Brain Connectivity and Autism Spectrum Disorder
by Wendy Hoke
Do individuals with Autism Spectrum Disorder (ASD) have any differences in their brain connectivity than the non-affected population? There is a common assumption based on certain studies that people with the disorders do in fact have differences in their brain connectivity. However, a recent look at these studies published in the April 2016 edition of “Current Opinion in Neurology,” suggest that certain factors may be skewing the results higher.
Purpose of the Review
The investigators took into consideration that many studies have found that those people diagnosed with ASD have different patterns of brain connectivity versus those individuals with more typical development. They noted that more recent studies do not completely support the conventional wisdom that individuals with ASD show higher connectivity within local brain regions and decreased connectivity between distant brain regions. The researchers looked at the various methods for measuring brain connectivity and how the use of varying metrics may contribute to the discrepancies in recent research.
What the researchers found was the certain methodological factors may be contributing to the differences between studies, such as the type of measurement used to assess connectivity, the brain region examined, the age of the individual with ASD, the band frequency, and the time interval.
Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder that features problems in language acquisition and social communication, in addition to repetitive behaviors and restricted interests. Much of the research surrounding ASD has sought to gain understanding of the neural underpinnings of the disorder by identifying biomarkers. These would be objective measures that relate to its core deficits and indicate a risk for ASD. However, a wide variety of methods for recording brain activity exist, as well as a broad array of extracting and delineating the data produced by those methods.
Why would brain connectivity be important in determining ASD? The measurements of brain connectivity suggest that brain regions connected to form brain networks serve either the brain’s resting state or the cognitive/behavioral performance state.
Hundreds of genes have been identified as conveying risk factors for ASD, and many of them converge on the synaptic pathways. At the microscopic level, the convergence suggests that synaptic connectivity may act as a potential neurobiological trigger for ASD. Yet, on the macroscopic level, the convergence may point to functional or axonal connectivity patterns as the most plausible mechanism for ASD.
The number of scholarly publications that have the keywords “brain connectivity” has exploded over the previous three decades. However, the number of scholarly publications with the words “brain imaging” has not grown, rather they have remained constant. This exponential growth in brain connectivity publications suggests the critical importance this relatively new field has in understanding the brain in a more comprehensive, integrated way.
According to the investigators, the inconsistent results between these studies results from the use of differing imaging techniques and modalities for constructing brain networks. In addition, they note the challenges inherent in the choice of method to reconstruct a brain network in order to test a hypothesis. In addition, researchers selecting one neuro-imaging technique that may seem to suit a study participant may actually filter out appropriate candidates who are particularly low-functioning. Therefore, those individuals with severe cognitive impairments who create noisy data may not be included in the sample. For instance, in individuals with ASD, cognitive levels and motor stereotypes should be considered when selecting imaging techniques that can be disrupted by motion or require the individual to lie still for long time periods.
The investigators concluded that more approaches using sophisticated electro-encephalography should be used. Doing so would more accurately connect causation data to the transfer of information between brain regions. This, in turn, could produce more accurate data sets regarding functional brain connectivity. In addition, further research into brain connectivity in relation to behavior is necessary to identify brain networks that could be the underlying core deficits associated with ASD.
In addition, while many studies used EEG/MEG methods to determine differences in functional connectivity in individuals with ASD, more research is needed to explore and identify those differences in early development. Finding these biomarkers of ASD risk would help significantly in earlier interventions with infants and young children. In addition, more research is needed in relation to brain connectivity in respect to behavior in order to find the underlying networks that may be at the root of ASD.
The investigators also recommended more investigation of EEG connectivity with relation to single nucleotide polymorphism, genotype measures and copy number variants. Lastly, they recommended deeper investigation into the tendency of the brain in the ASD individual to get stuck rather than be flexibly adaptive. This would hopefully begin to address the tendency of ASD individuals to present with restrictive interests or repetitive behaviors.