Chapter 40
PERCEPTION
MENTAL LIFE CONSISTS LARGELY IN THE DISCOVERY OF FACTS NEW TO THE INDIVIDUAL, AND IN THE RE-DISCOVERY OF FACTS PREVIOUSLY OBSERVED
You will remember the case of John Doe, who was brought before us for judgment on his behavior, as to how far it was native and how far acquired. We have since that time been occupied in hearing evidence on the case, and after mature consideration have reached a decision which we may formulate as follows: that this man's behavior is primarily instinctive or native, but that new attachments of stimulus and response, and new combinations of responses, acquired in the process of learning, have furnished him with such an assortment of habits and skilled acts of all sorts that we can scarcely identify any longer the native reactions out of which his whole behavior is built. That decision being reached, we are still not ready to turn the prisoner loose, but wish to keep him under observation for a while longer, in order to see what use he makes of this vast stock of native and acquired reactions. We wish to know how an individual, so equipped, behaves from day to day, and meets the exigencies of life. Such, in brief, is the task we have still before us.
Accordingly, one fine morning we enter our prisoner's sleeping quarters, and find him, for once, making no use of his acquired reactions, as far as we can see, and utilizing but a small fraction of his native reactions. He is, in short, asleep. We ring a bell, and he stirs uneasily. We {420} ring again, and he opens his eyes sleepily upon the bell, then spies us and sits bolt upright in bed. "Well, what . . ." He throws into action a part of his rather colorful vocabulary. He evidently sees our intrusion in an unfavorable light at first, but soon relaxes a little and "supposes he must be late for breakfast". Seeing our stenographer taking down his remarks, he is puzzled for a moment, then breaks into a loud laugh, and cries out, "Oh! This is some more psychology. Well, go as far as you like. It must have been your bell I heard in my dream just now, when I thought I saw a lot of cannibals beating the tom-tom". Having now obtained sufficient data for quite a lengthy discussion, we retire to our staff room and deliberate upon these manifestations.
"The man perceives", we agree. "By the use of his eyes and ears he discovered facts, and interpreted them in the light of his previous experience. In knowing the facts, he also got adjusted to them and governed his actions by them. But notice--a curious thing--how his perception of the facts progressed by stages from the vague and erroneous to the correct and precise. Before he was fully awake, he mistook the bell for a tom-tom; then, more fully aroused, he knew the bell. Ourselves he first saw as mere wanton intruders, then as cheerful friends who wished him no ill; finally he saw us in our true character as investigators of his behavior."
Following our man through the day's work and recreation, we find a large share of his mental activity to consist in the perception of facts. We find that he makes use of the facts, adjusting himself to them and also shaping them to suit himself. His actions are governed by the facts perceived, at the same time that they are governed by his own desires. Ascertaining how the facts stand, he takes a hand and manipulates them. He is constantly coming to know {421} fresh facts, and constantly doing something new with them. His life is a voyage of discovery, and at the same time a career of invention.
Discovery and invention!--high-sounding words, still they are applicable to everyday life. The facts observed may not be absolutely new, but at least they have always to be verified afresh, since action needs always to take account of present reality. The invention may be very limited in scope, but seldom does an hour pass that does not call for doing something a little out of the ordinary, so as to escape from a fresh trap or pluck fruit from a newly discovered bough. All of our remaining chapters might, with a little forcing, be pigeonholed under these two great heads. Discovery takes its start with the child's instinctive exploratory activity, and invention with his manipulation, and these two tendencies, perhaps at bottom one, remain closely interlinked throughout.
Some Definitions
_Perception_ is the culmination of the process of discovery. Discovery usually requires exploration, a search for facts; and it requires attention, which amounts to finding the facts or getting them effectively presented; and perception then consists in knowing the presented facts.
When the facts are presented to the senses, we speak of "sense perception". If they are presented to the eye, we speak of visual perception; if to the ear, of auditory perception, etc. But when we speak of a fact as being "presented" to the eye or ear, we do not necessarily mean that it is directly and completely presented; it may only be indicated. We may have before the eyes simply a _sign_ of some fact, but perceive the fact which is the _meaning_ of the sign. We look out of the window and "see it is wet to-day", though wetness is something to be felt rather than seen; {422} having previously observed how wet ground looks, we now respond promptly to the visual appearance by knowing the indicated state of affairs. In the same way, we say that we "hear the street car", though a street car, we must admit, is not essentially a noise. What we hear, in strictness, is a noise, but we respond to the noise by perceiving the presence of the car. Responding to a stimulus presented to one sense by perceiving a fact which could only be directly presented to another sense is exemplified also by such common expressions as that the stone "looks heavy", or that the bell "sounds cracked". or that the jar of fruit "smells sour". Sense perception, then, is responding to a stimulus by knowing some fact indicated by it either directly or indirectly. Perception that is not sense perception occurs when the fact perceived is not even indirectly presented to the senses at the moment. The fact is then presented by recall; yet the fact in question is not recalled. Recall not only gives you facts previously perceived, but may provide the data, the stimulus, for fresh perception. Putting together two recalled facts, you may perceive a further fact not previously known. Remembering that you took your umbrella to the office this morning in the rain, that it was fine when you left the office, and that you certainly did not have the umbrella when you reached home, you perceive that you must have left it at the office. Reading in the paper of preparations for another polar expedition, and remembering that both poles have already been discovered, you perceive that there is something more in polar exploration than the mere race for the pole. Perception of this sort amounts to "reasoning", and will be fully considered in another chapter, while here we shall focus our attention on sense perception.
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The Difference Between Perception and Sensation
If sense perception is a response to a sensory stimulus, so is sensation, and the question arises whether there is any genuine difference between these two. In the instance of "hearing the street car", the difference is fairly obvious; hearing the noise is sensation, while knowing the street car to be there is perception.
Sensation is the first response aroused by a stimulus, or at least the first response that is conscious. Perception is a second response, following the sensation, and being properly a direct response to the sensation, and only an indirect response to the physical stimulus. The chain of events is: stimulus, response of the sense organ and sensory nerve, first cortical response which is sensation, second cortical response which is perception.
Conscious sensation is the response of the part of the cortex that first receives the nerve current from the sense organ stimulated, the response of the "sensory area" for the particular sense stimulated. When the eye is stimulated, the nerve current first reaches a small portion of the occipital lobe, called the visual sensory area. Without that area there is no visual sensation. When the ear is stimulated, the conscious sensation is the response of a small portion of the temporal lobe called the auditory sensory area, and without this area there is no auditory sensation. But the presence of the visual sensory area is not enough to give the visual perception of facts, nor is the presence of the auditory sensory area enough to give auditory perception. The cortical regions _adjacent_ to the sensory areas are necessary for perception; if they are destroyed, the individual may still see, but not know the objects seen; or may still hear, but not recognize the words or tunes that he hears. If the cortical area destroyed is in the parietal {424} lobe, adjacent to the sensory area for the cutaneous and kinesthetic senses, he may still "feel" objects, but without being able to distinguish an apple from a lump of coal, or a folded newspaper from a tin pail.
