Psychological Review1994, VOL 101, No. 2,329-335

Copyright 1994 by the American Psychological Association, Inc.0033-295X/94/S3.00

James J. Gibson—An Appreciation

Ken Nakayama

Gibson conceived of a perceptual psychology very different from that taken by mainstream researchwork in vision over the past 30 years. Placing psychology in a biological and physical context andavoiding traditional disciplinary definitions, Gibson outlined a physics relevant to animate life.From this flowed his theory of affordances, his preoccupation with surfaces, and his interest in ani-mal locomotion. Visual motion played a decisive role in rounding out these views. His work here wasprophetic, anticipating neurophysiological discoveries on motion sensitivity and directly inspiringmore recent studies on higher order aspects of motion encoding. Gibson scrupulously avoided men-tion of internal representation. Yet, those researchers interested in such internal processes remaindeeply indebted to his enduring contributions.

I saw Gibson just once and immediately formed a negativeimpression. That was back in 1963.1 was a beginning graduatestudent in physiological psychology at the University of Califor-nia at Los Angeles (UCLA), eager to understand the mind andbehavior in terms of brain function. I attended Gibson's lecture,aware only that he was a well-known psychologist. Other thansome mention of slant perception, I recall little of the content. Ihad, however, an impression of a man impervious to new infor-mation, old fashioned, perhaps reactionary. But of course I wasyoung and opinionated, I knew very little about perception, andI had just learned of the spectacular discoveries of Lettvin(Lettvin, Maturana, McCulloch, & Pitts, 1959), Hubel andWiesel (1959), and others. Someone raised the issue of Lettvin'swork during the question period, and I was struck by Gibson'suninterested, dismissive tone. Either he did not seem to un-derstand the findings or, if he did, he thought them irrelevant."Hmm," I thought to myself, "another traditional psychologist,old fashioned, maybe on the defensive, not able to keep up withthings."

I walked out of the room somewhat puzzled. How could sucha renowned person have turned his back to such amazing re-sults? No matter. In the next several years, I was to follow mydream and set up a single-unit neurophysiology laboratory tomap receptive fields and to try to link the behavior of single cellsto visual psychophysics.

Just before leaving graduate school, I happened to comeacross Gibson's (1966) newly published book, The Senses Con-sidered as Perceptual Systems. Since Gibson's lecture in 1963,I had not thought much about Gibson. I did have the uneasyfeeling, however, that maybe I was missing something. Perhapsmy original opinion needed reassessment.

I bought the book, and I remember I could not stop reading

Thanks to Zijiang J. He, Jack Loomis, and Shinsuke Shimojo forstimulating conversations, to Shinsuke Shimojo for reading a draft ofthis article, and to the Air Force Office of Scientific Research for finan-cial support.

Correspondence concerning this article should be addressed to KenNakayama, Vision Sciences Laboratory, Department of Psychology,Harvard University, Cambridge, Massachusetts 02138.

it. It was hard to place. It did not seem scientific, but it readmore like a philosophical essay. Yet it really was not like con-temporary philosophy; it was more broadly synthetic. The lan-guage was very plain, free from the usual buzzwords or scientificjargon. Initially, the ideas had a distinct sense of the everydayabout them, a description of the earth below, sky above, cer-tainly not revealing any obvious hidden "secrets" about vision,at least not then. Nevertheless, Gibson's views did not seem un-reasonable, despite his claim to have a very radical position. Itwas his main point, the primacy of "perception" over "sensa-tion" that was generally the most memorable, but I also re-called, in passing, the emphasis of the mobile observer and theaccompanying change in the optic array. I remember conveyinga very tentative enthusiasm to a perception graduate student atUCLA. By then I realized that I really did not know much aboutperception and wanted his opinion. He scoffed, "Oh Gibson,that mystical stuff, where are the experiments? No testable hy-pothesis, no quantitative predictions."

