TACTUAL PERCEPTION

TACTUAL PERCEPTIONhardness of materials or the viscosity of liquids and perceivingthe shape of large three-dimensional objects.Included in the category of kinesthetic perception are thosecases of tactual perception where variations in cutaneous stimulation, though signifying contact or lack thereof between theskin surface and the external stimuli, do not inform the observerof their spatial or textural properties. One example would bethe identification of a raised pattern traced by a finger that iscovered by a thimble. Another would be discriminating betweenlengths of rods, the ends of which are held between the fingerand thumb. In both examples, the cutaneous stimulation servesonly to indicate contact with the stimulus, while variations inkinesthetic stimulation convey all of the spatial informationessential to performance of the task.1.3.Haptic PerceptionThe term haptic perception refers to tactual perception in whichboth the cutaneous sense and kinesthesis convey significantinformation about distal objects and events. Most of our everydaytactual perception and tactually controlled performance fallsinto this category.been the result. In much of his writing, Gibson equated passivetouch with what we have termed tactile perception. As one example, he wrote that "passive touch involves only the excitationof receptors in the skin and its underlying tissue. . . ." (Gibson,1962, p. 479). If passive touch is defined in this way, categories2 and 3 of Table 31.1 would be classified, along with categories4 and 5, as active touch since both involve more than cutaneousinformation.On other occasions Gibson hinged the distinction betweenactive touch and passive touch on whether the subject controlledthe pickup of information by way of efferent commands issuedto the muscles used in touching. On this basis, both categories2 and 3 of Table 31.1 would be classified as passive touch sinceneither involves active control of the touching process. Thissecond way of distinguishing between active and passive touchaccords with current usage in the motor control and perceptualadaptation literatures. Adopting i t in conjunction with our ownorganizational scheme, we obtain the following classificationof modes of touch, a s seen in Table 31.1: passive kinestheticperception, active kinesthetic perception, passive haptic perception, active haptic perception, and lastly, tactile perception,which can only be passive.2.2.2.2.1.Phenomenology of TouchPRELIMINARY CONSIDERATIONSActive and Passive TouchThe organization in terms of tactile, kinesthetic, and hapticperception adopted here recalls the distinction between activeand passive touch emphasized by Gibson (1962,1966). Becausethis dichotomy has been highly influential in the study of touch,it is of value to examine the relation between Gibson's distinctionand the present organizational scheme.Part of Gibson's motivation for advocating the study ofactive touch as opposed to passive touch was his disdain foratomism and introspectionism, a disdain expressed earlier byKatz (cited in Krueger, 1970, 1982). Gibson especially tookissue with the view that touch "sensations" can be construedas the building blocks of tactual perception. He believed thatlimiting the study of touch to probing the skin of a passiveobserver perpetuated the fallacious idea of sensation-basedperception. Gibson, like Katz, felt the important phenomena oftouch came into being when a n observer was permitted to actively explore a n object by touch. He drew some support for hisopinion from the fact (noted earlier by Weber, 184611978;Katz,cited in Krueger, 1970,1982) that when observers underwentpassive tactile stimulation they tended to describe their experience in terms of tactile sensations, whereas when they engagedin active tactual exploration they tended to describe their experience in terms of objects in space. Not only did Gibson questionthe focus of empirical research but he believed that the processof touch undergoes a fundamental change when the observeris given control over the "pickup" of information. When permittedto examine a n object actively, the observer does not attend tothe particular momentary sensations but rather seeks over timeand space the invariances in the stimulation that characterizethe object being explored.Unquestionably, there is merit in Gibson's position, particularly with regard to the tactual perception of three-dimensional objects. It is unfortunate, however, that Gibson did notdistinguish between active and passive touch in a consistentfashion, for some confusion in the subsequent literature hasThe observations of David Katz have contributed much to ourappreciation of the capabilites and richness of the sense of touch.His major work on the subject (Katz, 1925) is not available inEnglish, but several synopses (Katz, 1930; Krueger, 1970,1982;Zigler, 1926) convey a sense of his ideas, observations, andexperiments. Some of these will be presented within the appropriate sections of the chapter. What follows is a brief consideration of several phenomenological observations that occupied his interest. Although not central to a n understandingof the whole of tactual perception and performance, they doconstitute a n interesting set of facts that require explanation.Katz (1936; cited in Krueger, 1970,1982) and, before him,Weber (184611978) emphasized that most of our perceptual experience is of objects and events external to US rather than ofTable 31.l. Classification of Tactual ModesType of InformationAvailable to ObserverNocontrol1. Cutaneous information2. Afferent kinesthesis3. Cutaneous informationplusafferent kinesthesis4. Afferent kinesthesisControlplusefference copy5. Cutaneous informationplusafferent kinesthesisplusefference copyLabel ofTactual ModeTactile perceptionPassive kinestheticperceptionPassive hapticperceptionActive kinestheticperceptionActive hapticperceptionTactual modes vary depending upon the degree of observer control over thepickup of information and upon the type of information available (cutaneous,kinesthetic, or both). The left column indicates the different tactual modesthat result from the various combinationsof the two factors. The right columngives the label used to refer to these different modes.

