Availability of synchronous information in an additional sensory modality does not enhance the full body illusion

Abstract

The Full body illusion (FBI) is an illusion in which participants experience a change in self-location to a body that is perceived from a third-person perspective. The FBI is usually induced through experimenter generated stroking but can also be induced through self-generated stroking. In four experiments (three preregistered) we compared a self-generated stroking induction condition to a self-generated movement condition, where the only difference between conditions was the presence or absence of touch. We investigated whether the illusion reflects an all-or-nothing phenomenon or whether the illusion is influenced by the availability of synchronous information in an additional sensory modality. As a prerequisite, we investigated whether the FBI can also be induced using just self-generated movement in the absence of synchronous touch. Illusion strength was measured through illusion statements. Participants reported an equally strong illusion for both induction methods in Experiments 1, 2 and 3. In the third experiment, we additionally measured the time of illusion onset. Like the illusion strength measures, the illusion onset times did not differ between the two induction methods. In the fourth experiment participants only completed the self-generated movement condition. Again, they reported the FBI, demonstrating that the findings of Experiments 1, 2 and 3 were not dependent on the presence of a condition that used synchronous touch. Together, these findings confirm the hypothesis that the FBI is an all-or-nothing phenomenon and that adding additional multisensory synchronicity does not help to enhance the strength, onset time or onset probability of the illusion.

The Redundant Signals Effect and the Full Body Illusion: not Multisensory, but Unisensory Tactile Stimuli Are Affected by the Illusion

“…The wording of the illusion statements tends to be complex and previous research from our own lab suggests that participants may find it hard to capture their experiences in words when asked about them. Participants may also find it difficult to link their experiences to the statements in the questionnaire, resulting in different interpretations of the questions than intended by the experimenter (Swinkels et al, 2020b). This observation is in line with the idea that embodiment is rich and complex in nature but, at the same time, also elusive and hard to describe (Gallagher, 2006;Haggard and Wolpert, 2005;Longo et al, 2008).…”

“…Not only do experimenters use different scales, there also seems to be little agreement about which questions measure the illusion and which measure experimenter demand (e.g., Botvinick and Cohen, 1998;Kalckert and Ehrsson, 2012;Kokkinara and Slater, 2014;Lenggenhager et al, 2007;Maselli and Slater, 2013;Thür et al, 2019). Some studies use interviews or free recall in addition to illusion questionnaires to obtain a better insight into the illusion experienced by participants (Altschuler and Ramachandran, 2007;Longo et al, 2008;Swinkels et al, 2020b). Even though participants find it hard to capture their experiences in words when asked about them, previous research in our lab suggests that interviewing the participants may partially resolve some of the problems with questionnaires (Swinkels et al, 2020b).…”

“…Some studies use interviews or free recall in addition to illusion questionnaires to obtain a better insight into the illusion experienced by participants (Altschuler and Ramachandran, 2007;Longo et al, 2008;Swinkels et al, 2020b). Even though participants find it hard to capture their experiences in words when asked about them, previous research in our lab suggests that interviewing the participants may partially resolve some of the problems with questionnaires (Swinkels et al, 2020b). More specifically, in addition to obtaining insight in different characteristics of the illusion and specific experiences participants may have had, interviews may shed light on the interpretation of the questions and the usage of the scale and may in that way help to elucidate whether an illusion was experienced or not (see Note 1).…”

“…To create a multisensory stimulus these stimuli could also be presented simultaneously. During a FBI, both visual stimulation that is presented on the virtual body and tactile stimulation that is presented on the real body are experienced at the location of the virtual body (e.g., Lenggenhager et al, 2007;Swinkels et al, 2020b). This is not the case when no illusion is induced; in the asynchronous condition visual and tactile stimuli are typically experienced at separate locations, i.e., at the location of the virtual body and at the real body respectively.…”

“…To induce the illusion, the participant wears a head-mounted display (HMD) that shows a live feed of a connected camera that is positioned approximately 1.5 m behind the participant. The FBI can be induced by an experimenter who provides synchronous visual and tactile stimulation by stroking the participants on the back (Aspell et al, 2009;Lenggenhager et al, 2007), by means of a robotic stroking device (Salomon et al, 2013) or by the participants themselves using a robotic stroking device (Hara et al, 2014), neck stroking (Swinkels et al, 2020a) or stroking movements without touch (Swinkels et al, 2020b). Multisensory integration of synchronous signals originating from the different modalities is thought to underlie the illusion (Botvinick and Cohen, 1998;Ehrsson, 2012;Ehrsson et al, 2005).…”