The Core of Presence:
Presence as Perceptual Illusion
Abstract
Progress in understanding presence is inhibited by the
fact that we are unable to agree what it is we are talking about. What one
researcher means by presence is not the same as what another means, and from
this stems confusion about how to evaluate models of presence, how to
measure it, and how it relates to other psychological phenomena such as
mental imagery, attention and emotional engagement. If presence is a
phenomenon worthy of investigation, it has at least to be characterised in
a way that differentiates it from other phenomena already under long-term
investigation (such as those listed above). And this characterisation
should lead to ways of measurement that can, in the best case, cleanly discriminate
between changes in presence and changes in other phenomena. Over the two
decades of increasingly active presence research, several definitions of
what characterises presence have been put forward. We suggest that, of
these, there is one that meets the needs of future progress in the field:
electronically mediated presence is the perceptual illusion of being in an
external environment. The key word here is perceptual, not least because of
the implications it carries for measurement. We know how to measure the
effectiveness of perceptual illusions objectively, and we can expect to be
able to distinguish presence defined in this way from other psychological
phenomena.
1 Introduction: Is there a standard view of presence?
In a recent presentation, Biocca (May, 2003) made the
claim that presence researchers have been limited to a one-dimensional view
of presence, whereby mediated presence (in virtual environments) competes
with non-mediated presence (in the physical world) and that level of
reported presence reflects the frequency of switches between the two.
Although Biocca suggested that this is the standard model of presence, it
has to our knowledge only been given prominence as an explicit position
relatively recently (e.g. Slater et al., 2000). Be that as it may, views of
presence have definitely not been confined to a single dimension. We
ourselves first presented a two-dimensional model of presence at the Third
International Workshop on Presence in Delft in 2000 (Waterworth, E.L. and
Waterworth, J.A. 2000).
By our account, presence is primarily determined by the
balance between processing internal (conceptual) information and external
(perceptual) information, and it can be experienced in response to both the
physical (external real) and a virtual (externally modelled) environments.
In several later publications (e.g. Waterworth and Waterworth, 2001) we
expanded on this two-dimensional model and extended it to include a third
dimension. What we called the Focus dimension related to internal
(conceptual) versus external (perceptual) modelling; what we called the Locus
dimension related to presence in the physical versus a virtual world; and
what we called Sensus related to the level of conscious attention,
ranging from unconscious to fully alert.
Biocca’s latest position is actually quite close in some
ways to the one we have proposed for several years, but there are important
differences. Biocca proposes a two-dimension model, where the dimensions
are spatial updating and spatial attention. The first of these ranges from
virtual space to physical space, which is our Locus dimension (Waterworth
and Waterworth, 2000). The second discriminates between internal, mental
imagery space and external, perceptual space, which is our Focus dimension!
One key difference between Biocca’s new and our own longstanding position
is this: we insist that presence is primarily a perceptual phenomenon.
Biocca (May, 2003) identifies three poles at which
presence may be maximised – in
the real world, in a virtual world, and in an internal mental world. The
problem is that, by this account, we would always feel presence when
conscious. Presence seems to have become just another word for conscious
attention. In trying to solve the so-called book and dream-state problems
the baby of presence has been thrown out with the bathwater of conscious
attention: there is nothing left for the concept of presence to do. For us,
presence is about the present, the here and now in the physical or a
virtual world. The feeling one gets from absorption in an internal world (a
novel, a fantasy, or whatever) is quite different, which is why healthy
people almost never confuse the two (see Waterworth and Waterworth,
February 2003; Riva and Waterworth, April 2003; and below). Imagined worlds
are often not related to real time; a book can be put down, a line of
thought can be suspended until later. We suggest that presence must
be tied to the present, here and now, real time world – that is, the
perceived world of the body and its surroundings – or else we had better
stop calling it presence!
