Mr Fantastic Meets The Invisible Man: An Illusion of Invisible Finger Stretching

Mr Fantastic from The Fantastic Four can stretch his body beyond what is physically possible, whereas The Invisible Man can walk around unseen with light passing straight through his body. Fictional comic book characters often achieve impossible feats with the human body. But are these physically impossible bodies actually possible to experience in the human brain? The rubber hand illusion (Botvinick & Cohen, 1998), in which ownership over the fake hand is induced by synchronously touching the seen fake hand and the hidden real hand, demonstrates the importance of multisensory integration for body ownership. Top-down constraints governing bodily experience have also been identified, which prevent feelings of ownership (opposed to embodiment for tool-use, de Vignemont, 2011) over noncorporal objects (Tsakiris, Carpenter, James, & Fotopoulou, 2010). However, the extent to which appearance constrains body ownership has been challenged. We can experience different size and shaped bodies as our own, including invisible hands (Guterstam, Gentile, & Ehrsson, 2013) and comic book style elongated limbs (Preston & Newport, 2011, 2012). However, such stretching illusions typically use sophisticated technology to provide realistic visual feedback, which may not be necessary. This study (approved by the University of York Psychology ethics committee) further examined the extent of top-down constraints on body ownership by testing an invisible finger stretching illusion, in which synchronous visuotactile stimulation applied to the real fingers and an area of empty space elicits an illusion of elongating invisible fingers (Figure 1).

OPEN IN VIEWER

Figure 1.

Participants’ (N = 21, 1 M; two left handed; mean age = 19) right hand was hidden in a fixed location underneath a wooden platform with a cape covering their forearm (shoulder to wrist). In each 60-s trial (two per condition), the experimenter consecutively gripped the base of each finger (not the thumb) with her thumb and index finger and gently pulled while sliding her grip to the fingertip (∼1 Hz with ∼1 s between each stimulation). At the same time, she synchronously or asynchronously mimicked identical actions in empty space above the platform at a faster velocity extending beyond the length of the actual hand (depicted above left to right). Pulling tactile stimulation (Newport et al., 2015) was implemented to elicit feelings of the fingers stretching rather than simply owning longer fingers.

The invisible finger stretching illusion successfully manipulated perceived finger length (Figure 2). However, participants did not agree that their hand felt invisible (median = 0) and stretching but not invisibility correlated with proprioceptive drift. These results extend previous findings that we can experience ownership over a volume of empty space equivalent to the real hand size (Guterstam et al., 2013), and that we retain ownership over illusory stretched (visible) fingers (Newport et al., 2015).

OPEN IN VIEWER

Figure 2.

The invisible finger stretch illusion was measured with questionnaire responses (a) and proprioceptive drift (in separate trials), for which participants pointed to the felt position of the index finger before and after stimulation. Participants were asked to consider both lateral and vertical information in their judgment (b). Following synchronous touch, participants felt like their fingers were stretching (Bonferroni critical p = .016) (c) and judged the position of their right fingertip as closer to the illusory elongated fingertip compared to asynchronous touch (data from one participant excluded due to recording error) (d). These effects were not found for control questions or for questions asking about perceived hand invisibility (c). In the synchronous conditions, proprioceptive drift correlated with feelings of stretching (e) but not invisibility (f).

Feeling ownership over hands that we cannot see is an everyday occurrence and may underpin the experience of the invisible hand illusion (Guterstam et al., 2013). Indeed, full-body illusions using virtual reality suggest body transparency inhibits ownership (Martini, Kilteni, Maselli, & Sanchez-Vives, 2015). Alternatively, the illusion of elongating fingers may have just been a more salient experience and thus distracted participants’ from hand invisibility. Although stretchy fingers are also impossible, throughout life our bodies generally get bigger. Accordingly, it has been demonstrated that we can feel greater ownership over larger compared to smaller fake hands (Pavani & Zampini, 2007) and legs (Romano, Llobera, & Blanke, 2016), which may suggest that our body representation is particularly adaptable to increases in body size. Although perceptual reductions in body size have been observed using full-body illusions, in which naturalistic body proportions are maintained (Preston & Ehrsson, 2014).

We know that our brains are continually integrating sensory signals to create a coherent experience of the body and that this bodily experience is constrained by top-down knowledge of body appearance. However, it seems that despite these constraints our brain’s representation of our body is highly adaptable, to the extent that we can experience stretchy fingers like Mr Fantastic without direct vision of the body, even if we do not actually feel invisible like The Invisible Man.