Concept of the Stimulus in Psychology - Gibson

8/3/2019 Concept of the Stimulus in Psychology - Gibson

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THE CONCEPT OF THE STIMULUS IN PSYCHOLOGY 1JAMES J. GIBSON

Cornell Universityr seems to me that there is a weak link in the:hain of reasoning by which we explain experi-mce and behavior, namely , our concept of thestimulus. The aim of this paper is to find out wh atpsychologists mean by the term stimulus, with th ehope of deciding what they ought to mean by it.Af te r a short look at the history of the t e rm, I willtry to uncover th e sources of confusion in modernusage. In the end, perhaps, th e concept will beclarified. If not, certain contradictions will havebeen brought to light.The experimental study of the stimulus began inth e eighteenth century, so far as I can tell, with aninvestigation of the curiou s things that could bedone to make a frog's le g twitch. Th e experimentersdiscovered what is now called the nerve-musclepreparation. Galvani and later Volta gave theirnames to electricity as well as to physiology bytheir experiments. In the early nineteenth centuryJohannes Miiller applied these discoveries to thephilosophers ' problem of the human senses, thegates of know ledge. The n erves of sense, he pointedout, can be excited by a var ie ty of unnatura l agen-cies such as electrical cur ren t. Since the mind isacquainted only with the qualit ies specific to thesensory nerves, not with the stimuli , how it getsknowledge of the ma ter ia l wor ld became more puz-zling than ever. Later in the centu ry, Sherringtonw as to emphasize th e extent to which receptors arenaturally protected against such irrelevant stimuliby the structural specialization of sense organs.But meanwhi le i t had been discovered t ha t th e skinwould yield sensations only at certain discretepoints. Here was a fresh puzzle. The separatereceptor cells of all the sense organ s came to be seenunde r th e microscope, and the punctate charactero f the sensory process seemed to be established.Dur ing all this time, th e physical scientists werediscovering the laws of energy and tr iumphantlymeasuring it in its various forms, electricity, m o-mentum, light, heat, sou nd, and the results of chem-ical reaction. It becam e possible to measure certain1Presidential A d d r e s s to the Eas t e r n Psycholog ica l Asso-

c i a t i on , New York , New York , A p r i l 1960.

variables of energy at sense organs, at least th esimple ones like f requency and amoun t . Thresh-olds of reportab le sensation w ere established. Fech-ner , following Weber, conceived the grand schemeo f a measuremen t formula fo r consciousness, relat-ing its judged intensity to a simple variable of thes t imulus . Psychophysics was bora .Whatever could be controlled by an experimenterand applied to an observer could be thought of as astimulus. In the growing science of human psy-chology, it became evident that this was the inde-pendent variable of an experiment, to be isolatedand systematically varied. Much more complexthings than phy sical energies could be presented tothe sense organs wo rds for instance. These werealso called stimuli, although th e stimu lus conditionsmanipula ted, recency, f requency, meaningfulness,were vastly different from th e variables of the psy-chophysical ex periment.In the latter p a r t of the nineteenth century th econcept of the reflex arc was applied to the adaptivebehavior of animals. It had been thoug ht to ex-plain the strictly mechanical actions of the bodyever since Descartes. Reflexes had stimuli . Th esituatio ns of anima ls could be systematically alteredand the reactions obser ved. Organ isms obviouslyresponded to such stimuli , and the experimentercould apply them more freely than he could ventureto do with hum an beings . To shorten a long story,such exper iments came to be merged with humanexperiments and the outcome was a general stim-ulus-response psychology. This was a great suc-cess, especially in A m erica. Bu t stimuli for animalpsychologists were not the same as st imuli fo r sen-sory physiologists and stimuli were still differentf o r th e s tudents of perception and learning.Enough has been said to show t ha t in the twen-tieth century we have inherited a mixed batch ofideas about th e stimulus. W e constantly use theword but seldom define i t . We take i t for granted.We have behavior theory in full bloom, and percep-tion theory in ripened complexity, but who everheard of s t imu lus theory? A s a preliminary effortin this direction, I have made a survey of what

694


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THE CONCEPT OF THE STIMULUS IN P S Y C H O L O G Y 695writers seem to mean by the term. Some

it, but not many. My method was tof rom books. I then put them in

to one another. The ways of conceivinge stimulus are often in flat contradiction. Occa-

one book can be quoted against i tself . Theinto eight areas of disagreement and will treatseparately. In what follows, I will quote

comment, for the most part, keeping m yw n opinions to the end./. For Freud, the only use of the term stimulus

is discoverable in the Collected Papers (1949)to refer to a motivating force. This, after all, is

e dictionary meaning o f the wo rdsometh ingor impels to action. In ordinary speeche r e f e r to the stimulus of hunger or f e a r , which

ay compel extreme fo rm s of behavior. Freudnot of ten use the term, but when he does, a

is something to be satisfied or warded off.Psychologists and physiologists, however, haveth e term for the arousing of a sense

instead of a whole individual. But they dowholly agree about this. Some accept both

Neal Miller asserts that "any stimulusas some drive value" (Miller & Bollard, 1941, p .. However, Skinner believes that "a drive is

ot a stimulus," and that although "the term hase unfortunate connotation of a goal or spur to

we must not be misled by this popularof the word (1938, p . 3 7 5 ) . Here, then,area of disagreement in our way of con-

stimulus: does a stimulus motivate th eor does it merely trigger a response?

