Immersion, Presence, and Performance in Virtual Environments
2.1 Immersion and Presence
In reports of earlier studies we have made a distinction between immersion
and presence (Slater, Usoh and Steed,
1995). Immersion refers to what is, in principle, a quantifiable description
of a technology. It includes the extent to which the computer displays are
extensive, surrounding, inclusive, vivid and matching. The displays are
more extensive the more sensory systems that they accommodate. They
are surrounding to the extent that information can arrive at the
person's sense organs from any (virtual) direction, and the participant
can turn towards that direction receiving the appropriate directional sensory
signals. The notion of surrounding also includes the greater the reproduction
of the natural modes of sensory presentation (visual and auditory stereopsis
for example). They are inclusive to the extent that all external
sensory data (from physical reality) is shut out. Their vividness
is a function of the variety and richness of the sensory information they
can generate (Steuer, 1992).
Vividness is concerned with the richness, information content, resolution
and quality of the displays. Finally, immersion requires that there is match
between the participant's proprioceptive feedback about body movements,
and the information generated on the displays. A turn of the head should
result in a corresponding change to the visual display, and, for example,
to the auditory displays so that perceived sound direction is invariant
to the orientation of the head. Matching requires body tracking, at least
head tracking, but generally the greater the degree of body mapping, the
greater the extent to which the movements of the body can be accurately
reproduced.
Immersion also requires a self-representation in the VE - a Virtual Body
(VB). The VB is both part of the perceived environment, and represents the
being that is doing the perceiving. Perception in the VE is centred on the
position in virtual space of the VB - e.g., visual perception from the viewpoint
of the eyes in the head of the VB, an egocentric viewpoint.
Our general hypothesis is that presence is an increasing function of two
orthogonal variables. The first variable is the extent of the match between
the displayed sensory data and the internal representation systems and subjective
world models typically employed by the participant. Although immersion is
increased with the vividness of the displays, as discussed above, we must
also take into account the extent to which the information displayed allows
individuals to construct their own internal mental models of reality. For
example, a vivid visual display system might afford some individuals a sense
of "presence", but be unsuited for others in the absence of sound
(Slater, Usoh and Steed, 1994).
The second variable is the extent of the match between proprioception and
sensory data. The changes to the display must ideally be consistent with
and match through time, without lag, changes caused by the individual's
movement and locomotion - whether of individual limbs or the whole body
relative to the ground.
Immersion, in our view, is therefore an objective description of what any
particular system does provide. Presence is a state of consciousness, the
(psychological) sense of being in the virtual environment, and corresponding
modes of behaviour. Participants who are highly present should experience
the VE as more the engaging reality than the surrounding world, and consider
the environment specified by the displays as places visited rather than
as images seen. Behaviours in the VE should be consistent with behaviours
that would have occurred in everyday reality in similar circumstances.
It is important to realise that this model operates at many levels. Considering
the visual display as an example, at the most basic level the important
factors may be field of view, resolution, colour resolution, binocular disparity.
Corresponding behaviours from the point of view of presence are those autonomic
responses governed by the visual system such as vergence and accommodation
(Ellis, 1991). At a higher level
the realism of the content of the visual display may be considered - such
as whether objects behave in accordance with physical laws. Corresponding
behaviour with respect to presence may be concerned with observable gross
involuntary behaviour - such as the looming effect (when an individual ducks
in response to a flying object), or the experience of vertigo in response
to a virtual visual cliff. At the highest level such features as the realism
of the illumination may govern people's voluntary responses, such as evaluations
of their sense of "being there", or the "realism" of
the virtual environment.
2.2 Presence and Task Performance
It is sometimes argued that it is important to study presence because of
the potential relationship between presence and performance. For example,
in (Barfield, Sheridan, Zeltzer,
Slater, 1995) we find:
"Not only is it necessary to develop a theory of presence for virtual
environments, it is also necessary to develop a basic research program to
investigate the relationship between presence and performance using virtual
environments. ... we need to determine when, and under what conditions,
presence can be a benefit or a detriment to performance? ... When simulation
and virtual environments are employed, what is contributed by the sense
of presence per se?"
The question of the relationship between presence and performance goes to
the heart of why presence is important. The issue is not really that of
whether presence itself enhances performance. For example, an individual's
performance in word processing is usually superior using a modern point-and-click
user interface than under UNIX using "vi" - not of course because
of presence, but because of the former's superior user interface. In our
view presence is important because the greater the degree of presence, the
greater the chance that participants will behave in a VE in a manner similar
to their behaviour in similar circumstances in everyday reality. Hence if
an IVE is being used to train fire-fighters or surgeons, then presence is
crucial, since we want them to behave appropriately in the VE and then transfer
knowledge to corresponding behaviour in the real world. There could obviously
be cases where presence would diminish performance, just as being present
in a situation in real life using a machine with a poor "user interface"
similarly affects performance adversely.
Hence it is posing the wrong question to consider whether presence per
se facilitates task performance. Rather presence brings into play "natural"
reactions to a situation (which may or may not have something to do with
efficiency of task performance) - and the greater the extent to which these
natural reactions can be brought into play the greater that presence is
facilitated, and so on. It isn't really a question of how good the performance
is, but rather how it is grounded in presence.
We would nevertheless expect to find an association between presence and
performance for some tasks - precisely those tasks that benefit from immersion.
For the purposes of this study we postulated that increased "immersion"
would lead to improved "task performance" (to be defined below
in the context of this experiment). This is because the task involved comprehension
and memory of a complex three dimensional structure and events relating
to that structure, and we considered that performance would be enhanced
by an egocentric, stereo view based on a head-tracked HMD compared to an
exocentric screen based view. Since our overall hypothesis is that both
performance and presence are enhanced by immersion, we would therefore not
be surprised to find an association between performance and presence.