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THE CONCEPT OF THE STIMULUS IN PSYCHOLOGY1

JAMES J. GIBSON

Cornell University

r 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 the

stimulus. The aim of this paper is to find out whatpsychologists mean by the term stimulus, with thehope of deciding what they ought to mean by it.After a short look at the history of the term, I willtry to uncover the sources of confusion in modernusage. In the end, perhaps, the concept will beclarified. If not, certain contradictions will havebeen brought to light.

The experimental study of the stimulus began inthe eighteenth century, so far as I can tell, with aninvestigation of the curious things that could bedone to make a frog's leg twitch. The 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 knowledge. The nerves of sense, he pointedout, can be excited by a variety of unnatural agen-cies such as electrical current. Since the mind isacquainted only with the qualities specific to thesensory nerves, not with the stimuli, how it getsknowledge of the material world became more puz-zling than ever. Later in the century, Sherringtonwas to emphasize the extent to which receptors arenaturally protected against such irrelevant stimuliby the structural specialization of sense organs.But meanwhile it had been discovered that the skinwould yield sensations only at certain discretepoints. Here was a fresh puzzle. The separatereceptor cells of all the sense organs came to be seenunder the microscope, and the punctate characterof the sensory process seemed to be established.

During all this time, the physical scientists werediscovering the laws of energy and triumphantlymeasuring it in its various forms, electricity, mo-mentum, light, heat, sound, and the results of chem-ical reaction. It became possible to measure certain

1 Presidential Address to the Eastern Psychological Asso-ciation, New York, New York, April 1960.

variables of energy at sense organs, at least thesimple ones like frequency and amount. Thresh-olds of reportable sensation were established. Fech-ner, following Weber, conceived the grand schemeof a measurement formula for consciousness, relat-ing its judged intensity to a simple variable of thestimulus. 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 physical energies could be presented tothe sense organs—words for instance. These werealso called stimuli, although the stimulus conditionsmanipulated, recency, frequency, meaningfulness,were vastly different from the variables of the psy-chophysical experiment.

In the latter part of the nineteenth century theconcept of the reflex arc was applied to the adaptivebehavior of animals. It had been thought to ex-plain the strictly mechanical actions of the bodyever since Descartes. Reflexes had stimuli. Thesituations of animals could be systematically alteredand the reactions observed. Organisms obviouslyresponded to such stimuli, and the experimentercould apply them more freely than he could ventureto do with human beings. To shorten a long story,such experiments came to be merged with humanexperiments and the outcome was a general stim-ulus-response psychology. This was a great suc-cess, especially in America. But stimuli for animalpsychologists were not the same as stimuli for sen-sory physiologists and stimuli were still differentfor the students of perception and learning.

Enough has been said to show that in the twen-tieth century we have inherited a mixed batch ofideas about the stimulus. We constantly use theword but seldom define it. We take it for granted.We have behavior theory in full bloom, and percep-tion theory in ripened complexity, but who everheard of stimulus theory? As a preliminary effortin this direction, I have made a survey of what

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THE CONCEPT OF THE STIMULUS IN PSYCHOLOGY 695

modern writers seem to mean by the term. Somewriters define it, but not many. My method was tocollect quotations from books. I then put them inopposition to one another. The ways of conceivingthe stimulus are often in flat contradiction. Occa-sionally one book can be quoted against itself. Theissues interlock, of course, but I have separatedthem into eight areas of disagreement and will treatthem separately. In what follows, I will quotewithout comment, for the most part, keeping myown opinions to the end.

/. For Freud, the only use of the term stimulusthat is discoverable in the Collected Papers (1949)is to refer to a motivating force. This, after all, isthe dictionary meaning of the word—somethingthat arouses or impels to action. In ordinary speechwe refer to the stimulus of hunger or fear, whichmay compel extreme forms of behavior. Freuddoes not often use the term, but when he does, astimulus is something to be satisfied or warded off.

Psychologists and physiologists, however, havegenerally used the term for the arousing of a senseorgan instead of a whole individual. But they donot wholly agree about this. Some accept bothmeanings. Neal Miller asserts that "any stimulushas some drive value" (Miller & Bollard, 1941, p.59). However, Skinner believes that "a drive isnot a stimulus," and that although "the term hasthe unfortunate connotation of a goal or spur toaction," we must not be misled by this popularmeaning of the word (1938, p. 375). Here, then,is a first area of disagreement in our way of con-ceiving the stimulus: does a stimulus motivate theindividual or does it merely trigger a response?