II. Pavlov said that "a stimulus appears to bea given response as cause with ef-(1927, p. 10). This is a f o r t h r i gh t assertion.

Watson took as the whole a im of psychol-gy th e predicting of the response, given th e stim-specifying of the stimulus, given th e

p. 10). But contrast this with theof Hilgard and Marquis. "We refer to a

an instigator [and] no more is intendedthat the stimulus is in some sense the occasion

r the response" (1940, p. 73). Evidently whatWatson meant by a stimulus is notand Marquis meant, Nearly all psy-

now follow the second line. It is alloweda stimulus may cause a reflex, but not an act.

Wood w o r t h was one of the fi rst to emphasize thatthe stimulus does not in itself determine the re-sponse; facto rs in the organism intervene to helpdetermine it. The discussion of intervening vari-ables or mediating processes has by now fi lledvolumes.The same rule is taken to hold for experience. Itis allowed that a stimulus may cause a sensation,but not a perception. M. D. V er n o n , for example,states that "the nature of the percept is not . . .determined by the physical qualities of the stim-ulus, but is largely a funct ion of constructive ten-dencies in the individual" (1952, p. 47). But Ihave been arguing the opposite for some time, thatthe percept is in very good correspondence with thephysical variables of the stimulus. Can a stimulusbe taken as the sufficient cause o f a response, or canit not? This is a second area of confusion in ourconcept of the stimulus.

///. Skinner has recently noted that "we fre-quently define the stimulus by the very doubtfulproperty of its ability to elicit the response in ques-tion, rather than by any independent property ofthe stimulus itself" (1959, p. 3 5 5 ) . He suggests noremedy, however, for this doubtful scientific behav-ior, and he seems to be confessing a sin withoutpointing the way to salvation. In truth many psy-chologists do give a circular definition of the stim-ulus. Skinner himself believed in his first book that"neither term [stimulus or response] can be definedas to its essential properties without the other"(1938, p. 9). Neal Miller has said "a response isany activity by or within the individual which canbecome functionally connected with an antecedentevent through l e a rn i ng ; a stimulus is any event towhich a response can become so connected" (Miller& Bollard, 1941, p . 59). Miller, in fac t , has arguedthat this circular definit ion of the stimulus is notonly necessary but is theoretically desirable (Koch,1959, p. 239). He seems to have abandoned com-pletely th e specifying of a stimulus by variables ofphysical energy. But listen to Estes. "By stim-ulus, I r e fe r to environmental conditions, describ-able in physical terms without r e fe rence to the be-havior of an organism" (Koch, 1959, p. 455), andHayek says, "the distinction between d i f fe ren t stim-uli must be independent of the different effects theyhave on the organism" (1952 , p. 9).

Here is a disagreement. The student of psycho-physics will argue that w e must define ou r stimulus


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696 THE A M E R I C A N P S Y C H O L O G I S Tby cer ta in opera t ions of physical science, not by thejudgmen t s o f ou r subjec t . Otherwise how are weever to discover what stimuli c an be discriminatedand w h a t canno t? W h e n th e s t imulus is difficult tospecify in objective physical terms, however, inves-tigators tend to avoid th e difficulty and describe itas that which is r e sponded to, or t ha t which isperceived. A f ew g o f u r the r a nd , by a rgu ing tha tan exper imente r cannot define th e s t imu lus anywayexcept in t e rms of h is percept ion, reach a philosoph-ical position of subjectivism. There is an ancientpuzzle to which s tudents o f philosophy a re t rea ted w h e t h e r there exists any sound when a treecrashes in the fo res t wi th no living being the re tohear it. It is a question of how to conceive th eaudi tory s t imulus . It seems to rema in a puzz le fora good many psychologis ts .I t h i nk the central question is the fol lowing. Isa s t imulus that which does activate a sense organor that which c an ac t iva te a sense org an? Somewri ter s imply that a s t imulus no t cur ren t ly exc i tingreceptors is not a s t imulus at al l . Others imp lythat a s t imu lus need no t excite receptors to be calledsuch. They allow of potential stimuli . WitnessGuthrie 's assertion that s t imu l i a re "potential occa-sions" for the init iation of sensory activity, andtha t " the physica l s timul i , thoug h presen t , m a y n o tbe effective" (Koch, 19S9, p . 178) . Th e fo rmerconception al lows physical en erg y to be called as t imulus only when some response can be obse rved ;th e latter al lows of the possibil i ty that s t imulusenergy may be present wi thout necessa r i ly be ingresponded to. The latter seems the better concept.Wi t h th e fo rmer meaning, one could never speak ofa subthreshold s t imulus , and this i s a usefu l t e r m .A n effective s t imulus on one occasion may be i n -effective on another. A nd there are various re-sponse criteria by which a threshold can bemeasu red .Th e dis t inc t ion be tween effective and potent ia lstimuli is m ade by a few theorists, but i ts im plica-t ions have not been t raced , and the idea remainsundeveloped . The concept of a perm anen t envi ron-m e n t of objects is widely accepted , but not theconcept of a pe r m a ne n t e nv i r onm e n t of potentialstimuli.

Th e thi rd a rea of d i sagreement is this: must astimulus be defined independently o f th e response itproduces in physical terms rather than terms o fbehavior or sensory process?