Sense perception, then, is a response of areas adjacent to the sensory areas, and this response is aroused by nerve currents coming along "association fibers" from the sensory areas which are first aroused from the sense organs.
The whole chain of events, from the time the stimulus reaches the sense organ to the time the fact is perceived, occupies only a fifth or even a tenth of a second in simple cases, and the interval between the beginning of the sensation to the beginning of the perception is not over a twentieth when the fact is easily perceived. Since the sensation usually lasts for longer than this, it overlaps the perception in time, and the two conscious responses are so _blended_ that it is difficult or impossible for introspection to separate them.
But when an unusual fact is presented, perception may lag, though sensation occurs promptly. We may be baffled and confused for an instant, and have sensation without any definite perception; or, more often, we make a rapid series of _trial and error perceptions_. In one instance, a noise was first heard as distant thunder, and then, correctly, as somebody walking on the floor above. In another case, a faint sound was first taken for a bird singing, then for a distant locomotive whistle, and finally for what it was, the tinny noise of a piece of metal carried in the hand and brushing against the overcoat as the person walked; this series occupied not over five seconds. On touching an object in the dark, you may feel it as one thing and another till some response is aroused that fits the known situation and so satisfies you. Such trial and error perception can be observed very frequently if one is on the watch for {425} psychological curiosities; and it justifies the distinction between sensation and perception, since the sensation remains virtually unchanged while perception changes.
Another sort of shifting perception is seen in looking steadily at the "ambiguous figures" which were considered in the chapter on attention, the cube, staircase, and others; and the "dot figures" belong here as well. [Footnote: See p. 252.] In these cases the stimulus arouses two or more different perceptions, alternately, while the sensation remains almost or quite unchanged.
Perception and Image
The experiment with ambiguous figures also gives an answer to the question whether perception consists in the addition of recalled memory images to the sensations aroused by the present stimulus. If that were so, you should, when you see the upper side of the flight of stairs, see them as wooden stairs or stone stairs, as carpeted or varnished, with shadows on them such as appear on a real flight of stairs, with a railing, or with some other addition of a similar nature; and, when the appearance changes to that of the under side of a flight of stairs, the colors, shadows, etc., should change as well. The usual report is that no such addition can be detected, and that the subject sees no filling-in of the picture, but simply the bare lines--only that they seem at one moment to be the bare outline of the upper side, and at another moment an equally bare outline of the lower side, of a flight of stairs.
So again, when you "hear the street car", you do not ordinarily, to judge from the reports of people who have been asked, get any visual or kinesthetic image of the car, but you simply know the car is there. You will quite {426} possibly get some such image, if you _dwell_ on the fact of the car's being there, just as some persons, in talking to a friend over the telephone, have a visual image of the friend. There is no reason why such images should not be aroused, but the question is whether they are essential to perception of the fact, and whether they occur before or after the fact is perceived. Often they do not occur, and often, when they do occur, they follow the perception of the fact, being aroused by that perception and not constituting it.
Sometimes images are certainly aroused during the perception of a fact, and, blending with the present rather vague sensation, add color and filling to the picture.
Here is an instance of this which I once observed in myself, in spite of the infrequency of my visual images. Approaching a house through a wide field one winter night, and seeing a lamp shining out of a window towards me, I seemed to see the yellowish light touching the high spots in the grass around. I was surprised that the lamp should carry so far, and the next instant saw that the light spots on the ground were small patches of snow, lighted only from the clouded sky; and at this the yellow tinge of the spots vanished. I must have read the yellow color into them to fit the lamplight. The yellow was an image blending with the actual sensation. Colors tacked on to a seen object in this way are sometimes called "memory colors".
When this instance is considered carefully, however, it does not by any means indicate that the image produced the perception. I responded to the pair of stimuli--lamp shining towards me and light spots around me--by perceiving the spots as lighted by the lamp; and the color followed suit. I next saw the spots as snow, and the color vanished. It was a case of trial and error perception, with color images conforming to the perception.
Perception does not essentially consist in the recall of {427} images, but is a different sort of response--what sort, we have still to consider.
Perception and Motor Reaction
Possibly, we may surmise, perception is a motor response, completely executed or perhaps merely incipient, or at least a readiness for a certain motor response. This guess is not quite so wild as our customary sharp distinction between knowing and doing might lead us to think. When we say that reacting to a thing in a motor way is quite different from merely seeing the thing, we forget how likely the child is to do something with any object as soon as he sees what it is. We forget also how common it is for a person, in silently reading a word--which is perceiving the word--to whisper it or at least move his lips. To be sure, persons who read a great deal usually get over this habit, as the child more and more inhibits his motor response to many seen objects. But may it not be that the motor response is simply reduced to a minimum? Or, still better, may it not be that perceiving an object amounts to _getting ready_ to do something with it? May not seeing a word always be a getting ready to say it, even if no actual movement of the vocal organs occurs? May not seeing an orange consist in getting ready to take it, peel it, and eat it? May not perceiving our friend amount to the same thing as getting ready to behave in a friendly manner, and perceiving our enemy amount to the same thing as getting on our guard against him? According to this view, perception would be a response that adjusted the perceiver to the fact perceived, and made him ready to do something appropriate.
In spite of the attractiveness of this theory of perception, it is probably not the real essence of the matter. Just as perception may change while sensation remains the same, so there may be a hesitation between two motor responses {428} to an object, without any change in the way it is perceived; and just as a block may occur between sensation and perception, so also may one occur between perception of a fact and the motor response. In other words, perception of a fact may not spell complete readiness to act upon it. The best example of this is afforded again by cases of localized brain injuries.
It happens, in motor aphasia, that the subject hears and understands a spoken word--fully perceives it--and yet cannot pronounce it himself. And at that, there need be no paralysis of the speech organs. The brain injury has affected the motor speech-coördinating machinery, and deprived the individual of the power to get ready for speaking a word, even though he perceives it.