It was not until I became a postdoctoral student at the Uni-versity of California-Berkeley that at least one of Gibson's ideasbegan to sink in. Intending to record from the lateral geniculatenucleus, I often noticed that above the nucleus (in a then un-characterized extrastriate cortical area), it was very easy to iso-late single cells and that virtually all the neurons there were in-sensitive to form yet were clearly sensitive to motion. I thought,"Perhaps Gibson is right, maybe the function of motion is muchmore than the mere registration of moving objects in the world,but is also used for the perception of space and self motion.There seem to be too many neurons just for the registration ofmoving objects." Not too long afterward, I had the pleasure ofcollaborating with Jack Loomis. We read Gibson again andmade an effort to bridge the gap between his ideas and those ofBarlow (1961), who suggested that visual neurons should codemeaningful regularities in the environment, thus reducing re-dundancy. We postulated the existence of a class of velocity sen-sitive neurons that could obtain information about the bound-aries of surfaces during observer locomotion (Nakayama &Loomis, 1974). Such was the beginning of a growing apprecia-tion of Gibson's contributions and a recognition that, far frombeing old fashioned, his ideas about visual motion could pro-vide the foundation for entirely new research directions. I sus-

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pected that I would do well to reread Gibson from time to timebecause it was apparent that buried in this deceptively simpleprose were some of the most interesting thoughts on vision thatI had ever encountered. I would just have to reread these booksat intervals to grasp their full meaning.

Over the intervening decades, I have not been disappointed.This occasion to comment on Gibson's 1954 article, "The Vi-sual Perception of Objective Motion and Subjective Move-ment," certainly gives me another opportunity to both hear himout and to urge the reader to do so. The strength of Gibson'sshort article is that it anticipates and brings together issues andquestions that had been previously neglected and that have nowbecome recognized as critical questions. However, it also ap-pears that Gibson was severely constrained, having to write apiece to be both short and understood entirely on its own. Soby itself it lacks the depth of some of Gibson's longer writings,acquiring more significance in the broader context of Gibson'smore sustained thinking. Thus, to be fair to Gibson, I commenton his article in the larger context of his more systematic writ-ings on perception, primarily his books.

In his The Perception of the Visual World, Gibson (1950) pro-posed a psychophysics of vision, a research program thatdiffered markedly from the prevailing practice of visual psycho-physics. Traditional psychophysics began with Fechner, who in-vented an approach to link elementary physical quantities towhat he thought were the simplest mental events. The absoluteand differential threshold provided an operational, objectivemethod to establish a relationship between physical intensityand sensation. Later, the structuralists would attempt to un-derstand perception and higher processes in terms of elemen-tary sensations, as if to build mental molecules out of atomicsensations. Logical and reasonable as it may have sounded atthe time, the search for a "mental chemistry" failed. A descrip-tion of "elementary sensations" did not lead to an understand-ing of perception.

Although the specific enterprise of the structuralists did notsucceed, a much more broadly based program, motivated by asimilar desire to understand perception in terms of elementaryconstituents, has emerged in the past 30 years. Calling itself byvarious names, such as vision, visual science, and visual neuro-science, this program forms a large, vigorous, and growing in-terdisciplinary field embracing neurophysiology, anatomy, psy-chophysics, and perceptual psychology. It also reaches out tocomputer vision and cognitive psychology. It was boosted byHubel and Wiesel's (1959) description of visual receptive fieldsat various anatomical loci. Their catalogue of receptive fieldtypes held out the promise of a hierarchical progression, wherecells with simple properties would then go on to bestow suc-ceeding cells with complex, then hypercomplex, properties sim-ply on the basis of excitatory and inhibitory connections. Lett-vin et al. (1959) implied that the successive logical operations ofdifferentiation and generalization had the hallmarks of logicalthought. Extending this attractive idea to link it to the everydayfacts of perception, however, began to run aground fairly early.Hubel and Wiesel, wisely, sidestepped the issue as to the specificfunctional role of these cells and directed their energies towardanatomical investigations, using novel techniques to reveal theintricacies of cortical architecture with convincing detail. Thiswas accompanied and followed with the important discovery

that the posterior portion of the brain consisted of many mapsof the visual field (Allman & Kaas, 1974; Van Essen, 1985;Zeki,1978). However, this tremendous increase in knowledge has toldresearchers mainly about the "nuts and bolts" of the visual sys-tem, not about how visual perception itself might work or whatits specific functions might be.

