Touch is the first sensory system to develop in babies, and for good reason. Physical connection is vital to our social and cognitive development, especially in infancy. Of course, its importance continues throughout life, but as babies our sense of touch plants the seed for the quality and tenor of our social interactions later on.
A particular type of nerve, found only under hairy skin (on the arm or face, for example), is responsible for the rewarding emotional import of touch. They’re called C-tactile (CT) afferents, and scientists believe they’re crucial to the development of the human brain, too.
They might even play a role in autism, writes Francis McGlone in a comprehensive report published yesterday in Neuron. He and his colleagues in Liverpool and Sweden suspect that since CT afferents trigger the same brain areas that are implicated in autism studies, then autism might be detectable early on. It makes sense—many autistic people have trouble processing visual, tactic, or information because they’re prone to sensory overload. The otherwise simple act of looking someone in the eye or engaging in conversation is instead imbued with a very intense range of sensations.
Two Swedish scientists, Karl-Erik Hagbarth and Ake Vallbo, were the first to detect CT fibers in humans (they were previously discovered in cats, monkeys, and rats), using a technique called microneurography, in which a tungsten microelectrode penetrates the skin and records complex electrical signals underneath the surface.
Here’s Virginia Hughes, writing for Only Human:
Deciphering the code of these nerves is difficult and takes a lot of patience. “It’s like putting a microphone into a United Nations convention — there’s lots of different languages you’re going to be hearing,” McGlone says. “I think five people on this planet can record from C-tactile afferents.” These trained scientists can hear the language (that is, a certain pattern of electrical waveforms) of the CT afferents only when the skin is gently stroked.
In 2009, McGlone’s team found that when they stroked subjects’ arms at different speeds, specific velocities of stroking generated the most pleasant sensations—and those velocities correlated with the velocities that activated CT nerves. Three years later, they saw that stroking CT fibers triggered brain activity in the posterior insular cortex and the mid-anterior orbitofrontal cortex, both involved in emotion processing. These are the same brain areas detailed in studies on autism. Hairless parts of the palm, by contrast, stimulated non-emotional parts of the brain.
A separate team at Yale University is looking into the effect of stroking on the brains of children with autism. Other senses—taste, hearing, and seeing—are grounds for investigation, too.
Perhaps most intriguing about the touch aspect, though, is the idea that it could influence autistic people’s sense of self. Here’s Hughes again:
Stroking CT fibers also activated a brain region called the angular gyrus, which is involved in our internal representation of our body. (In studies of epileptic patients, stimulating this region leads to dramatic out-of-body experiences.) This result is intriguing, McGlone says, because it suggests that CT afferents are involved not only in our awareness of others, but in our physical sense of self.
Using touch as a gateway could help us understand how autistic people experience the world, and possibly the origins of their unique way of perceiving it.