Analogous disabilities occur in respect to other movements. It may happen, through injury somewhere near the motor area, though not precisely in that area, that one who clearly perceives a seen object is still quite incapable of handling it. He knows the object, and he knows in an abstract way what to do with it, but how to go about it he cannot remember. This type of disturbance is called "motor apraxia", and, like motor aphasia, it proves that there is a preparation that follows perception and still precedes actual movement. Paralysis of the motor area is different; then, the subject both perceives the object, and gets all ready to act upon it; only, the movement does not occur.
The truth seems to be that a series of four responses occurs in the brain, in the process of making a skilled movement dealing with a perceived object. First, sensation; second, perception of the object; third, coördinating preparation for the act; and fourth, execution of the act by the motor area arousing the lower motor centers and through them the muscles. The first response is like receiving signals {429} or code messages; the second deciphers the messages and knows the state of affairs; the third plans action; and the fourth sends out orders to the agents that perform the action.
The distinction between perception and preparation for action is sometimes rather difficult to draw. The twelve o'clock whistle means time to drop your tools, and it is hard to draw a line between knowing the fact and beginning the act. On the other hand, when my watch tells me the noon hour is almost over, some little time may be required before I get into motion. Where there is no block or inhibition, the chain of responses runs off with such speed as to seem a single response. But a block may occur at any one of several places. It may check the actual movement, as in the "delayed reaction", [Footnote: See p. 76.] and in cases where we itch to do something yet check ourselves. Here the preparation occurs, but the execution is checked. Sometimes the block occurs between perception and preparation, when we know a fact but find nothing to do about it or hesitate between two ways of acting. Sometimes, also, the block occurs between sensation and perception; a sudden loud noise will sometimes throw a person into a momentary state of confusion during which he is unable to recognize the noise.
Blocking of response at different stages can be illustrated very well in the case of anger. The irritating stimulus gives a prompt fighting reaction, unless checked at some stage. When the check prevents me from actually striking the offending person, but leaves me clenching my fist and gnashing my teeth, the chain of responses has evidently gone as far as readiness for action, and been blocked between that stage and the stage of execution. Probably the inhibitory influence here is anticipation of bad consequences. The block may occur one stage further back, when I say to myself that {430} I mustn't let myself get "all riled up" since it will spoil my morning's work; here, instead of substituting the clenched fist for actual fighting, I substitute a bored or contemptuous attitude for the pugnacious attitude. All this time I still am conscious of the offense done me. But suppose something leads me to try to look at the other person's behavior from his own point of view--then I perceive it in a different light, and it may no longer appear a personal offense to myself. I here get a substitute perception.
The process of blocking and substituting is the same process that we have seen in trial and error.[Footnote: See p. 408.] The response proving unsatisfactory, or promising to be unsatisfactory, is checked and a substitute response found. Other elements in the situation get a chance to exert their influence on the reaction. If perception of a fact were absolutely the same as preparing a motor act, we could not look over the situation, perceiving one fact after another, and letting our adjustment for action depend on the total situation instead of on the separate facts successively observed; nor could we perceive one fact while preparing the motor response to another fact, as is actually done in telegraphy, typewriting, reading aloud, and many other sorts of skilled action. In reading aloud, the eyes on the page keep well ahead of the voice; while one word is being pronounced, the next word is being prepared for pronouncing, and words still further ahead are in process of being perceived.
We conclude, accordingly, that perception of an object is not absolutely the same thing as motor response to the object, nor even as motor readiness to respond, although the transition from perception to motor readiness may be so quick that the whole reaction seems a unit. In reality, perception of the object precedes the motor adjustment, and is one factor in determining that adjustment.
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What Sort of Response, Then, Is Perception?
We can say this, that perception is knowing the fact, as distinguished from readiness to act. We can say that perception is an adjustment to facts as they are, while motor adjustment is a preparation for changing the facts. Perception does not alter the facts, but takes them as they are; movement alters the facts or produces new facts. We can say that perception comes in between sensation and motor preparation. But none of these statements is quite enough to satisfy us, if we wish to know something of the machinery of perception. What is the stimulus in perception, and what is the nature of the response?
It takes a collection of stimuli to arouse a perception. This collection is at the same time a selection from among the whole mass of sensory stimuli acting at any moment on the individual. Perception is thus a fine example both of the "law of selection" and of the "law of combination". [Footnote: See pp. 256, 263.] Perception is at once a _combining_ response and an _isolating_ response.
We perceive a face--that means that we take the face as a unit, or make a unitary response to the multiple stimuli coming from the face. At the same time, in perceiving the face, we isolate it from its background, or disregard the numerous other stimuli that are simultaneously acting upon us. If we proceed to examine the face in detail, we may isolate the nose and perceive that as a whole. We might isolate still further and perceive a freckle on the nose, taking that as a whole, or even observing separately its location, diameter, depth of pigmentation, etc. Even if we went so far as to observe a single speck of dust on the skin, in which case isolation would about reach its maximum, combination would still stay in the game, for we should either note {432} the location of the speck--which would involve relating it to some part of the face--or we should contrast it with the color of the skin, or in some similar way take the single stimulus in relation with other present stimuli. Perception is always a unitary response to an isolated assemblage of stimuli.
Consider these two opposite extremes: taking in the general effect of the view from a mountain top, and perceiving the prick of a pin. In the first case, combination is very much in evidence, but where is the isolation? There is isolation, since internal bodily sensations, and very likely auditory and olfactory sensations as well, are present but do not enter into the view. In the case of the pin prick, isolation is evident, but where does combination come in? It would not come into the mere reflex of pulling the hand away, but perceiving the pin means something more than reflex action. It means locating the sensation, or noticing its quality or duration or something of that sort, and so contrasting it with other sensations or relating it to them in some way. To perceive one stimulus as related to another is to respond to both together.
But in describing perception as a unitary response to an isolated assemblage of stimuli, we have not differentiated it from a motor response, for that, too, is often aroused by a few (or many) stimuli acting together. What more can we say? In neural terms, we can only repeat what was said before, that perception is the next response after sensation, being a direct response to a certain combination of sensations, and being in its turn the stimulus, or part of the stimulus, that arouses a motor adjustment, as it may also be the stimulus to recall of previously observed facts. In more psychological terms, we can say that sense perception is closely bound up with sensation, so that we seem to see the fact, or hear it, etc.; we perceive it as present to the {433} senses, rather than as thought of or as anticipated. Motor readiness is anticipatory, perception definitely objective. Motor readiness is an adjustment for something yet to be, while perception is an adjustment to something already present.
