Immersion
Immersion is the sense of being surrounded by a virtual environment, and, in the technical literature on virtual reality, the objective extent to which a system can deliver that sensation. There are different levels of immersion. Virtual Reality Head-mounted Displays can create immersion up to a degree, and as the technology improves the experiences tend to become more immersive.
A distinction that many researchers draw, and that matters for the rest of this article, is between immersion and Presence. In the framing associated with Mel Slater, immersion is a property of the equipment: it is what the hardware and software actually deliver to the senses, and it can be measured and compared between systems. Presence is the psychological response to that delivery, the subjective feeling of being in the virtual place even though you know you are not. Put simply, immersion is something a system has, and presence is something a person feels. The two usually rise together, but they are not the same thing, and treating them as interchangeable is a common source of confusion.
Immersion as an objective system property
Slater and Sylvia Wilbur set out an early version of this idea in 1997. They described immersion as the extent to which a display system can deliver an inclusive, extensive, surrounding, and vivid illusion of an environment to a participant.[1] Inclusive refers to how well the system shuts out the physical world, extensive to how many sensory channels it addresses, surrounding to how much of the space around the user it fills rather than presenting a single framed view, and vivid to the richness and resolution of what each channel delivers.[1]
In later work Slater defined immersion in terms of sensorimotor contingencies, the implicit rules we know about how to use our bodies to perceive: turning your head to see what is beside you, leaning in to look more closely, reaching out to a point of view. Under this account, one system is more immersive than another when it supports a superset of the valid perceptual actions of the other. As Slater puts it, the valid actions of the less immersive system "form a proper subset of those of" the more immersive one.[2] A headset that lets you bend down and look under a virtual table is more immersive than one that only lets you look around from a fixed point, which in turn is more immersive than a flat monitor. This gives immersion an objective ordering: you can in principle simulate a less immersive system inside a more immersive one, but not the other way around.[2]
Because immersion lives in the equipment, it can be stated as a list of measurable parameters. The properties that commonly come up include:
| Property | Why it affects immersion |
|---|---|
| Field of view | A wider field of view fills more of the user's vision and engages peripheral perception. Human horizontal vision spans roughly 200 to 220 degrees, while most consumer headsets cover only about 90 to 120 degrees, so a narrow field of view can feel like looking through a tube. |
| Resolution | Higher pixel density makes the image sharper and reduces the visible gaps between pixels (the screen-door effect), so detail in the scene reads as real rather than rendered. |
| Frame rate | A higher and steadier refresh rate keeps motion smooth. Dropped or low frame rates produce judder that is both visible and a known trigger for discomfort. |
| Latency | The delay between a head movement and the matching update on the display. Lower is better; in VR the figure people aim for is under about 20 milliseconds from motion to photon.[3] |
| Head and body tracking | Tracking the position and orientation of the head, and ideally the hands and body, so the viewpoint and limbs respond the way the real ones would. This is what actually supplies the sensorimotor contingencies above. |
| 3D audio | Sound placed at a specific point in space rather than fixed to the left or right channel, so a voice or footstep seems to come from a real direction and distance. |
Sensory channels
Immersion is built channel by channel, and the more of them a system addresses, and the more faithfully, the more immersive it is.
The visual channel usually does the most work. Stereoscopic images with a wide field of view, high resolution, and correct head-coupled motion are what most people mean when they call an experience immersive.
The auditory channel matters more than it gets credit for. 3D audio, also called spatial audio, processes sound so that it appears to originate from a location in three dimensions, the way it does in the real world. It does this with head-related transfer functions (HRTFs), which model how a sound arriving from a given direction is altered by the shape of the head and ears before it reaches each eardrum.[4] When audio cues line up with what the user sees, they reinforce the sense of being in the scene; when a sound is glued to the headphones instead of the world, the effect falls apart.
The haptic channel, the sense of touch and force, is the least developed of the three in consumer hardware. Most headsets convey it only as controller vibration. Richer haptics, such as gloves or vests that apply pressure or resistance, can deepen immersion further, but they remain uncommon outside research and specialist setups.