IV . For Pavlov a s t imu lus cou ld b e any th ing inthe te r res t r ia l wor ld . A ny event he could think ofto use in an exper iment he would ca l l a s t imulus ,and he employed tones, bells, th e sound of bubb l ingwater, l ights, rotating objects, pictures on a screen,ac id in the mouth, food, a sc ra tch on the back, orelectr ic shock. This common sense usage of theterm pers is ts among a good many behavior is ts .Spence has said that the te rm s t imulus means toh i m , "the physica l or world s i tua t ion, wi th it s d i f -fe rent aspects or features" (1956, p. 3 9 ) . For NealMiller any th ing tha t is d isc r iminable is a stimulusor, as he calls it, a cue, these terms having th e samemeaning . For Skinne r , a s t imulus is simply "a part,or modificat ion of a pa r t , of the env i ronment"(1938, p. 2 3 5 ) . To be sure , he says, i t must "referto a class of events th e m e m b e r s of which possesssome prope r ty in c om m on" (p . 3 4 ) . Because s t im-uli have this "gener ic na ture ," th e practice o f call-in g a be l l an audi tory s t imulus and a book a visua ls t imu lu s i s , as he puts i t , "f requent ly successfu l"(p . 2 3 5 ) , A ll these writers persist in bel ieving thatsomehow th e things of the env i ronment can stim-ulate us, and they re fuse to be worried by theparadox that only receptors at the skin of an indi-v idua l can ac tua l ly be s t imula ted .

This definit ion of the s t imu lus is considered na iveby perce ption psychologists. Stim uli are energies,no t objects. In Troland 's w ords , " the s t imu lus m aybe defined as the specific physical force, energy, oragency which br ings about th e stimulation of thegiven receptor system" (1930, p. 9). This concep-tion has the au thor i ty of a cen tury 's research on thesenses. In 1834, Johannes Miiller a rgued that as t imulus w as whatever exc i ted one of the "nervesof sense." To the m ode rn neuro phy siologist , astimulus is energy that depolarizes a living cell-especially, but no t exclusively, a nerve cel l . ForJennings in 1906, s tudying the ameba , a s t imuluswas a type of change in the immed ia t e env i ronm enttha t p roduced a change in behavior (1906, p. 19)and there existed precisely five types: chemical ,m echanical , ther m al, pho tic, or electr ical . W ood-wor th says that "a stimulus is any f o rm of ene rgyac t ing upon a sense organ and a rousing some activ-i ty of the organ i sm" (1929 , p . 2 2 3 ) . Koffka wishesto cal l st imuli "the causes of the excitations of oursense organs" (1935, p . 7 9 ) , bu t he , more than an yo the r theorist , faced up to the cont radic tory mean-ings of the te rm and proposed a fo rmal distinctionbetween the "proximal" s t imulus and the "distal"


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THE CONCEPT OF THE STIMULUS IN PSYCHOLOGY 697or "distant" stimulus. He made us consider theparadox that although perception and behavior seemto be determined by the distal object, they can infact only be aroused by the proximal stimulus.

Not all psychologists are willing to grapple withthis paradox and, in truth, it is baffling. If theproximal stimulus for a given object is altered withevery change of the observer's position in space, ifit is di f fe ren t on d i f fe ren t occasions, we are facedwith an absurdity. We must suppose that a count-less family of d i f fe ren t stimuli can all arouse thesame percept. Most behaviorists speak of the stim-ulus-object as if, by hyphenating two words withd i f fe ren t meanings, th e absurdity were removed.A s men o f co m m o n sense they see the need ofreducing to one the countless number of stimuli thatcan arouse a single percept, and in this surely theyhave a point. But perceptionists, being unable totake this easy way out, struggle to construct the-ories of how d i f fe ren t stimuli might arouse th e samepercept, the theories of perceptual constancy. Sofar , no theory has been agreed on. Is it possiblethat common sense is right without knowing it, andthat every family of proximal stimuli arising f romone object is, in a sense, one stimulus?

Here is a f ou r t h disagreement: do stimuli exist inthe environment or only at receptors? There is asuggestion that both usages of the term are some-how correct, but it has not been explained.

V. Osgood says that "a stimulus may be definedas that fo rm of physical energy that activates a re-ceptor" (1953, p. 12). But he does not tell uswhether he means by a receptor a single cell or amosaic of receptor cells, that is, a sense organ.Others besides Osgood are undecided about thisquestion, or have not thought about it. Hull knewwhat he thought. For him, the retinal image wasa pattern of stimuli (1943, p. 37) and a single lightray was a stimulus (p. 33). "A stimulus element isa stimulus energy which activates a single receptor-organ" (p. 349). This is straightforward. Wood-worth says that "of course the light entering the eyeand striking many rods and cones is a collection ofstimuli rather than a single stimulus," but in thenext paragraph he suggests that "the sudden cessa-tion of a light" is a stimulus (1929 , p. 2 8 ) . Kohlerwas fairly explicit on the question, saying that anorganism responds to "an objective constellation ofmillions of stimuli" (1929, p. 179) and Koffka alsoassumed that s t imul i on the retina or the skin were

local events (1935). But Nissen, on the otherhand, asserts that "a stimulus involves a pattern ofstimulation, spatial or temporal" (Stevens, 1951, p.374). Many other writers define stimuli as theoccasions for activation of a sense organ, not of areceptor cell, and speak as if a pattern were a stim-ulus. There is a vast difference between a patternof stimuli and a stimulus pattern, but we have notsufficiently thought about it. Is a "pattern" asingle stimulus or is it a number of separate stimuli?

