The question invol The cooperation of supposedly separate senses of touch and kinesthesis is an old and controversial problem in psychology (Gibson, 1966). It has been reformulated by Gibson in terms of a perceptual subsystem, haptic touch, and the problem then becomes one of defining the information in a combined input from the skin and the joints. Request PDF The Senses Considered as Perceptual Systems: The Revolutionary Ideas of Gibson's 1966 Book, 50 Years Later - Part 1 This editorial introduces the first part of a 2-part special.
Summary
Although Gibson (1979) did not explicitly discuss the perspectival appearing of the ecological environment, his important ecological approach to visual perception can accommodate both (a) the stream of visual-perceptual experience that flows at the heart of the visual system's total activity of ordinary visual perceiving (ordinary seeing), and (b) the dimension of the visual experiential stream that is the ecological environment's perspectival appearing to the visual perceiver. In the present article, perspectival appearing is located at the level of brain centers of the visual system, where processes are determined by the spatiotemporally structured visual stimulus flux. And the stream of visual experience is interpreted as itself possessing a kind of perspective structure (as does the visual stimulus flux), including variant and invariant features that the visual system isolates and extracts from experience, producing the perceiver's cognitive visual “awareness-of” (Gibson, 1979) the environment and self in the environment.
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Natsoulas, T. Perspectival appearing and Gilbson's theory of visual perception. Psychol. Res52, 291–298 (1990) doi:10.1007/BF00868060
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The sense of touch is one of the central forms of perceptualexperience. Thought to be the first sense to develop, touch occursacross the whole body using a variety of receptors in the skin. Itoften combines these signals with rich information made available bystretch receptors in the muscles and tendons as we actively move andexplore the world. Because of these unique features, touch raises manyinteresting philosophical issues. Its complex yet fundamental naturemakes it a central topic of discussion in debates about themultisensory nature of perception, the relation between perception andaction, and the connection between touch and bodily awareness.
1. Background and Terminology
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Touch is a fundamental form of non–visual perception, onethat plays a crucial role in nearly all of our sensory experiences (afeature noted by Gibson 1966). It is, unlike many of the other senses,more plausibly taken to be inherently multisensory, given thediversity of its constituent systems and forms of experience. Inaddition, it seems to have unique and philosophically interestingconnections to exploratory action and bodily awareness.
Before turning to these issues, it will be helpful to provide somebackground on the terminology typically used to talk abouttouch. These are standard terms used in both philosophy and cognitivescience, and they are (for the most part) neutral ways of talkingabout the diverse constituents involved in typical touchexperiences.
What we typically refer to as “the sense of touch” iswhat we will call “active” or “haptic”touch. This refers to touch that involves some movement. This movementcan be voluntary, exploratory movements of the hands and other sensorysurfaces involved in touch, or it can also refer to experiencesgenerated by objects moving against a stationary body. In most cases,haptic touch will involve the engagement of kinesthesis (awareness ofmovement) and proprioception (awareness of bodily position). Someapply the term “haptic” for any touch that involves theactivation of (the physiologically and functionally distinct)kinesthetic or proprioceptive systems (e.g., Loomis and Lederman1986). Touch mediated entirely through the skin will be referred to as“cutaneous touch”. The term “tactual” will beused as a broad term to refer to any form of touch experience (soit’s used in the same way that “auditory” and“olfactory” are used for hearing and smell ). For thefeatures and objects made available through touch the term“tangible” will be used (again, in the same way as“visible” is used for for features and objects madeavailable by vision).
2. Is Touch Multisensory?
Touch is often classified as one of the traditional five senses,along with sight, hearing, smell, and taste. Touch is, in severalways, seemingly different from these other senses, however. For onething, touch does not seem to have a single sense organ. The skin, ofcourse, is the most plausible candidate sensory organ, but the skinitself is not sensory. Instead, the skin contains many differentsensory systems. Many of them, like those that code for pain and itch,do not seem to be tied directly to the sense of touch. For instance,we don’t, at least in most contexts, seem to treat cutaneouspains as part of the sense of touch proper. The same seemstrue for itch, tingles, and twinges, though perhaps these seem moreclosely tied with touch. At any rate, they are not paradigm instancesof tactual perception, and if they seem more closely tied to touch,then this is something in need of explanation.
