Immersion, Place Illusion and Plausibility

Mel Slater, guru of the concept of presence,  has just released a paper explaining his theories about immersion and presence. (Warning: You really have to be wide awake to read it =)

He is explaining the concepts of Place Illusion (PI) and Plausibility Illusion (Psi) and gives a good definition of Immersion.

Place Illusion

Place Illusion is the sense of ‘being there’ (and nothing else), often called ‘presence’. He uses the word ‘illusion’ because you know that what you’re experiencing is not real. If you’re physically moving and your perception of the virtual environement changes (because a the system has updated the rendering to your new position), PI is maintained.

PI can be different in different modalities [visual, auditory…] . (…) Provided there are not inconsistencies between these modalities, there can be PI in one domain without there being PI in another. This point is
important because we regard PI not as a cognitive but as a perceptual phenomenon. (…) When PI breaks, it can quickly recover. (…)

Can PI occur in computer games as used on desktop systems? To what extent can you have a feeling of ‘being there’ with respect to a desktop virtual reality system? (…) The answer is ‘you cannot’. (…)

In the case of a desktop system the situation is quite different, the feeling reported as ‘being there’ if it comes at all is after much greater exposure, requires deliberate attention, and is not automatic – it is not simply a function of how the perceptual system normally works, but is something that essentially needs to be learned (…) PI may still be reported, but this is as a consequence of additional creative mental processing. It does not refer to the same qualia as for the first order systems.


Immersion is a technical capability of a VR system and nothing else. A user will experience PI if the system has a good level of immersion. Immersion provides the boundaries within which PI can occur.

We describe immersion not by displays plus tracking, but as a property of the valid actions that are possible within the system. Generally, system A is at a higher level of immersion than system B if the valid actions of B form a proper subset of those of A.” (…)

Parameters that determine the quality of the experience include the graphics frame rate (how long it takes to graphically render the currently visible portion of the virtual environment), the overall extent of tracking (apart from head tracking, how much of the rest of body movement is tracked), tracking latency (how long it takes before a head movement results in the correct change in displayed image), the quality of the images (how great are the brightness, spatial, colour and contrast resolutions), the field of view (how great is the visual field of view compared to what is possible in normal vision, and how much do the displays surround the participant), the visual quality of the rendered scene (how much do objects appear geometrically to appear like what they are supposed to depict, and how realistic is the illumination), the dynamics (how well does behaviour of objects conform to expectations) and the range of sensory modalities accommodated (and within each sensory modality the fidelity of its displays). (…)

By definition, one system would be more ‘immersive’ than another if it was superior on at least one characteristic above – for example, higher display resolution, or more extensive tracking, other things being equal. (…)

In this framework displays and interactive capabilities are inseparable. Consider for example the issue of display resolution. At first sight this may appear to have nothing to do with interaction (…), but in fact if the participant wants to examine an object very closely, then the extent to which this is possible will be limited by the resolution of the display. Relatively low visual display resolution will mean that the normal action of bringing an object closer in vision by moving the body, head and eyes closer to it, will
fail earlier than it would in physical reality, and at different times in different systems.

It should be noted that the level of immersion is completely determined by the physical properties of the system.

Plausibility Illusion

Plausibility Illusion is the illusion that what is apparently happening is really happening. This results from a sense that your actions have effects on the VE, that other events of the VE affect your sensations, and that these events are credible. If a virtual woman looks at a shy guy in the eyes, his heart rate might increase, he will blush etc. People with fear of public speaking will react with anxiety if speaking to a virtual audience. It is important to realise that Psi does not require physical realism.

Psi is therefore an illusion akin to PI – one that occurs as an immediate feeling, produced by some fundamental evaluation by the brain of one’s current circumstances – ‘is this real?’ Of course, at a higher cognitive level participants know that nothing is ‘really’ happening, and they can consciously decide to modify their automatic behaviour accordingly. (…)

If you are there (PI) and what appears to be happening is really happening (Psi) then this is happening to you! Hence you are likely to respond as if it were real. We call this ‘response-as-if-real’

This is why exposure therapies for phobias work.

When Psi breaks, it is unlikely to recover. Once you have ceased to accept the ‘reality’ (…) Psi does not usually reform again. (…)

Of course the participant knows that it is not happening, and this cognitive knowledge can certainly dampen down or entirely change responses away from the instinctive first reactions (to smile back at the character smiling at you, but then thinking – why am I doing this, there is no one there? – and consequently stop smiling and ignore the character). (…)

Psi is far more difficult to achieve than PI. (…) Psi requires a credible scenario and plausible interactions between the participant and objects and virtual characters in the environment – with very little room for error. A conclusion is that the area of Psi is now a more fruitful and challenging research area than PI.

Virtual Body

The body is a focal point where PI and Psi are fused. The concept of body ownership takes here a fundamental role. Several experiments have shown that you can transfer yourself partially in another body through perceptive illusions. When you are in a VE, you transfer your mind inside a virtual body.


There are also several interesting quotes :

As has been mentioned before (Pausch et al. 1996) [IVR] is much like both cinema and television in their early days, that were used essentially as a medium for theatre. Our approach is to treat virtual reality as something quite new with its own unique conventions and possibilities that provide a media where
people respond with their whole bodies, treating what they perceive as real.

There is a fundamental difference between an immersive and non-immersive system: In an ideal immersive system it is possible in principle to fully simulate what it is like to go into a non-immersive system.

Virtual reality can not only transform your sense of place, and of reality, but also the apparent properties of your own body.

This is an interesting framework to work with. But it makes me think that we are really far from understanding and achieving perfect presence. That’s also what makes our field interesting ! It’s not only interesting for VR, it’s also fascinating to use VR to understand the human being, how it perceives and acts in real life.

The paper has a lot more interesting information (the notion of virtual body for example) so you might want to take time to read it entirely.

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  1. Fascinating paper. Mel and his team have been working on VR and perception for a very long time now (I remember delivering their very first VR system, and helping them learn how to program it) and they have continued to amaze me with the quality of research that they do.

  2. Thanks for the article and link to Mel Slater’s publication.

    I think it will never really be possible to achieve a totally realistic virtual environment. Just think of force rendering. But it doesn’t really have to be perfect to achieve a quality where you can use it.

  3. You don’t have to get totally realistic to be believable. Anyway your brain has a quite limited perception of “reality” that it can be easily fooled by a ‘low res’ simulation.

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