During this time, psychophysics became informed by theseneurophysiologieal developments, and a parallel concept of anorientation selective spatial frequency channel emerged, onethat provided a link to the receptive fields of cortical neurons(DeValois & DeValois, 1990). Such channels could explain sim-ple detection and discrimination experiments, and some com-bination of channels provided some explanation of simple pat-tern discriminations as well as providing some foundation tounderstanding motion encoding and stereopsis. However, it wasa far cry from everyday perception. Marr (1980), keenly sensingthe crises in the playing out of the separate fields of physiologyand psychophysics, articulated a synthetic view of how thesefindings might fit into a larger conception of visual function.

Meanwhile, Gibson appeared to all but ignore these develop-ments. Early on he wrote, "The writer has elected to study psy-chophysics rather than psychophysiology because he believesthat it offers the more promising approach in the present stateof knowledge" (Gibson, 1950). Although using the term psy-chophysics, the psychophysics of Gibson was altogether differ-ent from the traditional variety just described. To review Gib-son's psychophysics, I consider what he meant by each compo-nent—the physical first, then the psychological.

Gibson's Physics

All through his long scientific life, Gibson the psychologistwould show a deep interest in physics, endeavoring to find abroad and more principled framework within which to placepsychology. However, for Gibson it is a physics not recognizableto physicists, at least not then. Gibson's physics is the physics ofthe everyday on the scale of the everyday. It spans the distancestraversed by animals, not those traversed by atoms or galaxies.It spans durations of the present, not infinitesimal instants orepochs. It deals with forces of the everyday and with its materi-als.

It is a physics more tied to macroscopic phenomenon, largelyignored by physicists. For example, against Eddington's (1928)The Nature of the Physical World, Gibson argued,

Some thinkers, impressed by the success of atomic physics, haveconcluded that the terrestrial world of surfaces, objects, places andevents is a fiction. They say that only the particles and their fieldsare "real." The very ground under one's feet is said to be "merely"the bombardment of molecules. (Gibson, 1966, p. 22)

Ground, the earth on which we stand, is physical and very realto Gibson and would become a cornerstone of his environmen-tal physics.

Thus, Gibson's physics differed in at least two significantways from conventional physics. It was a physics restricted toa scale comparable to the animal. In addition, it possessed aparticularity not usually associated with physics, consisting ofthose physical features of our earthbound environment relevantto animate life. Thus, it consisted of a firm ground below, but

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with numerous and often dramatic deformations and outcrop-pings (terrain), air and illumination above, and large bodies ofwater (oceans, rivers, puddles, etc.). Even more specific andmost emphasized in later work (Gibson, 1979) is the descriptionof the physical world exclusively considered in terms of its po-tential functional relation to an animal's existence. The focusremained rooted in the physical world, but it now became fusedwith psychology because Gibson felt a need for an environmen-tal physics denned specifically in relation to animate life. Forthis purpose, he developed his well-known theory of affor-dances:

The affordances of the environment are what it offers the animal,what it provides or furnishes, either for good or ill. The verb toafford is found in the dictionary, but the noun affordance is not. Ihave made it up. (Gibson, 1979, p. 127)

Gibson linked his ideas explicitly to evolutionary biology andbehavioral ecology, indicating that a particular set of affor-dances comprised an "ecological niche" for that animal.Affordances and niches to Gibson had an objective meaningfreed from mentalism and subjectivity: "The niche for somespecies should not be confused with what some animal psychol-ogists have called the 'phenomenal environment of the speciesin which the species is supposed to live'" (Gibson, 1979, p. 128).