Even if we focus only on those systems usually associated withtouch, we find a number of distinct sensory channels. Some of thesehave proven incredibly difficult to isolate and study (see e.g., thehistorical discussion in Gallace and Spence 2014). Indeed, we are justnow starting to understand the role and function of the most basicreceptor types involved in touch. Another methodological challenge, towhich we’ll return, concerns the deep connection between touchand exploratory action (nicely summarized in Jones and Lederman2006). For touch, many of the historically important empiricalinvestigations have focused on cutaneous touch, especially on mappingthe two–point threshold over the extent of the body. Thisthreshold is the minimal distance at which a subject can discriminatetwo distinct stimuli. Its study requires subjects to remain completelystill, while very small probes (like horse hair) are used to generatestimuli. It is much more difficult to measure touch in ecologicallysalient contexts where there are unconstrained movements using thewhole body.
This raises some interesting questions about what counts as thesense of touch. There have been various attempts to define touch. Oneattempt concludes that the typical means of unifying the systemsinvolved in touch fail to give anything like a coherent account. Theconclusion is that:
[T]his leaves us with something so heterogeneous that no criteria can unite its various characteristics and at the same time exclude characteristics of other senses. (Ratcliffe 2012: 3)
These discussions reveal the depth and difficulty of theproblem. Here are some reasons why the issue is so difficult. At thephysiological level, the afferent nerve channels that carryinformation about thermal properties are very similar to those thatcode for pain and itch, and they also differ significantly from thoseinvolved in the coding of pressure, shape, and vibration (for someempirical background, see Lumpkin and Caterina (2007); for importantrecent caveats, see Saal and Bensmaia (2014)). In other words, thermalchannels are physiologically and functionally more similar to thenociceptive channels involved in pain than they are to the channels involved indiscriminative touch (Schepers and Ringkamp 2010). And yet thermalawareness is often considered part of touch, indeed, it has often beenthought of as one of the central aspects of touch. Pains, on the otherhand, are almost never considered part of touch.
Isolating touch on physiological and functional grounds is thusdifficult. In addition, there is the problem that haptic touchinvolves many essential receptors that are located in the muscles,joints, and tendons, and not in the skin at all. Since these receptors arealmost always thought to be involved in coding for touch, there is theadditional problem that even the rich set of receptors in the skin arenot the only ones involved in typical touch experiences.
Given these facts, touch cannot be associated solely with the skinin any simplistic way. The skin contains a heterogeneous set ofdistinct receptor populations, and these populations differ in theirphysiological and functional properties. In addition, our ways ofclassifying elements as part of touch don’t seem sensitive toany of these differences in obvious ways. It thus seems clear that thesense of touch isn’t simply that form of perceptual awarenessconstituted by signals made available through the receptors in ourskin.
Perhaps because of this functional physiological diversity, thedominant view of touch in the recent cognitive science literature hastypically treated touch as inherently multisensory rather than asingle unified modality like vision and audition (for examples anddiscussion, see Loomis and Lederman 1986; Fulkerson 2011; Gallace andSpence 2014; Jones and Lederman 2006; Linden 2015).
Work in philosophy on the individuation of the senses has oftenfound touch to be a difficult case. This often has little to do withthe physiological and functional diversity alluded to above. Even ifwe focus only on the phenomenological features of tactual perception,or on the typical objects of tactual awareness, touch seems to possessa diversity that resists any single unified account. Aristotle notedthat touch, unlike the other dominant modalities, seemingly lacked asingle “proper sensible” (for a recent empiricaltreatment, see Kung 2005). On this account, a proper sensible is asensory feature only available in a single modality, one that wasconstitutive of that modality (Marmodoro 2014). Forvision color is the proper sensible. Any experience of coloris visual, since no other modality provides awareness of thatfeature. The proper sensibles were contrasted with the common sensiblesfound in more than one modality. Touch doesn’t have a singleplausible proper sensible. Instead it has multiple potential propersensibles, including pressure and temperature. This too might be astrong reason for treating touch as a collection of distinct sensesrather than a single modality.