Our perspective is as follows. First, the book problem
is not a problem! It is a confusion between sense of presence and emotional
and/or intellectual engagement in internal, imagined space (Waterworth and
Waterworth, February 2003). Presence has often been confused with
engagement because of the prevalent use of questionnaires (we discuss
measurement further in section 3 below). In these cases, metaphorical
expression is mistaken for literal. When I say I can smell the sea and feel
the wind when I read Moby Dick, I do not mean that literally. I do
not perceive the sea or the wind, through any sense modality. The text is
so engaging, the expression so vivid, that it is almost as if I do.
My intellect and my emotions are engaged as if I were perceiving it, as
if I were present, but I am not present and I do not feel presence.
Second, dream states are not like other cases of
internal modelling. We do not normally know, at the time, that the
environment we seem to be experiencing while dreaming is not real. In fact,
it is so real to us that our motor systems are immobilised while we dream,
otherwise we might damage ourselves and those around us In the rare cases
that this defence fails, the results are shocking: we may wake up in a
state of paralysis (failure to turn the defence mechanism off, or we may
act out deeds totally against our normal waking nature (failure to turn the
defence mechanism on); see for example, Ohayon et al. (1999). This is not
the case when we imagine a situation, whether while reading a book or not.
When awake, we do not confuse what we conceive in imagination with what we
perceive as the external world. It is our sense of presence that supports
the making of this distinction.
Both of these “problems” for a unified view of presence,
and their solution, relate to the evolutionary role of presence. Looking to
evolution is a key step in making further progress in the field, as we have
suggested in recent publications (Waterworth and Waterworth, February 2003;
Riva and Waterworth, April 2003).
2 An evolutionary view of presence
In its earliest evolutionary form presence was the
sense that something was happening outside the organism in the here and
now, something that could affect the organism, as opposed to being part of
the organism. Initially, this may have been based in sensation (in proto
consciousness) of something acting on the organism’s boundary with the
environment (Humphrey, 1992). Later on in evolutionary (and neurological)
terms, sensation led to perception, and presence emerged as the feeling (in
core consciousness) of being in an external, perceptible world in which
things happen in relation to the organism. Later still, internal modelling
(in extended consciousness) allowed attention to be directed towards
non-present, imagined worlds, experienced as being inside the organism
(specifically, in the head: Velmans, 2000). To be useful in assessing
possible scenarios, presumably their main evolutionary purpose, these
imagined events evoke similar emotional responses as external events would,
but not the same feeling of presence.
Once we could imagine situations and events, it then
became advantageous to discriminate imagined, internal, hypothetical worlds
from perceived events in the physical, external world – a discrimination
that we suggest is based on the evolved form of presence. It is interesting
to note that there may have been an intermediate period when this
discrimination was not reliably made in this way (Jaynes, 1976, 1990), and
when internal thoughts were perceived as the commands of Gods. But as
consciousness extended to encompass both the outside world and an evolving
internal, conceptual world, the survival advantages for organisms still
reliably able to make this discrimination are obvious, and presence emerged
in its current, evolved form. By this evolutionary view, presence does not
discriminate between the real and the virtual, but between the internal and
the external.
Clearly, evolution could not have equipped us to feel the
difference between what is really present externally and what we perceive
as present because of technological mediation. We can mostly tell the
difference with existing virtual environments because of technical, or
formal, limitations in the way the environment is coupled to the organism.
But still, virtual realities do attempt to engage the organism in the same
ways that the real world does, and they are more or less successful in
this. The extent to which they evoke presence is to a large degree the extent
to which they succeed. But this is not only a matter of emotional or
intellectual engagement – which can also be stimulated by imagined
situations. We may come to confuse a virtual experience with a real one
because they are, in principle and as far as the organism is concerned, the
same. They both evoke presence, the perception of a world surrounding the
organism. And this is why virtual reality is more effective in
psychotherapy than purely imaginal techniques, at least for certain
conditions.