Surfaces for Gibson are one of the most important genericphysical features of the physical environment:

The surface is where most of the action is. The surface is wherelight is reflected or absorbed, not the interior of the substance. Thesurface is what touches the animal not the interior.. . . If a terres-trial surface is nearly horizontal (instead of slanted), nearly flat(instead of convex or concave) and its substance is rigid (relative tothe weight of the animal), then the surface affords support.... Ifits surface of support with the few properties is also knee-highabove the ground, it affords sitting. We call it a seat in general, or astool, bench, chair. (Gibson. 1979, p. 23)

Thus, Gibson's physics is not about the physical world byitself, but only in its relation to potential physical actions ofanimals. Locomotion, a major focus of Gibson's 1954 Psycho-logical Review article, constituted one class of actions commonto all animals and was of major concern to Gibson. His interestcan be separated into two components. First is the issue of howlocomotion with respect to the physical environment is con-trolled, the issue of proprioception. Second are the conse-quences of locomotion for perception, the effect of a changingoptic array. When considered jointly, they would provide a ma-jor source of new ideas about vision.

Gibson's Psychology of Information Pickup

Hand in hand with Gibson's very different approach to phys-ics, his conception of perceptual psychology and the nature ofthe visual stimulus differed greatly from traditional psychophys-ics. Gibson's conception was to make a tight linkage betweenhis physics and his psychology. As such, he rejected the tradi-tional route on which most of researchers' current understand-ing of vision now rests. Referring to that tradition, he wrote,

The visual exposition of the sense of sight begins with the anatomyof the eye. There follows an account of the visual sensations, withemphasis on color, brightness and form as they are related to the

photosensitive cells of the retina.. . . The whole treatment is interms of human vision. We shall begin, however, with the questionof what eyes are good for. (Gibson, 1966, p. 155)

In short, for Gibson this meant getting information from thelayout of surfaces and controlling behavior in this environment,particularly locomotion. So rather than light intensity, spatialextent, spectral content (all of the physical variables of interestto vision researchers in the past), Gibson would be concernedwith surfaces in particular reference to the actions of animals.

Ignoring the conventional approach taken in perceptual psy-chology and psychophysics, with its preoccupation with visualangle in the two-dimensional retinal image and the needed re-construction of the third dimension by means of binocular par-allax, Gibson's space perception repudiates any scheme basedon Cartesian coordinate axes:

Visual space, unlike abstract geometrical space, is perceived by vir-tue of what fills it.. . . The surfaces, slopes, and edges of the worldhave correlates in the retinal image specifically related to their ob-jective counterparts by a lawful transformation. If this is correct,the problem of the restoration of the lost 3rd dimension in percep-tion is a false problem.. . . There is literally no such thing as theperception of space without the perception of continuous back-ground surface. This hypothesis might be called a "ground" theoryto distinguish it from the "air" theory. (Gibson, 1950, pp. 5-11)

Gibson thus saw light "itself," not as a stimulus but as a car-rier of information about the surrounding environment of sur-faces. Perhaps one of the most controversial ideas was that thepickup of information from surfaces was "direct," not mediatedby some kind of synthesis or set of inferences. What led Gibsonto this view was his notion of the higher order variable of theoptic array, a variable of optical stimulation that if directly mea-sured, would give an immediate and informative reading. Con-sider his well-known example of size. An object of a given sizecuts x units of texture elements with respect to the surface thatit sits on, no matter how far it is placed. No measurement ofdistance is necessary for an appreciation of equivalent size tobe registered. To obtain the slant of a surface, Gibson saw thegradient of texture as particularly important, providing a directmeasure of surface slant.

