VOL. 69, No. 6 NOVEMBER 1962

PSYCHOLOGICAL REVIEWOBSERVATIONS ON ACTIVE TOUCH '

JAMES J. GIBSON

Cornell University

Active touch refers to what is ordi-narily called touching. This ought tobe distinguished from passive touch,or being touched. In one case theimpression on the skin is broughtabout by the perceiver himself and inthe other case by some outside agency.The difference is very important forthe individual but it has not been em-phasized in sensory psychology, andparticularly not in the experimentalliterature.

Active touch is an exploratoryrather than a merely receptive sense.When a person touches something withhis fingers he produces the stimula-tion as it were. More exactly, varia-tions in skin stimulation are causedby variation in his motor activity.What happens at his fingers dependson the movements that he makes—•and also, of course, on the object thathe touches. Such movements are notthe ordinary kind usually thought ofas responses. They do not modify theenvironment but only the stimuli com-ing from the environment. Presum-ably they enhance some features ofthe potential stimulation and reduceothers. They are exploratory insteadof performatory. In this respect,

1 This research was supported in part bythe Office of Naval Research under Con-tract NONR 401 (14) with Cornell Uni-versity.

these touching movements of the fin-gers are like the movements of theeyes. In fact, active touch can betermed tactile scanning, by analogywith ocular scanning.

By means of active touch a greatmany properties of the adjacent en-vironment can be perceived in theabsence of vision. The blind must de-pend on it for most of their informa-tion about the world (Revesz, 1950).Nevertheless, despite its importance,the "sense of touch" (Boring, 1942,.Ch. 13; Geldard, 1953, Ch. 9-12) hasbeen studied by sensory physiologistsonly as a passive or receptive channel.It is treated as a part of cutaneoussensitivity. The sensitivity of theskin, which includes temperature andpain as well as touch, can be mosteasily investigated by the applying ofstimuli to the cutaneous surface. Ata perceptual level, Geldard (1957) hasapplied multiple stimuli to the skin andhas thereby demonstrated some of thevarious "messages" that the skin willtransmit but he is nevertheless con-cerned with a mosaic of receptors, notan exploratory organ. Katz (1925)and Revesz (1950) have argued, how-ever, that the hand is a kind of senseorgan as distinguished from the skinof the hand. Katz began the descrip-tion of tactual experience as it occursin life, and performed experiments on

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478 JAMES J. GIBSON

some of the remarkable discrimina-tions that are possible. Revesz, ob-serving the performances of the blind,proposed an unrecognized mode of ex-perience called haptics which goes be-yond the classical modalities of touchand kinesthesis. His interest is inphilosophy and esthetics. These twoseem to be almost the only investiga-tors who have given much thoughtto what is here called active touch.Their work has not been followed upby experimenters and the term itselfhas never been widely used, perhapsbecause it does not fit with the estab-lished ideas of what constitutes a sense.

The relation of touch to kinesthesis.It seems to have been assumed by psy-chologists that active touch is simplya blend of two modes of sensation,kinesthesis and touch proper. Thatis, touching merely combines the datafrom the feeling of movement and thefeeling of contact, the two sensationsbeing fused into one experience. Thisassumption, however, can be criticized.First, it fails to take account of thepurposive character of touching and,second, it fails to emphasize the multi-plicity of so-called kinesthesis.

The first objection is that the act oftouching or feeling is a search forstimulation or, more exactly, an effortto obtain the kind of stimulation whichyields a perception of what is being-touched. When one explores any-thing with his hand, the movementsof the fingers are purposive. Anorgan of the body is being adjustedfor the registering of information. Thelimbs and extremities are, of course,motor organs as well as sensing or-gans, whereas the eyes are only senseorgans, but the function of motor per-formance can be subordinated to thefunction of exploratory adjustment inthe case of the limbs.

Behavior theorists have recentlybeen emphasizing the importance of

what is variously called "feedback"from responses, or self-produced stimu-lation, or proprioception in general.They recognize that such inputs arenecessary for the purposive control orsteering of behavior. What they havenot recognized is that some of thestimulation produced by responsesyields objective information as wellas subjective. The stimulation pro-duced by sense-organ adjustment isof this sort. In active touch, as willlater be suggested, the flow of stimu-lation contains two components, oneexterospecific and one propriospecific.But these components are not thetraditional sensations of touch andkinesthesis respectively. The hy-pothesis will be advanced that thepurpose of the exploratory movementsof the hand is to isolate and enhancethe component of stimulation whichspecifies the shape and other charac-teristics of the object being touched.

