## 1. Background and Terminology


Touch is a fundamental form of non-visual perception, one
that plays a crucial role in nearly all of our sensory experiences (a
feature noted by Gibson 1966). It is, unlike many of the other senses,
more plausibly taken to be inherently multisensory, given the
diversity of its constituent systems and forms of experience. In
addition, it seems to have unique and philosophically interesting
connections to exploratory action and bodily awareness.


Before turning to these issues, it will be helpful to provide some
background on the terminology typically used to talk about
touch. These are standard terms used in both philosophy and cognitive
science, and they are (for the most part) neutral ways of talking
about the diverse constituents involved in typical touch
experiences.


What we typically refer to as "the sense of touch" is
what we will call "active" or "haptic"
touch. This refers to touch that involves some movement. This movement
can be voluntary, exploratory movements of the hands and other sensory
surfaces involved in touch, or it can also refer to experiences
generated by objects moving against a stationary body. In most cases,
haptic touch will involve the engagement of kinesthesis (awareness of
movement) and proprioception (awareness of bodily position). Some
apply the term "haptic" for any touch that involves the
activation of (the physiologically and functionally distinct)
kinesthetic or proprioceptive systems (e.g., Loomis and Lederman
1986). Touch mediated entirely through the skin will be referred to as
"cutaneous touch". The term "tactual" will be
used as a broad term to refer to any form of touch experience (so
it's used in the same way that "auditory" and
"olfactory" are used for hearing and smell ). For the
features 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 made
available 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 several
ways, seemingly different from these other senses, however. For one
thing, touch does not seem to have a single sense organ. The skin, of
course, is the most plausible candidate sensory organ, but the skin
itself is not sensory. Instead, the skin contains many different
sensory 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 cutaneous
pains as part of the sense of touch proper. The same seems
true for itch, tingles, and twinges, though perhaps these seem more
closely tied with touch. At any rate, they are not paradigm instances
of 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 with
touch, we find a number of distinct sensory channels. Some of these
have proven incredibly difficult to isolate and study (see e.g., the
historical discussion in Gallace and Spence 2014). Indeed, we are just
now starting to understand the role and function of the most basic
receptor types involved in touch. Another methodological challenge, to
which we'll return, concerns the deep connection between touch
and exploratory action (nicely summarized in Jones and Lederman
2006). For touch, many of the historically important empirical
investigations have focused on cutaneous touch, especially on mapping
the two-point threshold over the extent of the body. This
threshold is the minimal distance at which a subject can discriminate
two distinct stimuli. Its study requires subjects to remain completely
still, while very small probes (like horse hair) are used to generate
stimuli. It is much more difficult to measure touch in ecologically
salient contexts where there are unconstrained movements using the
whole body.


This raises some interesting questions about what counts as the
sense of touch. There have been various attempts to define touch. One
attempt concludes that the typical means of unifying the systems
involved in touch fail to give anything like a coherent account. The
conclusion 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 the
problem. Here are some reasons why the issue is so difficult. At the
physiological level, the afferent nerve channels that carry
information about thermal properties are very similar to those that
code for pain and itch, and they also differ significantly from those
involved in the coding of pressure, shape, and vibration (for some
empirical background, see Lumpkin and Caterina (2007); for important
recent caveats, see Saal and Bensmaia (2014)). In other words, thermal
channels are physiologically and functionally more similar to the
nociceptive channels involved in pain than they are to the channels involved in
discriminative touch (Schepers and Ringkamp 2010). And yet thermal
awareness is often considered part of touch, indeed, it has often been
thought of as one of the central aspects of touch. Pains, on the other
hand, are almost never considered part of touch.


Isolating touch on physiological and functional grounds is thus
difficult. In addition, there is the problem that haptic touch
involves many essential receptors that are located in the muscles,
joints, and tendons, and not in the skin at all. Since these receptors are
almost always thought to be involved in coding for touch, there is the
additional problem that even the rich set of receptors in the skin are
not the only ones involved in typical touch experiences.


Given these facts, touch cannot be associated solely with the skin
in any simplistic way. The skin contains a heterogeneous set of
distinct receptor populations, and these populations differ in their
physiological and functional properties. In addition, our ways of
classifying elements as part of touch don't seem sensitive to
any of these differences in obvious ways. It thus seems clear that the
sense of touch isn't simply that form of perceptual awareness
constituted by signals made available through the receptors in our
skin.