Thus, Gibson was concerned with obtaining informationabout surfaces in the world; he was not concerned with provid-ing an exact copy between the information about surfaces andone's visual experience. Not requiring a replica, he went on toask whether a psychophysical bottom-up approach to percep-tion is possible:

If, contrary to past teaching, there are exact concomitant variationsin the image for the important features in the visual world a psy-chophysical theory will be possible.. . . The question is not howmuch it resembles the visual world but whether it contains enoughvariations to account for all the features of the visual world. (Gib-son, 1950, pp. 61-62)

These views anticipate by over 25 years one of the most impor-tant tenets of computational vision, articulating the nature ofthe stimulus information required for vision (Marr, 1980).

As revolutionary and more immediately influential was Gib-son's suggestion that spatial gradients of motion were senseddirectly because in fact, these gradients of motion, like gradientsof texture, could provide direct information regarding surface

332 KEN NAKAYAMA

layout and extent. To appreciate the boldness of these assertionsabout motion, one has to understand the prevailing view of vi-sion at the time. Then and even now, vision is seen in termsof the static retinal image, a set of light intensities across thephotoreceptor mosaic, stimulating local retinal points, or"signs," from which perception must be built.

So sure was Gibson of his own views that as early as 1950, hecould turn his back to this established thinking and calmly yetprophetically assert,

Every photographer is aware that even a slight movement of hiscamera during exposure will shift the image on the film, for it ruinsthe picture. The same kind of shifting of the image on the retinaoccurs all the time during vision with the difference that vision isenriched rather than spoiled. (Gibson, 1950, p. 117)

The weight of the scientific tradition that Gibson had to repu-diate is described by Lombardo (1987):

Because the analogy between the retinal image and a static picturebecame popularized, the perceiver was not thought to be sensitiveto optical change, a property of optical change or an invariant ofoptical change as such. Only those instantaneous properties of onegiven retinal image were thought of as stimuli. Perceived change,such as movement always required memory images, integration,inference, or some process of metatemporal organization, (p. 216)

For Gibson, the memory image of the immediately precedingretinal image was deemed unnecessary: The observer could di-rectly sense change and the gradient of change.

Although the idea of a motion gradient did not receive anadequately precise definition by Gibson (it is more complicatedthan the usual gradient of scalar fields described by vector anal-ysis), his basic intuitions have proved to be largely correct, atleast mathematically. Evidently aware of contemporary ideas ofmathematical invariance and group structure, and citing Cour-ant and Robbins (1941) and Ernst Cassirer (1944) in his 1950book, Gibson argued that a higher order variable of the motionfield would be more explicitly informative about surface layoutthan motion itself. He also made the same claim for binoculardisparity, suggesting that the gradient of disparity rather thandisparity itself was more informative. In a highly influential setof mathematical articles in the mid 1970s, Koenderink and vanDoom (1976a, 1976b) examined the nature of motion parallaxand binocular parallax fields and were able to show that a cer-tain higher order component of the gradient of these fields wasindeed more explicitly informative about surface orientation.Since then, Koenderink in similar spirit has gone far beyondGibson, essentially creating a mathematics of vision, describinghow images from surfaces either vary or remain invariant withviewer position.

Gibson's Impact: Relative Motion and the Control ofLocomotion

For those younger scientists coming of age after the discoveryof motion sensitive neurons, Gibson's ideas might not seem par-ticularly startling until they realize that his detailed thinkingabout motion antedated the physiology. Only now has the phys-iology begun to catch up with and to address the class of ques-tions asked by Gibson, and only now has the physiology begunto discover otherwise unforeseen properties of visual neurons

that appear to vindicate his emphasis on motion and relativemotion.

