Disturbances of Embodiment in Schizophrenia and Psychosis: A Systematic Review of Rubber Hand Illusion Research

Abstract

Disturbances of self-experience have long been regarded as a core feature of schizophrenia. Classical psychopathological accounts described disruptions in the sense of self, agency, and bodily ownership as central to the disorder (Bleuler, 1911), while contemporary phenomenological approaches conceptualise schizophrenia as involving disturbances of the “minimal self” or pre-reflective sense of being a subject of experience (Sass & Parnas, 2003; Parnas & Handest, 2003). Such disturbances are thought to contribute to characteristic symptoms including passivity experiences, thought insertion, hallucinations, and impaired self-other differentiation. Understanding the mechanisms underlying altered self-experience has therefore become an important focus of schizophrenia research. The Rubber Hand Illusion (RHI; Botvinick & Cohen, 1998) has emerged as one of the most widely used experimental paradigms for investigating bodily self-consciousness and embodiment. In the classic paradigm, participants view a rubber hand being stroked synchronously with tactile stimulation applied to their own hidden hand. Under appropriate multisensory conditions, participants often experience ownership over the rubber hand. The illusion is typically assessed using subjective questionnaire measures of ownership and agency, behavioural indices such as proprioceptive drift, or both. Variants of the paradigm have expanded beyond passive visuotactile stimulation to include active visuomotor conditions designed to assess agency and action monitoring. The RHI is particularly relevant to schizophrenia because it provides an experimentally tractable model of bodily ownership and agency processes that are hypothesised to be disrupted in the disorder. Since the first application of the paradigm to schizophrenia by Peled et al. (2003), numerous studies have examined whether individuals with schizophrenia exhibit altered susceptibility to the illusion. Importantly, the literature has become increasingly heterogeneous, with studies differing in the paradigms employed (passive visuotactile versus active visuomotor), the outcome measures used (questionnaire ratings versus proprioceptive drift), the symptom dimensions examined, and the patient samples recruited. These methodological differences may contribute to inconsistencies in the reported findings and complicate interpretation of the overall evidence base. Furthermore, active and passive paradigms may index partially dissociable processes relating to ownership and agency, while subjective and behavioural measures may capture different aspects of embodiment. Although a previous small meta-analysis (of three studies) concluded that there was little evidence for a body ownership deficit in schizophrenia (Shaqiri et al., 2018), the evidence base has expanded considerably in recent years, including studies employing active movement paradigms and more sophisticated assessments of embodiment and agency. Consequently, an updated systematic review and meta-analysis is warranted. The present review aims to synthesise the available evidence concerning embodiment and action in schizophrenia using RHI paradigms, quantify the magnitude of observed effects where possible, and examine whether findings differ according to paradigm type, outcome measure, and symptom profile.