In the meantime, many rough tests of degree of attention have been devised for practical purposes. It is plain, for instance, that uniformity of performance, the maintenance of a steady level of achievement without marked variation from the average in either direction, indicates sustained attention, while the alternation of very good and very poor work indicates a vacillating attention. Tests of this kind are valuable so far as they go; but they cannot take the place of an exact psychological determination.
§ 83. Accommodation and Inertia of Attention We saw in § j6 that congruity with the contents of consciousness is one of the determinants of primary attention. It follows from this fact that, if two stimuli are presented for atten- tion at the same time, the one of which is and the other is § 83. Accommodation and Inertia of Attention 297 not congruent with ideas already present, they will reach the crest of the attention-wave, not together, but in suc- cession; the stimulus which fits in with the general trend of consciousness will outstrip its rival. We speak, in such cases, of a predisposition or accommodation of attention to a certain impression.
The fact of accommodation of attention may be illustrated by means of the arrangement shown in Fig. 44. A bell-metronome is fitted with a cardboard arc, whose radius is the length. /\ of the pendulum. Scale di- visions of 50 are laid off upon the circumference, the zero-point corresponding to the vertical position of the pendulum; an arrow-head of red paper serves as a pointer. The metronome is set to the rate of, say, 72 in the minute, and the bell rings at every complete os- cillation. In the instrument used by the writer, the bell sounds when the pointer is at 220.
The pendulum is released, and the observer is asked to say how far the pointer has travelled when the bell rings. In a first obser- vation, he is directed to attend to the moving pointer; the sound of the bell is secondary, ■ — it floats, so to speak, upon the main current of visual change. Under these conditions, the pointer carries the bell out; the sound is not heard, on the average, until the pendulum points to 300. In a second observation, he is directed to attend to the bell; the movement of the pointer is now secondary, — the expected bell-strokes stand out upon an indifferent shifting field. Under these conditions, the region of subjective coincidence lies between io° ari 150. It is evident that, when the pointer is the main object of attention, the bell 298 Attention stroke lags behind; and that, when the bell is the object of atten- tion, the pointer lags behind. In the first case, the pointer gets to 300 before the bell (which rang at 220) is heard; in the second, the bell is heard (at 220) when the observed position of the pointer is only some 150. The special accommodation of the attention may separate the two impressions by as much as io° of the scale.
The same result appears even if no special directions are given to the observer. A dial is furnished with a circular scale, and a pointer is made to revolve before it, very much as a clock-hand revolves before the face of the clock. Once in every revolution, when the pointer has reached a certain scale-mark, a bell-stroke sounds. The observer is asked to say where the pointer is when the bell rings: no further instruction is given. He accordingly follows the course of the pointer with his eye, and in the first revolution refers the sound to some region of the circle. The second revolution narrows this region; the third narrows it still more; until finally there are only a few scale divisions with which the bell appears to coincide. In the meantime, attention has been sharpening to the sound; an accommodation of attention has taken place; the observer is predisposed to hear the bell at a cer- tain instant. The instant comes; the bell-stroke rises at once to the higher conscious level; and there goes along with it the visual impression, not of the scale-mark with which it was objectively co- incident, but of a scale-mark that the pointer had already trav- ersed when the hammer struck the gong. It is as if there were a race to the top of consciousness, and the sound, outstripping the sight with which it started from the scratch, finishes abreast of a sight that had been given a certain handicap. The time- allow- ance that the sight receives is offset by the advantage accruing to the sound through accommodation of attention.
The time required for the accommodation of attention is about one and a half seconds. Hence, whenever quick and accurate observation is required in the psychological laboratory, it is customary to give a signal to the observer § 84. The Bodily Conditions of Attention 299 a short two seconds before the stimulus is presented. This is the rule when a single accommodation is required. If the stimuli are frequently repeated, the attention is able, within certain fairly wide limits, to adapt itself to their rate of succession. It is possible, for instance, to read a rhythmical form into sounds that are as little as a fifth of a second and as much as three seconds apart (§80). The attention can accommodate itself to any rate between these extremes.
Accommodation implies inertia; and we find, in fact, that it is easier to continue a certain direction of attention than it is to strike out on a new path. You can follow the move- ment of a single instrument in the orchestra better, when there has been solo-playing before, than when the whole group of instruments begin together; you can finish a con- versation, once begun, at a distance that would render the words of an unexpected question altogether inaudible; you can trace the upward course of a fire-balloon to a point at which it would otherwise be invisible. In the same way, it is difficult to break away from a current train of thought, and to give your full attention to a letter or a caller; and it is difficult to settle down again to your work after such an interruption. The description of inertia of attention must, unfortunately, be left in these general terms; no special investigation of its laws has as yet been made.
