Autism spectrum disorder (ASD) is a developmental disorder characterized by impaired social functioning, over-sensitivity to stimuli, and fixation on objects and routines (Bear et al, 2016). These symptoms are associated with changes in activity in certain parts of the brain, in comparison with typically developing children. One way of measuring activity in any given area of the brain is through Magnetic Resonance Imaging (MRI). MRI involves observing the amount of atoms in a particular area by applying energy to these atoms in order for them to jump to a higher energy state. As this source of energy is removed, the atoms will fall back down to their original energy state and release the amount of energy that was absorbed. The amount of energy emitted is recorded, and the strength of the signal is proportional to the amount of those atoms present in that area (Bear et al, 2016). This is an effective method of qualitatively observing activity in different regions of the brain. Previous studies using MRI have shown that there are significant changes in how the default mode network, a network in the brain active while you’re at rest, functions in individuals with ASD. Changes observed include an increased level of connections within the frontal lobe, the area of the brain involved in functions such as problem solving and speech.
An interesting aspect of this disorder is that most individuals show similar symptoms, but these symptoms can differ a great deal in severity. As a result, individuals with autism are said to be found at different places on a spectrum. As previously mentioned, typical MRIs offer only a qualitative view of brain function, meaning we cannot associate differing levels of activity with symptom severity. However, Jann et al. (2015) used arterial spin labeling magnetic resonance imaging (ASL MRI), which involved using an MRI technique to measure the amount of water (in blood) leaving the area through arteries. By assessing perfusion (the amount of blood circulating), a quantitative measure of the activity in the regions of the brain can be obtained, allowing for association between the level of activity observed and symptom severity.
An important finding in this study was increased perfusion in the frontotemporal lobe. This is indicative of underdevelopment in this region. As we age, neural pruning occurs in typically developing individuals. Neural pruning is the process through which we preserve connections in the brain that we use, and lose the connections we don’t use to make room for new ones or strengthen existing ones. If pruning does not occur, activity levels will stay as high as they were in childhood. In addition, decreased amounts of the major neurotransmitter in the brain that reduces activity (GABA) were found in the frontal, temporal and motor cortices, also implying increased activity in these regions. The level of perfusion in the frontal and temporal areas of the brain was found to be proportional to the level of impairment of functions such as language and social behaviour. Interestingly, this finding was consistent among all subjects, with or without ASD. The same relationship was found with perfusion in the medial orbitofrontal cortex, an area associated with the control of emotions.
This study is of importance because it was the first to quantitatively assess the link between symptom severity and perfusion in the associated areas of the brain in individuals with ASD using ASL MRI. This non-invasive technique will allow for easy data collection to deepen the body of research on ASD in all age groups and at every point on the spectrum.
References:
Bear, M. F., Connors, B. W., & Paradiso, M. A. (2015). Neuroscience: Exploring the Brain. Philadelphia, PA: Lippincott Williams & Wilkins.
Jann, K., Hernandez, L. M., Beck-Pancer, D., McCarron, R., Smith, R. X., Dapretto, M., & Wang, D. J. J. (2015). Altered resting perfusion and functional connectivity of default mode network in youth with autism spectrum disorder. Brain and Behavior, 5(9).