What increases immersion
In practice, raising immersion means improving the parameters in the table above: a wider field of view, higher resolution and frame rate, lower latency, more accurate and lower-lag tracking, the addition of positional sound, and support for movement of the whole body rather than just the head. Each of these expands either the fidelity of a sensory channel or the set of perceptual actions the system can honor, which is exactly what Slater's account predicts will make a system more immersive.
What breaks immersion
Immersion is fragile, and several failures pull a user out of the experience.
Tracking errors are among the worst. If the tracked viewpoint or a virtual hand drifts, jitters, or lags behind the real movement, the brain notices immediately, because the sensorimotor link it relies on has been violated.
A low or unstable frame rate has a similar effect. Motion that should be smooth turns into judder, which is distracting in its own right and, when the displayed motion does not match what the body feels, a cause of sickness.
High latency is closely related. When the delay between moving and seeing the result grows too large, the mismatch undermines the illusion and can cause simulator sickness, the cluster of symptoms (nausea, disorientation, eye strain) that comes from a conflict between the visual and vestibular senses.[3] In VR the comfort target is usually under about 20 milliseconds of motion-to-photon latency, and delays past roughly 50 to 60 milliseconds sharply raise the risk of discomfort.[3] Because simulator sickness forces people to stop, it is one of the more decisive ways immersion fails.
Beyond the technical faults, anything that reminds the user of the real world tends to break immersion: bumping into furniture, a notification, a controller running out of battery, or graphics and physics that behave in obviously wrong ways.
How immersion relates to presence
Immersion and Presence are linked but distinct. Immersion is the objective capability of the system; presence is the subjective experience it produces. In the model Slater developed, the experience of presence has two parts. The first is the Place Illusion, described as the strong illusion of being in a place in spite of the sure knowledge that you are not there.[2] The second is the Plausibility Illusion, the illusion that the events apparently happening are really happening, even though you know for certain that they are not.[2] Higher immersion makes both illusions easier to sustain, mainly by supporting the natural perceptual actions and coherent responses on which they depend.
A point that often surprises newcomers is that presence does not require photorealism. Slater showed that people can respond to a virtual scene as if it were real even when the graphics are plainly artificial: in a virtual restaging of the Milgram obedience study, participants showed real anxiety toward a character that could never be mistaken for a real person in appearance or movement.[2] So a more immersive system does not automatically feel more present, and a modest one can still produce a strong sense of presence if the sensory cues it does provide hang together. This is roughly the relationship the original stub described: in VR, being surrounded by the virtual world (immersion) is easier to achieve than the feeling of genuinely being inside it (presence), which is a bit more elusive. The distinction was stated plainly by Slater in 2018 in a paper whose title sums it up, "Immersion and the illusion of presence in virtual reality."[5]
See also
References
- ↑ 1.0 1.1 Slater, Mel; Wilbur, Sylvia (1997). "A Framework for Immersive Virtual Environments (FIVE): Speculations on the Role of Presence in Virtual Environments". https://direct.mit.edu/pvar/article/6/6/603/18157/A-Framework-for-Immersive-Virtual-Environments.
- ↑ 2.0 2.1 2.2 2.3 2.4 Slater, Mel (2009). "Place illusion and plausibility can lead to realistic behaviour in immersive virtual environments". https://pmc.ncbi.nlm.nih.gov/articles/PMC2781884/.
- ↑ 3.0 3.1 3.2 "Motion-to-photon latency". https://vrarwiki.com/wiki/Motion-to-photon_latency.
- ↑ "Generic HRTFs May be Good Enough in Virtual Reality. Improving Source Localization through Cross-Modal Plasticity". 2018. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5801410/.
- ↑ Slater, Mel (2018). "Immersion and the illusion of presence in virtual reality". https://bpspsychub.onlinelibrary.wiley.com/doi/10.1111/bjop.12305.