New research has shown that a defect in the brain explains why many people with autism recoil from hugs and physical touch - even from their parents.
The problem affects individuals with fragile X syndrome, a genetic defect that is the best-known cause of autism and inherited mental retardation.
Researchers from Northwestern University Feinberg School of Medicine have found that fragile X syndrome results in delayed development of the sensory cortex, the part of the brain that responds to touch.
This delay may trigger a domino effect and cause further problems with the correct wiring of the brain, it is believed.
According to the researchers, understanding how and when the function of the brain is affected in fragile X offers a target for a therapy to fix the incorrect development.
"There is a 'critical period' during development, when the brain is very plastic and is changing rapidly," said Anis Contractor, assistant professor of physiology at Feinberg and the lead author of the study.
"All the elements of this rapid development have to be coordinated so that the brain becomes wired correctly and therefore functions properly," Contractor added.
Working with a mouse model of fragile X, Contractor found the development of synapses, the sites where neurons communicate with each other, was delayed in the sensory cortex.
"The critical period may provide a window during which therapeutic intervention can correct synaptic development and reverse some of the symptoms of the disease," Contractor said.
People with this syndrome have debilitating sensory as well as cognitive problems.
"They have tactile defensiveness. They don't look in people's eyes, they won't hug their parents, and they are hypersensitive to touch and sound. All of this causes anxiety for family and friends as well as for the fragile X patients themselves. Now we have the first understanding of what goes wrong in the brain," Contractor said.
The sensory overload in people with fragile X results in social withdrawal, hyperarousal and anxiety. It shows up in early infancy and progressively worsens throughout childhood.
The study appears in the Feb. 11 issue of the journal Neuron.