Place Illusion and Plausibility Mel Slater Department of Computer Science, University College London Catalan Institute of Research and Advanced Studies (ICREA) University of Barcelona www.event-lab.org
Outline • A different look at immersion • Concepts • Experiments on Virtual Body • Discussion
Immersive Systems Immersive systems • can be characterised by the sensorimotor contingencies (SC) that they support. SCs refer to the • actions necessary for perception of the virtual world – you know to change gaze direction by moving your head
Immersive Systems Immersive systems can in principle be put into partial • orders. A system of order 1 has SCs that simulate those of physical reality. • (‘Simulation’ implies that it is not exact or necessarily complete). A system of order i < j can be used to simulate the SCs of j, but j • cannot be used to simulate the SCs of i. • For example, a desktop system can in principle be simulated within a Cave. Any immersive system of whatever order will have a set of • SCs that enable perception of the virtual world within that system. Desktop might be joystick and keys •
Qualia • Describe what it feels to see the colour red. • To see red and to know you’re seeing red • Eg, you may stop at a traffic light automatically, without consciously being aware of seeing the ‘redness’ of the light • But if you choose to become aware of it, you can • This is what philosophers and neuroscientists who study ‘consciousness’ call QUALIA.
Concepts – Place Illusion • Place Illusion ( π ) - refers to the illusion of being in the virtual place even though you know for sure that you are not there. It is a strong illusion • One that you cannot stop except by shutting out sensory input • You cannot stop it by ‘knowing’ that it is false • It gives rise to a feeling, a qualia of “ ‘being there’ but • knowing that you are not there” OMNIMAX, Disneyworld, very large display screens, • immersive virtual reality … all give rise to this illusion. It is like actually being there – it is not a concept that applies • to physical reality.
Place Illusion • π is not difficult to achieve • When perceiving an environment through sensorimotor contingencies that are approximately the same as in physical reality • “The basic claim of the enactive approach is that the perceiver’s ability to perceive is constituted (in part) by sensorimotor knowledge (i.e., by practical grasp of the way sensory stimulation varies as the perceiver moves).” (Action in Perception, A. Noe, p12). • With SCs similar to those of physical reality • PI will occur automatically – like many visual illusions nothing special has to be done to experience it
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π Can Break • When people perceptually probe they will inevitably break the illusion • Latency failure in head tracking • Look at something closer than resolution permits • Touch something and feel nothing • Feel something that should not be there (weight of helmet, cables, Cave walls) • SCs provide a framework in which π might occur • It is not deterministic • Our experience is that it quickly recovers
PI in Non-Immersive Systems Is there PI in Second Life or computer games (as used on • desktop systems)? If we consider PI as bound to normal SCs for perception • then the answer is ‘no’. Note that it is possible to simulate the playing of a • computer game itself inside an immersive system. In an immersive system you can pull up a chair, sit down, • switch on the computer, etc.. The limitation is today’s technology, but in principle it is possible. • Systems form a simulation hierarchy PI with respect to the immersive system and the response • to the desktop system must be qualitatively different.
Presence in Non-Immersive Systems • “The basic claim of the enactive approach is that the perceiver’s ability to perceive is constituted (in part) by sensorimotor knowledge (i.e., by practical grasp of the way sensory stimulation varies as the perceiver moves.” (Action in Perception, A. Noe, p12). • This does also happen in computer games, but the type of sensorimotor knowledge is different. To turn look to the left you use a joystick or key board presses, you • don’t turn your head. A whole new set of knowledge is established (eventually becoming • unconsciously competent), which establishes a particular SC set. Of course typically this relies on knowledge from normal everyday • SC (but it need not, the mappings could be arbitrary in principle – it only affects the time to expertise, or the learnability of the system).
PI in Non-Immersive Systems Does PI occur in the scenario of the computer game? • Yes, but it is conditional, your actions are bound to the SCs • that are possible. You can be present in SL, but it is at a different conceptual and • physical level. Let’s call an immersive system - first order. In a first order • system you can simulate physical reality and your SCs in relation to perceiving it are normal. A second order system can be simulated within a first order • system. It will have its own SCs. In any order system there can be engagement, interest, flow, … • Only in a first order system can there be PI. •
The Body The body is the fulcrum of ( π , ψ ) • In an immersive virtual reality there is something very simple that you can do… • Look down, and you will see your (virtual) body. • This is an action within the continuum of SC normal to that type of • reality. There is a continuity between your body and the rest of the • environment. Using normal actions you • know how to change your sensory stimulation as a function of your actions, and one action with no special status or discontinuity from other actions, is to look at yourself.
The Body • In a second order system such as a computer game, there are also means to look at ‘yourself’, within the set of SC defined by that world. • Now there is a two level relationship – you can in a first order system see your body, • while using normal SC actions be looking at yourself in the second order system.
The Body In other words within each SC system that is tied to a • particular form of virtual reality, there is in principle a way to perceive yourself as being part of that reality, • since you use the sensorimotor knowledge to look at yourself in a way no different from how you would look at anything else. Hence you must ‘be there’ in that reality. But since the realities form a hierarchy, • and the first order systems are simulations of physical • reality, it is only a first order system that can in a physical sense, • have the property to induce ‘being there’ (Place Illusion).
The Body Putting it another way: PI in a first order system is not a • mental inference, an emotional feeling, an extrapolation, a memory, an illusion of being there – it a property of the physics of the situation. There are lawful relationships between movements and • sensory stimulation, including the perception of your physical embodiment (self). There will be PI if those laws are similar to those of • physical reality, and your body is perceivable as being part of this reality. PI is not the whole story … •
Plausibility • P lau s ib i lity ( ψ ) is your subjective probability that observed events are ‘true’ • It is the illusion that situations and events are what they appear to represent • They are ‘really happening’ as they appear to be happening • Can apply in real life (‘con tricks’), theatre, cinema, etc and in mixed/virtual reality
ψ Can Break • Incongruence of events / appearances with expectation • Lack of correlation with own actions • Lack of consistency in The Stepford Wives, 1975, Bryan Forbes (Paramount) cause-effect relationships • …
Brain as Bayesian Inference Engine • People do not really believe • that they are in the virtual place ( π ) • Nor that what might appear to be happening is true ( ψ ) • These are illusions, • probably formed by the brain as a Bayesian engine, continually updating probabilities in the light of evidence • Cognitive knowledge overrides these automatic inferences possibly dampening down responses.
Hypothesis • Our major hypothesis is that • When π and ψ both occur • People will respond as if what is happening is real. • This is not deterministic – it says that π and ψ set a framework in which this might occur – but they are easily broken through probing • correlation between participant actions and responses is necessary – for both π and ψ
• Can a virtual body or limb feel like it is your own? Question
The Rubberhand Illusion (Botvinick and Cohen, Nature, 391: 756 1998) Armel and Ramachandran Proc Biol S ci. 270:1499, 2003
Rubber Hand Illusion • Can the same idea work when the hand and even the visual stimulation is entirely virtual? • Slater M, Perez-Marcos D, Ehrsson HH, Sanchez-Vives MV. Front Hum Neurosci. 2008;2:6. Epub 2008 Aug 20.
Virtual Arm Illusion
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