The question of how to individuate sensory modalities has recentlybeen of central importance in philosophy. In what follows, we willbriefly consider a few key accounts and how they might apply to thesense of touch.
One approach is to treat the individual sensorymodalities as specialized informational channels or “routesinto” an organism (Keeley 2002). According to this criterion, touch wouldagain seem to count as a collection of multiple sensory modalities,since touch involves a number of distinct informational channels. Theydiffer in the nature of the stimuli they are sensitive to, the natureof their activation profiles and functional properties, and even intheir evolutionary history.[1]
Another plausible view that in many ways is more amenable tounifying the sense of touch is to treat sensory modalities as aconventional kind, something we make use of but that isn't definedentirely by the relevant sciences (Nudds 2003). If this is right,then we shouldn’t expect there to be some underlyingphysiological unity or connection that grounds our conception oftouch. Instead, we use the talk of sensory modalities to mark what asubject might know on the basis of the experience. On this account,what we call the sense of touch is that form of awareness thattypically provides awareness of an object’s weight, texture, andtemperature, etc. Learning that Anna touched the vase conveysthe information that she was in a position to become aware of thewarmth and solidity of the vase, among the other typical tangibleproperties.
We could also reject entirely the idea that we can adequatelyclassify the senses along any single dimension. Instead, we couldclassify the senses using all of the available criteria, constructinga multidimensional space of possible sensory modalities (Macpherson2010). On this account, touch would be classified according to itstypical sensory organs, its representational content, the physicalstimuli to which it is sensitive, and its phenomenal character. Onthis view, human touch would seemingly occupy a larger expanse ofmodality space than the other major human senses. This would probablyreveal one kind of major difference in kind between human touch andthe other sensory modalities.
Another option is to argue that, while touch does have a disunifiedand heterogeneous functional and physiological profile, this is alsolargely true of the other sensory modalities. What each of thesystems that seem to be unisensory have in common then isn’tunity in any of the major criteria listed above (physiology, content,stimuli, etc.). Instead, it could be that each of the diversesystems involved in human touch function together to connect or bindsensory features together (Fulkerson 2014c). This binding thesis suggests one way inwhich the diverse systems involved in touch might hang together, evenif there is no single dimension on which touch is always unified.
3. Touch and the Other Senses
In addition to its own constituent systems, touch interacts withother modalities in interesting ways. This is important in the historyof philosophy especially because the most discussed interaction, orpotential interaction, concerned the connection between touch andvision. Both senses bring information about shape and size andlocation, but they seem to do so in very different ways. The centralquestion has long been the nature and strength of thesedifferences.
This question has received the most attention in large part becauseof Molyneux’s famous letter to Locke (see the entry on Molyneux’s problem). Molyneux asked whether whether a subject born blindcould, upon complete restoration of sight, tell a cube from a sphere(a difference learned through touch) using sight alone. This raisesmany questions about the transferability and connection between thespatial representations made available in touch and vision. There aremany assumptions underlying Molyneux’s question, which have longbeen investigated by philosophers and psychologists (Evans 1985;Campbell 1996; Hopkins 2005).
In addition, there has been considerable discussion of how touchand vision might differ in terms of their spatial features. Vision, itseems, provides a rich felt awareness of objects in a spatialfield–an area where there are potential objects but where nonecurrently reside (that is, we seem in vision to be able to see emptyspace). Touch, on the other hand, doesn’t seem to presentfeatures in this way. Instead, like audition, touch seems only tobring awareness of individual objects that each seem to occupy aspecific location.