A second objection is that the termkinesthesis itself means differentthings and should not continue toimply the existence of a unitary sense.Historically the word refers to move-ment of the body, and originally meantthe "muscle sense." But it is knownthat there is sensitivity to the positionof the body and of all its membersrelative to one another. This is anarticular sense, not a muscle sense,and the joints yield information aboutjoint position as well as joint rotation(Goldscheider, 1898). Probably thereis no "sense" of muscle contraction assuch, the joint initiating the spatialinformation and the muscle spindleshaving only reflex coordinating func-tions (Rose & Mountcastle, 1959).There is sensitivity for the uprightposture of the head, the linear andangular accelerations of the head, anda general "sense" of the equilibriumof the body. There is sensitivity ofsome kind for the force exerted by an

OBSERVATIONS ON ACTIVE TOUCH 479

individual, whether or not this is ac-companied by hypothetical "feelings ofinnervation" (Boring, 1942, Ch. 14).There is also, of course, visual sensi-tivity to the changes of position ofone's body in space. And there is pro-prioception in general, however thismay be defined, as well as somaes-thesia.

The single term "kinesthesis" can-not carry the burden of all the mean-ings that have been added to its origi-nal meaning. It fails to do justice todifferent inputs, to different combina-tions of these inputs, and to differentfunctions of these combinations.

Types and subtypes of anatomicalreceptors involved in active touch.Passive touch involves only the ex-citation of receptors in the skin and itsunderlying tissue, although the pat-terns may be elaborate. But activetouch involves the concomitant excita-tion of receptors in the joints andtendons along with new and changingpatterns in the skin. Moreover, whenthe hand is feeling an object, themovement or angle of each joint fromthe first phalanx of each finger up tothe shoulder and the backbone makesits contribution. And these inputsoccur relative to a continuous inputfrom the vestibular organs, along withthe cutaneous input from contact withthe ground. Presumably the feelingof an object by the hand involves thefeeling of the position of the fingers,hand, arm, body, and even the headrelative to gravity, all being integratedin some hierarchy of positional infor-mation.

The total flux of stimulation is enor-mously complex, but lawful modes ofcombination occur. Presumably themodes of combination of these inputsspecify the difference between touch-ing and being touched. Also certainspecial combinations probably carryinformation about the object being

touched. In short, the so-called senseof touch as it is most often employedby using the hands involves the playof inputs coming from the wholeskeleto-muscular system. Whether itis one sense or many is therefore amatter of dispute. It depends on whatone wishes to call a sense. There isno single organ or structure analogousto an ear or an eye. There is nounique quality of sensation analogousto warm, cold, pain, sour, sweet, orred. Active touch does not fulfill thesupposed criteria for a single sensemodality. Nevertheless it provides aquite definite channel of informationabout the external environment. It isa type of perception that is isolablefrom vision, audition, taste, and smell,and it needs to be studied in its ownright.

Mechanics of cutaneous stimulation,proximal and distal. The literal orproximal stimulus for touch, the im-mediate cause, is some sort of de-formation of the skin, not necessarilya simple depression of the surface asby a tactile "stimulator," but almostany deformation of the elastic tissue.Pulling on a thread glued to the skinis just as effective a stimulus as push-ing it in. The stimulus has to be achange in time, that is, a motion of theskin. Since the work of Nafe andWagoner (1941) it has been suspectedthat whenever the skin becomes abso-lutely motionless even in some abnor-mal conformation, the receptors ceasefiring and the stimulus becomes inef-fective. Outside of the laboratory thissituation would be rare. A preciselyconstant pressure of an object relativeto the skin would not occur, if only byreason of tremor or body sway.

The cause of elastic change of theskin's surface can be almost any me-chanical event. Some work must bedone, even if slight, but the actual me-

480 JAMES J. GIBSON

chanical happenings are so variousand complex that the textbooks ofmechanics do not begin to describethem. A common event is impact witha solid substance, but a puff of air ora drop of water will do as well. Inhairy regions of the skin, even theslightest bending of a hair gives astimulus. Some of the types of me-chanical events that stimulate the skin,in terms of everyday physics, may beexemplified as follows.

1. Brief events: pressure, push, slap,pat, tap, prick. Note that these varyin area as well as in duration.

2. Prolonged events without dis-placement : vibration, stretching,kneading, pinching.

3. Prolonged events with displace-ment : scratching, scraping, rubbing,sliding, brushing, rolling. Note thatthese moving deformations vary in thedegree of friction between the twosurfaces, and also in the degree oflateral stretching of the skin as wellas the degree of vertical depression.

These are relatively large scaleevents as compared to the micro-events that excite the actual end-organs imbedded in the tissue. Butthe latter are also mechanically com-plex. The ways in which mechano-receptors are made to fire nervousimpulses need not be considered here.The facts, so far as they are known,are to be found in the physiologicalliterature.

The external causes of these me-chanical events, the sources of poten-tial cutaneous stimulation, are the ob-jects and surfaces of the environment.They are at the distal end of a sequencethat leads to the excitation of nervecells. An objective surface of somekind and the cutaneous surface must"touch," either by movement of anobject with reference to the stationary

skin or by movement of the skin withreference to a stationary object.