Practised Perception
A fact perceived for the first time must needs be attended to, in order that it may be perceived. That is, the first and original perception of a fact is a highly conscious response. But the perception of a fact, like any other form of response, becomes easy with practice; the linkage of stimulus and response becomes stronger and stronger, till finally the stimulus arouses the perceptive response almost automatically. The familiar fact is perceived without receiving close attention, or even without receiving any attention. While your attention is absorbed in reading or thinking, you may respond to the sight of the flower in a vase on your table by knowing it to be there, you may respond to the noise of the passing street car by knowing what that is, and you may respond to the contact of your foot with the leg of the chair by dimly knowing what that object is. A great deal of this inattentive perception of familiar facts is always going on. Aside from sensation and from some of the reflexes, the perception of familiar facts is the most practised and the easiest of all responses.
The laws and sub-laws of learning apply perfectly to practised perception. The more frequently, the more recently, and the more intensely a given fact has been perceived, the more readily is it perceived again. The more a given fact is in line with the mental set of the moment, the more readily is it perceived. Sometimes it is so readily perceived that we think we see it when it isn't there. If you are hunting for a lost knife, anything remotely resembling {434} a knife will catch your eye and for an instant be perceived as the missing object.
The principle of _substitute stimulus_ applies remarkably well to practised perception. The first time you perceive an object, you observe it attentively, and expose your perceptive apparatus to the whole collection of stimuli that the object sends your way. The next time you need not observe it so attentively, for you make the same perceptive response to a _part_ of the original collection of stimuli. The response originally aroused by the whole collection of stimuli is later aroused by a fraction of this collection. The stimulus may be _reduced_ considerably, and still arouse the perception of the same fact. A child is making the acquaintance of the dog. The dog barks, and the child watches the performance. He not only sees the dog, and hears the noise, but he _sees_ the dog _bark_, and _hears_ the dog _bark_. This original perception is a unitary response to the combination of sight and sound. Thereafter he does not require both stimuli at once, but, when he hears this noise, he perceives the dog barking, and when he sees the dog he sees an object that can bark. In the same way, a thousand objects which furnish stimuli to more than one of the senses are perceived as units, and, later, need only act on a single sense to be known.
The stimulus, instead of being reduced, may be _modified_, and still arouse the same perception as before. A face appears in the baby's field of view, but away across the room so that it is a very small object, visually. The face approaches and gradually becomes a larger visual object, and the light and shadow upon it change from moment to moment, but it remains nearly enough the same to arouse essentially the same perception in the child. He comes to know the face at various distances and angles and under various lights.
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Again, the child holds a block in his hands, and looks at it square on, so that it is really a rectangle in his field of view. He turns it slightly, and now it is no longer visually a rectangle, but an oblique parallelogram. But the change is not enough to abolish the first perception; he sees it as the same object as before. By dint of many such experiences, we see a book cover or a door as a rectangle, no matter at what angle we may view it, and we know a circle for a circle even though at most angles it is really an ellipse in the field of view. A large share of practised perceptions belong under the head of "response by analogy",[Footnote: See p. 406.] since they consist in making the same response to the present stimulus that has previously been made to a similar but not identical stimulus. If every modified stimulus gave a new and different perception, it would be a slow job getting acquainted with the world. A thing is never twice the same, as a collection of stimuli, and yet, within wide limits, it is always perceived as the same thing.
Corrected Perception
Response by analogy, however, often leads us astray, in making us perceive a new object as essentially the same as something already familiar. First impressions of a new object or acquaintance often need revision, because they do not work well. They do not work well because they are rough and ready, taking the object in the lump, with scant attention to details which may prove to be important. It is easy to follow the law of combination and respond to a whole collection of stimuli, but to break up the collection and isolate out of it a smaller collection to respond to--that is something we will not do unless forced to it. Isolation and discrimination are uphill work. When they occur, it is {436} because the rough and ready response has proved unsatisfactory,
_Substitute response_ is the big factor in corrected perception, as substitute stimulus is in practised perception. When our first perception of an object gets us into difficulties, then we are forced to attend more closely and find something in the object that can serve as the stimulus to a better response. This is the process by which we isolate, analyze, discriminate.
Our old friend, the white rat, learned to enter a door only if it bore a yellow sign. [Footnote: See p. 304.] It was uphill work for him, hundreds of trials being required before the discriminating response was established; but he learned it finally. At the outset, a door was a door to the rat, and responded to as such, without regard to the sign. Whenever he entered a door without the sign, he got a shock, and scurried back; and before venturing again he looked all around, seeking, we may say, a stimulus to guide him; incidentally, he looked at the yellow disk, and this stimulus, though inconspicuous and feeble to a rat, finally got linked up with the entering response. The response of first finding and then following the sign had been substituted for the original response of simply entering.
In the same way the newly hatched chick, which at first pecks at all small objects, caterpillars included, learns to discriminate against caterpillars. In a practical sense, the chick, like the rat, learns to distinguish between stimuli that at first aroused the same response. It is in the same way that the human being is driven to discriminate and attend to details. He is brought to a halt by the poor results of his first rough and ready perception, scans the situation, isolates some detail and, finding response to this detail to bring satisfactory results, substitutes response to this {437} detail for his first undiscriminating response to the whole object.
The child at first treats gloves as alike, whether rights or lefts, but thus gets into trouble, and is driven to look at them more sharply till he perceives the special characteristics of rights and lefts. He could not describe the difference, to be sure, but he sees it well enough for his purposes. If you ask an older person to describe this difference, and rally him on his inability to do so, he is thus driven to lay them side by side and study out the difference still more precisely.
The average non-mechanical person, on acquiring an automobile, takes it as a gift of the gods, a big total thing, simply to sit in and go. He soon learns certain parts that he must deal with, but most of the works remain a mystery to him. Then something goes wrong, and he gets out to look. "What do you suppose this thing is here? I never noticed it before". Tire trouble teaches him about wheels, engine trouble leads him to know the engine, ignition trouble may lead him to notice certain wires and binding-posts that were too inconspicuous at first to attract his attention. A car becomes to him a thing with a hundred well-known parts, instead of just one big totality.
Blocked response, closer examination, new stimulus isolated that gives satisfactory response--such is, typically, the process of analytic perception.
Sensory Data Serving as Signs of Various Sorts of Fact
Among facts perceived, we may list things and events, and their qualities and relations. Under "things" we here include persons and animals and everything that would ordinarily be called an "object". Under "events", we include movement, change and happenings of all sorts. Under {438} "qualities" we may include everything that can be discovered in a thing or event taken by itself, and under "relations" anything that can be discovered by comparing or contrasting two things or events. The "groups" that we have several times spoken of as being observed would here be included under "things"; but the strict logic of the whole classification is not a matter of importance, as the only object in view is to call attention to the great variety of facts that are perceived.
