International Meeting for Autism Research (London, May 15-17, 2008): fMRI Investigation of Visual Search in Autism Spectrum Disorder

fMRI Investigation of Visual Search in Autism Spectrum Disorder

Saturday, May 17, 2008: 5:15 PM
Avize-Morangis (Novotel London West)
B. M. Keehn , Joint Doctoral Program Language and Communicative Disorders, San Diego State University and University of California, San Diego, San Diego, CA
L. Brenner , UCLA Department of Psychology, University of California, Los Angeles
E. Palmer , San Diego State University
A. J. Lincoln , Alliant International University, Los Angeles, CA
R. A. Müller , San Diego State University
Background: Previous studies investigating visual search in autism spectrum disorders (ASD) have demonstrated accelerated response times (RT) and enhanced search efficiency. However, the underlying neural mechanisms remain unknown. 
Objectives: To investigate the neurofunctional correlates of visual search in children with ASD and typically developing (TD) children, using an event-related functional magnetic resonance imaging (fMRI) design.
Methods: Participants were 13 individuals diagnosed with ASD and 13 age- handedness- and IQ-matched typically developing (TD) individuals. Four children with ASD, but none in the TD group, were excluded from the final sample due to excessive movement during fMRI scanning.  We used an event-related visual search paradigm, manipulating search difficulty by varying set size (6, 12, or 24 items), distractor composition (heterogeneous or homogeneous), and target presence (absent, present) to identify brain regions associated with efficient and inefficient search.
Results: Groups did not differ significantly in error rate. While the ASD group did not demonstrate accelerated response time (RT) compared to the TD group, they did evidence increased search efficiency, as measured by RT by set size slopes. fMRI results showed occipitotemporal activation in both groups. However, the ASD showed additional frontoparietal activation, which was not seen in the TD group.  Direct group comparisons (for both homogeneous and heterogeneous search conditions) confirmed greater frontoparietal activation in ASD compared to TD participants.
Conclusions: Enhanced activation in superior parietal and superior frontal regions in individuals with ASD suggests that accelerated performance in ASD may be related to enhanced top-down modulation of visual attention. While our finding of increased frontal recruitment during visual search in ASD may be consistent with enhanced top-down control of visual attention, it is surprising given previous theories of reduced prefrontal control in autism.
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