Functional Activation and Connectivity of Dorsal and Ventral Attention Networks in Autism Spectrum Disorder: An fMRI Study of Visual Search

Background: Prior studies have demonstrated that children and adults with autism spectrum disorder (ASD) evidence superior visual search abilities compared to typically developing (TD) individuals.  Corbetta and Shulman (2002; 2008) have proposed a dual-system model of visual attention for TD individuals: a dorsal frontoparietal network (bilateral frontal eye fields [FEF] and intraparietal sulci [IPs]) that mediates top-down, directed visual attention and a right-lateralized ventral frontoparietal network (inferior frontal gyrus [IFg] and temporal-parietal junction [TPJ]) that is responsible for reorienting attention to behaviorally-relevant stimuli.  Additionally, recent functional magnetic resonance imaging (fMRI) studies investigating visual search have suggested that TPJ suppression may reflect filtered visual processing and assist in reducing the influence of task-irrelevant distractor stimuli. 

Objectives: The goals of the present study were 1) to examine the fMRI activation in nodes of dorsal and ventral attentional networks during visual search in ASD and TD individuals, and 2) to investigate the functional connectivity within and between dorsal and ventral attentional networks, using activation-derived regions of interest from a visual search task.

Methods: Twenty children and adolescents with ASD and twenty age, handedness, and IQ matched TD individuals participated in an event-related fMRI visual search experiment.  Participants’ task was to indicate the presence or absence of a target (“T”) embedded within arrays of heterogeneous or homogeneous distractors that varied in set size (6, 12, or 24 items). Baseline trials consisted of a solitary target (target present baseline) or a single distractor (target absent baseline), displayed in the center of the screen.

Results: Behaviorally, there were no significant differences between ASD and TD groups for mean error rates or median response time (RT); however, individuals with ASD showed increased search efficiency for heterogeneous search trials compared to TD individuals, as measured by RT by set size slope.  FMRI results indicate increased activation of TPJ in individuals with ASD relative to TD individuals for heterogeneous versus baseline comparisons.  Functional connectivity MRI results indicate that individuals with ASD exhibit increased functional connectivity between TPJ and cerebellum.  Furthermore, correlations between TPJ-cerebellum connectivity reveal that increased functional connectivity is related to increased search efficiency (RT by set size slope) for ASD, but not TD, individuals. 

Conclusions: Contrary to previous reports, the current study did not find accelerated visual search RT in children and adolescents with ASD; however, individuals with ASD demonstrated increased search efficiency for the most difficult search trials as compared to TD individuals.  Functional activation and connectivity analyses revealed abnormally increased activation and connectivity of the ventral attentional network in ASD.  Decreased TPJ suppression relative to baseline in ASD may suggest abnormal top-down filtering of distractors during visual search.  Additionally, functional connectivity between TPJ and cerebellum was correlated with search efficiency, suggesting that increased search efficiency in ASD may be related to atypical functional connectivity between these regions.