autism and Asperger syndrome

Autism is a lifelong developmental disorder with multiple biological origins and very heterogeneous behavioural manifestations. Autism is conceptualized as a spectrum defined by core impairments in social communication, with a restricted range of activities and interest that vary with age and ability. The spectrum includes the severely impaired child of low IQ, who may be mute and exhibiting simple motor stereotypies, such as rocking. It includes the classically aloof child with echoed speech and insistence on sameness who, as well as showing some learning disabilities, shows islets of ability. The spectrum also includes very able and highly verbal individuals with less obvious symptoms. The absence of a delay in language acquisition frequently results in such cases being diagnosed much later in life. Such individuals have the same social communication problems although in a milder form, but retain narrow and obsessively pursued interests. Some of these people may crave social interactions without the understanding or ability to form relationships, have odd speech and body language, and be interested in unusual topics (e.g. registration numbers on lamp-posts). This latter picture describes the new diagnostic category of Asperger syndrome. The prevalence of autism spectrum disorder is thought to be in the region of 0.6 per cent and possibly up to 0.9 per cent of the population, with approximately three times more males affected than females. There is no evidence for an autism epidemic despite an increase in prevalence. Rather, earlier studies produced much smaller figures but had a very narrow definition of autism that only applies to a small proportion of cases with autism spectrum disorder.

As shown by twin and family studies, genetic causes are most likely to underlie autism and Asperger syndrome. Current genetic studies implicate locations on a number of chromosomes, e.g. 7 and 15. Given the relatively wide criteria of a broader phenotype which includes mild variants, the concordance is 82 per cent in identical twins and 10 per cent in fraternal twins. Non-genetic causes (e.g. viral encephalopathy), toxic influences early in fetal life, or specific neurological conditions (e.g. tuberous sclerosis, fragile X syndrome, Rett's syndrome) have also been implicated. Regardless of aetiology, genetic as well as non-genetic risk factors may adversely affect the development of certain brain systems and consequently the development of certain cognitive systems that are dysfunctional autism.

Because there is currently no biological marker, the diagnosis of autism is by behavioural criteria: qualitative impairments in social interaction and communication and the presence of restricted and repetitive interests and activities. Autism is associated with a variety of neurological signs (epilepsy, EEG abnormalities, mental retardation, motor stereotypies, and impairments). Various types of brain abnormality have been found, e.g. a decrease in Purkinje cells in the cerebellum and brain stem, and decreases as well as increases in various cortical and subcortical regions. However, the specific and defining features of autistic brains have not yet been identified. This is in part due to the heterogeneity of the condition, which makes it problematic to relate physiological and behavioural data.

In contrast, a much better consensus about the nature of the condition has been achieved at the cognitive level. At this level, the multiple biological causes and multiple behavioural consequences can be pulled together by postulating deficits and assets in a small number of cognitive functions. See Fig. 1.

One influential theory, which can be referred to as mind-blindness, has been particularly successful in explaining the very variable core symptoms in social communication in autism. Among the first reliable signs of autism is an absence of joint attention activities, e.g. a lack of spontaneous pointing and a lack of pretend play. These early difficulties signal a failure to attend to the contents of other minds, or to understand 'make-believe'. This is later manifested in an individual's failure to perform laboratory tasks of false belief, presented in dramatic form, in story form, in pictures or in animations. The crux of these tests is that the child has to understand that another person's behaviour can be predicted from a knowledge of his desires and beliefs. Children with autism, even high-functioning ones, seem unable to pass tests of belief attribution at the appropriate mental and chronological age. This is in contrast to other children with learning disabilities, such as Down's syndrome.

A mentalizing deficit explains the particular social communication difficulties that are typical of children and adults with autism, whilst allowing for a range of social and emotional behaviours which appear to function normally. For instance, children with autism are able to use instrumental gestures (to affect behaviour directly) much better than expressive gestures (to affect inner states). They are able to understand and engage in sabotage (to affect behaviour directly) better than in deception (to alter someone's belief). While able to remember messages verbatim, to give factual and honest answers to questions, and even to deliver learned speeches, they are curiously unable to respond to hints, to engage in gossip, to keep secrets, and to make confidences.

Faces are arguably some of the most important social cues, and people with autistic disorder have difficulties in remembering individual faces. Brain scans have shown that they do not activate the normal face-processing area. Whether this is a primary or secondary difficulty, arising from a lack of social orientation and a lack of learning about social agents, remains to be seen.

In individuals with Asperger syndrome, verbal ability and social adaptation tend to be higher than in autism. Compensatory learning of social communication skills can occur even in the absence of mentalizing early in life. In a proportion of cases mentalizing abilities are acquired with practice and through the application of logical inference. In a proportion of cases, mentalizing may be merely delayed. However, mentalizing acquired later in life appears to lack an intuitive basis, and tends to be fragile and effortful.

Evidence from neuroimaging has shown that compared to non-autistic individuals, people with autism display a different pattern of activation in the brain structures thought to underlie mentalizing, e.g. the medial frontal cortex, the temporoparietal junction, and the temporal poles adjacent to the amygdala. Some of these structures also show anatomical abnormalities and/or weak connectivity.

A number of other characteristic impairments on psychological tests have been found in autism which appear to be separate from social communication impairments. These notably include repetitiveness and inflexibility, stimulus-driven behaviour, and a lack of foresight and planning. These symptoms have been convincingly linked to frontal lobe dysfunction. People with autism also have unusual abilities, as reflected in a typical profile of intelligence test scores with peaks on tasks that require accurate perception of detail. Hypersensitivity to sensory stimuli and absolute pitch are often reported. In 10 per cent of autism cases, exceptional abilities are shown, most frequently rote memory, but also talents in art, music, language, and number. All these phenomena have been linked to a particular cognitive processing style, labelled weak central coherence, that favours enhanced perception of detail. This processing style appears to be part of the broader phenotype of autism, and deserves fuller investigation.


Fig. 1. A causal model of autism spectrum disorder. In the theory illustrated, the core impairments in social interaction and communication are explained by a single cognitive deficit in a mechanism with a complex neural basis. At all levels environmental influences interact with internal factors, either aggravating or ameliorating the condition, and increasing variability. This model can be readily changed to illustrate other theories by adding or substituting other putative cognitive deficits, e.g. theories that explain frequently associated impairments in executive functions, motor functions, and language. The framework can also be used to illustrate theories concerning the coexistence of well-functioning cognitive abilities, e.g. enhanced perceptual processing and high general intelligence.


(Published 2004)

— Uta Frith

Bibliography
• Baron-Cohen, S., Tager-Flusberg, H., and Cohen, D. J. (eds.) (2000). Understanding Other Minds: Perspectives from Autism (2nd edn.).
• Gillberg, C., and Coleman, M. (2000). The Biology of the Autistic Syndromes (3rd edn.).
• Happé, F., and Frith, U. (1996). 'The neuropsychology of autism'. Brain, 19.
• Klin, A., Volkmar, F., and Sparrow, S. (eds.) (2000). Asperger Syndrome.
• Wing, L. (1996). The Autistic Spectrum.

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