Now the question arises, by what signs or indications these various facts are perceived. Often, as we have seen, the fact is by no means fully presented to the senses, and often it is far from easy for the perceiver to tell on what signs the perception depends. He knows the fact, but how he knows it he cannot tell. A large part of the very extensive experimental investigation of perception has been concerned with this problem of ferreting out the signs on which the various perceptions are based, the precise stimuli to which the perceptions respond.
For example, we can examine objects by feeling of them with a stick held in the hand, and thus perceive their roughness or smoothness; but how do we sense these facts? It seems to us as if we felt them with the end of the stick, but that is absurd, since there are no sense organs in the stick. It must be that we perceive the roughness by means of sensations arising in the hand and arm, but to identify these sensations is a much harder task than to discover the objective fact of roughness.
Again, we distinguish the tones of two musical instruments by aid of their overtones, but elaborate experiments were required to prove this, since ordinarily we do not distinguish the overtones, and could simply say that the instruments sounded differently, and let it go at that.
Once more, consider our ability to perceive time intervals; {439} and to distinguish an interval of a second from one of a second and a quarter. How in the world can any one perceive time? Time is no force that could conceivably act as a stimulus to a sense organ. It must be some change or process that is the stimulus and that serves as the indication of duration. Most likely, it is some muscular or internal bodily change, but none of the more precise suggestions that have been offered square with all the facts. It cannot be the movements of breathing that give us our perception of time, for we can hold our breath and still distinguish one short interval from another. It cannot be the heart beat, for we can beat time in a rhythm that cuts across the rate of the heart beat. When a singer is accompanying himself on the piano, keeping good time in spite of the fact that the notes are uneven in length, and meanwhile using his feet on the pedals, what has he got left to beat time with? No one has located the stimulus to which accurate time perception responds, though, in a general way, we are pretty sure that change of one sort or another is the datum. With longer intervals, from a minute to several hours, the sign of duration is probably the amount happening in the interval, or else such progressive bodily changes as hunger and fatigue.
The Perception of Space
Stimuli for the perception of location are provided by all the senses. We perceive a taste as in the mouth, thirst as in the throat, hunger pangs as in the stomach. To a familiar odor we may respond by knowing the odorous substance to be close at hand. To stimulation of the semi-circular canals we respond by knowing the direction in which we are being turned.
We respond to sounds by knowing the direction from which they come, and the distance from which they come; {440} but it must be confessed that we are liable to gross errors here. To perceive the distance of the sounding body we have to be familiar with the sound at various distances, and our perception of distance is based on this knowledge. As to the direction of sound, experiment has proved that we do little more than distinguish between right and left; we are all at sea in attempting to distinguish front from back or up from down. Apparently the only datum we have to go by is the different stimulation given the two ears according as the sound comes from the right or left.
The remaining senses, the cutaneous, the kinesthetic and the visual, afford much fuller data for the perception of spatial facts. Movements of the limbs are perceived quite accurately as to direction and extent.
A cutaneous stimulus is located with fair exactness, though much less exactly on such regions as the back than on the hands or lips. If you were asked how you distinguished one point from another on the back of the hand, you could only answer that they felt different; and if you were further asked whether a pencil point applied to the two points of the skin did not feel the same, you would have to acknowledge that it did feel the same, except that it was felt in a different place. In other words, you would not be able to identify the exact data on which your perception of cutaneous position is based. Science has done no better, but has simply given the name of "local sign" to the unanalyzed sensory datum that gives a knowledge of the point stimulated.
In handling an object, as also in walking and many other movements, the cutaneous and kinesthetic senses are stimulated together, and between them furnish data for the perception of many spatial facts, such as the shape of an object examined by the hand. The spherical shape is certainly better perceived by this combination of tactile and kinesthetic {441} sensations than by vision, and the same is probably true of many similar spatial facts. That is, when we see a round ball, the visual stimulus is a substitute for the tactile and cutaneous stimuli that originally had most to do with arousing this perception.
In part by this route of the substitute stimulus, the sense of vision comes to arouse almost all sorts of spatial perceptions. Of itself, the retina has "local sign" since we can tell where in the field of view a seen object is, i.e., in what direction it is from us. This visual perception of location is so much more exact than the cutaneous or kinesthetic that it cannot possibly be derived from them; and the same is true of the visual perception of difference in length, which is one of the most accurate forms of perception. The retina must of itself afford very complete stimuli for the perception of location and size, as far as these are confined to the two dimensions, up-down and right-left. But, when you stop to think, it seems impossible that the retina should afford any data for perceiving distance in the front-back dimension.
The retina is a screen, and the stimulus that it gets from the world outside is like a picture cast upon a screen. The picture has the right-left and up-down dimensions, but no front-back dimension. How, then, does it come about, as it certainly does, that we perceive by aid of the eye the distance of objects from us, and the solidity and relief of objects? This problem in visual perception has received much attention and been carried to a satisfactory solution.
Consider, first, what stimuli indicative of distance and relief could affect a single motionless eye. The picture on the retina could then be duplicated by a painter on canvas, and the signs of distance available would be the same in the two cases. The painter uses foreshortening, making a man in the picture small in proportion to his distance away; {442} and in the same way, when any familiar object casts a small picture on the retina, we perceive the object, not as diminished in size, but as far away. The painter colors his near hills green, his distant ones blue, and washes out all detail in the latter--"aërial perspective", he calls this. His distant hill peeks from behind his nearer one, being partially covered by it. His shadows fall in a way to indicate the relief of the landscape. These signs of distance also affect the single resting eye and are responded to by appropriate spatial perceptions.
Now let the single eye move, with the head, from side to side: an index of the distance of objects is thus obtained, additional to all the painter has at his disposal, for the distant objects in the field of view now seem to move with the eye, while the nearer objects slide in the opposite direction. How much this sign is ordinarily made use of in perceiving distance is not known; it is believed not to be used very much, and yet it is the most delicate of all the signs of distance. The reason why it may not be much used by two-eyed people is that another index almost as delicate and handier to use is afforded by binocular vision.
When both eyes are open, we have a sign of distance that the painter does not use, though it is used in stereoscope slides. The right and left eyes get somewhat different views of the same solid object, the right eye seeing a little further around the object to the right, and the left eye to the left. The disparity between the two retinal images, due to the different angles at which they view the object, is greatest when the object is close at hand, and diminishes to practically zero when it is a few hundred feet away. This disparity between the two retinal images is responded to by perception of the distance and relief of the object.
