In my final post here, I would like to present some material from the final chapter of The First Sense on pleasant touch.
Perceptual experience is often more than merely descriptive or informational; it is also often evaluative and motivational. Offering an account of this component is critical to our understanding of touch, and perception generally. I will use the term “affect” to describe the felt pleasant and unpleasant aspects of an experience, and “affective touch” to refer to tactual experiences with a pleasant or unpleasant aspect.
There are (at least) two distinct ways that a perceptual experience can possess an affective character. In the first instance, a perceptual experience can cause a non-perceptual affective response in a subject. Call perceptual experiences of this kind “affect-causing.” Other perceptual experiences possess an affective character by presenting sensory features or objects as pleasant or unpleasant. Call these sorts of experiences “affect presenting.”
I don’t want to put too much stock in this distinction. It’s not as precise as it really needs to be, but it’s useful for now to rule out a certain class of cases from our explanatory target. Compare the following two cases:
Loaded Gun: Stan unexpectedly sees a loaded gun in his friend’s bag while at a playground filled with small children. This experience might create a strong affective response in Stan. However, feeling the gun in the bag or being told that a loaded gun was in the bag would have generated the same response. What was unpleasant about the experience wasn’t the way the gun *looked*. None of the visually salient properties of the gun were unpleasant. Rather, what was unpleasant was the information conveyed by the visual experience, and this information could have been attained in a variety of ways.
Bad smell: Nina comes home to find the garbage has been left sitting out, and its stench is filling her small apartment. Before she realizes not to, she inhales through her nose, filling it with the putrid, rotting air. She thereby comes to have a vivid olfactory experience of the garbage. And it smells bad. It causes immediate reactions in her, from covering her mouth and face with her arms to holding her breath until she is outside the room. She moves immediately to open the windows and tie up the trash bag, all while breathing as little as possible through her mouth. If asked, she would have preferred to avoid this experience entirely, and in all likelihood she will take immediate steps to prevent its recurrence.
I’m only interested here in the Bad Smell, affect-presenting type cases. Now, there is an important ambiguity remaining even in the affect presenting case. When the garbage smells awful, is it really the garbage that is perceived to be unpleasant, or is the unpleasantness a feature of how the garbage is experienced? Well, the answer is both, but I believe the experiential component is more fundamental, and grounds our affective experience of things in the world (the argument for this is discussed in more detail, though using slightly different terminology, in Aydede & Fulkerson 2014).
Empirical evidence, our affective judgments, and the phenomenology of affective perceptual experience strongly suggest that pleasant touch is not simply a nonsensory emotional reaction in us: when we have a pleasant touch experience, we often partly attribute the pleasant aspect to the sensory features of the object. It is the silk that feels good, the velvet that feels pleasant. However, it is also unlikely that the pleasantness is anything like a detected objective sensible property of the silk (that is, pleasantness is not among the basic tangibles we encounter when touching silk). Standard arguments from inter- and intrapersonal differences and the extreme context sensitivity of tactual affect, combined with their direct motivating influence on judgments and behavior, strongly suggest that affective qualities are highly relational, involving both a sensible quality and an experiential reaction. I suggest that the pleasant aspect of touch experience arises as incoming tangible signals are evaluated by a complex, subpersonal affective-motivational system associated with touch. Because this account reduces the felt qualities of affective touch to the functioning of this evaluative system, the view is a version of psychofunctionalism (the general version of this view has been developed and defended in recent and ongoing work with Murat Aydede).
Moving back to touch, there is one empirical roadblock that needs to be addressed. Much exciting work has been done recently on a newly discovered afferent channel that seems to mediate our awareness of pleasantness. This “C-tactile” (or CT) channel has many features that can make it seem like a genuine pleasantness detector.
Here is a description from Löken et al. (2009):
These results are, to the best of our knowledge, the first demonstration of a relationship between positive hedonic sensation and coding at the level of the peripheral afferent nerve, suggesting that C-tactile fibers contribute critically to pleasant touch. Soft brush stroking on hairy skin was perceived as most pleasant when it was delivered at velocities that were most effective at activating C-tactile afferents (1–10 cm/s), with a linear correlation between C-tactile impulse frequency and pleasantness ratings. In contrast, the response of myelinated afferents increased with faster velocities (30 cm/s) and showed no relationship with pleasantness ratings. The sweep of the brush over the skin surface activates a large number of tactile afferents and discharge of any given single unit is not sufficient for a pleasant percept. However, of all of the unit types tested, only the C-tactile afferent firing pattern correlated with average psychophysical ratings. In the palm, which lacks C-tactile afferents, we found no relationship between brush velocity and pleasantness ratings (p. 548).
One might conclude from this that affective touch does involve a channel that detects pleasantness. That is, one might think that CT fibers carry information about pleasantness and unpleasantness understood as sensible features of material objects. However, this would be a mistake. For one, it’s clear that some pleasant tactual experiences can be felt through the hands, lips, and genitals, where there are no CT fibers. So the CT system isn’t the whole story. But in addition, it isn’t at all clear that the CT system is carrying information about pleasantness understood as an objective sensory quality of external objects.
The correlation between CT activations and pleasant feelings is highly context sensitive and variable (as described in several other studies). There isn’t any single channel that simply detects and elicits experiences of tactual pleasures “out there.” While the correlations between gentle stroking and CT-fiber activations are strong in controlled laboratory settings, there is every reason to think that our feelings of pleasantness through touch are highly malleable complexes. This accords well with work on cutaneous pains, which seem to have both a sensory-discriminative component and an affective-motivational component. Affective touch also seems to essentially involve a discriminatory element (crucially, the A-delta fiber system must also be activated to generate pleasant experiences) combined with an affective-motivational component (likely mediated by CT activations). How exactly this system functions (and the kinds of affective experiences it generates) depends on the present and prior state of the overall tactual system, as well as on activations in other correlated systems involved in the evaluation of tactual inputs.
I will end by noting what an exciting time it is to be working on and thinking about non-visual modalities, multisensory interactions, and the affective richness of perceptual experience. There is so much good work coming out from so many overlapping fields, and there is a feeling of real progress being made.
I am again grateful to the brains behind Brains for inviting me to share some of my work in these areas.
Aydede, M., & Fulkerson, M. (2013). Affect: representationalists’ headache. Philosophical Studies. doi:10.1007/s11098-013-0206-7
Löken, L. S., Wessberg, J., Morrison, I., McGlone, F., & Olausson, H. (2009). Coding of pleasant touch by unmyelinated afferents in humans. Nature Neuroscience, 12(5), 547–548. doi:10.1038/nn.2312