The notion that a stimulus is what excites a cell,and is t he r e fo r e punctate, seems to many theoriststhe only rigorous definition. On this account Hullhad to introduce th e postulate of a ffe ren t neuralinteraction to explain molar behavior as distin-guished f rom molecular responses. The gestaltpsychologists had to develop the theory of sensoryorganization in order to explain perception. ButLashley once said thatthe stimulus to any reaction above the level of a spinalreflex involves not the excitation o f certain definite sensorycells but the excitation of an y cells of a system in certainratios, and the response may be given to the ratio eventhough the par t icula r cells involved have not p reviouslybeen excited in the same way (Murch ison , 1934, p . 4 76 ) .This passage suggests the idea that higher levelsof reaction require us to define higher orders ofstimulation. Lashley seems to be saying that aratio may be itself a stimulus, not just a relationbetween two stimuli. But note that the gestalttheorists, by conceiving all stimuli as local events,did not come to think in this way.

A controversy has long been going on over th equestion of how an individual could respond to arelation. It began with Kohler's evidence that achick will select th e brighter of two gray papersinstead of the absolute brightness of a particularpaper. Kohler thought it demonstrated a relationalprocess in the brain; Spence has gone to greatlengths to show that it could be explained in termsof absolute responses .to each piece of paper, subjectto the so-called principle of stimulus generalization.But the simplest explanation would be that theeffective stimulus in the experiment was the direc-tion of the difference in brightness in the field ofview. In line with this solution to the problem,students of vision conceive that a margin is a visualstimulus, perhaps the visual stimulus, and a marginin the array of light to an eye is strictly a ratio, thatis, a relation be tween measured intensities.


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698 THE A M E R I C A N P S Y C H O L O G I S THere is a fifth source of confus ion: when is a

pattern or relation to be considered a single stim-ulus an d when a number of separate stimuli?

VI. The notion that a stimulus can only besomething punctate is related to the notion that astimulus can only be something momentary. Thegestalt psychologists pointed out that a melody isperceived, but they never suggested that a melodyw as a stimulus. The notes of the melody weret aken to be the stimuli. B u t what about th e transi-tions between notes, or the "transients" of acous-tical engineering? Are they stimuli? The inves-tigators of speech sounds seem to think so, but theauditory literature of sensation is vague on thisquestion. And if a short transition is a stimulus,w hy not a long transition or temporal pa t t e r n?

In vision, experimenters have not been able tomake up their minds as to whether an optical m o-tion was a stimulus or a series of stimuli. Ther e t ina and also the skin are very sensitive to motion.It ought to be simple, but the facts of the strobo-scope and the phi-phenomenon have been inter-preted to imply that it is complex. Motion is takento be change of location, as it is in classical physics,and it is then reasoned that th e impression of loca-tion must be f un dam en t a l to any perception of achange of location.

On the other hand the generalization is fre-quently m et with that a stimulus is always a change.This is very confusing, in fact it is one confusionpiled on another. I think that writers who makethis assertion have in mind the experiments showingthat an unchanging stimulus soon ceases to be effec-tive for perception. They are thinking of sensoryadaptation. What changes in that case is not thestimulus but the process of excitation. For theretina, the skin, and the olfactory organ, sensoryadaptation does occur. For example, the steadyapplication of an image to a human retina, by themethod o f artificially stabilizing th e image, even-tuates in a wholly ineffective stimulus. But notethat the steady application of focusable light to ahuman eye does not. This stimulus never becomeswholly ineffective, even with the best voluntaryf ixation, because of slight movements of the eyei tself . This means that retinal stimulation is by nomean s the same thing as optical stimulation. Theyare different stages in the chain of events that leadsto vision. A "change in stimulation" means some-thing quite different when it is produced by some

adjustment of the sense organ itself than when it isp roduced by an external event.

Is optical motion, then, meaning a change in thepattern of focusable light to the eye, to be consid-ered a stimulus? Experiments based on this as-sumpt i on are beginning to appear. In the recentCornell research with optical transformation (Gib-son & Gibson, 1957) we not only think of this asa stimulus, w e have come to think of nonchange ofpattern as simply a special case. Stability, af te rall, is only def inab le as absence of motion. Sim-ilarly, a fo rm is def inab le as a nontransformation.In this conception, sequence is a dimension of stim-ulation whether or not change occurs.

Th e great virtue of this conception of sequence isthat it suggests a simple solution to the puzzle ofperceptual constancy. Tw o types of nonchange aredistinguishable, first, nonmotion of a pattern and,second, invariance of a pattern during motion. Theinvariant contained in a family of the perspectivesarising f rom a single object is a single stimulus.Hence there is only one stimulus for a single object,and the common sense opinion is right after all.Th e sixth conceptual issue is this: when does asequence constitute a single an d when a number o fseparate stimuli; also, can a single enduring stim-ulus exist throughout a changing sequence?