The relation between touch and agency reveals more interestingareas for further investigation. In particular, it seems plausiblethat the sense of touch has a closer connection to our agentialactions. This is partially a result of the fact that touch seems torequire active exploratory movements, and these movements are oftenguided and voluntary. Given these closeconnections, it is probably not surprising that touch has such a closeconnection to agency. One could even use of this close connectionbetween touch and agency to address epistemological problem ofperception (see Smith (2002), and the entry on the problem of perception). Whenwe press against a solid object, the resistance to our agential act ofpressing gives our experience a more solid epistemic foundation thanwhat we experience through the other sensory modalities. Only in touch do weseem to come into direct contact with reality, a reality that activelyresists our voluntary actions.
Philosophers have also been interested in the relation betweentouch and other sensory modalities. It seems plausible to think thattouch, unlike vision, does not have a full, 3 dimensional sensiblefield in external space. Instead, touch seems confined to the limitsof the body, and so the tangible field is, unlike in the othermodalities, defined by the limits and extent of the surface of thebody. This supposed difference in the nature of their spatialawareness marks a clear structural difference between touch andvision, even when they represent the same sets of objectivefeatures. Similarly, one could claim that touch also differs from theother senses in always involving bodily awareness(O’Shaughnessy 1989). Externally–directed touch awarenesson such a view would only be made possible against a background ofbodily awareness. This sort of view has been labeled “thetemplate view” (Scott 2001).
4. Tangible Qualities
Any discussion of a perceptual modality often turns to the natureof the perceptual qualities or features made available by thatmodality. A discussion of vision, for instance, naturally leads tometaphysical questions about the nature of the colors. A discussion ofaudition similarly would bring up questions about the nature ofsounds. Touch is unique in this respect, however, since there has beenlittle philosophical investigation of the nature of tangiblequalities.
One exception, already noted above, concerns the thermal qualitiesof hot and cold. But even here, little attention has been paid to themetaphysical nature of these qualities. This is understandable, giventhat touch seems to bring us into contact with ordinary materialobjects and their properties. There doesn’t seem to be anyserious worries about the nature of solidity, texture, vibration, andthe like. There is an important question about the structure of felttangible qualities, however. How are these ordinary tangible featuresrepresented or experienced?
One possibility is that, ultimately, all of the tangiblequalities, with the exception of hot and cold, can be reduced to thespatial properties of objects (Armstrong 1962). Defending such aclaim involves making an important distinctionbetween transitive and intransitive bodilysensation. The transitive sensations are those like warmth andpressure that have a sensory component as well as a real worldproperty. Intransitive sensations do not have anynon–sensational real world feature. This is one way to separateout pains, twitches, and tingles from perceptual touch proper. Thespatial view then states that nearly all of the objective tangiblefeatures made available through transitive sensation are relationalspatial properties. On this relational view of touch, all immediatetactual perception involves a relation holding between our body andobjects in contact with it. A rough surface is one that is solid, hashardness, and a certain uneven shape. A smooth surface only differs inhaving a regular shape. Roughness and smoothness can be analyzed interms of shape. A hard object is one that does not change its shape. Asoft object changes its shape under pressure. Liquidity is defined ashaving a particular shape in particular circumstances. Pressure playsa role here, so the view also requires a spatial account ofpressure. One possibility is that pressure is a quality that has atendency to cause a change in the shape of the body. Stickyness couldbe when something remains in contact with the same spot on the skin,slippery things do not. Despite the possibilities for reduction here,many of these moves feel ad hoc and unsupported by the empiricalevidence. A better account is needed.
A more recent view holds that tangible qualities are bestunderstood as intensive features that vary in intensity alonga single dimension of variation (Fulkerson 2014b). When we feelvibration, for instance, it seems to increase in intensity (in thiscase, in frequency) along a single qualitative dimension. This seemsto be true of most tangible features. They are not typically complex,but simples that vary along a single dimension. This view accords wellwith the empirical data and offers a unified explanation for thestructural connections between an otherwise disjoint collection ofsensory features. One worry for this view, however, again concerns ourawareness of hot and cold. Variations in intensity alone do not seemapt for capturing the nature of hot and cold experiences (seebelow). Rather than varying along a single dimension, our experiencesof hot and cold seem to vary in intensity in two directions around aneutral point (Gray 2012). Cold experiences are those that vary awayfrom neutral in one direction; our experiences of warmth vary awayfrom neutral in the other (increasing) direction. More importantly, itseems the nature of these movements depends more on changes in ourcurrent bodily state than the objective measures of temperature. Soonce again, thermal properties seem to pose additional difficultiesnot faced by other tangible qualities.
