The current growth in usage of virtual environments has given to some unique experiences and is mostly applied in specialist domains such as psychometry education and entertainment. Even though the direct benefit from VR in these domains are small they create a high impact.
- Dimensions Of Presence
- Factors Affecting Presence
- Measuring Presence
- Different presence leads to different experiences
This survey is about the 'Presence', which plays a major role in defining the success of VR. Most virtual reality researches are related to the concept of presence. Substantial research has been carried out to identify the factors that contribute to presence and develop methods for measuring it.
We are covering
- What constitutes presence and why is important
- Why should we measure it `
- Techniques available for measuring it
- Designing Experiences based on presence
Slater mentions, in literature the illusion that we are located inside the rendered virtual environment has been referred to as telepresence or presence - the “sense of being there” in the environment depicted by the virtual reality system. The origin of the concept of presence was rooted in teleoperator system, and is the feeling of being at the place of a remote physical robot that the user is operating (Minsksy 1980). In early 1990’s this idea was adopted by virtual reality, where instead of being at a remote physical environment, the users feel that they are present in a virtual environment with a sense of being at the place depicted by the system(Held and Durlach 1992). Witmer and Singer stated presence as the subjective experience of being in one place or environment, even when one is physically situated in another.
A deep sense of presence
Kent Bye, believes that it is important to understand how to combine an agency with embodiment to create a deep sense of presence. Bye states that we don’t just think with our minds but with our complete bodies, with which we don’t know how we are constructing the reality, but we have senses that do something with the holistic system of body and emotions. In his work he refers the Dale’s cone of experience, which is a visual model that is composed of eleven stages starting from concrete experiences at the bottom of the cone then it becomes more and more abstract as it reaches the peak of the cone. The arrangement in the cone is not based on its difficulty but rather based on abstraction and on the number of senses involved. He also says that, If we could mimic all of these things to our senses, then virtual reality is able to give a direct experience and is capable of converging objectivity and subjectivity. This in turn allows us to learn and retain information in a deeper and more intense way. And when creating virtual reality, we need to think about the different dimensions of what makes a good experience.
Figure 1 : Dale’s cone of experience
The concept of presence has been classified in different ways. Heeter classifies presence into 3 categories : personal, social and environmental presence. Here personal presence is a measure of the extent to which the users feel like they are a part of the VE when compared to the social presence which measures the extent to which other beings also exist in the VE. Social presence has a direct correlation with the interaction or conversation the user may have with the animate objects. Similarly environmental presence measures the extent to which the environment itself is able to react/interact with the user.
Another model developed by Bye classifies presence into four categories:
Active Presence : Active presence deals with expressing the agencies. The mechanics and the intuitiveness impacts active presence. The proximity of agencies and physical activity as well affects active presence.
Social and Mental: Mental presence deals with cognitive and social intelligence, languages, mental abstractions and social interactions. Some of the sectors that support social presence are puzzles, facial expressions and micro emotions.
Embodied Presence: The degree to which the body is in another world. The feeling of who you become and where you are some of the sectors that are affected by embodied presence.
Emotional Presence: This deals with the degree to which different kinds of emotions are experienced throughout the VE system.
Figure 2 : The four categories of presence in Kent Bye’s model
Biocca, distinguishes presence as self presence (Is This Body Really “Me”?), social presence and physical presence, and studied the effect of embodiment on the sensation of these presence types.
While the concept of presence has been widely discussed, only a few of the researchers have attempted to measure and relate it to its contributing factors. The following table shows the parameters known to influence the presence and the empirical findings.
(FPS, the frequency at which
the computer generated images changes
in response to user action
or to other dynamic aspects of simulation)
within the VE was significantly less
using an update rate of 5 and 10 Hz
when compared to update
rates of 20 and 25 Hz 
sense of presence in immersive VR
increases when chess pieces are arranged in
meaningful positions as compared to random
arrangements. This finding corroborates their
thesis that “focusing one’s attention on a
meaningful stimulus set supports one’s sense of presence” 
Table 1 : Parameters known to influence the presence and the empirical findings
The ground-breaking theoretical work by Seridian, Held and Bulach, suggested the factors thought to affect the sense of presence. Witmer & Singer  have categorized the identified factors and developed items for measuring presence, which was heavily based on the former work. The factors are:
- Control factors (CF) dealing with the user's interaction with the VE.
- Sensory factors (SF) deals with various aspects of the sensorial stimulation, such as multimodality,
- Distraction factors (DF), degree of distraction by objects and events in the real world
- Realism factors (RF), the degree of realism of the portrayed VE
They suggest that presence is dependant not only on immersion, but also of the user's involvement which is defined as "a consequence of focusing one’s energy and attention on a coherent set of stimuli or meaningfully related activities and events".
Figure 2 : Factors Hypothesized to contribute to a sense of presence by Witmer & Singer
Based on the above factors, the Witmer and Singer build 2 questionnaires, the first one is the Presence Questionnaire (PQ), which directly measures the degree to which subjects are sensitive to the factors presented above. The following are examples of questions presented in PQ, How much were you able to control events? (CF) How much did the visual aspects of the environment involve you? (SF) How aware were you of events occurring in the real world around you?(DF) How inconsistent or disconnected was the information coming from your various senses? (RF)
The second one is Immersive Tendency Questionnaire (ITQ) which tries to evaluate their tendency to become involved or immersed. The questions presented are like
"Are you easily disturbed when working on a task?"
Do you ever become so involved in doing something that you lose all track of time?
Slater  argues strongly that this questionnaire measures the user’s perception of system properties, rather than psychological presence. The problem is that two people may have different responses in the same environment although the system control parameters are the same.