§ 84. The Bodily Conditions of Attention. — Theories of attention are as plentiful as theories of affection. Some of them have been worked out with great ingenuity of detail. Since, however, all alike are in large measure speculative, we shall here simply indicate, in bare outline, Attention Attention what seems to be the most reasonable explanation of the attentive consciousness.
Neurologists are agreed that one nervous excitation may- influence another in two opposite ways: by helping and by hindering, or, in technical terms, by facilitation and by inhibition. We may take an elementary illustration in each case. If a weak cutaneous stimulus is applied to the hind foot of a decerebrised frog, there is no visible effect; the limb remains passive. But if at the same time a light is flashed into the eye, the leg-muscles may re- spond by a strong contraction. Here we must suppose that the two excitations, the cutaneous and the visual, i< have in some way reinforced each other; there is nervous facilitation. Again, a pressure applied to one part of the frog's body will touch off a croak; a strong pressure ap- plied to a different part will touch off a muscular contrac- tion. But if the two pressures are applied together, the frog does not both croak and move; he does nothing at *~ all; there is no response to the stimuli. Here we must suppose that the two excitations interfere with each other; there is nervous inhibition.
It seems plain that the conditions of the attentive con- sciousness are of these two kinds. The clear processes, at the crest of the attention-wave, are processes whose underlying excitations have been facilitated. Similarly, the obscure processes, at the lower level of consciousness, are processes whose underlying excitations have been in- hibited. The attentive consciousness is thus conditioned upon the interplay of cortical facilitation and cortical inhibition.
When, however, we ask for further details; when we try to form a picture of what is really going on in the § 84. The Bodily Conditions of Attention 301 cortex when facilitation and inhibition are at work; then we are thrown back upon speculation. Wundt, for in- stance, believes that there is a special cortical centre, in the frontal lobes, from which the inhibitions proceed.1 His opinion carries great weight, and is supported by a very considerable body of evidence. Nevertheless, the action of what he terms the apperception centre is con- fessedly hypothetical. Other psychologists believe that the processes of facilitation and inhibition are diffused more or less widely over the entire cortex. But their agreement extends no further. One recent theory main- tains, for example, that the clearness or vividness of a sensation is due to the complexity of cortical organisation, — to the numerous interconnections of the nervous ele- ments, the extreme variability of the resistances which they offer, and the number of alternative paths that may be opened in turn to the excitation-process. Another lays equal stress upon this complexity of organisation, but uses it in precisely the opposite way: a sensation is clear when its excitation-process is strictly local, and obscure when the excitation-process is spread abroad into many cross- paths and over many systems of nervous elements. No one can say which is right and which is wrong; no one can say, for that matter, if either is right or if both are wrong. We must therefore suspend judgment, until more is known of the physiological mechanism of inhibition and facilitation.
1 Wundt explains attention in terms of inhibition only, not of inhibition and facilitation.
302 Attention References for Further Reading §§ 75_84. Special works dealing with the psychology of attention are W. B. Pillsbury, Attention, 1908; E. Diirr, Die Lehre von der Auf- merksamkeit, 1907; T. Ribot, The Psychology of Attention, (1888) 1890. The first of these books gives a general account of the place of attention in the mental life; the second is written principally in the interests of education; the third offers a ' motor theory' of attention, which is worked out in improved form by J. M. Baldwin, Mental Devel- opment in the Child and the Race: Methods and Processes, 1906, ch. xv. A brief review of experimental investigations will be found in the au- thor's Lectures on the Elementary Psychology of Feeling and Attention, 1908, Lects. v.-vii.