5. Thermal Awareness
Philosophers have long been interested in the thermal system. Asnoted above, thermal properties are difficult to connect with othertangible features for both spatial and intensity views. On either view,it seems difficult to make sense of the unique structure and bodilyrole played by our thermoreceptive systems. One possibility for thisdifficulty may be due to the fact that thermal propertiesare secondary qualities. A secondary quality is one that (insome manner) crucially depends on our subjective awareness. The idea is that out in the world there really is no such thing as“hot” and “cold”. Instead, there are onlydiffering amounts of temperature or mean kinetic energy. We feelsomething much more structured than this through our skin (a fact mostkeenly developed in Gray 2012). We feel objects as very cold, to cool,and then on to neutral, and only within a fairly narrow totalrange. After this neutral point, objects take on an entirely differentcharacter, and start to feel warm, followed by hot (again, up to alimit). This space of thermal awareness involves a kind of inversionthrough a neutral point. But this neutral point is unique to humanexperience; there is no neutral point in the temperature takenobjectively. More importantly, the temperatures that we typically feelas neutral depends on the context and especially on the currentthermal conditions of our body and sensory surfaces.
This presents something of a problem for our naive conception ofthermal awareness. We generally tend to think that our awareness ofhot and cold provides objective or at least useful information aboutactual temperature. We use it to check baby bottles, determine if theheater is on, and make sure the bath isn’t going to scald us. Ifinstead, thermal awareness only gives us awareness of a purelysubjective experience only loosely connected with temperature, then itseems thermal awareness isn’t quite what we generally take it tobe. A warm stimuli feels cold when our bodies are already heated, butit feels hot when our bodies are in a cold state. This difference canbe experienced at once by simultaneously heating one hand in hot waterand cooling another in an ice bath, and exposing them both to aneutral water bath. The same water will feel hot and cold at the sametime.
Thermal awareness, and its unsteady connection to objectivetemperature, has been used to argue that sensory systems generallyaren’t in the business of (only) conveying objective sensoryinformation (Akins 1996). Instead, it seems to only conveyinformation about our current bodily state (Craig 2002). If this isright, then it might explain some well–known but puzzlingphenomena, such as metal feeling cool to the touch, and wood feelingwarm, even when they are of the same objective temperature. It alsoexplains our different levels of thermal awareness for body parts atdifferent temperatures.
6. Touch and Bodily Awareness
The sense of touch is closely connected to bodily awareness. Thisconnection is seemingly much stronger than what we find in the othersenses (though this is somewhat controversial, as we’ll see).This close connection is easy to understand. Whenever we touch, andespecially when we engage in active or haptic touch, we areforced to use our bodies. Setting aside the worries discussed aboveabout defining the organ of touch, there is some truth to the claimthat we touch with our whole body. After all, we touch using our skin,muscles, joints, and we can touch using nearly every surface along thewhole body.[2]Armstrong’s view, discussed above, suggested that touch wasalways a felt relation between our bodies and some objective featureconnected directly to us.
One possibility is that the sense of touch, while distinct fromproprioception, nevertheless always involves it (O’Shaughnessy1998, 2000; Ratcliffe 2008). Since proprioception is nothing otherthan awareness of the location and orientation of our own bodies,there is a strong sense in which touch nearly always involvesawareness of the body. Citing groundbreaking work on the relationbetween tactual and bodily awareness by Merleau-Ponty (2002), Gibson(1966), and Martin (1992), Ratcliffe (2012) offers a detailed overviewof the ways in which touch seems to bring both an awareness of thingsexternal to the body and of the body itself. The body is that bywhich we measure the features of things in the external world, and sois always present in our awareness of things through touch. There aremany reasons for thinking of this connection as relatively weak. Thereare plausible examples, like playing sports or quickly touchingobjects, where we seem to have external awareness through touch but nodirect bodily awareness (Scott2001).[3]
Another possibility is that, while touch indeed involves bodilyawareness, the nature and structure of this awareness is importantlydifferent from what we see in typical cases of mediatedperception. Touch does not involve awareness of the body as a part ofsome external feature (as in other cases of mediated perception) nordoes it involve any explicit cognitive attitude. Instead, it could bethat some bodily sensation is necessary for tactual perception(Richardson 2011). This view has the advantage both explaining whytouch necessarily involves bodily awareness but also why it differsfrom more typical cases of mediated perception. In particular, itdiffers from the template view by dropping any requirement for amatching of spatial or other contents between the tactual perceptionand the bodily awareness. For this reason, it more easily accommodatesextended touch.