Perhaps because of this functional physiological diversity, the
dominant view of touch in the recent cognitive science literature has
typically treated touch as inherently multisensory rather than a
single unified modality like vision and audition (for examples and
discussion, see Loomis and Lederman 1986; Fulkerson 2011; Gallace and
Spence 2014; Jones and Lederman 2006; Linden 2015).


Work in philosophy on the individuation of the senses has often
found touch to be a difficult case. This often has little to do with
the physiological and functional diversity alluded to above. Even if
we focus only on the phenomenological features of tactual perception,
or on the typical objects of tactual awareness, touch seems to possess
a diversity that resists any single unified account. Aristotle noted
that touch, unlike the other dominant modalities, seemingly lacked a
single "proper sensible" (for a recent empirical
treatment, see Kung 2005). On this account, a proper sensible is a
sensory feature only available in a single modality, one that was
constitutive of that modality (Marmodoro 2014). For
vision *color* is the proper sensible. Any experience of color
is visual, since no other modality provides awareness of that
feature. The proper sensibles were contrasted with the common sensibles
found in more than one modality. Touch doesn't have a single
plausible proper sensible. Instead it has multiple potential proper
sensibles, including pressure and temperature. This too might be a
strong reason for treating touch as a collection of distinct senses
rather than a single modality.


The question of how to individuate sensory modalities has recently
been of central importance in philosophy. In what follows, we will
briefly consider a few key accounts and how they might apply to the
sense of touch.


One approach is to treat the individual sensory
modalities as specialized informational channels or "routes
into" an organism (Keeley 2002). According to this criterion, touch would
again seem to count as a collection of multiple sensory modalities,
since touch involves a number of distinct informational channels. They
differ in the nature of the stimuli they are sensitive to, the nature
of their activation profiles and functional properties, and even in
their evolutionary 
history.[1]


 Another plausible view that in many ways is more amenable to
unifying the sense of touch is to treat sensory modalities as a
conventional kind, something we make use of but that isn't defined
entirely by the relevant sciences (Nudds 2003). If this is right,
then we shouldn't expect there to be some underlying
physiological unity or connection that grounds our conception of
touch. Instead, we use the talk of sensory modalities to mark what a
subject might know on the basis of the experience. On this account,
what we call the sense of touch is that form of awareness that
typically provides awareness of an object's weight, texture, and
temperature, etc. Learning that Anna *touched* the vase conveys
the information that she was in a position to become aware of the
warmth and solidity of the vase, among the other typical tangible
properties.


We could also reject entirely the idea that we can adequately
classify the senses along any single dimension. Instead, we could
classify the senses using all of the available criteria, constructing
a multidimensional space of possible sensory modalities (Macpherson
2010). On this account, touch would be classified according to its
typical sensory organs, its representational content, the physical
stimuli to which it is sensitive, and its phenomenal character. On
this view, human touch would seemingly occupy a larger expanse of
modality space than the other major human senses. This would probably
reveal one kind of major difference in kind between human touch and
the other sensory modalities.


Another option is to argue that, while touch does have a disunified
and heterogeneous functional and physiological profile, this is also
largely true of the other sensory modalities. What each of the
systems that seem to be unisensory have in common then isn't
unity in any of the major criteria listed above (physiology, content,
stimuli, etc.). Instead, it could be that each of the diverse
systems involved in human touch function together to connect or bind
sensory features together (Fulkerson 2014c). This binding thesis suggests one way in
which the diverse systems involved in touch might hang together, even
if 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 with
other modalities in interesting ways. This is important in the history
of philosophy especially because the most discussed interaction, or
potential interaction, concerned the connection between touch and
vision. Both senses bring information about shape and size and
location, but they seem to do so in very different ways. The central
question has long been the nature and strength of these
differences.


This question has received the most attention in large part because
of Molyneux's famous letter to Locke (see the entry on
 Molyneux's problem).
 Molyneux asked whether whether a subject born blind
could, upon complete restoration of sight, tell a cube from a sphere
(a difference learned through touch) using sight alone. This raises
many questions about the transferability and connection between the
spatial representations made available in touch and vision. There are
many assumptions underlying Molyneux's question, which have long
been investigated by philosophers and psychologists (Evans 1985;
Campbell 1996; Hopkins 2005).