II. Pavlov said that "a stimulus appears to beconnected with a given response as cause with ef-fect" (1927, p. 10). This is a forthright assertion.Similarly Watson took as the whole aim of psychol-ogy the predicting of the response, given the stim-ulus, and the specifying of the stimulus, given theresponse (1924, p. 10). But contrast this with thecaution of Hilgard and Marquis. "We refer to astimulus as an instigator [and] no more is intendedthan that the stimulus is in some sense the occasionfor the response" (1940, p. 73). Evidently whatPavlov and Watson meant by a stimulus is notwhat Hilgard and Marquis meant, Nearly all psy-chologists now follow the second line. It is allowedthat a stimulus may cause a reflex, but not an act.

Woodworth was one of the first to emphasize thatthe stimulus does not in itself determine the re-sponse; factors in the organism intervene to helpdetermine it. The discussion of intervening vari-ables or mediating processes has by now filledvolumes.

The same rule is taken to hold for experience. Itis allowed that a stimulus may cause a sensation,but not a perception. M. D. Vernon, for example,states that "the nature of the percept is not . . .determined by the physical qualities of the stim-ulus, but is largely a function 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 of 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. 355). 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 learning; a stimulus is any event towhich a response can become so connected" (Miller& Bollard, 1941, p. 59). Miller, in fact, has arguedthat this circular definition of the stimulus is notonly necessary but is theoretically desirable (Koch,1959, p. 239). He seems to have abandoned com-pletely the specifying of a stimulus by variables ofphysical energy. But listen to Estes. "By stim-ulus, I refer to environmental conditions, describ-able in physical terms without reference to the be-havior of an organism" (Koch, 1959, p. 455), andHayek says, "the distinction between different 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 we must define our stimulus

696 THE AMERICAN PSYCHOLOGIST

by certain operations of physical science, not by thejudgments of our subject. Otherwise how are weever to discover what stimuli can be discriminatedand what cannot? When the stimulus is difficult tospecify in objective physical terms, however, inves-tigators tend to avoid the difficulty and describe itas that which is responded to, or that which isperceived. A few go further and, by arguing thatan experimenter cannot define the stimulus anywayexcept in terms of his perception, reach a philosoph-ical position of subjectivism. There is an ancientpuzzle to which students of philosophy are treated—whether there exists any sound when a treecrashes in the forest with no living being there tohear it. It is a question of how to conceive theauditory stimulus. It seems to remain a puzzle fora good many psychologists.

I think the central question is the following. Isa stimulus that which does activate a sense organor that which can activate a sense organ? Somewriters imply that a stimulus not currently excitingreceptors is not a stimulus at all. Others implythat a stimulus need not excite receptors to be calledsuch. They allow of potential stimuli. WitnessGuthrie's assertion that stimuli are "potential occa-sions" for the initiation of sensory activity, andthat "the physical stimuli, though present, may notbe effective" (Koch, 19S9, p. 178). The formerconception allows physical energy to be called astimulus only when some response can be observed;the latter allows of the possibility that stimulusenergy may be present without necessarily beingresponded to. The latter seems the better concept.With the former meaning, one could never speak ofa subthreshold stimulus, and this is a useful term.An effective stimulus on one occasion may be in-effective on another. And there are various re-sponse criteria by which a threshold can bemeasured.

The distinction between effective and potentialstimuli is made by a few theorists, but its implica-tions have not been traced, and the idea remainsundeveloped. The concept of a permanent environ-ment of objects is widely accepted, but not theconcept of a permanent environment of potentialstimuli.

The third area of disagreement is this: must astimulus be defined independently of the response itproduces—in physical terms rather than terms ofbehavior or sensory process?