Fr-- Wundt's doctrine of attention, see Grundsuge der physiologischen Psychologic, iii., 1903, ch. xviii.; for his theory of a special brain-centre for attention, Principles of Physiological Psychology, i., 1904. 315 ff. Other physiological theories are given by H. Ebbinghaus, Grundsuge der Psychologic, i., 1905, 628 ff.; W. McDougall. The Physiological Factors of the Attention-Process, in Mind, N.S. xi., 1902, 316; xii., Important references are. further, G. T. Fechner, Elemente der Psy- chophysik, ii., (i860) 1907. ch. xlii.; G. E. MLiller, Zur Theorie der sinnlichen Aufmerksamkeit, 1S78; T. Lipps, Grundtatsachen des See- lenlebens, 1883, 125, 151; W. James, Principles of Psychology, i., 1890, ch. xi.; W. Wirth, Die experimented Analyse der BewusstseinspJiano- Recent work seems to show that there are two different types of the attentive consciousness. Some observers find that it always has the dual formation discussed in the text; others declare, just as definitely, that they can distinguish three or four simultaneous degrees of clearness. The results need confirmation; it appears, however, that the author, who belongs to the two-level type, has fallen in § 77 into the common psychological mistake of generalising his individual experience. It is some consolation that the multi-level modes of attention were first observed, under experimental conditions, in his own laboratory. Cf. L. R. Geissler, 'The Measurement of Attention, in Amer.Journ. Psych., Since § 82 was written, a first attempt at the measurement of atten- tion by the prescribed method has been made, with success, by L. R Geissler, op. at., 475 ff.
V PERCEPTION SPATIAL PERCEPTIONS § 85. The Sensory Attribute of Extent. — We said in § 12 that visual and^utaneous sensations are spreacUnit, areally^ into length and breadthj they appear as spatial extents. This elementary character of outspread or expanse is the foundation upon which all the forms of spatial conscious- ness^delicate_and refined as they are, have been built up. To realise it, we must go behind our adult modes of space- perception. The words ' area ' and ' extent ' naturally sug- gest to us some well-known surface, field or wall or table; and the surface has a definite form, a definite size, a definite distance, a definite position within the spatial field; its per- ception implies a whole space-psychology. We are now dealing, however, with extent of a more primitive kind: an extent that is merely expanse, without particular form, without recognised magnitude, without relation to other extents, neither near nor far, — an extent that is present as extended, and that is all. We can, perhaps, get the best idea of it by closing the eyes and observing the dark field: here is an outspread of black, or of dark red, but it is an outspread with no definite size or shape, and it lies neither on the eyeballs nor out in space. We get an ap- proximation to it when we open the eyes in a completely dark room, or face a bank of thick fog, or gaze through half-shut lids at the blue sky, though in these cases the effort to abstract from what we know of space is greater and oftentimes less successful.
304 Spatial Perceptions We may imagine, then, that the untrained eye sees the landscape as we ourselves see the field of the closed eyes. But the landscape is not uniform: there is usually a marked difference between what is above and what is below the jP j horizon; and the lower portion is variegated, made up of patches of colour which, at least in many cases, contrast with one another. The landscape is also, to a certain de- gree, in movement: clouds travel across the sky, and living creatures move about beneath the sky. Visual expanse, as the world of spatial stimuli is constituted, thus contains within itself the cues to localisation; colours are not only spread out, but they are also spread out here and there, spread out now here and now there. The perceptions of form and magnitude, and the perception of place or posi- tion, have their root in one and the same datum of extent.
Psychology has, unfortunately, been occupied rather with theo- ries of the origin and growth of space-perception than with the introspective study of psychological space itself. And we find, accordingly, the most radical disagreement among authorities. At the one extreme stand the statements that a certain roominess or volume " is discernible in each and every sensation, though more developed in some than in others,"1 and that " the accompaniment of a local sign or local characteristic is common to all sensa- tions "; 2 at the other, the statement that spatiality cannot be " an original attribute of the elements themselves, in any such way as intensity and quality of sensations are original attributes "; space implies the "arrangement of sensations," so that a sensa- tion with a spatial attribute is " psychologically impossible."3 The position taken in the text lies between these extremes. It grants 1 W. James, Principles of Psychology, ii., 1890, 135. We recur to the question of the third dimension in § 86; cf. supra, pp. 51, 94 f.
- M. von Frey, Die Gefiihle und ihr Verh'dltnis zu den E?npfindungen, § 85. The Sensory Attribute of Extent 305 to some sensations an original character of spatial outspread, and it makes localisation of these sensations a necessary consequence of qualitative differences within the total bidimensional field.
What sensations, then, have the attribute of extent) From his own introspection, the author would reply, with- U *jnL out hesitation, that visual sensations and sensations oi (I "P/d cutaneous pressure are spatial, and that sensations of hear.
ing and of smell are spaceless. He inclines to believe, further, that the other cutaneous sensations (warmth, cold4 pain), the organic pressures and pains, and all the sen. sations of the kinaesthetic senses are endowed with the spatial attribute, although they play parts of very varying importance in space-perception. Experiments on this mat ter are sorely needed; in particular, it seems impossible to say, from unaided observation, whether the taste-quail..■ ^. ties are extended or whether their spatial appearance 13 due to concomitant pressure.