Contact with the substratum. The"sense of touch" is not ordinarilythought to include the feeling of cu-taneous contact with the earth. Never-theless, the upward pressure of thesurface of support on some part ofthe body provides, for every terres-trial animal, a constant background ofstimulation. It is covariant with thecontinuous input of the statocyst re-ceptors of the inner ear already men-tioned. Together they provide whatthe ordinary person calls the "senseof support." The axis of gravity andthe plane of the ground provide thebasic frame of reference for tactualspace perception. Active touch, as willappear, yields clear perceptions of en-vironmental space in the absence ofvision.

The touching of the substratum isneither active nor passive touch asdefined above, i.e., neither the touch-ing of a stationary object nor beingtouched by a moving object. It isinstead a means of registering thestable environment with reference towhich both the movements of one'sbody and the motions of objects occur.

OBSERVATIONS OF THE ACT OFTOUCHING 2

In the simple experiments to be de-scribed, vision was always excluded.This was usually accomplished byhaving the seated observer put bothhands under a cloth curtain at thefront of a table, not by blindfoldinghim, or working in darkness. Hismanual activity could thus be observed.

When an object of any sort is placedin the hand of the observer or when

2 The collaborators in this work wereSusan Stanley, Lewis Greenberg, and JamesCaviness. The report has profited from acritical reading by Jesse Smith.

OBSERVATIONS ON ACTIVE TOUCH 481

his hand is placed on it, the followingfacts can be noted. He tends to bringthe other hand to the object if allowedto do so. He tends strongly to curvehis fingers around the object if per-mitted. He tends to move his fingersover the object in complex ways.Some of these are: tracing movementswith one fingertip or several fingertips,opposition of the thumb and otherfingers, and rubbing, grasping, orpressing movements of the fingers.Finally, he tends to name the objectif he can do so, or to compare it witha familiar object if he cannot. Non-sense objects are almost necessary forsuch observations since the observerdoes not persist after recognizingsomething.

The observer seems to be trying toobtain mechanical events at the skinat various places in various combina-tions. The movements by which heobtains them do not ever seem to betwice the same, but they are not aim-less. If the hand is conceived as asense organ, he seems to be adjustingit. He appears to be searching forstimulus information.

So much for the manual activity.Reports of experience during activetouch can also be obtained. Theytend to be radically different fromreports of experience during passivetouch. The following facts, noted bythe writer, are confirmed by otherobservers in this experiment, or areconsistent with earlier observations.

Unity of the phenomenal object.When feeling a single object with twofingers, only one object is perceivedalthough two separated cutaneouspressures occur. The separate "localsigns" are not noticed. This holds truenot only for finger-thumb oppositionbut also for other combinations offingers. The unitary perception oc-curs when all five fingers are appliedto an object, and even when two sepa-

rate hands are applied to the object.In fact, 10 different digital pressuresall at the same time yield a whollyunified experience. There are somelimitations on this unity, as can beobserved with Aristotle's illusion whena pencil is held between crossed fing-ers, but even then it is very hard toovercome, by the writer's introspec-tion. This fact needs further investi-gation.

Stability of the phenomenal object.When sliding the skin over a corneror protuberance of an object, the dis-placement of the cutaneous pressure,the "tactile motion," cannot usually benoticed. The object seems to remainstationary even though the impressionmoves relative to the skin. Not onlythe object but also the whole spaceof the table, chair, floor, and room isfelt as perfectly stable. This phenome-non has been studied in an experimentreported in the next section.

Rigidity or plasticity of the phe-nomenal object. When pressing afinger on a rigid surface or squeezingan object with the hand, it is difficultto notice the increase of intensity ofcutaneous sensation; instead the ob-server is primarily aware of the sub-stance and its resistance. Likewise,when pressing or squeezing a nonrigidobject (a lump of modeling clay, arubber ball, a handful of cloth) theobserver is aware of the yieldingness,elasticity, or softness of the substance,not of the (very different) intensityof the back pressure on the skin. Thisfact deserves further study. Presum-ably the degree of force exerted isbeing registered by the joint-and-ten-don receptors and the resulting degreeof skin-pressure is being registered bythe cutaneous and deep-tissue recep-tors, but the intensities are not experi-enced as such. Perhaps it is the rela-tion between these different intensitiesthat is registered. The proportion of

482 JAMES J. GIBSON

one to the other, and the relativetemporal buildup of these two, are dif-ferent for rigid and for nonrigidobjects. A possible hypothesis is thatproportion and buildup are registeredby the receptive system when it isoperating in an exploratory fashion.They constitute stimulus-informationabout the object, whereas the intensi-ties as such do not.

Shape of the phenomenal object.When the corners, edges, or otherprotuberances of a strange object arebeing felt, one can distinguish the pat-tern which these make to one anotherbut one cannot distinguish the patternwhich the various cutaneous pressuresmake to one another. One perceivesthe object-form but not the skin-form.The latter is, in fact, continuallychanging as the fingers move in vari-ous ways. It is almost completelyunreportable, whereas the pattern ofphysical corners and edges seems toemerge in experience. This fact isextremely interesting and two experi-ments arising from it will later bereported.