IV. For Pavlov a stimulus could be anything inthe terrestrial world. Any event he could think ofto use in an experiment he would call a stimulus,and he employed tones, bells, the sound of bubblingwater, lights, rotating objects, pictures on a screen,acid in the mouth, food, a scratch on the back, orelectric shock. This common sense usage of theterm persists among a good many behaviorists.Spence has said that the term stimulus means tohim, "the physical or world situation, with its dif-ferent aspects or features" (1956, p. 39). For NealMiller anything that is discriminable is a stimulusor, as he calls it, a cue, these terms having the samemeaning. For Skinner, a stimulus is simply "a part,or modification of a part, of the environment"(1938, p. 235). To be sure, he says, it must "referto a class of events the members of which possesssome property in common" (p. 34). Because stim-uli have this "generic nature," the practice of call-ing a bell an auditory stimulus and a book a visualstimulus is, as he puts it, "frequently successful"(p. 235), All these writers persist in believing thatsomehow the things of the environment can stim-ulate us, and they refuse to be worried by theparadox that only receptors at the skin of an indi-vidual can actually be stimulated.

This definition of the stimulus is considered naiveby perception psychologists. Stimuli are energies,not objects. In Troland's words, "the stimulus maybe defined as the specific physical force, energy, oragency which brings about the stimulation of thegiven receptor system" (1930, p. 9). This concep-tion has the authority of a century's research on thesenses. In 1834, Johannes Miiller argued that astimulus was whatever excited one of the "nervesof sense." To the modern neurophysiologist, astimulus is energy that depolarizes a living cell-especially, but not exclusively, a nerve cell. ForJennings in 1906, studying the ameba, a stimuluswas a type of change in the immediate environmentthat produced a change in behavior (1906, p. 19)and there existed precisely five types: chemical,mechanical, thermal, photic, or electrical. Wood-worth says that "a stimulus is any form of energyacting upon a sense organ and arousing some activ-ity of the organism" (1929, p. 223). Koffka wishesto call stimuli "the causes of the excitations of oursense organs" (1935, p. 79), but he, more than anyother theorist, faced up to the contradictory mean-ings of the term and proposed a formal distinctionbetween the "proximal" stimulus and the "distal"


or "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 different on different occasions, we are facedwith an absurdity. We must suppose that a count-less family of different stimuli can all arouse thesame percept. Most behaviorists speak of the stim-ulus-object as if, by hyphenating two words withdifferent meanings, the absurdity were removed.As men of common 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 different stimuli might arouse the 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 fromone object is, in a sense, one stimulus?

Here is a fourth 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 form 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. 28). Kohlerwas fairly explicit on the question, saying that anorganism responds to "an objective constellation ofmillions of stimuli" (1929, p. 179) and Koffka alsoassumed that stimuli 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 therefore punctate, seems to many theoriststhe only rigorous definition. On this account Hullhad to introduce the postulate of afferent neuralinteraction to explain molar behavior as distin-guished from molecular responses. The gestaltpsychologists had to develop the theory of sensoryorganization in order to explain perception. ButLashley once said that

the stimulus to any reaction above the level of a spinalreflex involves not the excitation of certain definite sensorycells but the excitation of any cells of a system in certainratios, and the response may be given to the ratio eventhough the particular cells involved have not previouslybeen excited in the same way (Murchison, 1934, p. 476).

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 thequestion of how an individual could respond to arelation. It began with Kohler's evidence that achick will select the 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 between measured intensities.


Here is a fifth source of confusion: when is apattern or relation to be considered a single stim-ulus and 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 melodywas a stimulus. The notes of the melody weretaken to be the stimuli. But what about the 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,why not a long transition or temporal pattern?

In vision, experimenters have not been able tomake up their minds as to whether an optical mo-tion was a stimulus or a series of stimuli. Theretina 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 the impression of loca-tion must be fundamental to any perception of achange of location.

On the other hand the generalization is fre-quently met 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 of artificially stabilizing the 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 voluntaryfixation, because of slight movements of the eyeitself. This means that retinal stimulation is by nomeans 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 isproduced 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-sumption are beginning to appear. In the recentCornell research with optical transformation (Gib-son & Gibson, 1957) we not only think of this asa stimulus, we have come to think of nonchange ofpattern as simply a special case. Stability, afterall, is only definable as absence of motion. Sim-ilarly, a form is definable as a nontransformation.In this conception, sequence is a dimension of stim-ulation whether or not change occurs.

The great virtue of this conception of sequence isthat it suggests a simple solution to the puzzle ofperceptual constancy. Two 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 from 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.

