Presence

See also: Social presence

Presence is the subjective sense of "being there" in a virtual environment: the feeling that you are physically and spatially located inside the simulated world rather than in the room where your body actually sits. It is one of the central goals of Virtual reality, and many researchers treat it as the quality that separates a convincing VR experience from merely looking at a screen. Presence is also one of the hardest aspects of VR to pin down, because it is a private perceptual response rather than something the hardware does directly.

A useful working definition comes from the academic literature: presence is the subjective experience of being in one place or environment even when you are physically situated in another. The user usually knows perfectly well that the experience is generated by a computer, yet still reacts as if the place and the events were real.

Presence versus immersion

Presence is often confused with Immersion, but in the research literature the two are kept distinct. Immersion is treated as an objective property of the system: the extent to which the technology can deliver sensory information and respond to the user, set by things like field of view, stereoscopic rendering, tracking quality, display resolution, and latency. Presence is the human response to that capability: the perceptual and psychological state the immersive system produces.

In this framing immersion is what the machine offers and presence is what the person feels. A more immersive system makes presence more likely, but the relationship is probabilistic rather than guaranteed: the same hardware can produce strong presence in one person or scenario and weak presence in another. A loose intuition that is sometimes used is that immersion is the sense of being surrounded by the virtual world, while presence is the sense of being within it and treating it as the place you are in. Presence shows itself most clearly when the involuntary, reflex parts of the brain take over: when a user instinctively reaches out to grab a virtual object, refuses to step off a virtual ledge, or feels their stomach drop while standing on a virtual rooftop.

Place illusion and plausibility illusion

The most influential modern account of presence comes from Mel Slater, who argued that the single word "presence" actually bundles together two separate illusions that can be present or absent independently of each other.

The first is place illusion (PI), the illusion of being in the place depicted by the VR despite sure knowledge that you are not really there. Slater ties place illusion directly to natural sensorimotor contingencies, meaning that you perceive the virtual world through ordinary bodily actions: turning your head, leaning, crouching, and looking around objects all change what you see and hear in the way they would in reality. Six degrees of freedom head tracking and display updates that follow body movement are what make place illusion possible.

The second is the plausibility illusion (Psi), the illusion that the events being depicted are actually happening, even though you know they are computer generated. Slater identifies three things that produce plausibility: the environment reacts to what you do, virtual entities sometimes refer or respond to you specifically, and the events match what you would expect from your real-world knowledge. Plausibility is about the unfolding situation rather than the geometry of the space.

The two illusions are described as orthogonal, meaning a VR experience can deliver one without the other. When both place illusion and plausibility illusion hold at the same time, participants tend to respond to the virtual environment realistically, with the same emotional and behavioral reactions they would have in a comparable real situation. This is why presence is treated as so important: it is the precondition for VR to actually move, train, or affect people.

Body ownership and embodiment

A closely related strand of research concerns the user's sense of having a body inside the virtual world. Embodiment refers to the feeling that a virtual avatar is your own body, and it is usually produced by seeing a life-sized virtual body from a first-person perspective combined with visuomotor or visuotactile synchrony, so that the virtual body moves when you move or is touched when you are touched.

This builds on the classic rubber hand illusion, in which a person can come to feel that a fake hand is their own when they watch it being stroked at the same moment they feel a stroke on their hidden real hand. In VR the effect extends to the whole body. A well-known experiment by Slater and colleagues showed that men given a first-person view of a virtual female body that substituted for their own could experience ownership of that body, an effect known as body transfer. A first-person viewpoint of the virtual body is generally enough to trigger the illusion, and it produces stronger ownership than a third-person view. Changing the kind of body a person is given can lead to measurable physiological, behavioral, and even attitudinal changes, including reductions in implicit racial bias when light-skinned participants are embodied in a dark-skinned virtual body. A strong sense of embodiment tends to reinforce presence, because having a believable body in the scene strengthens the feeling of really being there.

How presence is measured

Because presence is subjective, measuring it is an open research problem, and three broad families of method are used.

