Note: Brittany Travers’ lab uses identity-first language in alignment with the majority preference of the autistic community. This story reflects that preference.
By Charlene N. Rivera-Bonet, Waisman Science Writer
The same external sensory stimulus – a flashing light, a hug, or hearing one’s name – can provoke a different reaction in every person. Many autistic children, and some non-autistic children, can experience elevated sensory responses that can appear as a strong reaction to those sensory stimuli, or the opposite, a reduced reactivity to or interest to the stimuli. But what drives those differences?
In search of the neurobiological basis of elevated sensory responses, a recent study published in Molecular Autism by the lab of Brittany Travers, PhD, associate professor of kinesiology, found that the brainstem may play a role in elevated sensory responses in autistic children.
Elevated sensory responses can impact quality of life. They have been associated with decreased motor performance, increased core autism traits, increased anxiety, and decreased adaptive behaviors. “If the world is set up for you to notice specific things and respond to them in a specific way, and then that’s not how your brain and body are designed to work, there can be kind of a mismatch. And that’s often where we see sensory features impacting quality of life in autistic individuals,” says Olivia Surgent, PhD, a postdoctoral researcher and first author of the study.
The researchers wanted to understand how the brain, particularly the brainstem, relates to the sensory responses of autistic and non-autistic children.
The brainstem lays in the lower back half of the brain, where it connects the rest of the brain to the spinal cord. “It’s the gateway to the whole body,” as Travers describes it.
This location makes sense when you think about one of its key functions: processing sensory information. Sensory information comes in from the body, through the spinal cord, to the brainstem, and to the rest of the brain. Motor information takes the opposite route out.
In addition to sensory processing, the brainstem was one of the first brain regions to be associated with autism, which made it an ideal candidate for this study.
61 autistic and 72 non-autistic kids, with an average age of 8 years old were part of the study. Their caregivers filled out a report on the kids’ sensory patterns including hyperresponsiveness or increased responses to stimuli, hyporesponsiveness or decreased response to stimuli, enhanced perception, and sensory seeking for visual, auditory, taste, tactile, and vestibular systems. The kids then underwent magnetic resonance imaging (MRI) to collect images of their brains.
The researchers were interested in the connectivity within the brainstem, so they looked at the white matter tract microstructure. White matte tracts allow communication between different areas of the brain by serving as connecting fibers between them.
Because of the size and location of the brainstem, they had to develop a new imaging technique called TiDi Fused that allowed them to get more accurate results (read sidebar for details on this technique).
The structure of white matter tracts in the brainstem were found to be related to sensory responses in autistic children. This relationship was different for non-autistic children, suggesting that brainstem white matter microstructure may play a different role in the sensory responses of an autistic kid compared to a non-autistic kid. “We still don’t exactly know why that is, but it’s consistent with some of our other findings in the lab that suggest that maybe there’s some differences in how brain structure and brain wiring is aligned with behavior in autistic individuals compared to non-autistic individuals,” Travers says.
The biggest relationship with brainstem white matter was seen for two aspects of the sensory features: touch and hyporesponsiveness – a lack of or reduced behavioral response to a sensory stimulus.
Lastly, the authors wanted to see how much of the brainstem was contributing to sensory features compared to the rest of the brain. Their findings showed that sensory-brain relationships occurred mostly in the brainstem and cerebellum to a higher level than expected based on their size. “Only 7% of our search space was brainstem and cerebellum. But 21% of our findings were in those areas,” Travers explains.
Their study suggests that there may be potentially altered mechanisms for sensory responsiveness in autism that are heavily dictated by the white matter in the brainstem, a brain region involved in more reflex-like behaviors that requires less conscious control. “And what that makes me think is that interventions that ask any child to control their sensory responses is likely not to be effective. If these sensory responses are indeed so reflexive, any intervention that rewards or punishes based on thinking that a child can control them, it’s probably not going to be effective,” Travers says.
Follow-up studies from the lab are looking into brainstem gray matter, groups of brainstem cell nuclei, to characterize how the full brainstem complex is involved in sensory behavior in autistic children. “Being able to merge the information from [white matter and gray matter] together, will really help to paint a clearer picture of what’s going on,” Surgent says.
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