The recent "autism" brain scan study from the UK is a notorious example of an alarming reality in the world of autism research - the severely autistic are excluded from autism research. In that study autistic persons with intellectual disabilities were intentionally excluded and 16 of the 20 participants in the study actually had Aspergers diagnoses. The study press releases should have more accurately hyped the potential of the brain scan process involved to assist in diagnosing Aspergers. Of course that distinction will be moot once the DSM5 reduces the Pervasive Developmental Disorders to one Autism Disorder, expands the number of very high functioning persons who will be considered "on the spectrum" and removes those with Autistic Disorder and Intellectual Disabilities from the "spectrum".
The efforts of the DSM5 committees to remove intellectual disability from Autistic Disorder are assisted by studies like the brain scan study which already exclude the vast majority of persons with Autistic Disorder who also have intellectual disabilities. That study is far from unique as indicated in the comments of well known autism researcher and DSM5 panel member Catherine Lord Ph.D. in Social Policy Report, Autism Spectrum Disorders Diagnosis, Prevalence, and Services for Children and Families:
Aspergers Disorder will become the New Autism Spectrum Disorder in the DSM5 which will emphasize social and communication deficits but will do away with the implicit references to intellectual disability in the current DSM. The vast majority of those with Autistic Disorder today, the 75-80 % of those with Autistic Disorder who also have intellectual disabilities will, as in the research studies which have led to the New Autism Spectrum Disorder in the DSM5, simply be excluded.