A psychological field of space, a varied mental expanse that compels localisation, is furnished primarily by eye and skin (§§ 39< 50^ the two organs whose physical extent lies or^erflo^he^simultaneous operation of a number of spatial ^stimuli. How it comes about that the sensations from lese sensitive surfaces are ordered and arranged in cor- respondence with their external stimuli, we do not knoAv. The suggestion has been made that the arrangement is, in the last resort, a matter of habit: like impressions usually come together and are thus approximated in per- ception; unlike impressions usually come at a distance from one another and are thus separated in perception. Not only, that is, do qualitative differences within the total field give the general cue to localisation, but the run- ning together of like qualities and the holding apart of 306 Spatial Perceptions diverse qualities is also, in itself, localisation of a primitive and undeveloped sort. However this may be, the original psychological fields are those of sight and pressure. The visual field is the more homogeneous; indeed, it has been doubted whether the skin ever supplies a single field, — whether it does not rather give a number of heterogeneous, partial, though partially overlapping fields. Yet if you ob- serve yourself, not too analytically, as you lie comfortably in bed, breathing easily and free from organic disturbance, you may get the impression of a flattened, bidimensional field of pressure, astonishingly indefinite in form and size, but still unitary and single.1 § 86. The Third Dimension. — How, now, do we acquire the perception of depth, of distance away from us, of a third spatial dimension? This question must be asked and answered separately for the two great groups of spatial sensations, the cutaneous and the visual.
(i) Tactual Space. — In its first form, then, our question runs: Could an organism, of like origin and descent with man, but lacking eyes, perceive all three of the dimensions of space? And the answer seems to be that it could: primarily, because the skin can move, in all three objective dimensions, both upon external objects and upon itself. The blind organism of which we are speaking is, by 1 Stumpf raises the question ( Uebcr den psychologischen Ursprung der Raumvorstellung, 1873, 283) whether an observer who is entirely naive in matters of space-perception — as he puts it, a new-born baby — would per- ceive the pressure of a finger run round his body as a straight line or as a ring. Stumpf thinks that he would perceive it as a ring of pressure in three dimen- sions; Ebbinghaus (Grundziige der Psychologic, i., 1905,453) that he would get a large ring in two dimensions. The author, in the light of his own ex- periences of an unitary field of pressure, is disposed to believe that the per- ception would be that of two closely apposed lines (possibly fusing at their extremities), or perhaps of a single broad line, traversed in opposite directions, § 86. The Third Dimension 307 hypothesis, a moving organism. Hence the bidimensional field of pressure, which forms its stationary equipment of spatial consciousness, will be transformed into a bidimen- sional field of active touch; some preferred part of the skin — hand, finger-tips — will be used for the exploration of external objects; and as, in all such movement, the cutaneous impression is connected with complexes of artic- ular sensations, the right-left and up-down dimensions will be reduced in conciousness to a common spatial denomina- tor, and will be represented in terms of the sensations aroused by movement. But the organism has freedom of movement in the third or back-forth dimension as well; so that it gains a third set of experiences, of the same general kind as the other two, and yet distinguishable from them; it learns to perceive depth or distance away.
The shift of spatial emphasis from skin to joint, from cutaneous to articular pressure, would hardly be possible unless, as we suppose, the articular sensations are them- selves spatial in character. The general transition, from bidimensional to tridimensional space, must be favoured by the organism's ability to move and fold the skin upon itself: arms and legs may be crossed; the hand may be passed around as well as across the head or leg or arm; or the one hand may explore, in any direction, an object held by the other hand against the body. Moreover, the total movements of the organism, movements of locomotion, involve the third dimension of objective space; and their conscious representation may be derived, not only from the skin and the complex of joint, tendon, and muscle, but also from the kinaesthetic organs of the inner ear.
We have spoken as if the three dimensions of space were, in the world of stimuli, sharply distinct. In reality, they are conventional.
308 Spatial Perceptions It is possible to draw, through a given point, only three straight lines that lie at right angles to one another; and it has proved convenient to work out the science of geometry on the basis of this threefold system of coordinates. In the same way, it is con venient, when we are dealing with space-perception, to think of the organism as set in a space of three dimensions; and it is natural to consider two of these dimensions, the vertical and the horizontal (up-down and right-left), as lying in a frontal plane, and the third, the dimension of distance (back- forth), as lying in a sagittal plane. But the organism itself need know nothing of geometry; in per- ceiving the third dimension, it simply perceives objects as near or remote. And it must be remembered that the dimensions change, psyche logically, with every change of front executed by the organ- ism: turn to your left, and what was length becomes distance; lie on your back, and what was height becomes distance. This constant interchange of objective dimensions has undoubtedly helped towards the perception of tridimensional space.