In addition, there has been considerable discussion of how touch
and vision might differ in terms of their spatial features. Vision, it
seems, provides a rich felt awareness of objects in a spatial
field-an area where there are potential objects but where none
currently reside (that is, we seem in vision to be able to see empty
space). Touch, on the other hand, doesn't seem to present
features in this way. Instead, like audition, touch seems only to
bring awareness of individual objects that each seem to occupy a
specific location.


The relation between touch and agency reveals more interesting
areas for further investigation. In particular, it seems plausible
that the sense of touch has a closer connection to our agential
actions. This is partially a result of the fact that touch seems to
require active exploratory movements, and these movements are often
guided and voluntary. Given these close
connections, it is probably not surprising that touch has such a close
connection to agency. One could even use of this close connection
between touch and agency to address epistemological problem of
perception (see Smith (2002), and the entry on the
 problem of perception). When
we press against a solid object, the resistance to our agential act of
pressing gives our experience a more solid epistemic foundation than
what we experience through the other sensory modalities. Only in touch do we
seem to come into direct contact with reality, a reality that actively
resists our voluntary actions.


Philosophers have also been interested in the relation between
touch and other sensory modalities. It seems plausible to think that
touch, unlike vision, does not have a full, 3-dimensional sensible
field in external space. Instead, touch seems confined to the limits
of the body, and so the tangible field is, unlike in the other
modalities, defined by the limits and extent of the surface of the
body. Fardo *et al* (2018) offer a plausible empirical model for how this limited surface could generate rich spatial awareness. This supposed difference in the nature of their spatial awareness marks a clear structural difference between touch and
vision, even when they represent the same sets of objective
features. Similarly, one could claim that touch also differs from the
other senses in always involving bodily awareness
(O'Shaughnessy 1989). Externally-directed touch awareness
on such a view would only be made possible against a background of
bodily awareness. This sort of view has been labeled "the
template view" (Scott 2001).


## 4. Tangible Qualities


Any discussion of a perceptual modality often turns to the nature
of the perceptual qualities or features made available by that
modality. A discussion of vision, for instance, naturally leads to
metaphysical questions about the nature of the colors. A discussion of
audition similarly would bring up questions about the nature of
sounds. Touch is unique in this respect, however, since there has been
little philosophical investigation of the nature of tangible
qualities.


One exception, already noted above, concerns the thermal qualities
of hot and cold. But even here, little attention has been paid to the
metaphysical nature of these qualities. This is understandable, given
that touch seems to bring us into contact with ordinary material
objects and their properties. There doesn't seem to be any
serious worries about the nature of solidity, texture, vibration, and
the like. There is an important question about the structure of felt
tangible qualities, however. How are these ordinary tangible features
represented or experienced?


 One possibility is that, ultimately, all of the tangible
qualities, with the exception of hot and cold, can be reduced to the
spatial properties of objects (Armstrong 1962). Such a view could be 
connected to recent work on the spatial content of touch (for instance, 
views like those explored in Fardo *et al* 2018). Defending such a
reductive claim involves making an important distinction
between *transitive* and *intransitive* bodily
sensation. The transitive sensations are those like warmth and
pressure that have a sensory component as well as a real world
property. Intransitive sensations do not have any
non-sensational real world feature. This is one way to separate
out pains, twitches, and tingles from perceptual touch proper. The
spatial view then states that nearly all of the objective tangible
features made available through transitive sensation are relational
spatial properties. On this relational view of touch, all immediate
tactual perception involves a relation holding between our body and
objects in contact with it. A rough surface is one that is solid, has
hardness, and a certain uneven shape. A smooth surface only differs in
having a regular shape. Roughness and smoothness can be analyzed in
terms of shape. A hard object is one that does not change its shape. A
soft object changes its shape under pressure. Liquidity is defined as
having a particular shape in particular circumstances. Pressure plays
a role here, so the view also requires a spatial account of
pressure. One possibility is that pressure is a quality that has a
tendency to cause a change in the shape of the body. Stickyness could
be 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 empirical
evidence. A better account is needed.