Discussion. For all four of theperceptions described above, it is pos-sible to perform a sort of control ex-periment substituting passive touchfor active touch. The experiencesaroused are quite different. For thefirst, two different pressures appliedto two different loci on the skin of thehand yield two sensory impressions,not one. For the second, transloca-tion of the stimulus over the skinyields a perception of something mov-ing. For the third, increase in theintensity of skin-depression is report-able as such. For the fourth, a patternof simultaneous pressures is detectablewhen it is pushed into the passive skin.In all cases the sensory impressionscan be aroused by an experimenter,but when the observer himself bringsthem about they seem to disappear.

DIFFERENT WAYS OF PERCEIVINGMOTION OF AN OBJECT BY TOUCH

The objects of tactile perceptionconsidered so far have been essentiallystationary, either resting on a tabletop or in one or both hands. Howcan the motion of an object be per-ceived, and how can this be distin-guished from the motion of the handrelative to the object?

Translator^ motion over the skin.The simplest case is that when the ob-server's hand lies passively supportedbehind the curtain and an object ofsome sort is made to trace a path ofpressure across the skin. The path,direction, and speed of motion couldalways be described relative to theskin. They could equally well be de-scribed relative to the table top andto the space of the room.

For these observations various in-struments were used, a sharpenedpoint, a blade, a rounded stylus, a softbrush, and the wheel of a map-measur-ing instrument. The observer couldidentify the kind of instrument usedwithout ever seeing it. He distin-guished between brushing, rolling, andrubbing, and also between the blade,the point, and the rounded protuber-ance. There was always an objectivecomponent in the experience.

Friction at a single area of the skin.When a string is stretched over aregion of the skin and then pulled inone or the other direction, the velocityis perceived. In order to study thisphenomenon further, a yardstick slid-ing in a track was constructed on thetable top, on which the observer couldlower a fingertip, the hand being sup-ported on an adjacent motionless blockof wood. The yardstick could bemoved laterally by a rack-and-piniongear attached to a silent variable-speedclutch and a constant speed motor.When the observer lowered his finger

OBSERVATIONS ON ACTIVE TOUCH 483

on this surface, he could feel the stickand report whether or not it was ob-jectively moving, and the direction ofthe movement. He could estimate thespeed and the amount of displacementover a given interval of time with somesuccess. A speed of less than 1 mm.per second was detectable under theseconditions. In this case there was nopath of movement over the skin butonly directional stretching and frictionof a single patch of skin. There wasno change in the "local sign" of stimu-lated points. Nevertheless the percep-tion was that of a moving object.

Passive rotation of the joints of thelimb. With the apparatus described,when the wooden support for the handwas removed and the fingertip was al-lowed to rest on the yardstick, theobserver could still report accuratelywhether or not it moved, the directionof movement, its speed and the amountof displacement. Such results are pre-dictable from those of Goldscheider(1898, described in Geldard, 1953, Ch.12. See also James, 1890, Ch. 20).Only the receptors in the joints of thelimb are stimulated. In the situationdescribed, the joints of the finger,wrist, elbow, and perhaps the shoul-der are all involved. The perception,as before, was that of a moving object.A naive observer would assert that heperceived the movement "by touch"although in actual fact the only cuta-neous stimulation was a deformationof the skin of the finger, unchangedexcept by reason of slight tremor. Thewhole forearm was invisible behindthe curtain.

Evidently three anatomically differ-ent receptive systems, two in the skinand one in the joints, can be func-tionally equivalent for the experienceof objective motion when they arepassively stimulated by an externalagency.

Friction on the skin combined with

active movement of the arm. In activetouch, one runs the fingers over theprotuberances of the object. In thisoperation friction on the skin and ro-tation of the joints are combined, to-gether with voluntary contraction ofthe muscles. But in this case what oneexperiences is the movement of thehand, not an illusory motion of theobject. We wished to make an evenstronger test of this fact of phenomenalstability. Hence the apparatus wasused to determine whether the motionof the yardstick, or its stability, couldbe detected while the finger was beingactively moved along its surface, i.e.,rubbed over it.

The fingertip of the observer wasplaced at one end of the stick and hewas required to move his hand slowlyeither toward the right or the left.During this active movement the ob-ject was, on occasion, put into slowmotion in the same or the oppositedirection. It turned out that the ex-istence, direction, and approximatespeed of the object could be perceived,as before. Introspectively, there ap-peared to be a distinct separation ofthe movement of the hand from themotion of the stick. Both of theseseparate displacements were perceivedrelative to the space of the invisibletable top hidden behind the curtain,this being connected to the visible andtangible floor and chair. One can onlyconclude that the propriospecific com-ponent in the total stimulus input hadbeen separated from the exterospecificcomponent.