V I I . Users of the Rorschach test assume that as t imu lu s field can be either structured or, as theyput it, unstructured. I could find no explicit def-ini t ion of unstructured stimulation in the literaturebut only examples of the material to which the termis appl iedinkblots and other items used in theso-called projective tests. The idea of structuredstimulation comes f rom gestalt theory bu t onlyf rom a vague, tentative, and undeveloped hypothe-sis of gestalt theorythe external forces of organ-izat ion as distinguished f rom th e internal forces oforganiza t ion. Koffka, fo r example, was so pre-occupied with th e ways in which th e individualstructured his stimulus field that he scarcely con-sidered th e ways in which it might already havestructure (1935). In fact, he wrote sometimes asif it had none, as if all structure had to be imposedon it, because th e stimuli themselves were meaning-less points.This uncertainty about th e existence of structurein "the stimulus fo r perceived form still persists.B u t since Koffka 's time, and partly inspired b yhim, some experimenters are beginning simply to


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THE CONCEPT OF THE STIMULUS IN P S Y C H O L O G Y 699assume it, and to apply mathematics to the struc-tu re of a stimulus. They would not agree that aninkblot is in any sense an unstructured s t imulus .A picture has one structur e, an inkblot has another,bu t it does not lack structure. That can be saidonly of a film-color or the cloudless blue sky. Thestructure of an a r ray m ay have ambiguous or equiv-ocal components, as Koffka showed, but that is notthe same thing. The capacity of light to carryst ructu re to an eye may be impoverished or reducedexperimentally but i t r emains . The s t ruc tu re ofl ight may not specify anything familiar to the sub-ject, or to any observer, but it is a geometrical fact.The subject m ay be unab le to regis ter the s t ructurebecause it is nonsense to him, or he overlooks it ,o r he was not told to look fo r it, or his eyes aredefective, or he is too young, or for a dozen otherreasons, but it is still in the light. So, at least,some exper imenters would argue.W hat can be mean t by an u ns t ruc tu r ed s t imulusfield is thus a matter of disagreement. The seventhquestion is : how do we specify th e structure of astimulus?

VIII. The conception of stimuli as physicalenergies seems to imply that, in themselves, theyhave no significance or meaning. Especially if theyare considered to be only spots of ene rgy at briefmoments of time it is clear that they specify littleor nothing about the environment. Light, heat,mechanical, acoustical, chemical, and electrical en-ergy are far f rom being objects, places, events,people, words, and symbols, bu t nevertheless theyare the only stimuli that can affect receptors. Thistheory of the meaningless stimulus has been anaccepted doctr ine for a long, long tim e in the s t udyof th e senses. It leads to the notion of the sensedatumthe bare sensation, or raw sensory impres-sion, and thence to the persistent problem of howanimals and men can be supposed to perceive ob-jects, places, events, and one anothe r.Students of behavior, however, without question-in g th e doctrine of the empty stimulus, often act asi f they did not believe it . Beach speaks fo r com-parative psychologists when he says, in describinghow birds feed their offspring, "young birds exhibita gaping response which stimulates the parent toplace food in the nestl ing's m ou th" (Stevens, 1951,p . 415) . He takes i t fo r gran ted that l ight rays canspecify the event called gaping and refuses to wor ryabout it furth er. Students of perception do worry

about this question, but they are not consistent. Onthe one hand, they fi rmly assert that nothing getsinto the eye but l ight of variable wave length andintensity, not objects, or events, or facts of the en-vironment. On the other hand, they often say thatlight "carries" informat ion about th e environment ,or that s t imul i "provide" inform ation to the per-ceiver. If this is so, the stimu li m us t specify some-thing beyond themselves, and they cannot be emptyof mean ing .

A sort of comprom ise be tween th e in fo rmat ivestimulus and the em pty s t im ulus is provided by theuse of the term cue . According to Woodworth , "acue, as used in psychology, is a stimulus whichserves as a sign or signal of something else, the con-nection having previously been learned" (1958, p.60). Stimuli are conceived by analogy withmessages, or communicat ion in code. Brunswikt hought of stimuli as indicators of environmentalfacts , by analogy with pointer readings, emphasiz-ing, however, that they had only a probable connec-tion with the fact in question (1956). Boring hassuggested that stimuli m ay be taken as clues, andthis term points to Helmholtz 's theory of uncon-scious rational inference f rom the sense data(Harper & Boring, 1948).Merely to call th e st imulus a cue, sign, signal,message, indicator, or clue does not tell us w hat weneed to know . The question is to what extent doesthe stimulus specify its source, and how does it doso ? Is it possible that the use of these verbalmetaphors only prevents us f rom facing th e prob-l em? Or consider the use by modern in fo rmat iontheorists of a neutral term like input. When theycompare the org anism to a com mu nicat ion systemor to a black box, th e in ternal working of which hasto be discovered, are they avoiding th e obligation toconsider the environment of an organism and therelation of st imul i to the env i ronmen t?Th e p rob lem of the connect ion between stimul iand their natu ra l sources has not been taken se-riously by psychologists. Stimuli have not evenbeen classified from this point of view, but onlywith respect to the sense organs and the types ofenergy which carr y stimuli. It is a problem ofecology, as Brunswik realized when he wrote aboutthe "ecological validity" of cues (1956). I thinkth e problem has been obscured, and our recognitionof it delayed, by ou r fai lure to separate it into parts.The connection between natural stimuli and theirsources is not the same as the connection between


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700 THE A M E R I C A N P S Y C H O L O G I S Tsocial stimuli and their sources, for example, theconnection between words and the i r re fe rents . Thislatter problem, surely, is distinct. Semantics is onething, ecology is ano the r ; and a science of envi ron-mental stimuli may not prove to be as difficult as ascience of symbols, once we put our minds to i t .