A more recent view holds that tangible qualities are best
understood as *intensive features* that vary in intensity along
a single dimension of variation (Fulkerson 2014b). When we feel
vibration, for instance, it seems to increase in intensity (in this
case, in frequency) along a single qualitative dimension. This seems
to be true of most tangible features. They are not typically complex,
but simples that vary along a single dimension. This view accords well
with the empirical data and offers a unified explanation for the
structural connections between an otherwise disjoint collection of
sensory features. One worry for this view, however, again concerns our
awareness of hot and cold. Variations in intensity alone do not seem
apt for capturing the nature of hot and cold experiences (see
below). Rather than varying along a single dimension, our experiences
of hot and cold seem to vary in intensity in two directions around a
neutral point (Gray 2012). Cold experiences are those that vary away
from neutral in one direction; our experiences of warmth vary away
from neutral in the other (increasing) direction. More importantly, it
seems the nature of these movements depends more on changes in our
current bodily state than the objective measures of temperature. So
once again, thermal properties seem to pose additional difficulties
not faced by other tangible qualities.


## 5. Thermal Awareness


Philosophers have long been interested in the thermal system. As
noted above, thermal properties are difficult to connect with other
tangible features for both spatial and intensity views. On either view,
it seems difficult to make sense of the unique structure and bodily
role played by our thermoreceptive systems. One possibility for this
difficulty may be due to the fact that thermal properties
are *secondary* qualities. A secondary quality is one that (in
some 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 only
differing amounts of temperature or mean kinetic energy. We feel
something much more structured than this through our skin (a fact most
keenly developed in Gray 2012). We feel objects as very cold, to cool,
and then on to neutral, and only within a fairly narrow total
range. After this neutral point, objects take on an entirely different
character, and start to feel warm, followed by hot (again, up to a
limit). This space of thermal awareness involves a kind of inversion
through a neutral point. But this neutral point is unique to human
experience; there is no neutral point in the temperature taken
objectively. More importantly, the temperatures that we typically feel
as neutral depends on the context and especially on the current
thermal conditions of our body and sensory surfaces.


This presents something of a problem for our naive conception of
thermal awareness. We generally tend to think that our awareness of
hot and cold provides objective or at least useful information about
actual temperature. We use it to check baby bottles, determine if the
heater is on, and make sure the bath isn't going to scald us. If
instead, thermal awareness only gives us awareness of a purely
subjective experience only loosely connected with temperature, then it
seems thermal awareness isn't quite what we generally take it to
be. A warm stimuli feels cold when our bodies are already heated, but
it feels hot when our bodies are in a cold state. This difference can
be experienced at once by simultaneously heating one hand in hot water
and cooling another in an ice bath, and exposing them both to a
neutral water bath. The same water will feel hot and cold at the same
time.


Thermal awareness, and its unsteady connection to objective
temperature, has been used to argue that sensory systems generally
aren't in the business of (only) conveying objective sensory
information (Akins 1996). Instead, it seems to only convey
information about our current bodily state (Craig 2002). If this is
right, then it might explain some well-known but puzzling
phenomena, such as metal feeling cool to the touch, and wood feeling
warm, even when they are of the same objective temperature. It also
explains our different levels of thermal awareness for body parts at
different temperatures.


## 6. Touch and Bodily Awareness


The sense of touch is closely connected to bodily awareness. This
connection is seemingly much stronger than what we find in the other
senses (though this is somewhat controversial, as we'll see).
This close connection is easy to understand. Whenever we touch, and
especially when we engage in active or haptic touch, we are
forced to use our bodies. Setting aside the worries discussed above
about defining the organ of touch, there is some truth to the claim
that we touch with our whole body. After all, we touch using our skin,
muscles, joints, and we can touch using nearly every surface along the
whole body.[2]
Armstrong's view, discussed above, suggested that touch was
always a felt relation between our bodies and some objective feature
connected directly to us.


