The long sixth finger illusion: The representation of the supernumerary finger is not a copy and can be felt with varying lengths

Curved sixth fingers: Flexible representation of the shape of supernumerary body parts

“…In our previous study (Cadete & Longo, 2022), for the main questionnaire item "It felt like I had six fingers on my left hand", there was an effect size of dz= 1.26 in the analysis comparing the condition for normal sixth finger length with the control condition, which consisted in stroking the seen little finger instead of the empty space near the reflected hand, and does not induce the illusion.…”

“…To investigate whether supernumerary body parts can have independent features or are represented as a copy of the existing limbs, we tested if we can embody an illusory supernumerary sixth finger with varied lengths. We used the continuous sixth finger illusion, changing the visual-tactile stimuli to induce a sixth finger with altered lengths (Cadete & Longo, 2022). When stroking of the space where the sixth finger was perceived was made longer or shorter than actual finger size, participants reported feeling a long and a short sixth finger, respectively, and judged its length with half or double the size of the average little finger.…”

Examining constraints on embodiment using the Anne Boleyn illusion.

“…All participants were naïve to the purpose of the study, and no participants had been in any other body illusion experiments. A previous study using the Anne Boleyn illusion showed a large effect size for the primary illusion ( d = 1.76; Cadete & Longo, 2022). Given that we were examining variants of the illusion which may be less likely to occur, we aimed for a sample size of n = 33 to detect a medium effect size ( d = 0.6, alpha level 0.05; power 0.9) (G*Power 3.1; Faul et al, 2007).…”

“…Changes in perceived body size have been examined with the Pinocchio illusion (De Vignemont et al, 2005; Lackner, 1988) and other body illusions (Byrne & Preston, 2019). Recently, Cadete and Longo (2022) found that the Anne Boleyn illusion could change the perceived length of the illusory sixth finger, making it feel shorter or longer than a fifth finger. In a separate study, Cadete et al (2022) also found that participant can embody and perceive a curved sixth finger curved laterally toward the fifth finger at 180°.…”

Impact of supplementary sensory feedback on the control and embodiment in human movement augmentation

“…When the hand behind the mirror is touched while they see touch being applied to empty space next to their hand, questionnaires show that people report experiencing a supernumerary sixth finger being touched 174 , 175 . When the visual stimulus moves along a distance bigger or smaller than the tactile stimulus, people experience supernumerary fingers of different lengths 176 , with no change to the perceived length of the five actual fingers. This shows that the supernumerary finger is not just a copy of an existing one, but a distinct body part owning independent features.…”