Autism exists across a wide spectrum and there is debate as to whether, autism spectrum individuals with average IQ and typical speech onset (‘’Asperger’s syndrome’’) should be considered a distinct subgroup from those with delayed speech onset (“Autism”). Apart from the clinical relevance of this issue, the history of language acquisition is an important factor when deciding whether or not to merge these two subgroups in brain structural and functional magnetic resonance imaging (MRI) studies of autism, and may contribute to the inconsistent findings across research groups.
Objectives:
We investigated whether brain structure (in terms of cortical thickness or CT) in young adults with autism can be distinguished based on whether they have typical or delayed speech onset.
Methods:
There were three subject groups: 1) 13 autism spectrum individuals (AUT) with delayed speech onset (DSO) (defined as first words after 24 months and/or first phrase after 33 months old), 14 AUT with typical speech onset (TSO), and 15 typically developing individuals (TYP). All groups were matched on Wechsler IQ measures (FSIQ, VIQ and PIQ), age (mean 22 SD 6.1 years), gender and manual preference. The AUT groups were also matched on their diagnostic measures (ADI-R - Social, Communication and Repetitive behaviours scores).
T1-weighted MR images were obtained for all subjects on a 3 Tesla scanner. CT maps were derived from these MRI data for each subject and between-group statistical analyses were then performed. Ethical approval for the present work was obtained in accordance with the National Institute of Health guidelines.
Results:
AUT-DSO adults had increased CT relative to AUT-TSO adults in several cortical areas including bilateral cingulate and tranverse temporal gyri, as well as left insula, superior marginal, occipital and frontal gyri, but decreased CT in only the left postcentral and fusiform gyri. One possibility is that these CT differences are related to differences in age of speech onset (and less likely due to differences in core symptoms of autism, since diagnostic measures were matched between groups). Support for this claim comes from the fact that age of first phrase was correlated with CT in several of these above brain areas. Consistent with previous findings (Hyde et al., 2009), CT differences were also found between AUT-DSO and TYP in distributed cortical areas, however no CT differences were found between AUT-TSO and TYP.
Conclusions:
The novel CT differences found here between autism spectrum adults with typical versus delayed speech onset suggest that history of language acquisition may be a marker that distinguishes heterogeneous brain phenotypes within the autism spectrum. Thus, future studies should consider the history of language acquisition as an important factor to refine investigation of aetiological factors and interventions in autism.