The sixth conceptual issue is this: when does asequence constitute a single and when a number ofseparate stimuli; also, can a single enduring stim-ulus exist throughout a changing sequence?

VII. Users of the Rorschach test assume that astimulus field can be either structured or, as theyput it, unstructured. I could find no explicit def-inition of unstructured stimulation in the literaturebut only examples of the material to which the termis applied—inkblots and other items used in theso-called projective tests. The idea of structuredstimulation comes from gestalt theory but onlyfrom a vague, tentative, and undeveloped hypothe-sis of gestalt theory—the external forces of organ-ization as distinguished from the internal forces oforganization. Koffka, for example, was so pre-occupied with the ways in which the individualstructured his stimulus field that he scarcely con-sidered the 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 the stimuli themselves were meaning-less points.

This uncertainty about the existence of structurein "the stimulus for perceived form still persists.But since Koffka's time, and partly inspired byhim, some experimenters are beginning simply to


assume it, and to apply mathematics to the struc-ture of a stimulus. They would not agree that aninkblot is in any sense an unstructured stimulus.A picture has one structure, an inkblot has another,but it does not lack structure. That can be saidonly of a film-color or the cloudless blue sky. Thestructure of an array may have ambiguous or equiv-ocal components, as Koffka showed, but that is notthe same thing. The capacity of light to carrystructure to an eye may be impoverished or reducedexperimentally but it remains. The structure oflight may not specify anything familiar to the sub-ject, or to any observer, but it is a geometrical fact.The subject may be unable to register the structurebecause it is nonsense to him, or he overlooks it,or he was not told to look for 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 experimenters would argue.

What can be meant by an unstructured stimulusfield is thus a matter of disagreement. The seventhquestion is: how do we specify the 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 energy 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 from being objects, places, events,people, words, and symbols, but nevertheless theyare the only stimuli that can affect receptors. Thistheory of the meaningless stimulus has been anaccepted doctrine for a long, long time in the studyof the senses. It leads to the notion of the sensedatum—the 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 another.

Students of behavior, however, without question-ing the doctrine of the empty stimulus, often act asif they did not believe it. Beach speaks for 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 nestling's mouth" (Stevens, 1951,p. 415). He takes it for granted that light rays canspecify the event called gaping and refuses to worryabout it further. Students of perception do worry

about this question, but they are not consistent. Onthe one hand, they firmly assert that nothing getsinto the eye but light of variable wave length andintensity, not objects, or events, or facts of the en-vironment. On the other hand, they often say thatlight "carries" information about the environment,or that stimuli "provide" information to the per-ceiver. If this is so, the stimuli must specify some-thing beyond themselves, and they cannot be emptyof meaning.

A sort of compromise between the informativestimulus and the empty stimulus 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 communication in code. Brunswikthought 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 may be taken as clues, andthis term points to Helmholtz's theory of uncon-scious rational inference from the sense data(Harper & Boring, 1948).

Merely to call the stimulus a cue, sign, signal,message, indicator, or clue does not tell us what 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 from facing the prob-lem? Or consider the use by modern informationtheorists of a neutral term like input. When theycompare the organism to a communication systemor to a black box, the internal working of which hasto be discovered, are they avoiding the obligation toconsider the environment of an organism and therelation of stimuli to the environment?

The problem of the connection between stimuliand their natural 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 carry stimuli. It is a problem ofecology, as Brunswik realized when he wrote aboutthe "ecological validity" of cues (1956). I thinkthe problem has been obscured, and our recognitionof it delayed, by our failure to separate it into parts.The connection between natural stimuli and theirsources is not the same as the connection between


social stimuli and their sources, for example, theconnection between words and their referents. Thislatter problem, surely, is distinct. Semantics is onething, ecology is another; and a science of environ-mental stimuli may not prove to be as difficult as ascience of symbols, once we put our minds to it.

I have maintained that optical stimuli, for ex-ample, gradients of texture in the light 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,we face another unanswered question, the eighth:do stimuli carry information about their sources inthe world, and how do they specify them?

