Approximately 1/3rd of those with autism spectrum disorder also have an intellectual disability. A major unanswered neurobiological question, however, is what distinguishes autistic people with and without intellectual disability. As children, their intelligence quotient (IQ) will fluctuate a lot. Researchers previously distinguished 3 subgroups of autistic children with distinct patterns of intellectual growth from infancy (2–3½ years) to early adolescence (ages 9-12): Individuals were classified as either persistently high, having maintained a normal IQ, persistently low, having maintained an IQ less than 70, or changers, having displayed a change in IQ from intellectual disability to (n) normal IQ. There are established connections between the frontoparietal (FPN) and default mode (DMN) networks and cognitive ability. To determine if there were early-life differences in brain volume across these IQ trajectory groups, investigators focused on regions of the fronto-parietal and dorsolateral networks (FPN and DMN). Using structural MRI acquired at baseline, they compared the functional connectivity between the default mode network (FPN; 11 regions x 2 hemispheres) and the default mode network (DMN; 12 regions x 2 hemispheres) in three groups: 48 persistently high (18 female), 108 persistently low (32 female), and 109 changers (39 female). The regions of the FPN and DMN were defined by employing the networks found in Smith et al (Proc Natl Acad Sci USA 106:13040–5, 2009). Up to 3 early and intermediate-age IQ tests were used to classify subjects into IQ trajectory groups (mean age time 1: 3.2 years; time 2: 5.4 years; time 3: 11.3 years). At time 1, the Changer group showed DMN volumetric differences compared to both the Persistent Low and Persistent High groups. A change in DMN structure may be an early predictor of a shift in IQ trajectory, but there was no difference between the persistently high and persistently low groups. When compared to the persistently low and changers groups, the FPN showed that those in the persistently high group differed in a way that may have been related to their concurrent IQ and the absence of intellectual disability. There may be a distinction between children with autism whose low IQ remains stable over time and those whose IQ improves, based on differences in the volume of brain regions within the DMN during early childhood. Structural differences in brain networks highlight the distinct neural underpinnings of these three IQ-based subgroups of autism.
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