Asperger Syndrome: a simple matter of white matter?

By Ellis, H.D., and Gunter, H.L., (1999), Trends in Cognitive Sciences, Vol. 3, No. 5

Asperger syndrome, one of the Pervasive Developmental Disorders, is formally diagnosed on the basis of a cluster of cognitive, social and motor signs. It is also associated with poor visuo-spatial skills, good verbal performance, gauche social behaviour and clumsiness. Many of the difficulties evident in those with Asperger syndrome are closely associated with right-hemisphere dysfunction. In this respect they also resemble signs used to diagnose what has been labelled Nonverbal Learning Disorder. Here, these are treated as being the same or closely-related disorders with a possible common underlying aetiology; that is, a neurodevelopmental abnormality affecting white matter. This review examines the ability of this approach to account for a wide range of characteristics of the Asperger syndrome, and contrasts this with a theory-of-mind approach, which, although able to account for the primary features of Asperger syndrome, is less successful at explaining some of its secondary features.

In this article we present the thesis that , in many ways, AS can be explained as s deficit that primarily, but not exclusively affects right-hemisphere performance. We shall also endorse the suggestion that the basic underlying problem is a neurodevelopmental one centred on incomplete or, in some other respect, dysfunctional white matter.

Some evidence for right-hemisphere dysfunction in Asperger syndrome was provided by a SPECT scanning study of three individuals. This study revealed signs of abnormal right hemisphere function in all three cases. An MRI study of seven AS cases (with Tourette’s syndrome) also revealed mostly right-sided abnormalities. Neurological signs of right-hemisphere impairments (i.e. left-sided difficulties) have been described in two AS subjects; and children with right-hemisphere-related learning difficulties bear a striking similarity to those with AS.

The White Matter Hypothesis, however, has yet to be supported by any direct histological evidence. We do have some data showing that, at least in some, AS is associated with diminished right-hemisphere activity and that intercallosal connections can be abnormal.