A new study suggests that the brains of children with autism do not adapt to repeated touch or sound, even after several minutes. By contrast, the brains of most non-autistic people engage in a process called “habituation,” in which unimportant sensations – such as the sound of an air conditioner, or the feeling of a wool sweater against on bare skin – are tuned out, so that the brain can focus on new information.
According to the new study, some children with autism lack the process of habituation, which may explain their hypersensitivity to touch and sound. Shulamite Green, the lead investigator in the study, said that the brains of children with autism are “continuing to try to make sense out of these stimuli. It’s putting a lot of effort into this, and it’s very overwhelming and very exhausting.” Green is an assistant clinical professor of psychiatry and bio-behavioral sciences at the University of California, Los Angeles. The study’s findings suggest that “exposure therapy,” in which therapists help patients adapt by exposing them to increasing levels of an uncomfortable noise, would not be helpful for autistic children.
In conducting their research, Green and her colleagues studied the brain responses to sensory stimulation of 42 children with autism, and 27 children without the condition. The children were all between eight and eighteen years old, and had average or above-average intelligence. Parents filled out questionnaire’s evaluating their child’s responsiveness to sensory stimulation. Overall, the questionnaire showed that the non-autistic children were less sensitive than those in the autistic group.
The researchers split the autistic children into two groups based on their scores. The children whose scores showed them to be highly responsive to touch and sound were placed in one group, while those who scored as less sensitive were placed in another. Each child’s brain was then scanned while the child experienced a series of stimuli, each lasting fifteen seconds. The researchers monitored brain activity in the regions responsible for processing sound and touch, and in the amygdala, the region of the brain that filters sensory information. All of the children, both autistic and non-autistic, showed increased brain activity during the first two rounds of stimulation. Among the non-autistic children and autistic children with low sensory reactivity, brain activity dropped during the third and fourth rounds, and remained low. Among the autistic children with high sensory reactivity, the brain activity remained high for all six rounds.
The children were then exposed to two more rounds of sensory stimulation. The non-autistic children showed a slight increase in brain activity during the first round, but not during the second, suggesting that they recognized the stimuli as new, but tuned them out because of how similar they were to the previous ones. The highly responsive autistic children showed elevated brain activity throughout, suggesting their inability to habituate, or adapt. By contrast, the less sensitive autistic children showed no brain response to the new stimuli. According to Green, this suggests that they were either unable to recognize the stimuli as new, or that their brains had reduced the response to the original stimuli that they couldn’t activate in response to the new information.
To better understand the responses, the researchers analyzed changes in synchronized activity in the amygdala and the orbitofrontal cortex (OFC), which regulates the amygdala. Focusing on brain activity during the six original rounds of stimuli, the researchers found that the non-autistic children showed no changes from the first three rounds to the last three. Regarding the hyper-responsive autistic children, the researchers found that when one of the brain regions activated during the first three rounds, another deactivated, or vice versa. Green said this suggests that the OFC is trying to shut down the amygdala to help the brain adapt to the stimuli.
Green added that in the other, less hyper-sensitive autistic children, both regions of the brain increase their activity at first and then show opposing responses, possibly to avoid sensory overload. This suggests that autistic children who are not hypersensitive to noise and sound may still process the stimuli differently than non-autistic children. The results of the study were published in June in the American Journal of Psychiatry.
Green says that unpublished work from her team confirms that the study’s findings hold true even when anxiety is a factor. The researchers are currently studying how habituation varies with age in a larger group of children.