It will be recalled [Footnote: See pp. 253-254.] that when two utterly inconsistent {443} views are presented to the two eyes, as a red field to one and a green field to the other, the visual apparatus balks and refuses to see more than one at a time--the binocular rivalry phenomenon. But when the disparate views are such as are presented to the two eyes by the same solid object, the visual apparatus (following the law of combination) responds to the double stimulation by getting a single view of an object in three dimensions.
Esthetic Perception
Beauty, humor, pathos and sublimity can be perceived by the senses, though we might debate a long time over the question whether these characteristics are really objective, or merely our own feelings aroused by the objects, and then projected into them. However that may be, there is no doubt that the ability to make these responses is something that can be trained, and that some people are blind and deaf to beauty and humor that other people clearly perceive. Many a one fails to see the point of a joke, or is unable to find any humor in the situation, which are clearly perceived by another. Many a one sees only a sign of rain in a great bank of clouds, only a weary climb in the looming mountain.
"A primrose by the river's brim A yellow primrose was to him. And it was nothing more."
It would not be quite fair to describe such a one as lacking in feeling; he probably has, on sufficient stimulus, the same feelings as another man, and it would be more exact to say that he is lacking in perception of certain qualities and relations. He probably tends, by nature and training, to practical rather than esthetic perception. To see any {444} beauty in a new style of music or painting, or to sense the humor in a new form of humorous writing, you need to be initiated, to be trained in observing the precise qualities and relations that are depended on for the esthetic effect. A complex situation presents almost an unlimited range of facts that may be perceived; no one perceives them all, and which he shall perceive depends on his nature and training, as well as on his attitude or mental set at the moment when the situation is presented.
Psychology has not by any means been idle in this field of esthetics; it has developed experimental methods for determining the preferences of individuals and of social groups. But it must be confessed that the results offer little that can be succinctly summarized.
One curious result is that even the very simplest objects can produce an esthetic effect. You would scarcely suppose, for example, that a mere rectangle could produce any esthetic effect, or that it would make any difference what exact proportions the rectangle possessed; and yet it is found that some rectangles are preferred to others, and that the popular choice falls upon what the art theorists have long known as the "golden section", a rectangle with a width about sixty-two per cent, of its length. Also, however much you may like symmetry, you would scarcely suppose that it could make much difference where, on a horizontal line, a little cross line should be erected; and yet nearly every one, on being tested, will agree that the middle is the best point. These are merely a couple of sample results from the numerous studies in this field.
Social Perception
By the senses we perceive the motives and intentions of other people, their sincerity, goodness, intelligence, and {445} many other traits. We see them angry or bored, amused, full of energy. To be sure, none of these human characteristics is directly and fully sensed, but that is the case also with many characteristics of inanimate objects which, nevertheless, we perceive by aid of the senses. We perceive anger or sincerity in much the same way that we perceive moisture or smoothness by the eye. To experience the anger of another person is a complex experience, but a single element from this experience may come to serve as the sign of the whole condition. A good share of the child's undirected education consists in learning to perceive the intentions and characteristics of other people by aid of little signs. He learns to read the signs of the weather in the family circle, and he learns in some measure to be a judge of men.
I once saw an instructive little incident, in which an older boy suddenly grabbed the cap from a little boy's head, and held it out to the driver of a passing automobile, as if giving it to him. The man saw the joke, and drove on laughing, but the little boy took it seriously and was quite worried for fear the man would carry off his cap. An older child would have "seen into" the situation readily; he could not have been teased in that way. Many social situations which are "all Greek" to a little child are understood readily by an older person.
It would be very valuable if psychology could succeed in analyzing out the signs by which such a trait as intelligence or "will power" is perceived, so as to reduce such perception to a science; but it is very doubtful if this can be done. Some persons who probably have themselves a keen perception of such traits have put forward systems, based upon the shape of the face, etc. They probably think they perceive human traits according to their systems, but the systems fail in other hands, and are undoubtedly {446} fallacious. No good judge of character really goes by the shape of the face; he goes by little behavior signs which he has not analyzed out, and therefore cannot explain to another person.
You can tell very little regarding a person's intelligence from his photograph. This has now been pretty well established. Photographs of persons of various degrees of intelligence are placed before those who are reputed to be good judges, and their estimates compared with the test ratings, and there is no correspondence. You might just as well look at the back of the photograph as at the front.
Even with the person before you, you are likely to commit great errors. This sort of incident has happened. A young woman is brought before the court for delinquency, and the psychologist who has tested her testifies that she is of low intelligence. But the young woman is good-looking and graceful in her speech and manners, and so impresses the judge that he dismisses as "absurd" the notion of her being feeble-minded. He sets her free, on which she promptly gets into trouble again. Apparently the only way to perceive intelligence is to see a person in action, preferably under standard conditions, where his performance can be measured; that is to say, in an intelligence test.
Errors of Perception
The grocer needs to be assured of the accuracy of his scales, and the chemist of the high accuracy of his chemical balance; the surveyor needs to know about the errors that may creep into the process of measuring the length of a line or angle. All of them, using instruments to assist in accurate perception of facts, are concerned about the accuracy of their instruments. Now, we all use the senses in perceiving facts, and "errors of sense" therefore concern us {447} all. Some of the errors committed in sense perception can be laid at the door of the senses, and some rather belong in the sphere of perception proper.
If you come out of a cold room into a warm room, the latter seems warmer than it is; and if you come out of a dark room into a light room, the latter seems brighter than it is. These errors, due to adaptation of the temperature sense and of the retina, are properly classed as errors of sense.
If you are taking a child's temperature with a "minute thermometer", it is best to use your watch to tell you when the minute is up, for the minute, when you are simply waiting for it to pass, seems very long. But if you are "working against time", a minute seems short. The professor is shocked when the closing bell rings, and thinks that certainly the hour cannot be up; but some of the students have been consulting their watches for quite a long while, being sure the hour must be nearly over. These are scarcely errors of sense, but they are errors of perception.
Where we tend to err in one certain direction from the truth, as in the examples just cited, psychology speaks of a "constant error", and evidently the knowledge of such constant errors is of importance wherever the facts are of importance. In a court of law, a witness often has to testify regarding the length of time occupied by some event, and a knowledge of the constant errors in time perception would therefore be of considerable legal importance. They would need to be worked out in considerable detail, since they differ according to the desires and attitude of the witness at the time of the event.
Besides constant errors, there are accidental or variable errors, due to slight momentary causes. Both constant and variable errors can be illustrated by a series of shots at a target. The variable error is illustrated by the scatter of {448} the hits, and the constant error by the excess of hits above the bull's-eye, or below, or to the right or left. The constant error can be corrected, once you know what it is; if results show that you tend to shoot too high, you can deliberately aim lower. But the variability of any performance cannot be eliminated except by long practice, and not altogether even then.