In my own case, I was very interested in whether neurons inthe visual system of animals might analyze the moving image topick out surfaces for the moving observer. As mentioned earlier,with Jack Loomis and directly influenced by Gibson, we postu-lated the existence of higher order visual neurons that wouldtake differences in velocity between the center and surround ofa receptive field, independent of direction. Such cells couldhighlight the boundaries of surfaces for a wide range of observerand eye motions (Nakayama & Loomis, 1974). Our analysisrested on the fact that the velocity fields created by observertranslation could be mathematically distinguished from thosecreated by observer or eye rotation. Later, in collaboration withBarrie Frost (Frost & Nakayama, 1983), we looked for cells thatmight generalize velocity differences between center and sur-round and indeed found neurons with spectacularly complexproperties that were almost but not identical to those antici-pated. Essentially, all cells of the pigeon optic tectum fired onlywhen the motion of the center was in the opposite direction tothat of the surround, generalizing this property over a widerange of motion directions. In detail, it became quite clear thatsuch cells would not outline the edges of surfaces during mo-tion, say flight, because they would be unresponsive if centerand surround were both stimulated by motion in the same di-rection. Yet the eyes of the walking pigeon are mostly stationarywith respect to the environment because of the head-bobbingreflex. As such, these neurons could well serve to address one ofthe key questions asked in Gibson's 1954 article. He asked howwe can determine self motion from the motion of objects whenboth lead to image motion on the retina. These cells will distin-guish the motions of small objects from large background mo-tions at least during walking. Despite the highly specific andcomplex requirements for cell firing, the exact function of thesecells, however, remains unexplained. Nevertheless, without theGibsonian framework it is clear that we would never havelooked for or found cells with such remarkable properties.

More closely related to our original idea of outlining surfaceboundaries, John Allman and associates found cells in monkeyvisual areas that preferentially responded when the velocities ofcenter and surround were different (Allman, Miezin, & Mc-Guinness, 1985). More recently, and echoing Koenderink andvan Doom (1976b), cells with higher order properties of the ve-locity field (divergence or curl) have been identified in monkeyextrastriate cortex (Tanaka & Saito, 1989).

It is in the control of locomotion, however, that the most com-plete vindication of the Gibsonian outlook is evident, at least sofar. In his 1954 article, Gibson asked about the visual percep-tion of locomotion in a stable environment. Shortly thereafter,Gibson (1958) proposed that the focus of expansion, the pointin the velocity field where all motion vectors originate, consti-tuted an optical invariant that would identify for an observer hisown heading, his direction of motion with respect to a visualenvironment. Although this is not true if the animal rotates hiseyes during locomotion or moves in a curvilinear path (see Na-kayama, 1982), one can conceive of isolating the focus of ex-pansion from the added rotational component purely from op-erations of the velocity field itself or by taking account of eyerotations. Psychophysical studies have in broad outline con-

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firmed Gibson's imaginative hypotheses, showing that observ-ers can sense their own direction of motion in a computer sim-ulated display (Warren & Hannon, 1988). For complex condi-tions where the eye does not undergo pure translational motion,the sense of eye position is also required (Royden, Banks, &Crowell, 1992). As yet, however, researchers have not gone be-yond these psychophysical observations to show that humans oranimals actually use this information to perform real locomotortasks.

A more spectacular vindication of Gibson's approach con-cerns the timing of impending collision as one approaches sur-faces or when projectiles are moving toward observers. Sym-metrically growing images simulate a direct hit, a very primitivevisual spatiotemporal pattern to which human infants aredifferentially responsive (Yonas, 1981). Building on Gibson'stheory of invariances, Lee (1976) described the optical param-eter tau, the angular extent of an approaching object divided byits first time derivative. This variable provides exact informa-tion as to the time when the target will collide with the observer(see also Hoyle, 1957). It is remarkable that this variable pro-vides reliable information independent of distance or speed,thus providing not mediated but direct information of obviousbehavioral relevance. Lee later found that this variable couldaccount for the behavior of diving birds, who must streamlinetheir wings just before plunging into the water at high velocities(Lee & Reddish, 1981). Most recently, in a very exciting set ofphysiological experiments, Wang and Frost (1992) found neu-rons in the pigeon visual system that fire at a fixed interval oftime before an impending collision, independent of distance orspeed. This sequence of remarkable studies is perhaps one ofthe most dramatic of the recent success stories emerging fromthe Gibsonian framework.