Discussion. The ability of an in-dividual to distinguish objective mo-tion from subjective movement is atheoretical puzzle of long standing invisual perception (Gibson, 1954). Amovement of the eyes is never con-fused with a motion of an object eventhough both involve what seems to be

484 JAMES J. GIBSON

the same retinal stimulus. Evidentlythe problem is analogous in tactualperception, although the solution canhardly be quite the same. When anindividual actively moves his handrelative to his body, and thus relativeto gravity and the surface of support,he is in contact with the earth as wellas with the object, whether it be mov-ing or motionless. A receptive sys-tem of higher order is excited duringactivity than the system during passiv-ity. Hence the stimuli which have oneexcitatory capacity for a receptive sub-system in passive touch will have adifferent excitatory capacity in activetouch—a different specificity. Therelations between stimulation of thesubsystems become significant. A law-ful covariation of the inputs carriesinformation different from that carriedby isolation of the inputs, and yieldsdifferent perceptions. Johannes Miil-ler's law of the specific conscious qual-ity of each excited nerve is evidentlymuch too simple.

To lump one set of receptive sys-tems together as touch and another askinesthesis, then, is to obscure thefunction of the systems in combination.If both had their own unique quality ofexperience there would be some justi-fication for the traditional belief inseparate senses, but they do not. Ana-tomically different receptors may servethe same function and arouse the sameexperience, and, moreover, may servedifferent functions in different combi-nations. It might be better to dropthe term sense and speak instead of"esthesic systems."

It should be noted that nothinghas been asserted about performatorymovements of the hand, as when anobject is relocated or a tool is wielded.The perception is then subsidiary toaction and the receptive systems takeon still another function not here con-sidered, j

PERCEPTION OF THE TEXTURE, SUB-STANCE, AND SHAPE OF AN

OBJECT BY TOUCH

Katz (1925) showed that the tex-ture of a solid substance could be per-ceived by touch if there was relativemotion between the surface and theskin. Ordinarily this means rubbingthe fingers over the surface; perpen-dicular motion was very much lesseffective than lateral motion. A sub-stance could be identified by its tex-ture; the observers could distinguisha dozen kinds of paper including blot-ting paper, wrapping paper, and writ-ing paper. Katz also demonstratedthe perceptibility of other physicalproperties of a surface besides rough-smooth, including soft-hard, and vari-ous kinds of plasticity, elasticity, orviscosity.

Katz was primarily interested in thefine structure of the surface of anobject, and the substance of which itwas composed. He had less to sayabout the shape of a substance, whichis perhaps the main characteristic ofan object. Active touch, however, isan excellent channel of spatial infor-mation in that the arrangement of sur-faces is readily picked up. The solidgeometry of things is best got by feel-ing them. A number of observationsillustrate this assertion.

Curvature as against planarity of asurface. Departure of a surface fromthe plane can be judged by an observereither by holding his hand againstthe surface or by running his fingersover it. The opposite qualities of con-vexity and concavity are immediatelygiven. On a smaller scale, protuber-ances or indentations on a plane sur-face are identified, and this acuity istaken advantage of in Braille. More-over a surface with positive curvatureon one axis and negative curvature onthe other is perceived as saddle-shaped.

OBSERVATIONS ON ACTIVE TOUCH 485

The accuracy of all these judgmentscould be but has not been measured.

Slant of a surface with respect togravity. When the observer's handis placed on an adjustable palm-boardoutside the field of view, it is foundthat he can judge its slant with pre-cision. He can nullify this slant andadjust it to a normal plane. Thisholds for all three planes of space, thefrontal, the saggital, and the hori-zontal. Variable and constant errorsfor settings to the normal plane seldomexceed 2°-3°. Furthermore, the slantof a surface that is visible only canbe equated to the slant of a surfacethat is tangible only. The cross-modalmatching of slant is accepted as nat-ural by all observers, and the manualjudgment can be used as a criterionof visual perception (Gibson, 1950).

When the palm-board is maintainedat a slant from the frontal uprightplane for some time, "normalization"seems to occur, and a negative after-effect ensues. In this respect, thetangible slant of a surface behaves likethe visible slant of a surface (Berg-man & Gibson, 1959).

Parallelity of two surfaces. If apair of adjustable palm-boards is heldbetween the two hands, behind a cur-tain, they can be adjusted to the paral-lel position with small errors. Thedivergence or convergence of the twoplanes are felt as opposite qualities ofa simple spatial experience. This di-mension is also susceptible to normal-ization; after a few minutes of feelingdivergence (say) the setting acceptedas parallel is found to have shiftedseveral angular degrees toward diver-gence. This fact has been studied indetail (Gibson & Backlund, 1963).

