Altered Hierarchical Interactions in Visual Processing in Autism Spectrum Disorder

Post by Flora Moujaes

What's the science?

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by difficulties in social communication and interaction, restricted interests and repetitive behaviours. We still don’t fully understand the changes in brain function underlying ASD. There are two basic types of neurotransmission in the brain: excitation and inhibition. One theory that has gained traction in recent years is that ASD involves a higher neural excitation to inhibition ratio, resulting in an increase in neural response. However, findings in the literature have been inconsistent. We know that the brain is organized in a hierarchy of various regions and networks that interact: later processing stages may inherit and build upon earlier (input level) stages, and also influence and shape of earlier stages by top-down modulation. This means that the perceived increase or decrease in neural response in ASD could depend on the stage being measured. This week in The Journal of Neuroscience, Kolodny, and colleagues leverage the well-known hierarchical structure of the visual motion pathway in the brain to examine how fMRI response changes in adjacent stages of processing in ASD.

How did they do it?

The researchers measured sensory-driven fMRI responses in the visual cortex of 24 adults with ASD and 24 demographically matched neurotypical control adults. In the main task, simple visual stimuli were presented to participants during fMRI scanning. Moving stimuli (drift grating stimuli) were presented to the participants in blocks of either low (3%) or high (98%) contrast. Response to the stimuli was measured at two places in the visual cortex: (1) the visual cortex’s primary input region, V1 and (2) the higher-order motion-processing region, the middle temporal area (MT). These two regions were chosen as it is well established that the visual system is hierarchically organized, with information traveling forward from V1 to MT (feedforward projections) as well as backward (feedback projections). Two measures were used to ensure that participants were focusing on the center of the visual stimuli: (1) participants were asked to press a button when a green circle appeared at the center of the visual stimuli, and (2) eye-tracking data was collected so that eye movements could be monitored. A control task involving static non-moving stimuli was also conducted.

What did they find?

Does fMRI response magnitude in participants with ASD vary between adjacent stages of processing in the visual pathway? The researchers found that individuals with ASD showed reduced responses in V1 and increased responses in the MT area. Thus, ASD responses were reduced in a primary visual area but amplified in a subsequent higher-order area. This pattern was stimulus-specific, as V1 responses were reduced for moving stimuli but not for static stimuli.

Is there a relationship between early (V1) and higher order (MT) visual areas? There was a significant negative correlation between V1 and MT response to high contrast stimuli among individuals with ASD. This means that the same participants with lower V1 responses also showed higher MT responses. This could suggest a causal link between the decrease in V1 and the increase MT, and that the observed dissociation in neural response is driven by amplified suppressive feedback from MT to V1.  Is there a relationship between ASD symptom severity and visual cortex response? The researchers found that response magnitudes in both V1 and MT were differentially correlated with autism symptom severity, which suggests that response magnitude in the visual cortex in ASD may be clinically relevant.

What's the impact?

This is the first study to show a neural response dissociation between the adjacent stages of visual processing in ASD, as fMRI responses were reduced in a primary visual area but amplified in a subsequent higher-order area. This study uncovers a previously unknown cortical network alteration in autism. It also highlights that while neural response magnitude is altered in ASD, this altered response may be increased or decreased depending on the stage of the visual motion pathway. More research is needed to understand how these interactions between regions in the visual motion pathway may relate to the basic excitation/inhibition model of the brain.

Kolodny, et al. Response Dissociation in Hierarchical Cortical Circuits: a Unique Feature of Autism Spectrum Disorder. Journal of Neuroscience (2020). Access the original scientific publication here.