Autism: A Short Circuit in the Touch System?
WHY YOU SHOULD CARE
Because the answers to what causes autism — and how to screen and treat the disorder — might lie in the skin.
When Temple Grandin was growing up, her skin seemed to be crawling beneath her clothes. “Scratchy petticoats felt like sandpaper ripping off my skin,” said Grandin, an autism-rights advocate and animal scientist at Colorado State University. “There is no way a child is going to function in a classroom if his or her underwear feels like it is full of sandpaper.”
Many other individuals with autism are also extremely sensitive to touch, and some of the earliest descriptions of the disorder mention that infants with autism might cry or arch their backs when held.
It’s no longer one of those hippie, ‘give me a cuddle’ things. There are nerve fibers underlying this.
— Francis McGlone, cognitive neuroscientist
But why? Some researchers suspect that the answer might lurk in a system of nerves known as C-tactile afferents. While the fast, thick nerves beneath our fingertips help us distinguish between things like paper and plastic, or wood and steel, and the slower, thinner nerves nearby transmit pain, C-tactile (CT) afferents do something else entirely. These afferents, found beneath hairy skin, tell us about social interactions.
Last week, Francis McGlone, a cognitive neuroscientist, and his colleagues at Liverpool John Moore University published a commentary in the journal Neuron outlining current knowledge on CT afferents — including studies suggesting that they might play a role in autism and other developmental disorders. If so, then CT function could be assessed at birth or within days of birth to screen for these disorders, well before behavioral symptoms appear.
Although earlier studies have found that gentle stroking can lower stress and blood pressure levels, the neurobiological mechanisms have remained largely a mystery, until recently. “It’s no longer one of those hippie, ‘give me a cuddle’ things,” McGlone said. “There are nerve fibers underlying this.”
Scientists in the late 1930s were the first to detect CT afferents– in a cat, and then in rats and monkeys. In the late 1980s and early ’90s, Swedish scientists Ake Vallbo and Karl-Erik Hagbarth discovered that they also existed in humans after they inserted a tiny electrode into volunteers’ skin to record the electrical activity of the underlying nerves. One group of nerves — the CT afferents — fired only when the skin was gently stroked.
Nerves that fire in response to pain, temperature and texture relay information about our physical environment, warning us about dangerous surfaces, for example. But why would we need a system of nerve fibers that respond selectively to gentle touch?
McGlone’s group unearthed some clues in a Nature Neuroscience study in 2009, in which a robotic brush gently stroked different parts of volunteers’ bodies at various speeds. The speeds the volunteers rated as most pleasant matched those that activate CT afferents. Three years later, McGlone scanned volunteers’ brains while the robotic brush stroked a hairy region of the forearm, innervated with CT afferents, and the hairless palm, which lacks them. The somatosensory cortex, which processes touch, lit up when subjects’ palms were stroked.
Could autism be the result of a touch system gone haywire?
But lightly touching the forearm activated the posterior insular cortex and mid-anterior orbitofrontal cortex, both involved in processing emotions, and the angular gyrus, responsible for maintaining our physical sense of self. (A Nature study in 2002 found that stimulating the angular gyrus in epileptic patients triggered powerful out-of-body experiences.)
Earlier studies have also linked these three brain regions to autism, a complex spectrum of disorders whose features typically include impaired communication, poor social engagement and repetitive behaviors. Many of the 70 million people who fall on the autism spectrum also have trouble processing visual, tactile and other sensory information. An Autism Research study in 2012 found that brain regions responsible for sensory processing in individuals with autism had dampened responses to pleasant and neutral textures, and heightened responses to unpleasant textures.
Today, McGlone is working to identify the neurotransmitters that process the brain’s responses to gentle touch.
Could autism be the result of a touch system gone haywire?
McGlone thinks it’s possible. When gentle strokes activate CT afferents, we experience pleasant, rewarding feelings, making us more likely to seek such caresses in the future — a beneficial behavior, McGlone says, since these touches might play a crucial role in social development, especially early in life. For example, cuddling can facilitate mother-child bonding, essential for children’s well-being and survival. But gentle touch wiring, including CT afferent signaling, might go awry in autism. “It’s like building the Taj Mahal,” McGlone said. “If you don’t lay the foundation down, there will be consequences later on.”
McGlone worries about children who don’t receive enough nurturing touch, especially in the U.S., where many women receive only an eight-week-long maternity leave from work. (In the U.K., maternity leaves can extend up to a year.) “It’s an absolute abuse,” he said. But he notes that the touch system could become impaired in the womb, as well.
Research by Kevin Pelphrey, a neuroscientist at Yale University, also hints that autistic individuals have an impaired touch system. In 2012, his group scanned the brains of 19 adult volunteers while they received slow or fast brushstrokes on their forearm. Slow strokes activated brain regions involved in social behaviors — but volunteers who scored high on tests of autistic traits showed the lowest activity in these regions.
“The argument has been made that there are many autisms. I think that’s an overstatement,” Pelphrey told SFARI.org. His take? There may be countless genetic causes of autism, but they cause the same short-circuits in brain regions that allow us to recognize the emotions of others and express our own. Those same regions are the ones activated by gentle touch.
Scientists might be able to scan CT afferent function at birth or within days of birth, well before symptoms appear.
If CT afferents play a role in autism, researchers could develop drugs that activate the receptors at the ends of these fibers. Scientists might also be able to scan CT afferent function in high-risk individuals — like those who have a sibling with autism — at birth or within days of birth, well before symptoms appear (at around 2 to 4 years of age).
But McGlone notes that the link between autism and CT afferents still remains hypothetical and that the Neuron paper is a perspective piece, not a controlled study. “It enabled us to put together a list of ideas and hypotheses that are anchored on a very firm scientific basis showing that humans have a system of nerve fibers that respond positively to gentle touch.”
Such a drug might even be able to treat anorexic patients, who also dislike being touched.
More broadly, McGlone worries about how a growing reliance on social media — and tendency to regard touch as invasive — could affect our social development.
“There’s hostility toward touch,” he said. “We’re demonstrating that we do have system of nerve fibers that’s maximally activated during exactly the kind of behavior between two lovers or a mother and her infant. It’s a basic biological need.”