SOME POSITIVE HYPOTHESES

Can anything useful be salvaged from these var-ious contradictory usages and definitions? No onecould be blamed for being pessimistic about it.S. S. Stevens, who has thought hard and long aboutstimuli, concluded that it is futile even to attempt ageneral definition of the stimulus in psychology.Psychology as a whole, he says, can be equated withthe problem of defining the stimulus, that is, givinga complete definition of the stimulus for a givenresponse. To be able to do so would require thatwe specify "all the transformations of the environ-ment, both external and internal, that leave theresponse invariant." And "for no response havewe yet given a complete definition of the stimulus"in this sense (Stevens, 1951, pp. 3If.). If I under-stand him, what Stevens chiefly had in mind 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.

But, unlike Stevens, I have hopes, and even somepositive hypotheses to suggest. Once the contra-dictory assumptions about stimulation are madeexplicit, we can try to resolve them. For one thingwe might search for an invariant component in thebewildering variety of functionally equivalent stim-uli. Perhaps there is an invariant stimulus for theinvariant response, after all. Many sorts of higherorder variables of energy may exist, only awaitingmathematical description. They will have to bedescribed in appropriate terms, of course, not assimple functions of frequency and amount. Wemust not confuse a stimulus with the elements used

for its analysis. We must learn to conceive anarray not as a mosaic of stimuli but as a hierarchyof forms within forms, and a flux not as a chain ofstimuli -but as a hierarchy 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 measure what anorganism is doing, not how all its muscles are con-tracting. With this kind of observation on theresponse side there should be a corresponding kindof observation on the stimulus side. We should tryto discover what an organism is responding to, notwhat excites all the little receptors. Of course allthe muscles may be contracting and all the re-ceptors may be excited, but observation at thatlevel is the job of the physiologists.

The same recommendation can be made for thestudy of perception. The gestalt theorists havedemonstrated the fact of molar experience, but theydid not look for molar stimuli. These may verywell exist outside the laboratory and, with in-genuity, can perhaps be isolated in the laboratory.If so, we shall have a new and powerful kind ofpsychophysics.

This conception of molar stimuli is not whollynew. Forty-five years ago, E. B. Holt was con-vinced that cognition, along with behavior, was aconstant function of stimulation. In this he agreedwith Pavlov and Watson. But Holt emphasizedthat the stimulus oj which cognitive behavior was afunction was more abstract and more comprehen-sive than the stimulus of classical psychophysics.As one passes from reflexes to behavior, the effec-tive stimulus "recedes," as Holt put it (1915,passim). By the recession of the stimulus he meantthat it seems to be located far out in the environ-ment rather than close by in the receptors. And healso meant that as cognition develops, the stimulusof which it is a function 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 made. Learningwould then involve not only an alteration of be-havior but also an alteration in the effective stim-ulus. Presumably its molar character has gone upa stage in the hierarchy.

Potential Stimuli. Evidently the hypothesis ofpotential stimulation, accepted casually by sometheorists, has quite radical but unrecognized impli-


cations. We have long acknowledged the almost un-limited possibilities for new responses in learningtheory; why not equally vast possibilities of newstimuli? The environment, so considered, wouldconsist of a sort of reservoir of possible stimuli forboth perception and action. Light, heat, sound,odor, gravity, and potential contacts with objectssurround the individual. But this sea of energy hasvariables of pattern and sequence which can beregistered by sense organs. They can be explored,either at one station-point or by moving around inthe environment. The fields of radiating sound andodor, together with the flux of light rays reflectedfrom surfaces, make it possible to respond to thingsat a distance. The changes of pattern in time serveas controlling stimuli for locomotion and manipula-tion. The variables and covariables and invariablesof this stimulus environment are inexhaustible.

Surprisingly 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 is thistrue? I think it is because psychologists take forstimuli only the variables of physics as they standin the textbooks. We have simply picked thewrong variables. It is our own fault. After all,physicists are not primarily concerned with stimuli.They have enough to do to study physical energieswithout 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 waitfor 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. And if 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 soundspectrograph seems to have shown that certainhigher order variables of acoustic energy are thecritical constituents of speech and the stimuli forhearing it. These newly discovered invariant pat-terns of sound are completely physical, even if theyhad not previously been studied in physics. Whatwas needed to understand the psychophysics ofhearing words was not more psychology but morephysics.

