Functional Magnetic Resonance Imaging in Intellectual Impairment and Autism Spectrum Disorder

Background:  

A major objective of research into autism spectrum disorders (ASD) has been to delineate the affected neural systems, paving the way for the rational development of new therapies. Functional magnetic resonance imaging (fMRI) of the brain has played a key role in this research, but its application has been limited to affected individuals with preserved or even superior intellectual functioning. This is despite research consistently indicating that many individuals with autism also have co-morbid intellectual impairment.  Such individuals are generally excluded from fMRI studies for practical reasons related to difficulties obtaining informed consent, tolerability of the scanner environment and the lack of suitable fMRI tasks which can be completed by intellectually less able individuals. While this is understandable it leads to an unacceptable situation where the existing fMRI research concerns individuals who are not necessarily representative of the broader autism population. 

Objectives:  

The objective of this study was to demonstrate that it is feasible to reliably use a functional MRI task in individuals with intellectual impairment ± co-morbid autism spectrum disorder (ASD) in a task of implicit emotion processing.

Methods:  

We conducted a functional magnetic resonance imaging study using an implicit emotion processing task to examine participants’ differential response to images of emotion-laden human faces compared to neutral-expression faces (N=10; ASD=5; age: 25 ± 2.5 years).  Diagnosis of ASD was confirmed using the Autism Diagnostic Observation Schedule (ADOS).  Prior to undergoing the scan, participants were able to practice the task on a laptop and also to practice on a mock scanner (where there were no time pressures).  The block-design fMRI task consisted of 6 blocks of 6 faces of an emotional (Ekman fear faces) or neutral (Ekman neutral faces) type.  Each face was shown for 5000ms.  Prior to, and between, each block participants were asked to observe a fixation cross.  To ensure the participants were attending to the stimuli presented, they were asked to depress a trigger each time they viewed a face.  The images were acquired on a Siemens 3T scanner and analysed using SPM5.

Results:  

All 10 participants were reliably able to perform the task both in rehearsal and in the real scanner environment.  Analysis of the data using voxel based morphometry showed that individuals with ASD had less grey matter in the posterior cingulate (Brodmann area 31).  Analysis of the fMRI data revealed reduced signal in the ASD group in areas previously described as associated with emotion processing, confirming findings from studies with non-intellectually impaired individuals.

Conclusions:  

In this study we have demonstrated that it is possible to undertake functional magnetic resonance studies in individuals with intellectual impairment, who were previously considered unable to participate in such studies.  We have also replicated findings of grey matter tissue loss in individuals with intellectual impairment and co-morbid ASD compared to intellectually impaired controls; and decreased cerebral function in brain regions previously linked with emotional processing deficits in non-intellectually impaired individuals with ASD.