Distance between two surfaces. Thespan of the hands holding an objectbetween them is experienced vividly.The tangible extent can be matched

with a visible extent or vice versa.Likewise the distance between thethumb and one or more of the opposa-ble fingers is perceived, and is accu-rately matched with the width of avisible object. According to Kelvin(1954) the cross-modal size compari-son is made with no less accuracy thaneither unimodal size comparison alone.The finger-span of one hand can beimmediately felt as equal or unequalto the finger-span of the other hand,and this comparison has been utilizedby Kohler and Dinner stein (1949) instudying a phenomenon they termedthe kinesthetic figural aftereffect. Inthe many repetitions of this experi-ment the equivalence of the two handsfor perceiving a spatial extent seemsto have been taken for granted withoutits being studied in its own right.

Plane-angle, or edge of a surface.To exploratory touch, a junction oftwo planes constituting an edge is verynoticeable. The angle is perceptibleas varying from sharp to dull. At theextreme case of a knife-edge, sharp-ness is more accurately judged bytouch than by vision.

Edge-angle, or corner of a surface.Where two edges intersect, or wherethree or more planes adjoin, a corneris produced. In the case when theangles of a convex corner are acuteit is apt to be called a point. Cornersand edges seem to be the main identi-fying feature of many objects. Thefingers appear to seek them out andexplore them.

Discussion. Planarity, curvature,slant, parallelity, span, edge, and cor-ner may be conceived as variables ofsolid geometry. They are all percep-tible to active touch, and appear to besimple qualities of experience. Sincethey are amenable to physical meas-urement, psychophysical experimentswould be possible, but few have beenperformed.

486 JAMES J. GIBSON

FIG. 1. The six forms to be identified by touch.

TACTUAL FORM PERCEPTION :A COMPARISON OF ACTIVE

AND PASSIVE TOUCH

The study of the perception of ob-jects by touch has been largely con-fined to investigations of two dimen-sional form or pattern on the skin,with passive stimulation (e.g., Zigler& Barrett, 1927). They are in thetradition of experiments on cutaneouslocalization and the two point thresh-old, the assumption being that the skinis first of all a receptor mosaic withpatterns of excitation being projectedto the brain. The forms pressed intothe skin were called solid or outlinedepending on whether the edges of asurface or the edges of a metal stripwere used; the latter gave betterdiscrimination. The identification offorms in this manner is not very ac-curate, and there is some uncertaintyeven about circles, squares, and tri-angles.

Everyone knows, however, that abetter way to perceive a thing of thissort is to run one's fingers over theedges. The fact is often taught to be-ginning students, and is even demon-strated in laboratory exercises (Lang-feld & Allport, 1916) but a search ofthe experimental literature failed tolocate its source. Lashley (1951), forexample, asserts as if it were commonknowledge that the same shape whichcan scarcely be distinguished withpassive touch "can readily be distin-guished when explored by tactile scan-ning" (p. 128). An experiment wasperformed to quantify this assertion,since no authority for it could befound. Each of a set of tactual forms

had to be matched to its visual equiva-lent. The accuracy with which thiscould be done was compared when theform was pressed into the palm of thehand and when it was held above thepalm and explored by the fingertips.

MethodThe sources of stimulation were bent

metal strips with handles, the edges havingthe forms shown in Figure 1. They werein fact small "cookie cutters" with a meandiameter of 1 inch. The six used werechosen for being mutually different in aboutthe same degree. No two had the samenumber of corners or points. Each subjectwas shown the set of stimulators and theequivalent set of numbered drawings hungon the curtain in front of him. He thenput his hand palm up on the table behindthe curtain and was either touched by theobject or was allowed to touch it for sev-eral seconds. In the one condition, he wasinstructed to open and relax his hand; in theother to cup the fingers upward and pre-pare to feel the edges. He then had toidentify the form by number. The stimu-lator was not necessarily presented in thesame orientation as the drawing.

The six forms were presented five timeseach (but the subject was told not to expectequal frequencies) and under two conditions,making a total of 60 trials. The order wasrandom. No preliminary practice or knowl-edge of results was given. Twenty subjectswere tested.

Results. A chance level of judg-ments would be 1/6 or 16.7%. Forpassive touch the mean frequency ofcorrect matches was 49%. For activetouch the mean frequency was 95%.The difference is significant.

In the active condition, the subjectwas permitted to explore the edges inany manner he chose. One style wasto use all five fingertips simultaneously,another was to move the tip of the

OBSERVATIONS ON ACTIVE TOUCH 487

index finger around the edge, andothers were intermediate. All sub-jects, however, actively moved theirfingers and thus obtained relative mo-tions between the skin and the object.

In the passive condition, the formwas pressed down on the palm bymanual action of the experimenters sothat a certain amount of unsteadinesswas inevitable. When subsequently,with 20 different subjects, a mechani-cal lever system was used to apply thestimulation the mean frequency of cor-rect matches fell to 29%.

Some subjects reported that theytried to count the number of cornersor points they could feel, and that thiswas easier with the active than withthe passive condition.