For another example consider the 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 the variables of an optical texture do accountfor the seeing of a surface, and that by manipulat-ing 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 not more theoriz-ing about cues but more attention to geometricaloptics.

Effective Stimuli. An 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 judgments—whichever 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, the stage of matura-tion, the capacities for sense organ adjustment, thehabits of attention, the activity in progress, and thepossibilities of educating the 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, facts 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 motorequipment. From this point of view, it seems tome, the senses begin to make sense.


Stages of Specificity. Johannes Muller began thestudy of the way in which the modes of experienceare specific to the excitations of nerve fibers. Sher-rington and others showed how the excitations offibers were generally specific to the patterns of thestimulus. Ecological physics will tell us the extentto which the proximal stimuli are specific to theirsources in the world. If experience is specific toexcitation, and excitation to stimulation, and stim-ulation to the external environment, then experiencewill be specific to the environment, within the limitsof this chain of specificities. The first two stageshave long been under investigation. The last isripe for study. There has been a controversy overwhether or not visual stimuli can specify theirobjects (for example, Cantril, 1950), but it can besettled, for the facts are discoverable, and argu-ments should await evidence.

The Informative Capacity of Molar Stimuli. Ifthe structure and sequence of stimulus energy canbe analyzed, potential stimuli can be described andarranged in a hierarchy. There will be subordinatestimuli and superordinate stimuli, of lower orderand higher order. So conceived it is reasonable toassume that stimuli carry information about theterrestrial environment. That is, they specifythings about objects, places, events, animals, people,and the actions of people. The rules by which theydo so are to be determined, but there is at leastenough evidence to warrant discarding the oppositeassumption under which we have been operatingfor centuries—that stimuli are necessarily and in-trinsically meaningless.

Natural Stimuli, Pictorial Stimuli, and CodedStimuli. I have suggested that, instead of continu-ing to employ the careless analogies of our presentloose terminology for stimuli—cues, clues, signals,signs, indicators, messages, inputs, and the like—we make a systematic study of the laws by whichstimuli specify their sources. We need to know thelaws of stimulus information. Almost certainlythese will not be the laws which govern the trans-mission of information in human systems of com-munication. The natural world does not literallycommunicate with the sense organs. The potentialphysical stimuli arising from an event are not to becompared to the physical stimulus arising from theword for that event. We cannot hope to understandnatural stimuli by analogy with socially coded stim-uli, for that would be like putting the cart beforethe horse. Just this, however, is what we tend to

do when we speak of the "signs" for depth percep-tion and the "messages" of the senses. We cannotafford to speak of coded information for the senseorgans when we mean stimuli, for some of theseare coded and some are not.

A systematic study of the specifying power ofstimuli will put the problem of meaning in percep-tion on a new footing. It will take several forms,depending on the kinds of relations discovered.My guess is that there will be at least three,corresponding to the stimuli from things, frompictures, and from words. It is true that men,besides learning to perceive objects, also learn toapprehend things by way of perceiving picturesand words. These mediated perceptions get mixedwith direct perceptions in the adult. But we shallhave to disentangle them before we can have acomplete theory of human perception.

CONCLUSION

The foregoing distinctions and assumptions seempromising to me. But I would agree that a stimu-lus theory cannot be established by merely assert-ing it. The scientific question is whether all thesenew kinds of stimuli exist. I suggest that we lookfor them in the environment and then try to bringthem into the laboratory.

It is still true that the stimulus is the primeindependent variable of a psychological experiment.I quote from Underwood (1957):

One may vary more than one stimulus condition in a givenexperiment . . . but to draw a conclusion about theinfluence of any given variable, that variable must havebeen systematically manipulated alone somewhere in thedesign. Nothing in analysis of variance, covariance, Latinsquares, Greco-Latin squares, or Greco-Arabic-Latin squareshas abrogated this basic principle (p. 35).

If Underwood is right, the secret of a good ex-periment is to discover the relevant stimulus beforedoing the experiment. The moral of my argumentis that a systematic search for relevant stimuli,molar stimuli, potential stimuli, invariant stimuli,specifying stimuli, and informative stimuli willyield experiments with positive results. Perhapsthe reservoir of stimuli that I have pictured is fullof elegant independent variables, their simplicityobscured by physical complexity, only waiting to bediscovered.

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