I have maintained that optical stimuli , fo r ex-am ple, grad ients of texture in the l ight to an eye,specify environmental objects by the relation ofprojection. To me this is not at all the same as therelation by which words specify objects, which Iwould call one of coding. But however this may be,w e face another unanswered ques t ion, the e ighth:do stimuli carry information about their sources inthe world, and how do they specify them?

SOME POSITIVE HYPOTHESESCan anything usefu l be salvaged from these var-ious cont radic tory usages and defini t ions? No onecould be blamed for being pessimistic about it .S. S. Stevens , who has tho ugh t hard and long abou tst imuli , concluded that i t is futile even to a t tempt ageneral definition of the stimulus in psychology.Psychology as a whole, he says, can be equated w i thth e problem of defining th e stimulus, that is , givinga complete definition of the s t imulus for a givenresponse. To be able to do so wou ld requ i re tha t

we specify "all the t ransfo rm at ions of the envi ron-men t , both exte rna l and internal , that leave th eresponse invariant." A nd "for no response havewe yet given a comple te definition of the s t imulus"in this sense (Stevens, 1951, pp. 3 I f . ) . If I u nd e r -s tand him, what Stevens chiefly had in m ind is thepuzzle of constancy . He was saying that we do notknow how to specify, in the chaos of literal prox-imal-energy stimulation, the actual cause of a givenresponse. This is a discouraging truth.

B u t , unlike Stevens, I have hopes, and even somepositive hypotheses to suggest. Once the contra-d ic tory assumptions about s t imula t ion a re madeexplicit , we can try to resolve them . For one thingwe might search for an invariant component in thebewi lder ing var ie ty of func t iona l ly equivalent stim-uli. Perhaps the re is an invariant stimulus for theinvar ian t response, af ter al l. Ma ny sorts of higherorder variables of ene rgy m ay exist , only awai t ingmathematical description. They will have to bedescribed in appropr ia te te rms, of course, not assimple funct ions of f requency and a m ou n t . W emus t not confuse a stimulus with th e elements used

fo r its analysis. W e must learn to conceive ana r r ay not as a mosaic of s t imul i but as a hierarchyof fo rms within fo rms , and a flux not as a chain ofs t imuli -but as a hie r a r chy of sequences withinlonger sequences.

Molar Stimuli. Ever since Tolman, behaviortheorists have been agreeing that psychology is con-cerned with molar responses, not molecular ones.Accordingly we try to observe and measu re wha t anorganism is doing, not how all its muscles are con-tracting . W ith this kind of observation on theresponse side there should be a cor responding kindo f observation on the stimulus side. W e should tryto discover what an organism is responding to , notwhat excites all the little receptors. Of course allthe muscles m ay be contracting and all the re-ceptors may be excited, but observation at thatlevel is the job of the physiologists.The same recommendat ion can be made for thes tudy of perception. The gestalt theorists havedemons t r a t ed th e fact of molar experience, bu t theydid not look for molar stimuli. These m ay verywell exis t outs ide the labora tory and , wi th in-genui ty , can perhaps be isolated in the l abora tory .If so, we shal l have a new and powerfu l kind ofpsychophysics.

This conception of molar stimuli is not whollyn e w . Forty-five years ago, E. B. Holt w as con-vinced that cognition, along with behavior, was aconstant funct ion of s t imu l a t ion . In this he agreedwith Pavlov and Watson. B u t Holt emphasizedtha t the s t imulus oj which cogni tive behavior was afunct ion w as m o r e abstract and more comprehen-sive than th e stimulus of classical psychophysics.As one passes f rom reflexes to behav io r , th e effec-t ive stimulus "recedes," as Holt put i t (1915,passim). By the recession of the s timulus he m eantthat i t seems to be located far out in the environ-ment rather than close by in the receptors. A n d h ealso meant that as cognition develops, the stimulusof which it is a func t ion recedes more and more .Following this suggestion, one might conclude thata change in response implies a change in the stim-ulus to which the response is ma de . Learningwould then involve not only an alteration of be-havior but also an alteration in the effective stim-u lus . Presumably i t s mola r charac te r has gone upa stage in the hie ra rchy.Potential Stimuli. Evidently the hypothesis ofpoten tial st im ula tion , accepted casual ly by sometheorists, has quite radical bu t unrecognized impli-


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THE CONCEPT OF THE STIMULUS IN PSYCHOLOGY 701cations. We have long acknowledged the almost un-l imi ted possibilities fo r new responses in learningtheory; w hy no t equally vast possibilities of news t imul i? The environment, so considered, wouldconsist of a sort of reservoir of possible stimuli fo rboth perception and action. Light, heat, sound,odo r , gravity, and potential contacts with objectss u r r o un d th e individual. B ut this sea of energy hasvariables of pattern and sequence which can beregistered by sense organs. They can be explored,e i ther a t one station-point or by moving around inthe environment. The fields of radiating sound andodor , together with the f lux of light rays reflectedf rom surfaces, make it possible to respond to thingsat a distance. The changes of pattern in time serveas controlling stimuli for locomotion and manipula-t i on . The variables and covar iab les and invariablesof this stimulus environment are inexhaustible.Surpr i s ingly little has been written about poten-tial stimuli. The sensory physiologists, of course,have read their physics and chemistry. But phys-ical science portrays a sterile world. The variablesof physics make uninteresting stimuli. Why i s thist r u e ? I think it is because psychologists take fors t imul i only th e variables of physics as they standin the textbooks. We have simply picked thewrong variables. It is our own fault. Af t e r all,physic i s t s a re not primarily concerned with stimuli.They have enough to do to study physical energieswi thou t worrying about stimulus energies. I thinkthat we will have to develop the needed disciplineon a do-it-yourself principle. It might be calledecological physics, with branches in optics, acous-tics, dynamics, and biochemistry. We cannot waitfo r the physical scientists to describe and classifypotential stimuli. The variables would seem tothem inelegant, the mathematics would have to beimprovised , and the job is not to their taste. Butit is necessary. A nd i f successful , it will provide abasis for a stimulus-response psychology, whichotherwise seems to be sinking in a swamp of inter-vening variables.