The Perceptual Primacy of Surfaces

For Gibson, surfaces were all important, both in the realm ofthe physics relevant to animate life and in the psychology ofinformation pickup. Not surprisingly, the traditional approachof psychophysics and neurophysiology, with its preoccupationwith visual angle, luminance, and spectral content, ignored sur-faces. They were not forgotten, however, by perceptual phenom-enologists, who created many demonstrations showing the im-portance of surface phenomena (Kanizsa, 1979;Metelli, 1974).Yet, so buried was this awareness of surfaces that Marr's (1980)theoretical formulation of the 2.5-dimensional (2.5 D) sketch(essentially a revival of surfaces) became a high point in his am-bitious enterprise to "explain" vision. It is of interest to quoteMarr directly: "For all these reasons, the emergence during theautumn of 1976 of the idea of the 2.5 D sketch, which first ap-peared in Marr and Nishihara. . . was for me the most exhila-rating moment of the whole investigation" (Marr, 1980, p. 269).

Over the past 7 years and with the close collaboration of sev-eral colleagues, I have developed a very strong interest in sur-faces. Our work, however, did not start from Gibsonian princi-ples or outlook but began as a set of phenomenological explora-tions. Yet, a set of conclusions emerged from our work that Ithink show clear parallels to ideas promulgated by Gibson yearsearlier. First, we argued that the pickup of information aboutsurfaces is more or less an autonomous process, accomplished

before object recognition (Nakayama, Shimojo, & Silverman,1989), which may begin as early as cortical area V1 (Nakayama& Shimojo, 1990a, 1990b). Second, and strongly echoing Gib-son's philosophical views on the primacy of perception over sen-sation (Gibson, 1966), we showed that observers are more im-mediately responsive to surface properties in an image, not fea-tures. Thus, surface shape, not features, determinesperformance in visual search (He & Nakayama, 1992), visualtexture segregation (He & Nakayama, 1994), and apparent mo-tion experiments (He & Nakayama, in press; Shimojo & Na-kayama, 1990). Finally, and more directly influenced by Gibsonthrough Koenderink and van Doom's (1976c) formal descrip-tion of the sampling of images from different vantage points, wedeveloped a theoretical framework to understand the perceptuallearning of surfaces (Nakayama & Shimojo, 1992).

Locomotion in a World of Surfaces?

In reviewing Gibson and his successors, I note that two of hismost persistent interests, locomotion and the pickup of infor-mation about surfaces, have been, in at least one respect, curi-ously isolated from each other. To my knowledge, the control oflocomotion and the perception of a world of surfaces have notbeen considered together. Perhaps this is because the parametersrelevant to control the direction of locomotion (the focus of ex-pansion) and the estimation of the time of collision (tau) havebeen considered as image properties with only an indirect rela-tion to the composition of surfaces. Furthermore, more recentanalyses within the Gibsonian framework have identified otherpotential optical parameters that could control locomotion. Forexample tau-dot (the first derivative of tau) provides informa-tion to control braking (Lee, 1976; see Yilmaz& Warren, 1992);changing image orientation has been identified as an optical pa-rameter for a pilot landing an aircraft on a straight runway(Loomis & Beall, 1992). In all of these cases, one might bedrawn to the conclusion that locomotion could be controlledindependently of a surface representation. To my knowledge,this supposition has never been explicitly raised or tested. Re-searchers need to ask whether locomotion in general must beconsidered within a world of perceived surfaces or whether theremight be primitive processes of motor control, perhaps the onescited here, that might be driven by image information alone.

Summing Up

It should be evident from what I have written that I hold Gib-son in the highest esteem. I should. At critical moments in myscientific career, I have benefited immensely from his theoreticalperspective. However, his influence of course is far broader.

