Neural Connectivity in Autism: A Systematic Review of the Neuroimaging Data

Background:

Over the past several years investigators have uncovered evidence for abnormalities in functional and structural brain connectivity in individuals with autism. From this work several models have been proposed to characterize these abnormalities, including 1) general under-connectivity, 2) local over-connectivity with long-distance under-connectivity, and 3) abnormal connectivity in those systems that underlie social processing but intact connectivity in systems that do not underlie social processing.

Objectives:

Systematically review the literature that directly examines structural and functional connectivity in individuals with autism in an effort to evaluate the major theories that have been proposed to account for abnormal neural connectivity in autism.

Methods:

PubMed, Medline, and PsycInfo were searched online and key journals were searched manually for articles on neural connectivity in autism. Seventy-seven articles were identified.  Eighteen articles involved direct comparisons of functional or structural connectivity (via DTI, fMRI, or fcMRI) between individuals with autism and controls, and were systematically reviewed.

Results:

Thirteen studies provided direct support for long-distance under-connectivity in autism, whereas three studies provided evidence inconsistent with long-distance under-connectivity.  Three studies provided direct support for local over-connectivity, whereas six studies did not support local over-connectivity (3-null, 3-underconnectivity).  Six studies provided direct support for the social impairment model of connectivity, whereas nine studies provided evidence inconsistent with the social impairment model.

Conclusions:

Of the theories examined, the long-distance under-connectivity model currently has the strongest support.  To date, there is little support for a consistent pattern of local over-connectivity in autism.  Finally, the data strongly suggest that abnormalities of connectivity are not limited to the neural systems that underlie social processing.  The implications of these findings will be addressed in the context of research on other disorders associated with abnormalities in neural connectivity (e.g., schizophrenia, dyslexia), as well as methodological challenges associated with examining neural connectivity in disordered populations.