The effect of global intellectual ability on the relationship between autistic features and prefrontal cortical folding

Background: Magnetic resonance imaging (MRI) studies have suggested that aberrant patterns of cortical folding may be associated with the development of autism spectrum disorders (ASD).  However, these studies have considered only individuals without global intellectual impairment, i.e. with an IQ>70.   Around 70% of people with ASD are considered to be intellectually impaired, therefore the existing literature is not representative of the general autistic population. 

Objectives: We set out to determine whether the relationship between autistic features and the degree of prefrontal cortical folding differs between individuals with and without global intellectual impairment. 

Methods: 87 male individuals receiving special educational assistance were recruited from across Scotland.  The participants were divided into two groups: those with global intellectual impairment (IQ<70, n=34) and those without global intellectual impairment (IQ>70, n=53).  Autistic features were assessed using the Social Communication Questionnaire (SCQ) and each participant received a structural MRI scan.  Prefrontal cortical folding was determined using automated methodology to measure the gyrification index (GI), a standard measure of cortical folding.  The relationship between SCQ score and prefrontal GI was examined separately in each group and tests of interaction employed to determine if the relationship differed between the groups. 

Results: SCQ scores for the group as a whole ranged between 3 and 33.  A significant negative relationship was seen between SCQ score and GI among the intellectually impaired participants, whereas no significant relationship was seen in the non-intellectually impaired group (r = -0.36, p=0.03 and r=0.03, p=0.81 respectively).  This difference between the groups was significant (F=3.82, p=0.05). 

Conclusions: These results suggest that different, or additional, brain structural features may be associated with the expression of autistic characteristics in people with intellectual impairment compared to those without.  Results from non-intellectually impaired groups may not necessarily apply to the broader population of people with ASD.