One possibility is that the sense of touch, while distinct from
proprioception, nevertheless always involves it (O'Shaughnessy
1998, 2000; Ratcliffe 2008). Since proprioception is nothing other
than awareness of the location and orientation of our own bodies,
there is a strong sense in which touch nearly always involves
awareness of the body. Citing groundbreaking work on the relation
between tactual and bodily awareness by Merleau-Ponty (2002), Gibson
(1966), and Martin (1992), Ratcliffe (2012) offers a detailed overview
of the ways in which touch seems to bring both an awareness of things
external to the body and of the body itself. The body is that by
which we measure the features of things in the external world, and so
is always present in our awareness of things through touch. There are
many reasons for thinking of this connection as relatively weak. There
are plausible examples, like playing sports or quickly touching
objects, where we seem to have external awareness through touch but no
direct bodily awareness (Scott
2001).[3]


Mattens (2017) has argued that philosophers have typically focused too
much on external objects and object perception in their work on touch. The fine-grained
manipulation and exploration of objects with human hands is not, according
to Mattens, the best way to think about touch. Instead, tactile awareness serves 
primarily as a kind of protective signaling system that warns about incoming threats (cf. the Bodyguard Hypothesis defended by De Vignemont 2018).
On this view, touch is primarily about bodily awareness, about what is happening with
and to our bodies, and only derivatively about the nature of external object.


 Another possibility is that, while touch indeed involves bodily
awareness, the nature and structure of this awareness is importantly
different from what we see in typical cases of mediated
perception. Touch does not involve awareness of the body as a part of
some external feature (as in other cases of mediated perception) nor
does it involve any explicit cognitive attitude. Instead, it could be
that some bodily sensation is necessary for tactual perception
(Richardson 2011). This view has the advantage both explaining why
touch necessarily involves bodily awareness but also why it differs
from more typical cases of mediated perception. In particular, it
differs from the template view by dropping any requirement for a
matching of spatial or other contents between the tactual perception
and the bodily awareness. For this reason, it more easily accommodates
extended touch.


Extended touch, or awareness of distal objects and features through
a cane or other tool, can be used to argue for a related but distinct
view of the relation between perceptual touch and bodily awareness
(Fulkerson 2012). That we experience through touch objects and
features located some distance from the body raises questions about
the mediating role of bodily awareness. Given the nature of extended
touch, it seems implausible to hold that there is anything like a
matching of the contents of perceptual touch (or the features made
available through touch) with the content or features involved in the
bodily awareness. Instead, perceptual touch seems to depend on bodily
awareness *informationally*. The idea is that both bodily
awareness (proprioception and kinesthesis especially) and
discriminative touch make use of the very same sensory
inputs. Perceptual touch is the result of extracting distal
information from the more proximal bodily information, for use by
dedicated downstream systems (cf. Serino and Haggard 2010). Since all
perceptual touch will be the result of such extraction, there will
always be bodily information available for awareness.


These various views all take a stand on the extent to which touch
brings direct awareness of external features and objects, and to what
extent such awareness is mediated by awareness of the body. In
addition, they are usually forced to take a stand on the spatial
contents of perceptual touch. On views according to which all external
tactual awareness is ultimately a form of bodily awareness, it is
simply not possible to have extended touch experiences. On other views
that hold a less restrictive dependence between perceptual touch and
bodily awareness, there is an additional burden to explain how distal
touch is possible. These choices have analogues in the empirical
literature. Some, for instance, want to make a clear distinction
between "interoception" and "exteroception"
 (Craig 2002). According to this view, many aspects of touch,
though primarily its thermal properties and most forms of pure bodily
sensation (itch, tingles, twinges, etc.) are primarily directed not at
the external world, but at the present state of our bodies. This is
part of how our bodies maintain homeostasis. This results in
the possibility of very different functional roles for certain aspects
of bodily awareness, especially as they function in the generation of
perceptual touch.


## 7. Touch and Action


As noted in the beginning, touch seems to have both a passive and
an active nature. In its more passive forms, touch involves cutaneous
activations across the surface of the body. These include bodily
sensations of hot and cold, pressure, vibration, and the like. In
addition, these activations have a limited, entirely bodily spatial
character. This form of touch has played an important role in the
early empirical study of touch, especially in determining the nature
and acuity of our sensory receptor populations. This has often been
accomplished through the measure of the two-point threshold
mentioned above. This distance varies across the body and is a
reliable guide to the density and spatial resolution of many tactual
receptors on the 
 body.[4]
 While these passive forms of touch are
important, especially in the empirical investigation of touch, in
typical, unconstrained touch, we use every part of our bodies to
actively explore the environment. We use our hands and arms and
fingers, we move our trunks and legs, and we actively feel with nearly
every surface of our body. Touch in ordinary use involves specialized
movements and grips. Such prehensile manipulation through touch is
often distinguished from stereognosis, or object recognition through
touch. In addition, these movements recruit and make use of many
receptive systems that seem inherently active. For instance, haptic
touch involves feedback from our movements, along with information
from our motor activities (both in motor planning and efference
copies). Perhaps most importantly, it also includes information from
receptors in our muscles and joints. All of these elements interact
and play a critical role in forming and developing our sense of
touch. For this reason, touch, especially of the active, haptic
variety, seems like an ideal model for views of perception according
to which perception is essentially a form of action, or at least a
form 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 motor
areas (Gibson 1966). The wider haptic system so understood involves
the entire body and brings direct awareness of things in the
environment by engaging and actively connecting together a number of
distinct sensory networks. Many later theorists have been influenced
by this compelling account of touch.