Our three-layer, evolutionary model of presence (Riva
and Waterworth, 2003) suggests that maximal presence arises when proto
consciousness, core consciousness and extended consciousness are focused on
the same external situation or activity. As hinted at above, proto
consciousness deals mostly with bodily orientation in the world (Damasio,
1999); in a VR the proto-presence layer is mostly addressed through body
tracking and sensori-motor coupling. Core consciousness deals with the
perceptual world of the here and now (Damasio, 1999); in a VR the
core-presence layer is addressed mostly through the vividness of the
various displays. Extended consciousness, as Damasio puts it (1999, page
195), “goes beyond the here and now of core consciousness, both backward
and forward”. In a VR, the extended-presence layer is addressed through the
content. When the other layers are integrated with core consciousness,
intense presence is experienced. But when they are not integrated, presence
is less strong (“unfocused”). This will happen if, for example, the body
tracking (proto layer) has significant deficiencies, such as lag, or if the
semantic content (extended layer) directs attention away from the display,
towards the internal world of the imagination (cf. the “book problem”).
When we imagine a past situation, a possible future
situation, or a fictional situation portrayed in a book, we experience some
of the same emotional reactions as if we were really in the situation, by
what Damasio calls the “as-if body route”. According to Damasio, this is a
key aspect of our evaluations of hypothetical situations. But, this is not,
for us, presence, since we never normally confuse the feeling of being in
an external situation with the “as if” feeling of imagining a situation.
But when extended consciousness is focused on the present, external
environment, presence is enhanced. Taken together, this accounts for the
fact that in a VR content may enhance or detract from the feeling of
presence.
Some researchers suggest that presence is a purely
formal matter, a product of the level of immersion provided by the
technology of virtual reality. Slater (2003) recently made this claim most
clearly, suggesting that this is a way to resolve the terminological
confusion surrounding presence. We obviously sympathise with this aim, and
with Slater’s position that reading a book cannot evoke a significant
degree of presence. For Slater, as for us, presence is a product of
perceiving and interacting with an external environment, whether real or
virtual. But presence arises from attending to an external world. Even if
the form of a VR is such that high presence could be experienced, by our
view it will not be if the immersant is not attending to that world, if her
mind is elsewhere – perhaps reliving a vivid experience in her imagination.
Form is necessary but not sufficient for presence.
For us, maximal presence arises from an optimal
combination of form and content. The form must provide the means for a
convincing perceptual illusion, but the content should be integrated with
(and so attract attention to) the form for the illusion to happen
convincingly. Unlike Biocca, we do not think we always feel presence when
attending to something, internal or external, nor that the internal and
external are always competitive in producing presence. On the contrary, as
we said earlier, the internal, “imaginal” content may either enhance or
detract from the overall sense of presence. An example of enhancement is a
typical computer game, where game designers strive to ensure that content
and form are well integrated. Unlike Slater (2003), we do not think that we
necessarily feel presence if the right form for a convincing perceptual
illusion exists irrespective of content.
We find some experimental support for the views
presented here in findings that presence sometimes varies with emotional
content, and sometimes with form (Olsson et al., in preparation; Waterworth
and Waterworth, in press). But this is an area where further research is
needed (and is being conducted in, for example, the EMMA project; Alcaņiz
et al., 2002). Clarifying the relation between presence and emotion is,
however, complicated by the question of how we measure presence, to which
we turn in the next section.
3 Measuring presence: more than questionnaires?
We can measure a psychological phenomenon in a variety
of ways according to our view of what it is and what drives it, and this is
reflected in the varied ways in which presence measurement has been approached.
Measurement may be based on technological adequacy, subjective reports of a
phenomenon, objective tests that depend on the existence of the phenomenon,
or physical measures of the phenomenon happening, amongst others.
If we accept the idea that presence is a perceptual
phenomenon, how do we measure it? From the perspective of experimental
psychology, it does not seem adequate merely to ask an observer whether an
illusion was experienced. Rather, we should look for a way of testing
whether features of the display materials were reported in a way that is
consistent with actually perceiving the illusion, not just imagining it
(“as if”). Familiar examples include reports that indicate that equal
stimuli are seen as unequal, that motion is perceived in stationary
displays, that ever-rising pitch is heard in continuous patterns of sound,
or that objectively straight lines are really seen as bent. It is not that
the observer can imagine these things, but that the observer actually
perceives things that way. In other words, we should not ask for an opinion
or judgement about whether an illusion is experienced, but rather
for a report that depends on the illusion being experienced.