Extended touch, or awareness of distal objects and features througha cane or other tool, can be used to argue for a related but distinctview of the relation between perceptual touch and bodily awareness(Fulkerson 2012). That we experience through touch objects andfeatures located some distance from the body raises questions aboutthe mediating role of bodily awareness. Given the nature of extendedtouch, it seems implausible to hold that there is anything like amatching of the contents of perceptual touch (or the features madeavailable through touch) with the content or features involved in thebodily awareness. Instead, perceptual touch seems to depend on bodilyawareness informationally. The idea is that both bodilyawareness (proprioception and kinesthesis especially) anddiscriminative touch make use of the very same sensoryinputs. Perceptual touch is the result of extracting distalinformation from the more proximal bodily information, for use bydedicated downstream systems (cf. Serino and Haggard 2010). Since allperceptual touch will be the result of such extraction, there willalways be bodily information available for awareness.
These various views all take a stand on the extent to which touchbrings direct awareness of external features and objects, and to whatextent such awareness is mediated by awareness of the body. Inaddition, they are usually forced to take a stand on the spatialcontents of perceptual touch. On views according to which all externaltactual awareness is ultimately a form of bodily awareness, it issimply not possible to have extended touch experiences. On other viewsthat hold a less restrictive dependence between perceptual touch andbodily awareness, there is an additional burden to explain how distaltouch is possible. These choices have analogues in the empiricalliterature. Some, for instance, want to make a clear distinctionbetween “interoception” and “exteroception” (Craig 2002). According to this view, many aspects of touch,though primarily its thermal properties and most forms of pure bodilysensation (itch, tingles, twinges, etc.) are primarily directed not atthe external world, but at the present state of our bodies. This ispart of how our bodies maintain homeostasis. This results inthe possibility of very different functional roles for certain aspectsof bodily awareness, especially as they function in the generation ofperceptual touch.
7. Touch and Action
As noted in the beginning, touch seems to have both a passive andan active nature. In its more passive forms, touch involves cutaneousactivations across the surface of the body. These include bodilysensations of hot and cold, pressure, vibration, and the like. Inaddition, these activations have a limited, entirely bodily spatialcharacter. This form of touch has played an important role in theearly empirical study of touch, especially in determining the natureand acuity of our sensory receptor populations. This has often beenaccomplished through the measure of the two–point thresholdmentioned above. This distance varies across the body and is areliable guide to the density and spatial resolution of many tactualreceptors on the body.[4] While these passive forms of touch areimportant, especially in the empirical investigation of touch, intypical, unconstrained touch, we use every part of our bodies toactively explore the environment. We use our hands and arms andfingers, we move our trunks and legs, and we actively feel with nearlyevery surface of our body. Touch in ordinary use involves specializedmovements and grips. Such prehensile manipulation through touch isoften distinguished from stereognosis, or object recognition throughtouch. In addition, these movements recruit and make use of manyreceptive systems that seem inherently active. For instance, haptictouch involves feedback from our movements, along with informationfrom our motor activities (both in motor planning and efferencecopies). Perhaps most importantly, it also includes information fromreceptors in our muscles and joints. All of these elements interactand play a critical role in forming and developing our sense oftouch. For this reason, touch, especially of the active, hapticvariety, seems like an ideal model for views of perception accordingto which perception is essentially a form of action, or at least aform of experience that involves action in a unique manner.