The connections between touch and exploration run deep. To cite
just one example, subjects have a remarkable ability to determine the
size and shape of many large objects simply by hefting and wielding
the objects through space (Turvey 1996). These actions provide a rich
and distinctive form of awareness that cannot be generated or
recreated through skin activations alone. These dynamic touch
experiences reveal the strong connection between touch and action.


Many theorists now think of perception as inherently active. This
is 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 and
Turvey, would seem to provide strong evidence in favor of such
views. Or at the least, if the case is to be made, one would expect it
to be strongest for the sense of touch in its dynamic forms. To cite
here just one prominent example: Noe (2004) defends an account of
perception that takes it to be essentially a form of action. He begins
his discussion by arguing that vision is "touch like",
involving sampling and exploratory probing of the environment. The
active nature of touch is never in question, and can be used in this
context as a model for understanding other forms of perception. It is
true that haptic touch seems inherently active, but many detailed
questions remain about the metaphysical relations between action and
touch (for instance, whether action is merely causally necessary or
constitutive of touch). This is likely to be an area of ongoing active
research.


## 8. Pleasant Touch


A final area of increasing philosophical investigation concerns the
role of pleasure and pain in perception, something typically referred
to as
 "affect".[5]
 The question is central to our full
understanding of the richness and complexity of perceptual
experience. While perception is often assumed to be entirely receptive
and descriptive, it is just as often evaluative and motivational. When
we smell something awful or see something graphic, we have intense
reactions to these experiences, and are directly motivated to act in
various ways. These forms of affective perceptual experience seemingly
bridge the gap between experience, emotion, and evaluative
judgment. For these reason, there are many importantly different
accounts available for explaining the valenced nature of these
experiences.


Touch again is an excellent source for such investigation. Our
tactual experiences often seem to have a felt pleasant or unpleasant
character. This isn't simply an
associated but distinct state of pleasure or pain that accompanies the
perceptual experience, but part of the perceptual experience
itself. Paradigm examples include the pleasure derived from delicious
food or the awfulness of certain bad smells. Here again, touch is
ready for increased investigation. In recent
years, researchers have discovered a specialized class of afferent
nerve channels that seem to be responsible for the experience of
pleasantness (Loken et al. 2009; McGlone et al. 2012). These
channels, called CT-Afferents, are maximally responsive to
slow, regular activations like those generated by a feather pulled
gently across the arm. These channels seem like pleasant versions of
the famous C-fibers implicated in pain experiences.


While the discovery of these afferents has been an exciting
development in our understanding of affective perceptual experience,
they also raise many questions. How can a receptor "code for
pleasantness"? Are there other similar receptors for pleasant
touch in glabrous skin (the smooth, hairless skin of the lips and
palms where they are no CT-afferents). What do these receptors
mean for our understanding of pleasure and pain and affective
experience more generally? And returning to where we started, we can
seriously wonder whether and to what extent the CT system seems to be
a 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, the
CT system also doesn't appear to have any direct discriminative
function. CT interactions reveal much about the complex ways in which
emotions and motivation can come to be closely connected to perceptual
experience. They can also be seen as a bridge to a better
understanding of affiliative touch, the close form of caring touch
that forms an essential element of social bonding and human
development, especially of the immune system (Field 2003). This should
prove to be an area of increasing research.



In addition, the unique aesthetic properties of touch have recently been explored by Korsmeyer (2019, 2020). In particular she has, explored the ways in which direct contact with objects mediated by touch can confer a sense of authenticity and value to an object that it cannot get through any other modality. This connects with recent trends in tangible art installations that encourage rather than prohibit direct contact and exploration.