A Continuous Illusion of Having a Sixth Finger

Our body is central to our sense of self and personal identity, yet it can be manipulated in the laboratory in surprisingly easy ways. Several multisensory illusions have shown the flexibility of the mental representation of our bodies by inducing the illusion of owning an artificial body part or having a body part with altered features. Recently, new studies showed we can embody additional body parts such as a supernumerary finger. Newport et al. recently reported a novel six-finger illusion using conflicting visual and tactile signals induced with the mirror box to create the illusory perception of having a sixth finger for a brief moment. In this study, we aimed to replicate this result and to investigate whether the experience of embodiment of a sixth finger could be prolonged for an extended duration by applying continuous visual–tactile stimulation. Results showed that a continuous illusion of having a sixth finger can be clearly induced. This shows that the six-finger illusion does not reflect merely a momentary confusion due to conflicting multisensory signals but can reflect an enduring representation of a supernumerary finger.

Metacognitive Awareness in the Sound-Induced Flash Illusion

In the sound-induced flash illusion, discrepant numbers of stimuli in the visual and auditory modalities are integrated to create illusory visual percepts. While this phenomenon has been extensively studied over the last two decades, relatively little is known about the role of metacognition in this illusion. Specifically, when incongruent audiovisual signals yield the same behavioral reports as conditions with congruent audiovisual signals, is confidence in the perceptual judgment the same across conditions? To probe this question, we conducted an experiment where 22 observers viewed from 1-4 flashes and 1-4 beeps on each trial, and reported three things: (1) the number of perceived flashes, (2) confidence in the judgment about the number of flashes, and (3) confidence in whether the number of beeps and flashes were the same or different. In our exploratory analyses, we paired conditions of incongruent and congruent audiovisual signals which produced the same report about the number of flashes. Results showed that in several condition pairs, confidence in the perceived number of flashes was higher for congruent audiovisual signals than incongruent audiovisual signals. For confidence judgments about whether the number of auditory and visual signals presented were the same or different, confidence ratings were similar for all condition pairs but one. These findings provide preliminary support for the hypothesis that metacognition may be able to index differences between multisensory illusions and congruent multisensory information, but (in most circumstances) may be unable to index differences in the underlying causal structure which produces the sensory signals.

Cue Integration for Continuous and Categorical Dimensions by Synesthetes

For synesthetes, sensory or cognitive stimuli induce the perception of an additional sensory or cognitive stimulus. Grapheme–color synesthetes, for instance, consciously and consistently experience particular colors (e.g., fluorescent pink) when perceiving letters (e.g.,u). As a phenomenon involving multiple stimuli within or across modalities, researchers have posited that synesthetes may integrate sensory cues differently than non-synesthetes. However, findings to date present mixed results concerning this hypothesis, with researchers reporting enhanced, depressed, or normal sensory integration for synesthetes. In this study wequantitativelyevaluated the multisensory integration process of synesthetes and non-synesthetes using Bayesian principles, rather than employing multisensory illusions, to make inferences about the sensory integration process. In two studies we investigated synesthetes’ sensory integration by comparing human behavior to that of an ideal observer. We found that synesthetes integrated cues for both continuous and categorical dimensions in a statistically optimal manner, matching the sensory integration behavior of controls. These findings suggest that synesthetes and controls utilize similar cue integration mechanisms, despite differences in how they perceive unimodal stimuli.

Body Schema Illusions: A Study of the Link between the Rubber Hand and Kinesthetic Mirror Illusions through Individual Differences

Background. The well-known rubber hand paradigm induces an illusion by having participants feel the touch applied to a fake hand. In parallel, the kinesthetic mirror illusion elicits illusions of movement by moving the reflection of a participant’s arm. Experimental manipulation of sensory inputs leads to emergence of these multisensory illusions. There are strong conceptual similarities between these two illusions, suggesting that they rely on the same neurophysiological mechanisms, but this relationship has never been investigated. Studies indicate that participants differ in their sensitivity to these illusions, which provides a possibility for studying the relationship between these two illusions. Method. We tested 36 healthy participants to confirm that there exist reliable individual differences in sensitivity to the two illusions and that participants sensitive to one illusion are also sensitive to the other. Results. The results revealed that illusion sensitivity was very stable across trials and that individual differences in sensitivity to the kinesthetic mirror illusion were highly related to individual differences in sensitivity to the rubber hand illusion. Conclusions. Overall, these results support the idea that these two illusions may be both linked to a transitory modification of body schema, wherein the most sensitive people have the most malleable body schema.

How Long is Your Arm? Using Multisensory Illusions to Modify Body Image from the Third Person Perspective

Updating body representations from the 3rd person perspectives (3PP) seems to require viewing the real body, unlike when viewing from a 1st person perspective. Here, 3PP updating was investigated through induction of a physically impossible multisensory illusion in which participants viewed real-time 3PP video of themselves having their arm pulled until it stretched to twice its normal length. The illusion elicited the subjective experience that the participant's own arm had been stretched and caused an overestimation of reaching distance, although actual reaches were unaffected. Multisensory illusions from the 3PP can alter body image when applied to real bodies.