Approach	What is measured	Examples
Self-report questionnaires	The participant's rated experience after (or during) exposure	The Witmer and Singer Presence Questionnaire (PQ) and Immersive Tendencies Questionnaire (ITQ); the Slater-Usoh-Steed questionnaire
Behavioral measures	Spontaneous, involuntary reactions to virtual events	Flinching, ducking, postural responses, reluctance to step over a virtual edge, reaching to steady oneself
Physiological measures	Bodily signals associated with arousal and stress	Heart rate, skin conductance, skin temperature

The most widely cited questionnaire is the one by Witmer and Singer, who built both a presence questionnaire and an immersive tendencies questionnaire and reported them to be internally consistent and reliable. Questionnaires are easy to deploy but have been criticised, notably by Slater, for relying on participants to introspect and rate something they may not be able to put into words.

Behavioral and physiological measures try to get around that by watching what the body does rather than asking. A landmark study by Meehan and colleagues placed participants in a "pit room," a virtual environment with a deep hole in the floor that participants had to look into or walk near, and measured their stress reactions. The work found that change in heart rate tracked the experience of presence best, with change in skin conductance a weaker indicator and skin temperature not satisfying the criteria; it also reported that adding a real physical ledge as passive haptic feedback raised presence. Physiological signals only work as presence measures when the virtual scene is designed to provoke a response, which is why height and threat scenarios are common in this research.

What builds presence

In practice, building presence means giving the brain a consistent, responsive sensory experience so that the involuntary parts of the perceptual system accept the virtual world. The main contributors are:

• Low latency. The delay from moving your head to the display updating, known as motion-to-photon latency, has to be very small. Presence in an HMD is generally considered achievable when motion-to-photon latency is below roughly 20 milliseconds; above that the world feels like it lags behind the user.
• Accurate, consistent tracking. Six degrees of freedom head tracking with sub-millimeter accuracy and no jitter lets the user perceive the scene through natural movement, which is the basis of place illusion.
• High-quality display. A wide field of view, high resolution with no visible pixel structure, low persistence to avoid motion blur, and a refresh rate of 90 Hz or more to eliminate flicker all reduce the cues that remind the brain it is looking at a screen.
• Spatial 3D audio. Sound that is positioned in three dimensions around the user, so that sources can be localised and change correctly as the head turns, adds environmental cues that match natural listening. Studies have found that spatialized audio raises both self-reported and physiological indices of presence compared with non-spatial sound.
• Interaction and reactivity. An environment that responds to the user and behaves the way the user expects supports the plausibility illusion. Being able to touch, grab, and affect objects, and seeing the world react, makes the events feel like they are really happening. Matching physical feedback, such as passive haptics, can reinforce this.

What breaks presence

Presence is fragile. Slater describes place illusion as essentially "always on" during a session but subject to occasional failures, which he and others call breaks in presence (BIPs): moments when the user is jolted out of the virtual world and back into awareness of the real room. Common causes include:

• Tracking glitches and lag. Dropped frames, stutter, jitter, or visible latency contradict the user's movements and snap them out of the experience. High latency is also a primary cause of cybersickness.
• Simulator sickness. Nausea, disorientation, and eye strain caused by sensory conflict not only ruin comfort but also reduce presence; people who report more simulator sickness symptoms tend to report less presence.
• Breaks in plausibility. When the virtual world behaves in a way that violates expectations, for example a character that fails to notice the user or an object that reacts wrongly, the plausibility illusion collapses. Slater notes an asymmetry here: if place illusion fails it usually re-forms, because the sensorimotor conditions that produced it are still in place, but if plausibility fails it tends not to recover, because once a user realises an event is not really responding to them they lose belief in it.
• Intrusions from the real world. Bumping into a real wall, snagging a cable, or hearing a real-world noise reminds the user where their body actually is and breaks the illusion.

Augmented reality

Presence matters for Augmented reality as well, though the goal is different. In AR the aim is the seamless integration of virtual content into the physical environment, so that virtual objects appear to genuinely occupy the user's real space. As in VR, the virtual content has to align with the user's expectations and stay correctly registered to the world as the user moves. One way of describing fully convincing AR is that the user should not be able to tell virtual objects apart from real ones.

See also

• Immersion
• Virtual reality
• 3D audio
• Motion-to-photon latency
• Simulator sickness

Further reading

• Henrik Ehrsson Lab publications on body ownership and embodiment: http://www.ehrssonlab.se/publications.php