Consider, for example, the physics (that is to saythe acoustics) of speech sounds. As recently as1951, in the Handbook oj Experimental Psychology(Stevens, p. 869), the fact that a word is percept-ually the same when whispered as it is whenshouted was taken to prove that the physical char-acteristics of sound waves, frequency, intensity, andso on, cannot tell us about speech. Speech percep-tion would require a psychological theory, not phys-

ical measurement. But the invention of the soundspec t rog raph seems to have shown that certainhigher order variables of acoustic energy are thecri t ical constituents of speech and the stimuli fo rhearing it. These newly discovered invariant pat-terns of sound a re completely physical, even if theyha d no t previously been studied in physics. Whatw as needed to understand th e psychophysics ofhear ing words was no t more psychology b u t morephysics.

Fo r another example consider th e optics of anarray of light. The physical variables applying tothe point source and the image point do not explainthe seeing of a surface. But my own work showsthat th e variables of an optical texture d o accountfo r the seeing of a surface, and that by manipulat-in g textures an experimenter can produce syntheticperceptions of objects (Gibson, Purdy, & Lawrence,1955) . Gradients, patterns, and other invariantsare not part of existing geometrical optics, but theyare physical facts . What was needed for a psycho-physics of visual perception was no t more theoriz-in g about cues but more attention to geometricaloptics.

Effective Stimuli. A n effective stimulus can nowbe defined. It is one which arouses receptor activ-ity, or recorded neural impulses, or sense organ ad-justments, or overt responses, or verbal judgmentswhichever criterion one chooses. Note that theidea of fixed innate thresholds of sensation is re-jected. It always was a myth, for every psycho-physical experimenter knows that the thresholdobtained depends on the method used and theresponse criterion chosen.

In short, whether or not a potential stimulusbecomes effective depends on the individual. Itdepends on the species to which he belongs, on theanatomy of the sense organs, th e stage of matura-tion, the capacities for sense organ adjustment, thehabits of attention, the activity in progress, and thepossibilities of educating th e attention of the indi-vidual. Such facts make up the field of perceptualdevelopment and perceptual learning. At the lowerlevels they are called facts of sensory physiology;at the higher levels, fac t s of attention or explora-tion, but they are all one problem. Animals seemto be driven to make potential stimuli effective.They use their receptor equipment, probably, in asgreat a variety of ways as they use their motorequ ipment . From this point of view, it seems tom e, the senses begin to make sense.


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702 THE A M E R I C A N P S Y C H O L O G I S TStages of Specificity. Johannes Muller began th es t udy of the way in which th e modes of experienceare specific to the exc itat ions of nerve f ibers . Sher-

r ington and others showed how the exci ta t ions offibers were generally specific to the patterns of thest imu lus. Ecological physics will tell us the extentto which the proximal s t imuli are specific to theirsources in the world. If experien ce is specif ic toexcitat ion, and excitat ion to s t imula t ion, and st im-ulat ion to the external envi ron m ent , then exper iencewi l l be speci f ic to the env i ronm ent , wi th in the l imi tsof this chain of specificities. The first two stageshave long been under investigation. The last isripe for study. There has been a controversy overwhether o r not v isual s t imuli can speci fy thei robjects ( fo r example , Cant r i l , 1950) , but i t can besettled, fo r the facts are discoverable , and argu-ments should awai t ev idence.

The Informative Capacity of Molar Stimuli. Ift he s t r uc tu r e and s equence o f s t imulus ene rgy canbe analyzed, potential st imuli can be described andar ranged in a hierarchy. There will be subo rd ina t es t imuli and supe ro rd ina t e s t imul i , of lower o r de ran d higher o rder . So conceived it is reasonable toassume that s t imul i carry information abou t th eterrestrial env i ronmen t . That is, they specifythings about objects , places, events , animals , people,and the actions of people. The rules by which theydo so a re to be de t e rmined , bu t there is at leastenough evidence to wa r r an t d i sca r d ing th e oppositeassumpt ion un de r which we have been operat ingfo r cen tu r i e stha t s t imuli are necessar i ly and in-tr insically meaningless .Natural Stimuli, Pictorial Stimuli, and CodedStimuli. I have suggested that, instead of continu-ing to employ th e careless analogies of our presentloose terminology for s t imul icues , clues, signals,signs, indicators , messages, inputs , and the likew e m a k e a systematic s tudy of the laws by whichstimuli specify their sources. We need to know thelaws of s t im ulus i n fo rma t ion . A lmos t ce r ta i n lythese will not be the laws which govern the trans-mission of i n fo rmat ion in human sys t ems of com-mun ica t i on . The na tu r a l wo r ld does not literallycommunicate with the sense organs. The potent ia lphysical s t imuli ar is ing f rom an event are not to becompared to the physical s t imulus ar is ing from theword fo r that event . W e cann ot hope to unders tandnatural st imuli by analogy with socially coded stim-uli , for that would b e l ike put t ing th e cart befo r ethe horse. Just this , however , is w h a t we tend to

do when we speak of the "signs" for depth percep-t ion and the "messages" of the senses. W e can notafford to speak of coded i n fo rmat ion for the senseorgans when w e mean s t imul i , fo r some of theseare coded and some are not .

