At the age of six, Jack Craven started telling his mother he wanted to die. “God made a mistake when he made me,” he would say. “Why can’t I just die?” His mother, Lori Craven, says she didn’t even know that kids his age could think such things: “Can you imagine your child saying that?”

Jack, now 12, has sensory processing disorder (SPD). It’s a contentious diagnosis. Some doctors will argue that it doesn’t really exist, while those who recognise it estimate that sensory issues affect between 5 and 16 per cent of us.

A caress feels like sandpaper tearing their skin

For some people, this means they are over-sensitive to lights or sounds. But there are others for whom a caress feels like sandpaper tearing their skin, and there are babies who will scream and won’t sleep unless they are held tightly and bolt upright. It can make what many of us take for granted as ‘normal’ life practically impossible.

As a toddler, Jack had found it unbearable to be anywhere loud: “There was a lot of screaming if it was noisy,” Lori recalls. “Actually, there was just a lot of screaming from him.” At school, “he was like a deer in headlights”.

Jack is a bright boy, but the environment was so overwhelming he couldn’t perform well. He would come home and tell Lori that the other kids were saying he was “stupid”, “a dummy”. Now she schools him at home herself. Every day, in everything they do, they have to consider Jack’s sensitivities, she says.

Despite the difficulties, some adults and some parents of kids with SPD report an upside. As well as being more reactive to physical sensations, they’re also more sensitive to other people’s emotions. To researchers, this is intriguing.

Could sensory processing help explain the personalities of people we might otherwise just think of as ‘sensitive’? Might people with SPD even be at the extreme end of a spectrum of sensitivity that developed as an evolutionary advantage?

“How am I feeling in this room right now? Horrible. This is a terrible room. I’m trying not to focus on the lights and let them bother me. I’m trying not to listen to that echo – because I’m hearing my voice in my throat, and in the air and bouncing off the walls. And we are sitting with this gap behind us, so I’m floating in the middle of the room and a piece of me is going, ‘I hope this doesn’t screw me up for when I have to be on stage’… I guess the door is locked? No one’s coming in, right?”

For Rachel, an echo is as attention-seizing as scraping nails

It’s a little after midday on a bright cold Tuesday in Chicago, andRachel S Schneider is describing what it’s like for her to be in what to most people would seem a bland ante-room to the hall in which she will shortly be speaking. If being in here is so discomfiting, the flight from New York, where she lives, must have been torture? She nods.

Schneider was diagnosed with SPD in 2010, when she was 27. For her, it means she’s particularly over-sensitive to sights and sounds. No one likes a spotlight in the face or the sound of nails on a blackboard, but to Schneider a glow can feel like a spotlight.

For her, an echo is as intrusive and attention-seizing as scraping nails. She also struggles with a lesser-known sense called proprioception – the sense of where your body parts are in space. For this reason, she thinks, she enjoys pressure on her body (“I’m a hugger!”).

She’s been hyper since she burst into the room, fuelled by excitement (relating to the symposium she’ll shortly be addressing) and anxiety (relating to her talk, the travel, this interview, this room). When we start talking about recent neurological research into SPD, she practically bangs her fist on the table. “It was pivotal!” she says. “PIVOTAL! When I first heard about it, I was so excited, I wanted to throw a parade!” She pauses for a moment. “And I don’t like parades.”

The parade would have been for Professor Elysa Marco at the University of California, San Francisco (UCSF). Marco is a paediatric neurologist and is now regarded as a leading expert on SPD. Go back six years, though, and she hadn’t even heard of it. She was, however, starting to think more about the role of sensory problems in the symptoms of many of her young patients.

In her consulting rooms, she was seeing kids with a range of brain-related difficulties. “What I realised was that the families were coming in, and I would want to talk about the kids’ seizures or their headaches or their language problems in the case of, say, kids with autism,” she says. “And the parents wanted to talk about that also.”

They couldn’t get a shirt on their kids because they would scream bloody murder

“But what they really wanted to talk about was the minute by minute, the day by day, which was so hard as they couldn’t get their kids into the shower to wash their hair because the kids wouldn’t let them touch their heads, or they couldn’t get a shirt on them because they would scream bloody murder. Or they couldn’t make soup in the kitchen with the blender because the kid would cover their ears and run out the door.”

In the 1960s, Jean Ayres, an occupational therapist and educational psychologist working in California, first identified SPD (or “sensory integration disorder”, as she called it then) as something distinct. For people who accept SPD as a disorder, what unifies patients is that while their sense organs work normally, their brains do not respond typically to the data those organs send on. Some people with SPD are “under-responsive” (they crave the stimulation of one or more senses), while many are “over-responsive” to one or two senses, or more. Some are under-responsive in some senses and over-sensitive in others.

Lucy Jane Miller was one of Ayres’ students, and has researched SPD for more than 30 years. Now a professor of paediatrics at Rocky Mountain University of Health Professions in Colorado, and the founder of the SPD Foundation, she has developed assessment scales for diagnosis, co-ordinated research into therapies and done all she can to spread the word that SPD exists. But the recognition she has long fought for – a listing in the Diagnostic and Statistical Manual of Mental Disorders (or DSM, sometimes called the ‘bible’ of psychiatrists and psychologists) – still eludes her.

In the summer of 2008, Miller gave a presentation at the UC Davis Medical Investigation of Neurodevelopmental Disorders Institute about problems with sensory processing. Elysa Marco was in the audience. “It was as though a big bright light turned on for me,” Marco recalls. “I was very excited. I thought: okay, this is the way I need to think about and study my kids.”

After the talk, she went up to Miller and told her she was already planning a study at UCSF to image the brains of children with autism; perhaps she could use brain imaging to investigate SPD too? Miller helped Marco get the funding she needed, and the research that followed changed Rachel Schneider’s life.

The first experiments, whose results were published in 2013, used magnetic resonance imaging (MRI) to look at the brains of kids with SPD and a group without any disorder. The results showed measurable differences in brain structure, primarily in areas towards the back of the brain that connect regions involved in processing visual, aural and touch data. (A bigger follow-up supporting these findings was published in 2016.)

A second paper, published in 2014, compared children who had SPD with autistic children, because there are overlaps between the two conditions. According to some estimates, as many as 90 per cent of people with autism have sensory processing problems, but Marco wanted to confirm that SPD could also occur without autism.

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