The Slater-Usoh-Steed (SUS) questionnaire is based on variations on one of three themes: a sense of being in the VE, the extent to which the VE becomes the dominant reality, and the extent to which the VE is remembered as a place. Participants rate each of the six questions on a 1-to-7 scale and the number of 6 and 7 responses is counted to produce the score for the SUS. An example question is " During the time of the experience, did you often think to yourself that you were actually in the office space?
Some researchers have identified problems in which the subjective measures produce unstable and inconsistent responses depending on a participant’s prior experience . Many have suggested problems with questions that directly ask participants their sense of presence, because this term is not well known to the general public and requires either the participant to guess or the researcher to explain in detail either of which could influence the participant’s response.
However, they can be used in conjunction with other measures, such as the subject's behavior and /or performance in a VE, as well as physiological measurements. This point is emphasized in , noting that body movement has to be coherent with the spatial structure of the environment the subject experiences as "reality”.
Most of the researchers on presence mention Slater’s famous work which involves a "pit room". In this experiment the participant walks into a virtual room with an 18m precipice at its center. In fact, the person stands and walks in a system, in which a wooden ledge is positioned at the place where the virtual edge lies in the VE. This meant to corroborate visual signals, indicating that they are standing in a room with a dangerous precipice. Slater  interestingly note that heart rate increases when subjects reach the virtual precipice, suggesting that physiological measures might be an indicator of presence (see also ).
A number of physiological indicators including skin conductance and temperature, muscular tension, change in heart rate, and pupillometry have been suggested as presence measures . The idea is that these indicators can deliver continuous information regarding the effects of specific environmental stimuli or events experienced in a VE. As these measures vary from person to person, the experiment must measure the base level first and produce results based in the changes compared to that base line
One of the studies shows the usefulness of pupil dilation as a marker of autonomic responses and the feasibility of evaluating this in VR hardwares that are available commercially. The participants showed greater skin conductance and pupil responses to the spider, and the impact depended on the closeness of the stimuli to the participant, with a stronger reaction when the spider was near to the virtual self.
Recently, studies have started using fMRI (functional magnetic resonance) to study brain activity associated with the sensation of presence .
Simple reflex actions may show that participants feel as if they are in the virtual environment. The actions may include reaching for a virtual object, interacting with avatars, turning away or closing the eyes when presented with an approaching object [11, 12]. Another example, if a virtual ball was tossed at the user’s head and the user ducks, that would indicate a sense of presence in the virtual environment.
For instance, in the "pit room" mentioned earlier, the fact that users walk carefully when close to the visual cliff seems a reasonable indicator that they feel present in the VE. Slater performed such a study in which participants were shown a real radio. Then they were asked to enter into VE with the radio at the same position. During the experiment, the actual radio was shifted to different positions and switched on, and the participants were requested to point to the location of the radio. Slater expected that the more present the participant, the more likely he would point to the location of the virtual radio rather than the real radio. He found a significant correlation between the participant’s behaviour and the presence questionnaire.
Freeman in his study examined the participant’s postural response as a possible measure of presence. The experiment involved the degree to which participants swayed back and forth
while watching a video shot from the hood of a rally car racing around a track. The premise was that the more a participant felt present in the video, the more he would feel as if he were moving and the more postural adjustments he would make. A magnetic tracking device measured the participants’ movements.
A simple rough evaluation of the user performance is not enough to judge presence in virtual environment. If locomotion has to be evaluated, this has to be carried out by a precise analysis and modeling of sensori-motor coupling level. One example of such behavior is oculomotor behavior. Ocular behaviors have long been suggested as presence measures . Measures of visual system behavior may provide a wealth of information regarding attention, alertness, eye-trackers, and electro-oculograms (EOGs) have the potential to be useful tools in the isolation of presence invoking stimuli. These visual indicators may serve to identify which elements of the VE capture attention.
Effects of Real reality over Virtual Reality:
To understand the effect of presence (actual/stimulated )an experiment conducted by UBC’s department of psychology was observing the result of contagious yawning. The phenomenon in which people tend to yawn contagious during events reminding them of yawning. However it has also been observed that people yawn less when they are aware there are others around. In the experiment by UBC participants were allowed to witness yawning events in a Virtual environment and has produced about 38% positive results. This number matches the results of actual reality.
In another event where people were aware that there is a researcher present in the room with the participants, it was observed that they yawned less. This could further be explained to prove that the notion of actual reality directly affects the presence in VE.
We happen to observe multiple possibilities from Heeter, Bye and Biocca to categorise presence. The most elaborate being Bye’s model which has been categorised into four parts: Active, Social and Mental, Embodied, and Emotional. The theoretical work by Seridian, Held and Bulach suggest four factors : control, sensory distraction and realism factors which are thought to affect the sense of presence. These factors helped scientists like Witmer and Singer build ways in which presence could be measured. One of the simplest being a series of questionnaires that measure the degree and sensitivity of presence to the participants. Other methods include measuring sensory motor reflexes, behavioural measure and physiological measure.
From these methods it has now become possible to quantify presence. It is also understood that how presence is directly connected to experience in VE. The ability to measure and quantify something yields new possibilities including the ability to control presence. A curious likelihood would to try dynamic presence. An ability to control and change the immersive experiences based on the user’s current state of mind. This would lead in building much richer storytelling experience.
The abstract concept of categorising presence still leads to a gap where it becomes difficult to categorise the intensity of each category. It would be interesting to devise methods to help in understanding intensity to a micro level, which would in turn give granular level control to the VE.
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- Historical Context of Virtual Reality - Kent Bye's SVVR 2017 Keynote
- Studying Social Presence in VR
- The effect of update rate on the sense of presence within virtual environments
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