Objectives: To determine whether existing functional neuroimaging studies have shown: (1) an overall differential engagement of the visual system in autism; and (2) whether any differential visual system activity in autism is general across tasks or specific to particular visual domains.
Methods: We performed a PubMed search to identify PET and fMRI studies utilizing visual tasks with autistic and non-autistic groups. Of a total of 95 studies, 22 were rejected because of low power (n<10), 21 because of partial brain coverage, 11 because results were not reported in a standard anatomical space, and 9 because only between-group contrasts were presented. The remaining 32 articles reporting 1251 within-group foci in a standard anatomical space were included. First, Activation Likelihood Estimation (ALE) maps reflecting the regions of convergence for all tasks were computed within and between groups (FWHM = 8mm, pFDR=0.05). Second, the contrasts were classified according to their domain specificity (face, object, and written language tasks) and the within- and between-groups activation maps recomputed.
Results: First, the ALE results for the meta-analysis of all coordinates for both groups revealed activity in occipito-temporal (BA 17, 18, 19, 37), frontal (BA 6, 8, 9, 46, 47), parietal (BA 7, 40) and insular cortex. Between group comparisons revealed higher activity in the visual system (BA 17, 18, 19, 37) and lower activity in frontal areas (BA 6, 9, 44) in autistics. Second, comparison maps for each domain revealed specific between-group differences. In the face processing domain, greater primary visual and antero-medial fusiform activity, combined with reduced inferior frontal gyrus ALE values were seen in autistics. In the object processing domain, a pattern of higher ALE values in extrastriate cortex, superior parietal lobule and precuneus combined with lower values in inferior and medial frontal gyri were seen in the autistics. For tasks involving written language, higher activity was seen in the reading network (BA 17, 37, 22 and 45) in autistics.
Conclusions: This literature meta-analysis demonstrates greater activity in the autistic visual system for a range of tasks. Moreover, domain-specific activity increases in visual areas specialized for face, object, and language tasks were observed in autism. An overall stronger engagement of the visual system is therefore a task-independent and robust finding in autism and is possibly related to their well-documented enhanced visual abilities.