PERCEPTUAL ORGANIZATION AND COGNITIONthe more proximal stages and processes that intervene betweenthe distal stimuli and our higher brain centers. This is notsurprising when there is and can be no perceptual representationof an intervening stage, like the retina. However, there arenumerous examples where "externalization of experience" occursin spite of "subsidiary awareness" (Polanyi, 1962) of one ormore intervening stages. With'touch, for example, the receptivesurface of the skin, unlike that of the eye, is represented withinperceptual space, yet frequently when the skin is touched theperceptual experience is of a n object external to the perceptualboundary of the body. For example, when one probes a surfaceusing a stylus held in the hand, one's awareness is not of thevibrations felt in the hand, but of the surface being explored.Similarly, when one stirs a viscous fluid, one has the experienceof fluid a t the end of the s t i m n g rod rather than of sensationsper se in the fingers, joints, and muscles.Katz (cited in Krueger, 1982) observed that the senses varyin the degree to which the resulting percepts are experiencedas part of the self (are "phenomenally subjective") or externalto the self (are "phenomenally objective"). Vision is the mostobject-sided sense, for most visual experience is referred to perceptual space beyond the bodily self. At the opposite extremeare the interoceptive senses (hunger, thirst, etc.) and pain(Krueger, 19821, for the perceptual experience is of "sensations"within the phenomenal body. This distinction between the phenomenally subjective and phenomenally objective is purely adescriptive one, there being little theoretical import in it. It ismost useful in the description of touch experience, for the senseof touch is intermediate between vision and the interoceptivesenses in terms of how often perceptual experience is referredto either the subjective or objective poles. The objective pole isfavored when the subject is allowed to explore an object activelyand when the skin surface making contact with the object isthat normally used for tactual exploration (Gibson, 1966;Krueger, 1970, 1982; Weber, 184611978). Conversely, the subjective pole is favored when body loci such as the inside of thenose or ear are passively touched (Gibson, 1966; Krueger, 1970,1982; Weber, 184611978). Especially interesting are the situations where one part of the body touches another. If one part,like the fingertip, favors the objective pole while the secondfavors the subjective pole, the resultant touch impressions arereferred primarily to the latter (Krueger, 1982). If one part ismoved across the surface of another, the objective pole dominatesfor the moving part and the subjective pole for the stationarypart.When vision and touch are used together, the "externalization" of tactual experience becomes especially compelling(Krueger, 1970), presumably because vision, which is the moreobject-sidedof the two senses, dominates over touch. (See Chapter25 by Welch & Warren for a discussion ofintersensory integrationand conflict.) When one touches an object with a probe whileviewing the tip of the probe, one "feels" the probe making contactalmost a s if i t were one's fingertip. Similarly, when one is manipulating an object with pliers or tongs, one has an impressionof "touching" the object that is not unlike grasping i t with thebare hand. Yet another example is the experience that skilledtechnicians have in working under a dissecting microscope:The dissecting tools come to be felt a s extensions of one's handsand fingers. It is this sense of transparent operation ("operatorpresence" or "telepresence") that Corker, Mishkin, and Lyman(1980) and Minsky (1980) believe is essential for or results fromthe skilled operation of remote manipulators, such a s thoseused in space exploration, in medical applications, and in thehandling of hazardous materials (for an insightful discussionof externalization and telepresence see Dennett, 1978).3.3.1.TACTILE PERCEPTIONTactile Pattern Perception3.1 .I. Spatial and Temporal Filtering of Cutaneous Processing. Like human vision, computer image display systems,television, and other image processing systems, the cutaneoussense is limited in its pattern sensing capability by both itsspatial sensitivity and its temporal sensitivity. Surely otherfactors, such as attentional and cognitive resources, do a t timeslimit tactile perception (see Loomis, 1981b; Millar, 19811, butspatial sensitivity and temporal sensitivity are of special importance, for they place quantifiable limits on perceptual capacityand dictate how best to transmit information through the cutaneous sense; when these other limiting factors are minimized,the way to achieve optimal information transfer is to matchthe spatial and temporal display parameters to the spatial andtemporal sensitivities of the cutaneous sense.3.1.1.1. Linear Systems Analysis. The concepts of linearsystems analysis and, in particular, of Fourier analysis arevaluable for understanding how cutaneous spatial sensitivityand temporal sensitivity exert their effects on tactile patternperception, just as these concepts have contributed greatly toan understanding of vision (Campbell & Green, 1965; Cornsweet,1970; de Lange, 1958; Ginsburg, 1978; Graham, 1980; Kelly,1979; Schade, 1956; Sekuler, 1974; Watson, 1983; Watson &Ahumada, 1983; Wilson & Bergen, 1979). In the context oftouch, the essential idea is that the earlier stages of cutaneousprocessing can be conceived as constituting a spatiotemporalfilter acting upon the stimulus that is applied to the skin. Someof the spatial and temporal information within the stimulus isattenuated partially or totally as a conse

Tactile perception refers to perception mediated solely by vari- ations in cutaneous stimulation. Two examples are the perception of patterns drawn onto the back and speech perception by a "listener" who senses speech information by placing one hand on the speaker's jaw and lips