Abstract
The rubber hand illusion (RHI) is a perceptual illusion, whereby a fake hand is recognized as one’s own hand when a fake hand and felt real hand are stroked synchronously. RHI strength is mainly assessed using a questionnaire rating and proprioceptive drift (PD). PD is characterized by the proprioceptively sensed location of the participant’s own hand shifting toward the location of the fake hand after RHI. However, the relationship between the two measures of hand ownership and location remains controversial due to mixed findings: some studies report correlations between them, while others show that they are independent. Here, we demonstrated significant PD without RHI using delayed visual feedback. In this RHI study, video images of the fake hand were delivered to the subjects, and four delay intervals of visual feedback (80, 280, 480, and 680ms) were introduced. In four of six conditions, the delay interval was fixed throughout the condition. In the other two conditions, four delays were delivered in a predetermined order (i.e., serial condition; higher predictability) or in a pseudo-random order (i.e., random condition; low predictability). For the four conditions with a fixed delay, the questionnaire ratings and PD declined significantly when the delay interval exceeded circa 300ms. In both the serial and random conditions, no illusory ownership of the fake hand was reported in the questionnaire. In contrast, greater PD was found in the random condition but not in the serial condition. Our findings suggest that hand ownership and localization are caused by distinct multisensory integration processes.
Highlights
A sense of body ownership is an experience of the body as a part of the self (“our body is our own”), which is of critical importance to self-consciousness (Gallagher, 2000)
The current study examined how the predictability of visual feedback delay under rubber hand illusion (RHI) affects ownership and perceived location of the hand
In the four conditions with fixed delays, ownership ratings showed that RHI was induced when nearly synchronous visuo-tactile stimulation was delivered in the 80 ms condition, and longer delay intervals beyond 300 ms abolished RHI (i.e., 480 and 680 ms conditions)
Summary
A sense of body ownership is an experience of the body as a part of the self (“our body is our own”), which is of critical importance to self-consciousness (Gallagher, 2000). Because the RHI strength from the questionnaire correlates with the PD magnitude, earlier studies assumed that hand ownership and proprioceptive localization of the hand have a common underlying process of multisensory integration (Botvinick and Cohen, 1998; Tsakiris and Haggard, 2005). Rohde et al (2011) found that greater PD without a sense of ownership over the fake hand occurred during intermittent asynchronous stroking but not during prolonged asynchronous stroking. They argued that PD was mainly caused by attenuation effects of prolonged asynchronous stimulation on visuoproprioceptive integration of the hand (e.g., Holmes et al, 2006), rather than by the facilitatory effects of synchronous stimulation. Intermittent asynchronous stroking interrupts the accumulation of evidence, which leads to greater PD
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