Experimental psychology has taken great pains in measuring the accuracy of different sorts of perception. How small a difference in length can be perceived by the eye, how small a difference of weight by the hand--these are sample problems in this line.
For example, to measure the fineness with which weights can be perceived when "hefted" in the hand, you take two objects that are alike in size and appearance but differing slightly in weight, and endeavor to decide which is the heavier just by lifting them. You try repeatedly and keep track of the number of errors, using this number as a measure of the accuracy of perception. Now, if one weight were twice as heavy as the other (one, for example, weighing 100 grams {449} and the other 200), you would never make an error except through carelessness; but if one were 100 and the other 120 grams, you would make an occasional error, and the number of errors would increase as the difference was decreased; finally, comparing 100 and 101 grams, you would get almost as many wrong as right, so that your perception of that small difference would be extremely unreliable.
ERRORS IN PERCEIVING SMALL DIFFERENCES OF WEIGHT (From Warner Brown)
Difference 20 16 12 8 4 8 2 1 grams
Errors 1 2 5 18 28 81 89 44 per hundred trials
The weights were in the neighborhood of 100 grams; each weight was compared with the 100-gram weight, and each such pair was lifted and judged 1400 times. Notice that the per cent of errors gradually increases as the difference becomes smaller.
The smaller the difference between two stimuli, the more numerous the errors in perceiving it, or, the less perceptible it is, and there is no sharp line between a difference that can be perceived and one that is too small to be perceived. That is the first great result from the study of the perception of small differences.
The second great result is called _Weber's law_, which can be stated as follows: In the same sort of perception, equal relative (not absolute) differences are equally perceptible. For example, from the preceding table we see that 28 per cent. of errors are made in comparing weights of 100 and 104 grams; then, according to Weber's law, 28 per cent, of errors would also be made in comparing 200 grams with 208, or 500 with 520, or 1000 with 1040 grams, or any pair of weights that stood to each other in the ratio of 100 to 104. Weber's law is only approximately true for the perception of weights, since actually fewer errors are committed in comparing 500 and 520 than in comparing 100 and 104 grams; but the discrepancy is not extremely great here, and in {450} some other kinds of perception, as especially in comparing the brightness of lights or the length of seen lines, the law holds good over a wide range of stimuli and only breaks down near the upper and lower extremes. We are familiar, in ordinary life, with the general truth of Weber's law, since we know that an inch would make a much more perceptible addition to the length of a man's nose than to his height, and we know that turning on a second light when only one is already lit gives a much more noticeable increase in the light than if we add one more light when twenty are already burning.
A third great result of this line of study is that different sorts of perception are very unequal in their fineness and reliability. Perception of brightness is about the keenest, as under favorable conditions a difference of one part in one hundred can here be perceived with very few errors. Visual perception of length of line is good for about one part in fifty, perception of lifted weight for about one part in ten, perception of loudness of sound for about one part in three. But the perception of small differences in the pitch of musical tones is keener still, only that, not following Weber's law in the least, it cannot be expressed in the same way. A person with a good ear for pitch can distinguish with very few errors between two tones that differ by only one vibration per second, and can perceive this same absolute difference equally well, whether the total vibration rate is 200, 400, or 800 vibrations per second.
Illusions
An error of perception is often called an "illusion", though this term is commonly reserved for errors that are large and curious. When one who is being awakened by a bell perceives it as a tom-tom, that is an illusion. An {451} illusion consists in responding to a sensory stimulus by perceiving something that is not really there. The stimulus is there, but not the fact which it is taken to indicate. Illusion is false perception.
The study of illusions is of value, not only as showing how far a given kind of perception can be trusted, but also as throwing light on the process of perception. When a process goes wrong, it sometimes reveals its inner mechanism more clearly than when everything is running smoothly. Errors of any kind are meat to the psychologist.
Illusions may be classified under several headings according to the factors that are operative in causing the deception.
1. Illusions due to peculiarities of the sense organs.
Here the stimulus is distorted by the sense organ and so may easily be taken as the sign of an unreal fact.
Separate the points of a pair of compasses by about three-quarters of an inch, and draw them across the mouth, one point above it and the other below; you will get the illusion of the points separating as they approach the middle of the mouth (where the sensory nerve supply is greatest), and coming together again as they are drawn to the cheek at the other side.
Under this same general head belong also after-images and contrast colors, and also double vision whenever for any reason the two eyes are not accurately converged upon an object. The fact that a vertical line appears longer than an equal horizontal is supposed to depend upon some peculiarity of the retina. Aside from the use of this class of illusions in the detailed study of the different senses, the chief thing to learn from them is they so seldom are full-fledged illusions, because they are ignored or allowed for, and not taken as the signs of facts. An after-image would constitute a genuine illusion if it were taken for some real {452} thing out there; but as a matter of fact, though after-images occur very frequently--slight ones practically every time the eyes are turned--they are ignored to such an extent that the student of psychology, when he reads about them, often thinks them to be something unusual and lying outside of his own experience. The same is true of double images. This all goes to show how strong is the tendency to disregard mere sensation in the interest of getting objective facts.
2. Illusions due to preoccupation or mental set.
When an insane person hears the creaking of a rocking-chair as the voice of some one calling him bad names, it is because he is preoccupied with suspicion. We might almost call this an hallucination,[Footnote: See p. 375.] since he is projecting his own auditory images and taking them for real sensations; it is, at any rate, an extreme instance of illusion. In a milder form, similar illusions are often momentarily present in a perfectly normal person, as when he is searching for a lost object and thinks he sees it whenever anything remotely similar to the desired object meets his eyes; or as when the mother, with the baby upstairs very much on her mind, imagines she hears him crying when the cat yowls or the next-door neighbors start their phonograph. The ghost-seeing and burglar-hearing illusions belong here as well. The mental set facilitates responses that are congruous with itself.
3. Illusions of the response-by-analogy type.
This is probably the commonest source of everyday illusions, and the same principle, as we have seen, is operative in a host of correct perceptions. Perceiving the obliquely presented rectangle as a rectangle is an example of correct perception of this type. Perceiving the buzzing of a fly as an aeroplane is the same sort of response only that it happens to be incorrect. If the present stimulus has something in {453} common with the stimulus which has in the past aroused a certain perception, we may make the same response now as we did before--especially, of course, if the present mantel set favors this response.
