Inducing Illusory Ownership of a Virtual Body

Immersive virtual environments can break the deep, everyday connection between where our senses tell us we are and where we are actually located and whom we are with. The concept of 'presence' refers to the phenomenon of behaving and feeling as if we are in the virtual world created by computer displays. In this article, we argue that presence is worthy of study by neuroscientists, and that it might aid the study of perception and consciousness.

In the "rubber-hand illusion," the sight of brushing of a rubber hand at the same time as brushing of the person's own hidden hand is sufficient to produce a feeling of ownership of the fake hand. We shown previously that this illusion is associated with activity in the multisensory areas, most notably the ventral premotor cortex (Ehrsson et al., 2004). However, it remains to be demonstrated that this illusion does not simply reflect the dominant role of vision and that the premotor activity does not reflect a visual representation of an object near the hand. To address these issues, we introduce a somatic rubber-hand illusion. The experimenter moved the blindfolded participant's left index finger so that it touched the fake hand, and simultaneously, he touched the participant's real right hand, synchronizing the touches as perfectly as possible. After approximately 9.7 s, this stimulation elicited an illusion that one was touching one's own hand. We scanned brain activity during this illusion and two control conditions, using functional magnetic resonance imaging. Activity in the ventral premotor cortices, intraparietal cortices, and the cerebellum was associated with the illusion of touching one's own hand. Furthermore, the rated strength of the illusion correlated with the degree of premotor and cerebellar activity. This finding suggests that the activity in these areas reflects the detection of congruent multisensory signals from one's own body, rather than of visual representations. We propose that this could be the mechanism for the feeling of body ownership.

Where are my hands? The brain can answer this question using sensory information arising from vision, proprioception, or touch. Other sources of information about the position of our hands can be derived from multisensory interactions (or potential interactions) with our close environment, such as when we grasp or avoid objects. The pioneering study of multisensory representations of peripersonal space was published in Behavioural Brain Research almost 30 years ago [Rizzolatti G, Scandolara C, Matelli M, Gentilucci M. Afferent properties of periarcuate neurons in macaque monkeys. II. Visual responses. Behav Brain Res 1981;2:147-63]. More recently, neurophysiological, neuroimaging, neuropsychological, and behavioural studies have contributed a wealth of evidence concerning hand-centred representations of objects in peripersonal space. This evidence is examined here in detail. In particular, we focus on the use of artificial dummy hands as powerful instruments to manipulate the brain's representation of hand position, peripersonal space, and of hand ownership. We also review recent studies of the 'rubber hand illusion' and related phenomena, such as the visual capture of touch, and the recalibration of hand position sense, and discuss their findings in the light of research on peripersonal space. Finally, we propose a simple model that situates the 'rubber hand illusion' in the neurophysiological framework of multisensory hand-centred representations of space.