There is an interesting parallel here with the
psychological phenomenon of synaesthesia, where a stimulus in one modality
produces a consistent sensation in that and another modality. Everyone can
probably carry out a thought experiment, where they imagine the days of the
week, say on a calendar, each having its own colour. Or, we could all do
the same with a few numbers. For example, you might imagine 1 as yellow, 2
as red, 3 as brown, 4 as blue, 5 as green, and so on, and you might do this
fairly consistently on different occasions (if you have a good memory). If
you were shown a number by an experimenter, you could name a colour, and if
you were asked if you experienced the colour, you might say that you do (in
a sense, you do, because imagining colour is an experience). But, of
course, unless you really are a natural synaesthete of the right variety,
you do not really perceive the number as coloured! This is an analogous
distinction to the one between an imagined scene and a (real or virtual)
perceived scene. A true synaesthete does not imagine colours in letters or
the days of the week, she really perceives them. So it is with all
perceptual illusions and so, we suggest, it is with presence. Imagined
presence is not presence.
The problem with questionnaires becomes clearer if we
imagine questioning people about synaesthetic experiences. It is well known
that many people who are not true synaesthetes claim to have such
experiences (Cytowik, 1989). When asked if they associate the days of the
week (or musical notes, or whatever) with colours, they will claim, not
entirely falsely, that they do. But this association is not perceptual, it
is imaginary or metaphorical. Such questions cannot distinguish between
synaesthesia and metaphorical thinking. To report such experiences of
mental imagery is simply to report “the literary mind” (Turner, 1996) in
operation. In the same way, questions about presence cannot distinguish
between metaphor and perception, a distinction we believe to be crucial to
progress in the field.
The analogy between presence and synaesthesia can be
taken further, to suggest objective tests of presence. Ramachandran and
Hubbard (2001) devised several procedures to distinguish true synaesthesia
from imagined (metaphorical) number colouring. In one example, a visual
display showed an array of many black “5”s, with a shape (say, a triangle)
of equal-sized black “2”s embedded in it. If you are not synaesthetic it
will take you a long time to identify what and where the shape is. For a
number-colour synaesthete, the shape perception is instantaneous, because a
triangle (formed by the “2”s, seen as, say, red) will “pop out” against a
background of “5”s (seen as, say, green). It is not possible to fake
synaesthesia with this test, and there is no danger of its being confused
with the results of a vivid imagination!
A similar test for presence as perceptual illusion
might go like this. An identical room, as far as possible, might be
described in text and created in a virtual reality. When the items in the
room were “viewed”, in imagination or in the VR, from a certain perspective
they would form a “hidden” shape of some kind. In the case of the written
text, finding the hidden shape would be a slow and mentally demanding
process (if possible at all, which is doubtful). In the VR, the shape would
“pop out” automatically if the observer is attending to the environment.
This test obviously depends on the difference between conceiving in the
imagination and perceiving in the environment, between being there and
imagining you are there.
If presence is, as we suggest, the feeling of
involvement with the here and now of the present environment, then an
obvious problem with questionnaire administration is its retrospective
nature. Observers may not remember the ongoing experience accurately. By
their nature and the time of administration, the questions must tap the
contents of extended consciousness, what is recalled from the experience,
not the core experience itself. We know that memory is affected by emotion
in complex ways; remembered experiences may not be accurate reflections of
the experience at the time it occurred. As far as possible, we should
measure experiences through behaviour as it occurs, which is what we are
trying to achieve with some sort of “perceptual pop-out” test.
From the perspective of neuro-psychology, and thus
taking a non-dualistic stance on the nature of subjective experiences, we
would expect that if presence really is different from other psychological
phenomena it should be possible, at least in principle, to observe a
specific type of brain activity that is reliably associated with other
observations and subjective reports that suggest that the phenomenon is
taking place. For neuro-psychology, if the phenomenon cannot be revealed in
brain images, at least in principle, it does not really exist. We suggest that
it should be possible, through functional brain imaging, to distinguish
between imagining a textually described scene and experiencing a perception
of a real or VR scene, in line with our suggestion that the former is not a
case of presence, while the latter two both are.
