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Two Spectra or Continua?
Autism and schizophrenia are two complex neurodevelopmental disorders/conditions that have been extensively studied in the field of psychiatry. Although ASD and schizophrenia share some overlapping symptoms and risk factors, they are distinct neurodevelopmental conditions with different diagnostic criteria and clinical presentations.
Historical Note
Both autism and schizophrenia have a long history, but their understanding and diagnostic criteria have evolved over time.
The term ‘autism’ appeared in the medical/clinical literature long before it was applied by Kanner to a ‘unique syndrome’ in 1943. In 1911 a Swiss psychiatrist Eugene Bleuler published “Dementia praecox, oder Gruppe der Schizophrenien”, in which he considered the special features of the symptoms of ‘infantile imbecility’: their dissociation, splitting; and coined a new term to describe them – “schizophrenia” (fr. Gk “skhizo” - “split”, and “phren” - “mind”). In the same paper Bleuler introduced another new term - “autism” (fr. Gk “auto” - “self”) to describe one of the features of schizophrenia (so-called “4A’s: association, affectivity, autism, ambivalence).
Although Bleuler introduced the term "autism" for schizophrenic patients, Kanner believed that these conditions were fundamentally dissimilar. Specifically, Kanner identified childhood schizophrenia as a form of psychotic disorder that exhibited a distinct pattern: it would typically manifest with a period of relative normalcy, only to be followed by a progressively deteriorating course. Through his elucidation of the disparities between autism and childhood schizophrenia, Kanner shed light on the intricate nature of these psychiatric conditions. His astute observations not only significantly contributed to a more comprehensive understanding of these disorders but also paved the way for further research and advancements in the field of psychiatry.
For example, in the 1970s, schizophrenia in infancy, childhood, and adolescence comprised a group of related and overlapping syndromes characterized by withdrawal, regression, and dissociation. Diagnostic criteria included (1) withdrawal from the environment, (2) disturbance of thought and speech, (3) inappropriate affect, (4) alteration in mobility, (5) disorientation, (6) preoccupation, (7) retardation, (8) disintegration of body image, (9) resistance to change, and (10) anxiety. About 1.9% of schizophrenic patients were less than 15 years old. Patients responded poorly to treatment; about one third achieved some social adjustment. There was no correlation between prognosis and therapy used, but apparently the earlier the disorder appeared, the worse was the outlook (Chan et al. 1971).
There were developed several diagnostic systems, designed to differentiate infantile autism and early childhood schizophrenia. However, while the autistic scales devised by Rimland, Polan and Spencer, Lotter, and the British Working Party correlated significantly, the degree of correspondence (35%) indicated that several children obtained high autistic scores in one system but low scores in another. The BWP's term “schizophrenia” has more correspondence with the term “autism” used by others than with Rimland's “schizophrenia.” In the DeMyer-Churchill categorical system (early schizophrenia, primary autism, secondary autism, and non-psychotic subnormal), “primary autism” most resembles Rimland's concept of infantile autism (DeMyer et al. 1971).
The diagnostic conflation between autism and schizophrenia was not definitively resolved until the publication of the DSM-III in 1980. Subsequently, diagnostic criteria have undergone refinement, enabling better identification and classification of individuals with these conditions.
Although the DSM-5 no longer recognizes schizophrenia subtypes as separate diagnostic categories, it is important to acknowledge the historical existence of five subtypes: paranoid, disorganized (hebephrenic), undifferentiated, residual, and catatonic. These subtypes were initially introduced to elucidate the diverse manifestations of schizophrenia, a subject of study for well over a century.
Definitions: Two Spectra
Autism Spectrum Disorder is a group of complex clinically distinct neurodevelopmental disorders/conditions with both genetic and environmental components and many varied patterns of persistent deficits in social communication and social interaction, and myriad patterns of restricted and repetitive activities and interests. However, the diagnosis of ASD is based on behaviours: social communication impairments, and restricted, repetitive patterns of behaviour with three levels of severity described as levels of necessary support (1) support; 2) substantial support; 3) very substantial support). (Many researchers have expressed doubts about the reliability of the ASD diagnosis, given the wide heterogeneity in phenotypical manifestations (Waterhouse & Mottron 2023).)
Schizophrenia is a chronic mental disorder characterised by a combination of hallucinations, delusions, disorganised speech and behaviour, and negative symptoms such as, e.g., diminished emotional expression or avolition. However, in DSM-5, the formal name for ‘schizophrenia’ has changed to ‘Schizophrenia Spectrum Disorder’ (SSD). A spectrum approach acknowledges heterogeneity of the condition and recognises a wide range of features, duration, and levels of severity within the schizophrenia spectrum.
The neurodevelopmental hypothesis
The concept that disruptions during early brain development play a role in the development of schizophrenia, commonly known as the neurodevelopmental hypothesis, has gained significant acceptance. However, it is important to note that schizophrenia is considered distinct in terms of its classification, pathophysiology, and clinical presentation from other conditions such as ASDs, ADHD, and intellectual disability, which typically present in childhood and are grouped together as ‘neurodevelopmental disorders’.
Continua
The Neurodevelopmental Continuum
An alternative view suggests that neurodevelopmental disorders, including schizophrenia, should not be viewed as separate and distinct conditions. Instead, it is more accurate to conceptualize them as lying on an etiological and neurodevelopmental continuum, with the major clinical syndromes reflecting the severity, timing and predominant pattern of abnormal brain development and resulting functional abnormalities. It has also been proposed that, within the neurodevelopmental continuum, severe mental illnesses occupy a gradient of decreasing neurodevelopmental impairment as follows: intellectual disability, ASDs, ADHD, schizophrenia and bipolar disorder (Owen & O’Donovan (2017).
Intellectual disability – ASDs – ADHD – schizophrenia and bipolar disorder
The Autism-Schizophrenia Continuum
Humans develop a variety of different ways to perceive and understand the world around them, even when exposed to the same sensory information. According to the Bayesian framework, these differences may stem from the varying importance placed on prior knowledge versus new external inputs when making predictions. Recent advancements in computational psychiatry suggest that ASD and SSD exist on opposite ends of a predictive continuum. It is possible that the adoption of a specific inferential style is influenced by dispositional factors associated with autistic and schizotypal traits.
In a study conducted by Tarasi et al. (2023), the role of the ASD-SSD dimension in shaping the neuro-behavioural markers underlying perceptual inference was directly investigated. The researchers employed a probabilistic detection task and simultaneously recorded EEG data to examine whether neurobehavioral patterns related to prior processing were influenced by ASD and SSD traits in the general population. The results of the study indicated that an individual's position along the ASD-SSD continuum influenced the predictive strategies they employed in decision-making. Those closer to the positive schizotypy pole tended to adopt a predictive approach that heavily relied on prior knowledge, while those closer to the ASD pole favoured strategies that prioritized sensory evidence.
These findings shed light on the significance of the weight assigned to prior knowledge as a marker of the ASD-SSD continuum. This knowledge could potentially be valuable in identifying individuals at risk of developing mental disorders and enhancing our understanding of the mechanisms contributing to the onset of symptoms observed in ASD and SSD clinical forms.