Discussion. A common belief hasbeen verified but, in a sense, no claimcan be made for the conclusiveness ofthe comparison. It could be arguedthat the fingertips are more "sensitive"than the palm of the hand, meaningthat localization is better and the two-point threshold lower. This is true.The comparison is not very meaning-ful. The real point of the experimentis that tactual form perception in onemeaning of that term does not dependon the pattern of local signs on theskin. If any number of fingertipsfrom one to five can be used indiffer-ently, the chaotic pattern of local signsresulting can hardly be the sole basisof the perception. With active touchno forms existed on the skin, but onlya changing pattern of pressures. Evi-dently the registering of "form" bytouch is not as straightforward a proc-ess as experimenters have supposed.Another experiment may help to clar-ify the matter.

TEANSPOSABILITY OF FORM ONTHE SKIN

Gestalt theorists have emphasizedthe fact that a form can be moved over

the retina, completely changing the setof receptors stimulated, without chang-ing the perception. The contributionto perception of the anatomical patternof excitation becomes a puzzle. Thesame is true of the skin.

Along with the experiment just de-scribed, another was carried out at thesame time in order to compare passivestatic form perception with passivemoving form perception. In thesetrials the "cookie cutter," instead ofsimply being pressed into the palm ofthe hand, was pressed and continuallyrotated clockwise and counterclock-wise with a twisting motion. Thesame area of skin was stimulated inboth cases and the same anatomicalreceptors were available, but in thelatter case the form of stimulationchanged from one instant to the next.

Judgments of the form of the objectwere more accurate when it was ro-tated than when it was not, the meanfrequencies of correct matches being72% and 49%, respectively. The dif-ference is significant at the 2% level.To the writer's introspection, the cor-ners of the object seemed to "standout" better when it was twisted. Inshort, the form of the object seemedto become clear when the form of theskin-deformation was most unclear.

Discussion. Continuous change ofthe proximal stimulus with no changeof the rigid object constituting thesource of the stimulus is an old prob-lem in visual perception. Why doesthe perception correspond to the formof the object instead of to the form ofthe stimulus? Evidently a quite simi-lar problem arises for tactual percep-tion. The paradox is even more strik-ing, for tactual perception correspondswell to the form of the object whenthe stimulus is almost formless, andless well when the stimulus is a stablerepresentation of the form of theobject. A clear unchanging percep-

488 JAMES J. GIBSON

lion arises when the flow of sense im-pressions changes most.

It might be that the skin does nothave as its primary function the regis-tering of form as this has usually beenconceived. Impressions of form andlocation relative to the skin might bequite separate from and incidental tothe use of the extremities as senseorgans. The informative stimuli mightwell be incorporated in the seeminglycomplex motions and transformationsof the skin. There must exist relationsof separation and proportion whichremain invariant over time, and whichare specific to the object. They donot represent the shape of the objectbut they specify it. The solution tothe problem of object perception intouch would then be that continuouschange in the proximal stimulation isaccompanied by nonchange, that is, theset of invariant relations. The formeris not noticed; the latter is separatedout and attended to.

The role of exploratory finger move-ments in active touch would then be toisolate the invariants, that is, to dis-cover the exterospecific component inthe flux of stimulation. Only thuscould the above paradox be resolved.

RELATION BETWEEN TOUCHAND VISION

A number of similarities and differ-ences have been noted in the courseof these observations between activetouch and visual perception. Many ofthese have been pointed out before byother observers such as Katz (1925,1935) but a few of them have not pre-viously been emphasized.

Surface perception. Solid surfacesare tangible and visible. This canonly be because they have mechanicalinertia in the first place, and becausethey reflect light with scattering in thesecond place. (A few visible surfaces

such as smoke are not tangible and afew tangible surfaces such as plateglass may not be visible, but most sur-faces are both.) The physical textureof a surface, the various kinds ofroughness, can be observed either byvision or by touch. The color of asurface is visible only, not tangible,and consequently the pigment-textureof a surface (the differential coloringof marble or of a picture) is the same.On the other hand, if color is intangi-ble temperature is invisible. Eachsense has its special sensitivities to theproperties of a surface but they alsohave some in common. The bark ofa tree looks rough without touchingit and feels rough without seeing it.The expert can identify the tree byeither sense alone.

The visible array of surfaces ex-tends for miles of course, and the tan-gible array only to the length of man'sarm. But if surfaces determine spaceperception, both touch and vision arespatial senses.

Object perception. A detached solidsubstance with a topologically closedsurface is perceptible as such to boththe hands and the eyes. By virtue ofphysics, the eyes can encompass theclosed contour of a very large object(depending on its distance) while thehands can explore only the surface ofan object of limited span. The unityof the visual perception is said to bebased on the "figure-ground" phe-nomenon, the simultaneous registeringof the whole contour, whereas theunity of the tactual perception has tobe based on either cutaneously sepa-rated impressions or on successiveimpressions. Vision is said to registeronly the front surface of an objectwhile touch registers the front andthe back at the same time. In theserespects vision and touch seem to dif-fer. Nevertheless these contrasts areexaggerated; succession enters into

OBSERVATIONS ON ACTIVE TOUCH 489

the operation of both senses. The eyesnormally fixate in succession just asthe fingers explore in succession. Withtwo eyes, and by changing one's stand-point, more of an object than its frontsurface is perceived. As regards theunity of separated impressions fromthe two hands, there is a visual anal-ogy in the unity of separated impres-sions from the two retinas.