A systematic s t udy of the speci fying power ofstimuli will put the problem of meaning in percep-tion on a new foot ing . It will take several forms ,depending on the k inds of relat ions discovered.My guess is that there wi l l be at least three,cor responding to the s t imul i f rom things, frompictures, and f rom words . It is t r ue that men ,besides learning to perceive objects , also learn toapprehend things by way of perceiving picturesand words . These mediated percept ions get mixedwith direct perceptions in the adul t . But we shallhave to disentangle them before we can have acomplete theory of human percept ion .

C O N C L U S I O NThe foregoing distinctions and assumptions seempromis ing to me. But I would ag r ee that a s t imu-lus theory cannot be established by merely asser t -in g it. The scientific question is whethe r all thesenew k inds of s t imu li exis t. I suggest that w e look

for them in the en v i r o n men t and then t ry to br ingthem in to th e l abo r a to ry .

It is still true that the stimulus is the primeindependent var iable of a psychological experiment.I quote f rom Un de r w o o d ( 1 9 5 7 ) :On e m ay va ry more t h an one s t imulus condition in a givenexper iment . . . but to d r aw a conclusion ab ou t theinfluence of any given var iable , t ha t va r i ab le mus t havebeen systemat ical ly manipulated alone somewhere in thedesign. Nothing in analysis of var iance, covariance, Latinsquares , Greco-Lat in squares, or Greco-Arabic-Lat in squareshas ab roga t ed th i s basic principle (p . 35) .

If Underwood is r ight , th e secret of a good ex-pe r imen t is to discover th e r e levant s t imulus beforedoing the experiment. The moral of my argumentis that a sys temat ic search for relevant s t imuli ,molar st imuli , potential s t imuli , invariant stimuli,specifying s t imuli , and inform at ive s t im uli w i llyield experiments with posit ive results . Perhapsthe reservoir of s t imuli that I have p ictured is fullof elegant independent variables, their simplicityobscured by physical complexity, only waiting to bediscovered.

REFERENCESB R U N S W I K , E. Perception and the representative design of

experiments. Berkeley: Univer . California Press, 1956.


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t r a n s fo rma t i on s and the perception of r ig id mot ion . J.exp. Psychol., 1957, 54, 12S-138.

GIBSON, J. J., PUSDY, J., & L A W R E N C E , L. A me thod ofcontrolling st imulation for the study of space perception:The optical t unne l . /.exp. Psychol, 1955, 50, 1-14.

HARPER, R . S., & BORING, E. G. Cues. Amer. J. Psychol.,1948, 61, 343-351.

HAYEK, F . A . The sensory order. Chicago: Univer. Chi-cago Press, 1952.

HILGARD, E. R., & MARQUIS, D. G. Conditioning an d learn-ing. New York: A ppleton-Century -Crofts, 1940.

HOLT, E. B. The Freudian wish. New Y o r k : Holt, 1915.HULL, C. L. Principles of behavior. New York: Appleton-

Century-Crof t s , 1943.JENNINGS, H. S. Behavior of the lower organisms. New

York: Columbia Univer. Press, 1906.KOCH, S. (Ed.) Psychology: A study o f a sc ience. Vol. 2.

New York: McGraw-Hill, 1959.KOPFKA, K. Principles of gestalt psychology. Ne w Y o r k :

Harcourt, Brace, 1935.K.8HLEH, W . Gestalt psychology. New York : Live r igh t ,

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MILLER, N. E., & D O L L A R O , J. Social learning and imitation.New Haven: Yale Unive r . Press, 1941.

MURCHISON, C. Handbook of general experimental psy-chology. W orcester: Clark Univer. Press, 1934.

O S G O O D , C. E. Method and theory in experimental psy-chology. New York: Oxford Univer. Press, 1953.

P A V L O V , I. P. Conditioned reflexes. (Trans, by G. V.A n r e p ) L o n d o n : Oxford Unive r . Press , 1927.SKINNER, B. F. The behavior of organisms. New York:

Apple ton-Century-Crof t s , 1938.SKINNER, B. F. Cumulative record. New York: Apple ton-

Century-Crof t s , 1959 .SPENCE, K. W. Behavior theory and conditioning. New

Haven: Yale Univer. Press, 1956.STEVENS, S. S. (Ed.) Handbook of experimental psychol-

ogy. New York: W iley, 1951.T R O L A N D , L. T. Psychophysiology. Vol. 2 . New York:

Van Nostrand, 1930.U N D E R W O O D , B . Psychological research. New York:

Appleton-Century-Crofts, 1957.VERNON, M. D. A further study of visual perception.Camb ridge : Cam bridge Unive r . Press , 1952.

WATSON, J. B. Psychology from the standpoint of abehaviorist. Philadelphia: Lippincott , 1924.

WOODWORTH, R. S. Psychology. New York: Holt, 1929.WOODWORTH, R. S. Dynamics of behavior. New York:

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Concept of the Stimulus in Psychology - Gibson