In fact, for sheer breadth, incisiveness, originality, and influ-ence, I cannot imagine anyone more qualified to be recognizedas the most important perceptual psychologist of the last 100years. Spanning many levels—philosophy, physics, behavior,specifics of the stimulus—the sweep is without parallel. Thenthere is the obvious originality: surfaces, texture, invariance,motion, the moving observer, and ecological optics, to mentiona few. Moreover, I would argue that Gibson's influence in per-ception, psychophysics, neurophysiology, and computer visionruns very deep, although not always fully acknowledged.

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That said, and aside from noting my own first impressions ofGibson, I have avoided one aspect of Gibson's views that havereceived the greatest criticism. Gibson and especially his follow-ers have scrupulously avoided reference to any form of internalrepresentation. Whether this reflects a defensible ideological po-sition as articulated by his followers (Turvey, Shaw, Reed, &Mace, 1981), a pragmatic ordering of research priorities as in-dicated by Gibson himself (1950), or a fundamental naivete assuggested by Marr (1980, p. 30), this almost blatant disinterestin the face of steady and often brilliant progress in the fields ofneuroscience and psychophysics strikes me as a major limita-tion, particularly now. Nonetheless, I have turned a blind eye tothis uncompromising stance because, early on, I decided thatGibson's ideas were just too good to pass up. For whatever validreasons Gibson may have had against internal representation,the discovery of new and unexpected forms of internal repre-sentation have been the happy result.

So, Gibson's influence has had some paradoxical features.Throughout his career, he has more or less ignored internal rep-resentation, the very thing that most of us non-Gibsonians havebeen looking for. Yet, he has had quite a few admirers fromwithin "our" ranks. Why should this be so?

The answer is apparent as soon as researchers realize that thesearch for internal representation cannot proceed in isolation,divorced from behavior or from an analysis of that which has tobe represented. Recall that Marr (1980) outlined different levelsof explanation required for complex processes like vision. Theywere (a) the computational level, (b) the algorithmic level, and(c) the level of implementation. In grudging acknowledgment,Marr noted that Gibson's main contribution was restricted tothe computational level but faulted Gibson for underestimatingthe difficulties posed beyond. My own view is that even Marr,his followers, and most of contemporary visual science mightbe comparably faulted for underestimating difficulties at thecomputational—I prefer the word functional—level.

Whatever the faults or limitations of studies conducted atthese various levels, those of us who have chosen to study visionare very fortunate in having such rich traditions from which todraw. As a field, we need to think hard about internal represen-tation and to continue to look to physiological findings. We alsoneed the approach advocated by Gibson: clear and originalthinking about the nature and purpose of visual function linkedto a rigorous analysis of the optical information required.

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Received September 10,1993Accepted September 23,1993 •

P&C Board Appoints Editor for New Journal:Journal of Experimental Psychology: Applied

In 1995, APA will begin publishing a new journal, the Journal of Experimental Psychology: Applied.Raymond S. Nickerson, PhD, has been appointed as editor. Starting immediately, manuscripts shouldbe submitted to

Raymond S. Nickerson, PhDEditor, JEP: AppliedDepartment of PsychologyTufts UniversityMedford, MA 02155

The Journal of Experimental Psychology: Applied will publish original empirical investigations inexperimental psychology that bridge practically oriented problems and psychological theory. Thejournal also will publish research aimed at developing and testing of models of cognitive processingor behavior in applied situations, including laboratory and field settings. Review articles will beconsidered for publication if they contribute significantly to important topics within applied experi-mental psychology.

Areas of interest include applications of perception, attention, decision making, reasoning, informationprocessing, learning, and performance. Settings may be industrial (such as human-computer interfacedesign), academic (such as intelligent computer-aided instruction), or consumer oriented (such asapplications of text comprehension theory to the development or evaluation of product instructions).