At any rate, our view is that this perception of a third dimension is due to analogy. The two original dimensions of cutaneous space are translated into characteristic complexes of articular sen- sations; and a third characteristic complex of articular sensations gives rise — by help of the skin's movement upon itself — to a third perceived dimension. It is in some such way that the con- genially blind achieve their direct perception of a tridimen- sional space (§ 90). They are, however, at a disadvantage as compared with our imaginary organism; for the central nervous mechanism which in man subserves space-perception is essentially a visual mechanism; sight-space dominates touch-space; and the human being who is deprived of sight is therefore deprived of much more than his eyes; he loses also no small portion of his brain.
(2) Visual Space. —If you fix your eyes steadily upon some object in the field of vision, — a tree, let us say, seen through the open door, — the surrounding objects appear in their proper shapes and places; the space-values of the field are entirely normal. But if, now, you hold up a pen- § 86. The Third Dimension 309 cil, at arm's length, between the eyes and the point of fixa- tion, you find that it doubles, that you see two pencils. And if, after this experience, you consider the field of vision somewhat more carefully, you will find that it shows a good deal of doubling: the tip of the cigar in your mouth splits into two, the edge of the open door wavers into two, the ropes of the swing, the telegraph pole, the stem of an- other, nearer tree, are all doubled. So long, that is, as the eyes are at rest, only certain objects in the field are seen single; the rest are seen double.1 The images of the former fall upon what are called corresponding retinal points; those of the latter upon non-corresponding or dis- parate points.
Think of the two retinas as slipped, the one over the other, and as held together by a pin driven through the superimposed foveas (p. 88). The two pin-holes then represent corresponding points, the retinal points stimulated by the point in objective space which the eyes, at any given moment, are fixating. Let other pins be driven, vertically, through the two retinas, at any points round about the fovea: in the rough, every pair of pinholes will repre- sent a pair of corresponding points. Now it is clear, if you work the matter out by help of diagrams, that, when the eyes are in a certain fixed position, only a certain number of the points in objective space can be imaged upon corresponding retinal points. The sum-total of these, singly seen and correspond- ingly imaged, objective points is called the horopter. If, for instance, the eyes are directed straight forward to the horizon, the 1 Our habitual disregard of double images is one of the curiosities of binocu- lar vision. It is due in part to the fact that the eyes are in constant movement, so that the various objects in the field are successively fixated; in part to the indefiniteness of indirect vision (p. 83); in part to the suppression of the one or the other image by retinal rivalry (p. 320). Apart, however, from these peripheral factors, it is due, perhaps mainly, to cortical set or adjustment; we mean, expect, are disposed to see singly things that are objectively single.
3io Spatial Perceptions horopter may be a plane surface which is practically identical with the surface of the ground upon which the observer stands; if they are directed upon a point at finite distance in the median plane, it may consist, in theoretical construction, of a horizontal circle which passes through the two eyes, and of a vertical straight line which lies in the median plane and passes through the fixation- point.
Suppose, now, that the images of some object in external space fall upon retinal points that are al- most, but not quite, in cor- respondence. The object is seen as single; for the corresponding points are not points in the mathe- matical sense; a point on the one retina corresponds to a small area on the other. Suppose, again, that the images fall on retinal points that are just a little further removed from correspon-' dence. The object is still seen as single; but it is now seen as extending in the third dimension. That is to say, tridimensional vision, the vision of the object as solid, is a sort of halfway house between single and double vision; to see a thing solid is a compromise between seeing it as spatially one and seeing it as spatially two.
But why should this combination of disparate retinal images take place at all? Why should not disparity of Fig. 45. Horopter Model, showing the horopter as a horizontal circle and a vertical through the fixation-point.
§ 86. The Third Dimension 311 images mean, at once and always, that we see the object double? These are difficult questions; and we can no more answer them, in any ultimate sense, than we can say, for instance, why light of a certain wave-length is seen as red and not as blue. But we can at all events give a prox- imate answer; we can show under what conditions the combination of the disparate images is effected. Human vision is binocular vision; the two eyes work together as a single organ. Now the two eyes are like two separate observers, who view the objects in the spatial field from somewhat different standpoints; so that, within certain limits of distance, the one eye sees a given object some- what differently from the other eye (binocular parallax). There can be no cooperation between them unless their separate views are reconciled and combined; and recon- ciliation is, consists in, tridimensional vision.