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Virtual reality


Six-Finger Illusion: better understanding our perception of our body and our avatar.

Inria - J.C. Moschetti © Inria - J.C. Moschetti

Is it possible to create the illusion that our right hand has six fingers? More generally, can we believe in structural changes to our bodies in a virtual reality environment? What mechanisms do we use to adopt our virtual body or avatar? If our virtual avatar is different to our real self, how do we perceive the differences and how can they influence our self-perception and behaviour? The Hybrid and MimeTIC teams at the INRIA Rennes – Bretagne Atlantique research centre set up a new experimental platform to examine these questions.

A new experimental platform to study “virtual embodiment” and avatars in virtual reality environments

In order to study this topic, Hybrid and MimeTIC developed a novel virtual reality experimental platform that enables a user equipped with a VR headset to observe their virtual body (or “avatar”), and in particular, their (virtual) hands. These hands can be displayed with properties that are either the same as or different to their own hands. Specifically, for this project, a simulated sixth finger was added to the right hand, in order to study how this radically and structurally different hand would be assimilated by the user as though it were their own.

Persuasive initial results

This study revealed that participants readily adopted this virtual hand, despite it being structurally significantly different to their real hand. 
The techniques used to “generate the illusion of a sixth finger” appear to work well. For example, participants demonstrated good behavioural responses:  when asked to raise their virtual sixth finger, more than 95% did indeed attempt to move their hand and a real finger (in most cases the ring finger). The study questionnaires also revealed good subjective results: participants reported a strong feeling of control over the sixth finger and a strong sense of adoption of the six-fingered hand.

Understanding the perception, representation and adoption of the body in virtual reality

Virtual reality technologies are shedding new light on fundamental topics relating to the perception, representation and adoption of our bodies. VR can be used to simulate and test highly original experimental situations in perfectly controlled, safe conditions. 
Over the past few years, as these technologies have been perfected and become accessible to a wider audience, surprising new findings have been published regarding the concept of virtual embodiment. Mel Slater (Barcelona Univ., Spain),  Jeremy Bailenson (Stanford Univ., USA) and Betty Mohler (Max Planck Institute) have all published interesting research in this field. 
This study is consistent with this movement. It innovates in realistically simulating a major structural modification of the body in virtual reality in order to observe and study its impact on the subject’s sense of adoption and control of their body.

Study sequence

From a technical perspective, the platform consisted simply of a VR video headset and a computer on which to run the virtual reality simulation. To smooth the “transition” between the real and virtual environments, a number of “tangible” elements are present in both worlds, including a wooden shelf that users can see (as it is replicated in the virtual scene) and touch (by placing their hand on it).
The six-fingered hand was modelled by a professional computer graphics designer. The extra finger was inserted between the ring finger and little finger. Its movement is controlled (by interpolation) based on the movements of the two adjacent fingers: when the user curls the ring finger and/or the little finger, the sixth finger also curls.
To enhance the illusion, the experiment prompts the user to “feel” the sixth finger. To achieve this, the researchers applied a technique used to create another well-known illusion: the “rubber hand” illusion. Using a small paintbrush, a researcher brushes each finger in turn. The paintbrush and its movements are visually reproduced in the virtual scene.
To simulate the effect on the sixth finger, the virtual paintbrush brushes the sixth finger in the image in the VR, headset, while in reality the researcher simultaneously brushes the user’s ring finger.  Thus, the user experiences a physical, tactile sensation as the virtual paintbrush touches the sixth finger.
The researchers used several types of observation to gauge the illusion produced, including subjective questionnaires and behavioural reaction assessments. They asked subjects to raise their virtual fingers one by one, and in particular, measured their movements when asked to raise the sixth finger: would subjects remain immobile?  Which finger would they raise?
The scientists studied and analysed several conditions, in particular a second “degraded” experimental condition in which the sixth finger was present but stiff and inanimate, and was never brushed with the paintbrush... as if the finger were “dead”. This served as a control condition to check that the sense of control and adoption was significantly weaker when the sixth finger did not move and was not subjected to tactile stimulation.

Context and applications

The macro-context is the emergence of affordable VR headsets (Occulus Rift, HTC Vive, Playstation VR) and virtual reality applications designed for consumers: virtual communities, social media, video games, new media and interactive movies, etc. Major industry players including Sony, Google, Samsung, Facebook and Microsoft are investing massively in this area.  The consumer VR market is forecast to be worth tens of billions of dollars by 2020.
In most such applications, users are represented by an avatar, whose physical and morphological attributes may be very different to the user’s own. Numerous other VR applications also represent the user as a virtual avatar, including teaching and physical training applications. Another area in which VR is used is film-making with special effects, in which actors are very often required to embody characters with very different bodies to their own: animals, mutants, etc. 
The need to research the topic of VR representations (avatars), their adoption, their ability to influence self-perception and self-representation or even the user’s behaviour in a VR environment has never been more critical.

This novel experimental platform is now available for third-party use, enabling further research in fields concerned by the issues raised, including neuroscience, cognitive science and psychology. 
It may also be useful to medical personnel wishing to study the clinical potential of VR technologies.

Keywords: Six finger illusion Avatar Virtual reality Hybrid Anatole Lecuyer INRIA Rennes - Bretagne Atlantique Mimetic