Sensing the self:

To many researchers, the affective touch system suggests a compelling mechanism at autism’s roots. Touch is one of the dominant modes of perception and social interaction in the earliest weeks and months of a baby’s life. “A whole lot of your world is coming to you through caregiver touch — there’s a whole lot of cuddling, cradling, rocking,” Cascio says. If babies’ perceptions of these touches are altered in some way, it could transform how they situate themselves in the world and learn to interact with others. Those changes, in turn, could account for autism’s hallmark social challenges.

Most researchers interviewed for this article subscribe to some version of this idea but admit it is still tentative. “We really don’t have strong evidence for it yet,” Cascio says. What evidence they do have falls somewhere along a three-link chain of logic.

The first link is the observation that affective touch seems crucial for delineating our sense of ‘self.’ To explore that idea, some researchers have turned to the ‘rubber-hand illusion,’ in which an experimenter strokes a participant’s hand and a stuffed rubber glove at the same time until the participant mistakes the fake hand for her own. In typical people, the illusion is strongest when the stroking speed and textures involved elicit the peak response of C-tactile fibers. “You make an almost unconscious-to-the-individual change, and that makes a big change in their perception,” says Aikaterini Fotopoulou, a cognitive neuroscientist at University College London.

Yet another hint that affective touch is important to self-definition comes from people who have had a stroke and feel one of their arms is not their own. In a study of seven people who lost the ability to recognize their left arm, Fotopoulou and her colleagues stroked that arm to activate the participants’ C-tactile fibers. The participants then reported reconnecting with their ‘lost’ limbs. “They start saying things like, ‘Well, after you touched it, I said to my arm: Come, I welcome you back,’” Fotopoulou says.

The second link is more theoretical: If affective touch can redraw a person’s boundaries such that they mistake a fake hand for their own, perhaps it is responsible for drawing those boundaries to begin with. This link in the chain holds that our entire sense of body ownership may be one grand rubber-hand illusion imparted from all that cuddling we got as babies. “I put my leg there, or my fingers there, and then there is a response. I say, ‘Oh, that’s me,’” says Anna Ciaunica, a philosopher of mind at University College London who works with Fotopoulou.

The third link connects these two ideas to autism. Cascio and others have found that autistic people are less susceptible to the rubber-hand illusion than neurotypical people are, suggesting their sense of self is somehow less flexible. That rigidity might explain the strong response many of them have to touch. “If you have a very clear border of your own body, then of course everything else that touches you will bother you,” Boehme says. Many autistic people also say they relate their feelings about touch directly to their sense of self. Kirsten Lindsmith has written about this in her blog: “When I shake a person’s hand, I feel as though a tiny part of myself — my awareness, my consciousness, my identity — is commandeered by their touch, and I no longer feel fully autonomous.” Dehennin also says she experiences that sensation: “I often feel like I’m not ‘in’ my body; deep pressure helps that.”

Several imaging studies also suggest that autistic people have an altered sense of affective touch. In 2012, for example, Cascio led a series of experiments in which a lab assistant stroked autistic and typical adults’ forearms with a soft cosmetics brush, bumpy burlap or scratchy plastic mesh. Both groups described each texture much in the same way, but brain imaging revealed that they processed the sensations differently: The autistic group showed more activity than controls in brain regions associated with discriminative touch and less in those associated with affective touch.

Most interesting, Cascio says, was that burlap in particular lit up social brain regions in the controls, even though burlap has no obvious social significance. She interprets this activity as subconscious deliberation — that is, the burlap touch could be considered positive or negative depending on social cues. “We’re seeing processing in those regions that would make us think that they’re trying to figure out how pleasant or unpleasant it feels,” she says. The social brain areas of autistic participants, however, don’t seem to show this internal deliberation. Or if they do, as Cascio’s newer work suggests, they do so after a delay.

“Maybe this is actually getting at a biological marker that gets us a better understanding of the causes of autism and, at the very least, a very early detection of autism.” Kevin Pelphrey

In another experiment, autistic people and controls both said they liked the sensation of being stroked rhythmically on the arm or hand with a watercolor paintbrush. “A lot of the field would be like, ‘Well, that’s kind of a dead end; maybe touch isn’t affected in autism,’” says Pelphrey, one of the researchers. But brain scans again showed clear distinctions between the groups. Stroking the forearm, rich in type C afferents, lit up social brain areas in the controls, but stroking the palm, which contains predominantly type A nerve fibers, had no such effect. In autistic participants, location didn’t matter; their social brain activity remained at a constant level in between the extremes shown by the typical participants. “Individuals with autism showed the middle response for everything,” Pelphrey says.

Autistic people also appear to process pain differently, reflecting possible differences in their type C nerve fibers. In 2017, Cascio’s lab affixed a small heating pad, about 1 inch in diameter, to the calves of autistic and neurotypical volunteers. They then brought the temperature to an agonizing 49 degrees Celsius for 15 seconds. (The pad was not hot enough to burn the skin.) Both groups rated the pain 7 out of 10. But once again brain imaging offered a nuanced picture. In brain areas that respond to pain, such as the anterior cingulate cortex, insula and thalamus, the reaction in the neurotypical people lasted 30 seconds, lingering after the heat was removed. In autistic people, it abated after only 10 seconds, even though heat was still being applied. “It really looks like, when you look at the data, that something’s turning the pain response off,” Cascio says.