On the other hand, we do not think it likely that the
difference between presence in a virtual environment and presence in the
physical one would be revealed through functional brain imaging. Since they
are both cases of the same thing: perceptual attention directed to a world
outside the organism. In our terms, they are both cases of presence, and so
should not be distinguishable in this way. Attempts to detect shifts
between the real and the virtual are in our view doomed to failure,
although non-specific discontinuities of perceptual attention might be
revealed.
4 Conclusions
If presence is worth researching, it must have
measurable effects not wholly explainable in other terms. If presence is a
distinctive psychological state, it will have a distinctive profile in
terms of brain activity; a profile which can be detected through brain
scanning – at least in principle. We should also be able to devise
objective psychological tests of presence that do not relying on subjective
opinions.
We suggest that, of the various ways of viewing
presence that have been discussed, only one fits the bill. At its core,
presence is the feeling we get from attending perceptually to the present
world (in both time and space) outside ourselves. Mediated presence is
primarily the perceptual illusion of being in an external, sharable world.
It is not an internal, imaginary “thought experiment”. As our three-layer
model of presence makes clear (Riva and Waterworth, April 2003), presence
is often more than just perception, and mediated presence more than just a
perceptual illusion. Unlike the classical, visual illusions found in
psychology textbooks, contributions from other layers of consciousness can
either contribute to, or detract from, the strength of the illusion of
being present.
At its core, presence – in physical or virtual
environments – is perceptual. The perception of an environment is not the same thing as imagining
the environment. If we could have the same experience in an imagined a room
as in a VR or a real room - if they are all equally cases of presence - why
should we spend money on expensive and still imperfect VR equipment? Why
not just employ a good writer? Why would we be researching presence? And
why would virtual reality be more effective in psychotherapy than purely
imaginal techniques? It is sometimes said that VR works because the world
is virtual. In some sense, this is true, but it tends to lead to the non
sequitur that the imagined is the same as the virtual. We are all interested
in presence not because reality is virtual, but because – for the organism
if not the intellect – VR is real, in a way that mental imagery is not.
As a research community, we need to agree on a
characterisation of presence that supports progress in the field. Presence
is still a vague concept; researchers in the area agree that there is
something important conveyed by the term, but differ widely on exactly what
that something is. The main area of contention, we suggest, is how
inclusive or exclusive the definition should be. As a community, we can
characterise presence however we like, but a view that does not distinguish
presence from other psychological phenomena, such as emotional engagement,
imagination, attention, or consciousness in general will not do the job –
it is too inclusive to be useful. For our current project EMMA (Alcaņiz at
al., 2002) we have adopted the following definition as a basis for research
on the relationship between mediated presence and emotion, in the context
of mental health interventions: Presence is the subjective sensation of being there in a mediated
environment yielding a perceptual illusion of non-mediation.
The shared, social aspect of perceptual worlds is, we
suggest, a feature of presence that follows from this definition, and which
is also compatible with our evolutionary account of its origins. As
suggested earlier, it is slow and laborious to extract details from purely
mental spaces, they only have one direct observer, and they are
idiosyncratic to the person concerned. My experience of a novel, or any
imagined space, is not the same as yours. Presence, as we have
characterised it, is something that can be shared, by two or more observers
perceiving the same external world (although, of course, their experiences
will not be identical). This supports co-operation in coordinated action in
the world. This is not the case with internal visualisations, which must be
externalised in some form to be shared. The more concrete, the more
directly perceptual, the form, the more similar will be the experience.
This paper has attempted to illustrate why viewing
presence as primarily a perceptual phenomenon will support progress in
developing both useful models and necessary tests of the experience of
presence. Our view is that it is better to have a definition of presence
that excludes internal, conceptual representations – such as those
underlying the experience of mental imagery – since otherwise presence
becomes synonymous with conscious attention. If we wish to understand how
technological features, emotional engagement, and other factors influence
mediated presence, we obviously need to characterise presence in a way that
is neither synonymous with other factors, nor excludes them from presence
research.
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