Comparison of the autism and schizophrenia spectrums

Abstract

Although they share a number of clinical features, autism and schizophrenia are usually distinguished by their different ages of onset and certain discriminating features such as major impairments to communication in the former and positive psychotic symptoms in the latter. However, the recognition that these conditions are part of broader spectrums of impairment has led to the definition of disorders which do not show such marked and discriminating features, such as autism spectrum disorders (ASD) and schizotypal personality disorder (SPD). Reviewing the historical development of these concepts and areas of potential overlap or difference between them revealed that they have both shared and discriminating features, but no study to date has directly compared them. Three experiments were therefore conducted to compare ASD and SPD using clinical, neuropsychological and functional magnetic resonance imaging (fMRI) techniques. In the clinical experiment, standardised measures were used to determine if it was possible to distinguish between the groups, and to allow their quantitative comparison. It was possible to distinguish between ASD and SPD in most cases, although 17% of the population tested met criteria for both conditions. This ‘comorbid’ (CM) group were therefore considered separately. When a single diagnosis could be allocated, there were clear overlaps of clinical features between the conditions and each condition showed more traits of the other than were seen in controls. The overlaps were most prominent for negative schizotypal traits which did not differ between the groups. The CM group were more affected than either the ASD or SPD groups across multiple domains. All groups had high levels of previously undiagnosed psychopathology. In the neuropsychological experiment, tests of social cognition, executive function and central coherence / local-global processing bias were employed. The similarities between the ASD and SPD groups were striking. Both showed similar evidence of impairment in social cognition and executive function, although there was some evidence of greater impairment in working memory in the ASD group. Differences were seen using a test of local-global processing bias, although these were potentially confounded by differences in general intellectual ability. Two fMRI tasks were conducted: a working memory task (a letter based n-back task) and a social judgment task (where individuals made judgements of either gender or approachability from a picture of a face). The former did not distinguish between the ASD and SPD groups. In the latter, individuals with SPD showed significantly greater activation than the ASD group in several brain regions known to be associated with social cognition, with the controls scoring in-between the two. Although they show marked clinical and brain functional overlaps, the results of the fMRI task of social judgement suggest that it is correct to consider ASD and SPD as separate diagnostic entities. The findings are consistent with the idea that, although both conditions are associated with impairments in understanding the mental states of others (mentalising), the mechanism which underlies these differs between the groups, with ASD associated with hypo-mentalising and SPD associated with hyper-mentalising.