The projected hand illusion: component structure in a community sample and association with demographics, cognition, and psychotic-like experiences

Abstract

The projected hand illusion (PHI) is a variant of the rubber hand illusion (RHI), and both are commonly used to study mechanisms of self-perception. A questionnaire was developed by Longo et al. (2008) to measure qualitative changes in the RHI. Such psychometric analyses have not yet been conducted on the questionnaire for the PHI. The present study is an attempt to validate minor modifications of the questionnaire of Longo et al. to assess the PHI in a community sample (n = 48) and to determine the association with selected demographic (age, sex, years of education), cognitive (Digit Span), and clinical (psychotic-like experiences) variables. Principal components analysis on the questionnaire data extracted four components: Embodiment of "Other" Hand, Disembodiment of Own Hand, Deafference, and Agency-in both synchronous and asynchronous PHI conditions. Questions assessing "Embodiment" and "Agency" loaded onto orthogonal components. Greater illusion ratings were positively associated with being female, being younger, and having higher scores on psychotic-like experiences. There was no association with cognitive performance. Overall, this study confirmed that self-perception as measured with PHI is a multicomponent construct, similar in many respects to the RHI. The main difference lies in the separation of Embodiment and Agency into separate constructs, and this likely reflects the fact that the "live" image of the PHI presents a more realistic picture of the hand and of the stroking movements of the experimenter compared with the RHI.

Deficits in Agency in Schizophrenia, and Additional Deficits in Body Image, Body Schema, and Internal Timing, in Passivity Symptoms

“…For example, embodiment of the “other” hand is proportionally related to disembodiment of one’s real hand, with the total embodiment of both being equal to one single hand (44, 45). A similar balance also exists with the sense of agency (46). Disembodiment (of limbs) and reduced agency (over actions) are clinical features of persons with passivity symptoms, so performance on such measures are of particular interest.…”

“…Participant were exposed to each condition once (3 min each), with the order of presentation being counter-balanced across participants. A 20-item questionnaire assessing the subjective experience of the illusion was administered after each condition (46); adapted from Ref. (43).…”

“…Each item was rated on a 7-point Likert scale ranging from −3 (strongly disagree) to +3 (strongly agree). A recent PCA (46) identified that the following components could be extracted from the questionnaire, assessing body image (“Disembodiment of own hand” and “Embodiment of the ‘Other’ hand”), and the sense of agency (“Agency over the ‘Other’ hand,” and “Loss of agency over own hand”) in both synchronous and asynchronous conditions. Table 1 shows the 20 items (Embodiment items 1–8, Disembodiment 13–17, Agency 9–10, and Loss of agency 11–12).…”

The Effect of Visual, Spatial and Temporal Manipulations on Embodiment and Action

“…These restrictions can be overcome by using a live video image of the participant’s hand. In the video-version of the RHI (the “projected hand illusion (PHI)”; Graham et al, 2015), the rubber hand is replaced by a live video image of the participant’s own hand. As well as allowing precise manipulation of the synchrony of seen and felt brush strokes, the PHI allows the congruency between seen and felt movements to be manipulated.…”

“…In the PHI, an unmanipulated video image of the participant’s hand is typically either projected onto the surface of a table (e.g., Tsakiris et al, 2006) or shown via a display screen embedded within a table (e.g., Graham et al, 2015). A disadvantage of this set-up is that the viewed hand is in a different plane to the real hand, which may require additional computation factors for the brain to overcome.…”