A good instance of this type is the "proofreader's illusion", so called, perhaps, because the professional proofreader is less subjcet to it than any one else. The one most subject to it is the author of a book, for whom it is almost impossible to find every misspelled word and other typographical error in reading the proof. Almost every book comes out with a few such errors, in spite of having been scanned repeatedly by several people. A couple of misprints have purposely been left in the last few lines for the reader's benefit. If the word as printed has enough resemblance to the right word, it arouses the same percept and enables the reader to get the sense and pass on satisfied. {454} Before we began to pore over books and pictures, the lines that we saw usually were the outlines of solid objects, and now it requires only a bare diagram of lines to arouse in us the perception of a solid object seen in perspective. An outline drawing, like those of the cube and staircase used to illustrate ambiguous perspective, is more readily seen as a solid object than as a flat figure.
Another illusion of this general type dates away back to Aristotle. Cross two fingers, perhaps best the second and third, and touch a marble with the crossed part of both fingers, and it seems to be two marbles; or, you can use the side of your pencil as the stimulus. In the customary position of the fingers, the stimuli thus received would mean two objects.
A much more modern illusion of the same general type is afforded by the moving pictures. The pictures do not actually show an object in motion; they simply show the object in a series of motionless positions, caught by instantaneous photography. The projector shows the series of snap-shots in rapid succession, and conceals them by a shutter while they are shifted, so as to avoid the blur that would occur if the picture were itself moved before the eyes. But the series of snap-shots has so much in common with the visual stimulus got from an actually present moving object that we make the same perceptive response. {455} The same illusion in a rudimentary form can be produced by holding the forefinger upright three or four inches in front of the nose, and looking at it while winking first the one eye and then the other. Looked at with the right eye alone it appears to be more to one side and looked at with the left eye alone it appears to be more to the other side; and when the one eye is closed and the other simultaneously opened, the finger seems actually to move from one position to the other.
4. Illusions due to imperfect isolation of the fact to be perceived.
Here belong, probably, most of the illusions produced in the psychological laboratory by odd combinations of lines, etc. A figure is so drawn as to make it difficult to isolate the fact to be observed, and when the observer attempts to perceive it, he falls into error. He thinks he is perceiving one fact, when he is perceiving another. The best example is the Müller-Lyer figure, in which two equal lines are embellished with extra lines at their ends; you are supposed to perceive the lengths of the two main lines, but you are very apt to take the whole figure in the rough and perceive the distances between its chief parts. You do not succeed in isolating the precise fact you wish to observe.
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The Müller-Lyer Illusion
The most familiar form of this striking illusion is made with arrow heads, thus
In attempting to compare the two horizontal lines one is confused so as to regard the line with outward-extending obliques longer than that with inward-extending obliques, though, measured from point to point, they are equal. The same illusion occurs in a variety of similar figures, such as
where the main lines are not drawn, but the distances from point to point are to be compared; or such as
where the two distances between points are again to be compared. Angles, however, are not necessary to give the illusion, as can be seen in this figure
or in this
In the last the lengths to be compared extend (_a_) from the right-hand rim of circle 1 to the left-hand rim of circle 2, and (_b_) from this last to the right-hand rim of circle 3. The same illusion can be got with squares, or even with capital letters as
or
or
where the distances between the main vertical lines are to be compared.
Here is an another form of the same illusion
the middle lines being affected by those above and below.
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Though these illusions seem like curiosities, and far from every-day experience, they really do enter in some degree into almost every figure that is not perfectly square and simple.
Any oblique line, any complication of any sort, is pretty sure to alter the apparent proportions and directions of the figure. A broad effect, a long effect, a skewed effect, may easily be produced by extra lines suitably introduced into a dress, into the front of a building, or into a design of any sort; so that the designer needs to have a practical knowledge of this type of illusion.
Extra lines have an influence also upon esthetic perception. The esthetic effect of a given form may be quite altered by the introduction of apparently insignificant extra lines.
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Esthetic perception is very much subject to the law of combination, and to the resulting difficulty of isolation.
One of the most interesting illusions, not being visual, can {459} only be described and not demonstrated here.
It is called the "size-weight illusion", and may be said to be based on the old catch, "Which is heavier, a pound of lead or a pound of feathers?" Of course, we shrewdly answer, a pound's {460} a pound. But lift them and notice how they feel! The pound of lead feels very much heavier. To reduce this illusion to a laboratory experiment, you take two round wooden pill-boxes, one several times as large as the other, and load them so that they both weigh the same; then ask some one to lift them and tell which is the heavier. He will have no doubt at all that the smaller box is the heavier; it may seem two or three times as heavy. Young children, however, get the opposite illusion, assimilating the weight to the visual appearance; but older persons switch over to the contrast effect, and perceive in opposition to the visual appearance. What seems to happen in the older person is a motor adjustment for the apparent weights, as indicated by their visual appearance, with the result that the weight of larger size is lifted more strongly than the weight of smaller size; so that the big one comes up easily and seems light, the little one slowly and seems heavy.
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EXERCISES
1. Outline the chapter.
2. Show that the law of combination accounts both for many correct perceptions, and for many illusions.
3. Through which of the senses are spatial facts best perceived?
4. "At first, the baby very likely perceives a ball simply as something for him to handle and throw; but, through the medium of blocked response, he comes to perceive it more objectively, i.e., as an object related to other objects, and not simply related to himself." Explain and illustrate this statement.
5. Give an example from the field of auditory perceptions where "isolation" is very much in evidence.
6. Can you see any law analogous to Weber's law in the field of financial profit and loss? Does a dollar gained or lost _seem_ the same amount, without regard to the total amount possessed?
7. Trial and error perception. Go about the room with closed eyes, and identify objects by touching them with the hands. Notice whether your first impression gives place to corrected impressions.
8. Perception of form by "active" and "passive" touch. With the eyes closed, try to distinguish objects of different shapes (a) by letting them simply rest upon the skin, and (b) by handling them. What senses coöperate in furnishing data for "active touch"?
9. Binocular parallax, or the differing views of the same solid object obtained by the two eyes. Hold a small, three-dimensional object a foot in front of the face, and notice carefully the view of it obtained by each eye separately. A pencil, pointing towards the face, gives very different views. What becomes of the two monocular views when both eyes are open at once?
10. Binocular compared with monocular perception of "depth" or distance away. Take a pencil in each hand, and bring the points together a foot in front of the face, while only one eye is open. When the points seem to be nearly touching, open the other eye, and see whether the two points still seem to be close together. Repeat.
REFERENCES
Discussions of perception that are in some respects fuller than the present chapter can be found in C. H. Judd's _Psychology, General Introduction_, 2nd edition, 1917, pp. 162-194; in Titchener's _Textbook of Psychology_, 1909, pp. 303-373; and in Warren's _Human Psychology_, 1919, pp. 232-269.
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