Abstract
Humans have evolved an elaborate system of self-consciousness, self-identity, self-agency, and self-embodiment that is grounded in specific neurological structures including an expanded insula. Instantiation of the bodily self has been most-extensively studied via the ‘rubber hand illusion’, whereby parallel stimulation of a hidden true hand, and a viewed false hand, leads to the felt belief that the false hand is one’s own. Autism and schizophrenia have both long been regarded as conditions centrally involving altered development of the self, but they have yet to be compared directly with regard to the self and embodiment. Here, we synthesize the embodied cognition literature for these and related conditions, and describe evidence that these two sets of disorders exhibit opposite susceptibilities from typical individuals to the rubber hand illusion: reduced on the autism spectrum and increased in schizophrenia and other psychotic-affective conditions. Moreover, the opposite illusion effects are mediated by a consilient set of associated phenomena, including empathy, interoception, anorexia risk and phenotypes, and patterns of genetic correlation. Taken together, these findings: (i) support the diametric model of autism and psychotic-affective disorders, (ii) implicate the adaptive human system of self-embodiment, and its neural bases, in neurodevelopmental disorders, and suggest new therapies and (iii) experimentally ground Bayesian predictive coding models with regard to autism compared with psychosis.Lay summary: Humans have evolved a highly developed sense of self and perception of one’s own body. The ‘rubber hand illusion’ can be used to test individual variation in sense of self, relative to connection with others. We show that this illusion is reduced in autism spectrum disorders, and increased in psychotic and mood disorders. These findings have important implications for understanding and treatment of mental disorders.
Highlights
An evolutionary approach to understanding human psychiatric disorders compels determination of what cognitive adaptations have evolved along the human lineage, and how these traits become subject to effects of extreme development, trade-offs, mismatches and genetically based conflicts that lead to maladaptation
All studies of psychotic-affective conditions, which included analyses, showed faster or stronger effects of the illusion associated with these conditions, for at least 1 test: 6 of these analyses compared schizophrenia subjects versus controls, 4 analysed levels of schizotypy, psychoticism or psychosis proneness, one analysed borderline personality subjects, and one analysed subjects with c9orf72-mutation induced frontotemporal dementia
The illusion was increased among individuals with eating disorders or with borderline personality, two conditions commonly considered as situated on the psychotic-affective spectrum, and after pharmacological treatment with the psychosis-inducing agents ketamine and amphetamine
Summary
An evolutionary approach to understanding human psychiatric disorders compels determination of what cognitive adaptations have evolved along the human lineage, and how these traits become subject to effects of extreme development, trade-offs, mismatches and genetically based conflicts that lead to maladaptation. Among the most remarkable of human-evolved and human-elaborated psychological phenotypes are those associated with the self, which encompass self-awareness, spatial-temporal location, identity, agency and embodiment [2,3,4,5] These semantic and psychological self-constructs are subserved by neural structures and networks that, over the past decade, have become sufficiently well understood for empirical analyses that connect neurological adaptations to psychiatric conditions, in the context of quantitative, predictive models of neural activity The term ‘autism’ derives from the Greek autos (2#Ò ), for ‘self’ The core of this disorder, as originally described by Kanner [7], is extreme aloneness, and psychological separation between one’s self and others: The outstanding ‘pathognomonic’, fundamental disorder is the children’s inability to relate themselves in the ordinary way to people and situations from the beginning of life. There is from the start an extreme autistic aloneness that, whenever possible, disregards, ignores, shuts out anything that comes to the child from the outside
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