain composer ‘He always overdoes the drums!’—but the drum-rhythm is itself a temporal perception. We cannot point, then, to a separate class of intensive perceptions.

[§ 28]. The Perception of Distance.—A complete psychology of perception would contain an analytical treatment, up to the limits of our present knowledge, of all the various perceptions, qualitative, temporal and spatial, as well as complex, that occur in experience. Such a treatment is here out of the question. We must pick and choose; and as a sample of perception at large we shall consider the perception of distance. We seem, quite immediately and directly, to see distances; we see that our friend is coming nearer, we see that he has passed the bridge, we see that he is entering the gate, we see when to shake hands with him. Yet there is no sensation of distance, and there is no specific stimulus to distance. What, then, really happens?

In the first place, there are plenty of visual cues to distance. We take familiar things to be far off if they look small, and near by if they look large; the size of the men and vehicles in the street makes us realise the height of the building we are gazing down from. We take things to be far off, again, if they are hazy and bluish, near by if they are clearly outlined and varied in colour; everyone knows or has read of the deceptive nearness of distant mountains in clear dry air. We notice the distribution of light and shade; a morning or evening landscape, a shaded face or sphere, looks deeper, more solid, more plastic, than the landscape at high noon or the outline drawing. We notice the course of boundary lines and the visibility of surfaces; that is nearer which cuts across the rest or blots part of it out; the telephone wire is thus nearer than the elm, and the elm is nearer than the house. We notice the number of objects that the eye must traverse to arrive at its goal; and the more numerous the objects, the farther off do we take the goal to be; the town looks near, we say, but there are all those fields, and the wood, and the churchyard, and half-a-dozen farmhouses to pass, and then the outlying houses; it must be a good two miles. We get various hints from movement; a crawling train or car is far away; and if we are looking at a near object and move the head to one side, distant objects move in the same direction, while if we are looking at a far object and move the head, near objects go in the opposite direction; and so on. All these things—linear perspective, aerial perspective, chiaroscuro, interposition, number, movement—are, however, secondary affairs; they represent short-cuts to the meaning of distance (p. 123); they do not lead us to the perception of distance itself. At the same time, we should bear in mind that these secondary processes were there, ready to take up the burden of meaning, all the while that the perception was forming.

Having thus cleared the ground, we naturally appeal to experiment; but unfortunately the first step that we take lands us in difficulties. It is found that, when all the cues above mentioned are ruled out, the estimation of distance is still possible; and many psychologists believe that it depends upon kinæsthetic sensations set up in and about the eye. Each eyeball is slung in its orbit upon six muscles; and the contraction of these muscles is, naturally, greater for convergence of the eyes upon near objects than for their convergence upon far; so that the sensations of convergence seem fitted to play a part in the perception of distance. If only one eye is used, these sensations may be replaced by others, derived from the muscular system, within the eyeball, that adjusts or accommodates the lens for clear vision at different objective distances. The sensations of accommodation, though, in ordinary binocular vision they are entirely subordinate to the sensations of convergence, can nevertheless—within a lesser range of distances—play the same part in perception. Unfortunately, as was hinted just now, the results of these experiments are disputed; we shall come back to them, and to the possible rôle of the kinæsthetic sensations, later on.

Meantime, what is to be said of the eyes themselves, and of the impression that a solid object, a tridimensional stimulus, makes upon them? If you hold up a closed book, back towards you, in the middle line of the face, and if you observe it alternately with the right and left eye, you will find that the two views do not tally; the left eye sees the back and the cover to the left, the right eye sees the back and the cover to the right. If you now make outline drawings of the two views, mount them upon a suitable card, and look at them through a stereoscope,—which, as you know, combines them into a single view,—lo! you have before you a solid book, the back near you, and the edges away in space. It is as if the two eyes had reconciled their conflicting views, and the result were depth or solidity.

But is not this the very thing we were in search of? have we not at last got at the secret of visible depth? No; we are rather at the crucial point of our discussion. For this binocular picture, the image seen in the stereoscope, cannot be, of its own nature and in its own right, deep or solid, unless there is a depth-sensation; and that conclusion goes against everything that we know both of sensation and of the stimuli that arouse sensation. To avoid it, some psychologists call in the kinæsthetic sensations from the muscles of the eye. Depth or distance, they say, is psychologically a blend or fusion of visual and kinæsthetic sensations. Our binocular view of the book, its appearance to the two eyes, is in itself flat; but we run the eyes over it, and the muscular sensations thus blend with the visual. Nay more, even if we hold our eyes fixed, there is still a tendency to move them; and this tendency, now ingrained in our nervous system, is enough to realise the perception. Indeed, if experiment fails in every case to show the sensations of convergence and accommodation, that is just because the fusion is so long-established and so ingrained; we perceive distance, the fusion itself; we can hardly expect to recover the kinæsthetic sensations that originally entered into it; the wonder rather is that they should ever appear, that experiment should be able to reveal them at all.

No one can say positively that this hypothesis is wrong; but it is difficult to believe that the blend of visual and kinæsthetic sensations should yield a result so different from either,—namely, the perception of space. It seems safer to say that the binocular picture, the appearance of the book to the two eyes or the combined image of the stereoscope, carries the immediate meaning of depth or voluminousness. The picture is not itself deep or solid; but we cannot help perceiving it as deep and solid; and this pressure is laid upon us by what we have called racial heritage, an inherited disposition of the nervous system: the brain meets the impression halfway. The binocular picture thus becomes the core or nucleus of the perception; and the meaning of depth is carried by a nervous disposition that has no correlate in sensation or image. The kinæsthetic sensations may then very well serve, as a secondary context, to give precision and accuracy to the perception, to develop the perception of crude voluminousness into the perception of definite distances. As to the nervous disposition, we can only say that it has been set up by the same biological causes that have made the organism a motor organism, one that moves freely in space; beyond that general statement we cannot go.

So far we have dealt with the space of sight; but there is also a space of touch; and we have next to ask whether the perception of distance can be couched in terms of touch alone. Our appeal lies to those who are born blind. Observations show that, in their case, the direct perception of solidity, of plasticity, is rare and fleeting; it arises, perhaps, when they clasp a child to their breast, or when they have been trained by long manipulation to distinguish objects of various shapes and sizes; it does not form a permanent item of their mental furniture. The blind behave as if they perceived distance; they avoid obstacles,—near obstacles by the pressure or temperature of the air reflected back upon their face, and remote obstacles by sounds; they can be taught geometry, and they measure objective distances by pacing; but the meaning of distance seems always to remain abstract, very much as the meaning of light and colour must remain abstract; there is no realising perception of distance. The brain mechanism which is ready to act at once at the behest of sight thus seems to be lacking where touch alone is present; even the perception of crude volume, of depth, has to be built up afresh by the individual. The blind live mainly in a world of sounds; touch is employed, as a rule, only for special and limited purposes, such as dressing, reading, handicraft; and their world is therefore not pervasively spatial, like the world of the seeing.

Go back now, for a moment, to the objection raised on p. 124! We have, as a matter of fact, been led to the belief that the meaning of depth is carried, in the last resort, by a brain-habit. But how differently does this sentence read before and after the discussion! You have learned something of the difficulties of the study of perception; you see why it is necessary to look at perception historically, developmentally; you have been taken behind the obvious visual cues to the perception itself; you have seen how the kinæsthetic sensations and the binocular picture may be made the subject of experiment. Even the bare outline that the narrow compass of the present book allows should convince you that the objection was duly answered.