Spatial properties of objects. Cer-tain variables of the shape of a de-tached object, slant, curvature, edgeand corner are independently visibleand tangible. The equivalence of thetwo modes of perception for judg-ments of the object is such that dif-ferences got by one sense are equatedto differences got by the other. Thestimulus information for these cross-modal judgments, one depending onmechanical and the other on luminousenergy, might seem hopelessly discrep-ant, but the fact is that the judgmentscan be made.

Whether both vision and touch canregister form is an ambiguous ques-tion, as the experiments showed. Theanswer depends on what is meant byform. Drawings and pictures on aflat surface are sensed only by vision.The conformation of the edges of anobject, however, is sensed by bothvision and active touch. The tan-gible and visible features of thingsabstracted by solid geometry (slant,curvature, edge, corner) did not seemany more complex to introspectionthan the solely visible features ofthings abstracted by plane geometry(triangle, square, circle).

A new set of shapes has now beendevised for the study of object-percep-tion by active touch. They consist often solid sculptures, or free-forms,made of plastic, the surfaces beingcurved, with no planes, edges, or cor-ners. They are intended to be feltwith two hands (and are called "feel-

ies"). Approximately one-half of eachsurface (the "rear") is convex; theother half (the "front") consists ofsix convexities with intermediate sad-dles or concavities. In general, thereare five protuberances around a cen-tral protuberance, but no object issymmetrical, either radially or bi-laterally. They cannot, therefore, bedistinguished from one another bycounting. Each is readily discrimi-nated from every other by vision ofthe "front" surface. They are alsomutually distinguishable by feeling, al-though with some error and hesitationfor an unpracticed observer.

Replicas of the ten objects are avail-able, made from the same molds. Itis therefore possible to present oneobject to the hands and the same or adifferent object to the eyes simultane-ously. Preliminary results indicatethat the cross-modal matching of thesenovel preceptions is possible even fora naive observer, and that practice canbring about errorless judgments in allobservers so far tested.

Passivity and activity in touch andvision. In passive touch the individualmakes no voluntary movements. Simi-larly, in passive vision he makes noeye movements, which means that hemust voluntarily fixate his eyes on apoint specified by the experimenter.Neither state is natural to an individ-ual. In a tactual situation, the ob-server will explore with his fingersunless prevented and, in a visual situ-ation, he will explore the focussablelight, fixating, accommodating, con-verging and pursuing. Both sensesare normally active. The passive stim-ulation of the skin or the retina isnecessary for the study of the receptor-cells in the skin or the retina, but theexperiences resulting are atypical.

In active touching and looking theobserver reports experiences of a quitedifferent order. They correspond to

490 JAMES J. GIBSON

the environment instead of to theevents at the sensory surface. Theexperiences noted with passive stimu-lation can scarcely be noticed, if atall. The effective stimuli are presum-ably different, being relations andcombinations selected from a flux ofpotential stimuli. Moreover, the ex-periences of active touching and look-ing are more nearly equivalent; theyare more alike to introspection thanthose of passive touch and vision.

In general, the foregoing surveysuggests that vision and touch havenothing in common only when they areconceived as channels for pure andmeaningless sensory data. When theyare conceived instead as channels forinformation-pickup, having active andexploratory sense organs, they havemuch in common. In some respectsthey seem to register the same infor-mation and to yield the same phe-nomenal experiences.

CONCLUSIONS AND SUMMARY

A series of observations, both intro-spective and behavioral, confirms thedistinction between touching and beingtouched. The former is a channel fora great variety of information aboutthe environment, but whether it shouldbe considered one or several senses isa matter of definition. The simpleformula that it consists of passivetouch plus kinesthesis is insufficient.The hypothesis of two components ofstimulation, one exterospecific and onepropriospecific, is more promising.

Many properties of surfaces and ob-jects can be registered by active touch.The accuracy of such judgments canbe measured and formal experimentsare needed. The observations suggestthat the variables of solid geometrycan be more profitably studied thanthose of form. Object constancy ischaracteristic of tactual perception. In

several respects, tactual and visualperception are alike.

In general, experimenters have notrealized that to apply a stimulus to anobserver is not the same as for anobserver to obtain a stimulus. Obtain-able stimuli can be controlled and sys-tematically varied, just as appliedstimuli can be. Psychophysical ex-periments are possible for tactualperception as well as for cutaneoussensitivity if appropriate methods aredevised for controlling the obtainablestimuli.

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(Received July 30, 1961)