Given its active nature and dependence on exploratory movement,many have taken touch to be best understood as an extended “haptic system” that includes both surface receptors and motorareas (Gibson 1966). The wider haptic system so understood involvesthe entire body and brings direct awareness of things in theenvironment by engaging and actively connecting together a number ofdistinct sensory networks. Many later theorists have been influencedby this compelling account of touch.
The connections between touch and exploration run deep. To citejust one example, subjects have a remarkable ability to determine thesize and shape of many large objects simply by hefting and wieldingthe objects through space (Turvey 1996). These actions provide a richand distinctive form of awareness that cannot be generated orrecreated through skin activations alone. These dynamic touchexperiences reveal the strong connection between touch and action.
Many theorists now think of perception as inherently active. Thisis a large, diverse group, one that includes motor theorists,enactivists, and many others (see the entry on embodied cognition). Touch,especially of the dynamic and interactive kind described by Gibson andTurvey, would seem to provide strong evidence in favor of suchviews. Or at the least, if the case is to be made, one would expect itto be strongest for the sense of touch in its dynamic forms. To citehere just one prominent example: Noë (2004) defends an account ofperception that takes it to be essentially a form of action. He beginshis discussion by arguing that vision is “touch like”,involving sampling and exploratory probing of the environment. Theactive nature of touch is never in question, and can be used in thiscontext as a model for understanding other forms of perception. It istrue that haptic touch seems inherently active, but many detailedquestions remain about the metaphysical relations between action andtouch (for instance, whether action is merely causally necessary orconstitutive of touch). This is likely to be an area of ongoing activeresearch.
8. Pleasant Touch
A final area of increasing philosophical investigation concerns therole of pleasure and pain in perception, something typically referredto as “affect”.[5] The question is central to our fullunderstanding of the richness and complexity of perceptualexperience. While perception is often assumed to be entirely receptiveand descriptive, it is just as often evaluative and motivational. Whenwe smell something awful or see something graphic, we have intensereactions to these experiences, and are directly motivated to act invarious ways. These forms of affective perceptual experience seeminglybridge the gap between experience, emotion, and evaluativejudgment. For these reason, there are many importantly differentaccounts available for explaining the valenced nature of theseexperiences.
Touch again is an excellent source for such investigation. Ourtactual experiences often seem to have a felt pleasant or unpleasantcharacter. This isn’t simply anassociated but distinct state of pleasure or pain that accompanies theperceptual experience, but part of the perceptual experienceitself. Paradigm examples include the pleasure derived from deliciousfood or the awfulness of certain bad smells. Here again, touch isready for increased investigation. In recentyears, researchers have discovered a specialized class of afferentnerve channels that seem to be responsible for the experience ofpleasantness (Löken et al. 2009; McGlone et al. 2012). Thesechannels, called CT-Afferents, are maximally responsive toslow, regular activations like those generated by a feather pulledgently across the arm. These channels seem like pleasant versions ofthe famous C-fibers implicated in pain experiences.
While the discovery of these afferents has been an excitingdevelopment in our understanding of affective perceptual experience,they also raise many questions. How can a receptor “code forpleasantness”? Are there other similar receptors for pleasanttouch in glabrous skin (the smooth, hairless skin of the lips andpalms where they are no CT–afferents). What do these receptorsmean for our understanding of pleasure and pain and affectiveexperience more generally? And returning to where we started, we canseriously wonder whether and to what extent the CT system seems to bea part of touch. The CT system seems, unlike that for cutaneous pain,to be a genuine component of the touch system; on the other hand, theCT system also doesn’t appear to have any direct discriminativefunction. CT interactions reveal much about the complex ways in whichemotions and motivation can come to be closely connected to perceptualexperience. They can also be seen as a bridge to a betterunderstanding of affiliative touch, the close form of caring touchthat forms an essential element of social bonding and humandevelopment, especially of the immune system (Field 2003). This shouldprove to be an area of increasing research.
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Matthew Fulkerson<[email protected]>
Matthew Fulkerson<[email protected]>