Introduction

The term "virtual reality (VR)" suggests a technology that involves an artificial or illusory "reality" that is experienced by the user as a substitute for true reality. Although many professionals now prefer the term "virtual environment," the more popular term implies a particular experiential reaction about how a user will engage and adapt to the constructed world. Many feel that this reaction is a defining aspect of VR [1]. The bulk of studies of these experiential reactions are contained in the literature on "presence." Presence has been defined as the sense of being present in a particular environment [2]. This concept originally grew out of work with remote controlled robots and the idea of "telepresence." Such robots transmit sensory information from a remote environment to their operator and manipulate the remote environment based on the operator's controls. Telepresence refers to the feeling that the operator is actually present in the place where the remote device is located because of the sensory information they experience and their interactive control of the device. When the robot is eliminated and a person experiences sensations and interactions derived from a computer generated virtual environment, we have "virtual presence."
 

Purpose

In attempts to capture a greater understanding of this experience and how it is created, the presence literature has explored many ideas about what presence is and how is should be measured. These ideas have mostly been cognitively or environmentally based, generally ignoring the emotional aspects of presence. We suggest that presence has a critical emotional component which creates further implications about our understanding of it and its further study.
 

Methods

We will explore our assertions about presence by presenting an overview of the presence literature, focusing on the types of presence that have been described and the ways researchers have attempted to measure it. We will then compare this to an overview of some of the literature on emotions, focusing of how we understand emotions and how they have been measured.
 

Results

Review of the presence literature reveals several different definitions and descriptions of presence. Although it has often been defined as the feeling of being present in a virtual environment, there are many variations on this theme. Schloerb divides presence into "subjective presence" and "objective presence" [3]. Subjective presence is the "probability that a person perceives that he or she is physically present in the given environment" (emphasis added). Objective presence is the "probability that the specified task is completed successfully." In contrast to such a utilitarian definition, Heeter describes three types of presence "subjective personal presence," "social presence," and "environmental presence" [4]. Personal presence is "the extent to which and the reasons why you feel like you are in a virtual world." Social presence is "the extent to which other beings (living or synthetic) also exist in the world and appear to react to you." Environmental presence is "the extent to which the environment itself appears to know that you are there and to react to you." Zahorik and Jenison even defined presence as "successfully supported action in the environment," referring to J. J. Gibson's ecological view of perceptual veridicality [5]. This definition avoids the need for a subjective sense of presence by suggesting that the effectiveness of the coupling of perceptions and action between the user and the (virtual) environment define presence.

In addition to varying definitions, the presence literature also includes references to related terms. Many authors emphasize the importance of "immersion." Slater and Wilbur define this as an objective measure, describing it as "the extent to which the computer displays are capable of delivering an inclusive, extensive, surrounding, and vivid illusion of reality to the senses of a human participant" [6]. Witmer and Singer define it more subjectively, stating it is a "psychological state characterized by perceiving oneself to be enveloped by, included in, and interacting with an environment that provides a continuous stream of stimuli and experiences" [7]. They also emphasize the importance of "involvement" which they define as a "psychological state experienced as a consequence of focusing one's energy and attention on a coherent set of stimuli or meaningfully related activities and events." Depending on the author and their sense of what creates virtual reality, one finds different emphases placed on different aspects of presence.

These different emphases have created different ways of attempting to measure presence. These attempts can be broadly classified into two groups: objective and subjective measures. Objective measures involve monitoring the impact of a virtual environment on less consciously controlled reactions such as startle responses or physiologic measures such as pulse or respiration. Subjective measures involve asking directly or indirectly about feelings in the virtual environment. Examples of direct questions include: "How strong was your sense of presence, 'being there,' in the virtual environment?" or "If your level of presence in the real world is '100' and your level of presence is '1' if you have no presence, rate your level of presence in this virtual world" [8]. Other studies ask for subjective comparisons: "Did your sense of being in the scene increase, decrease or stay the same over the course of the experiment?" [9]. The most complete example of this type of work is the presence questionnaire developed by Witmer and Singer which has demonstrated reliability in their studies [7].

This brief overview of the presence literature suggests that there is no single agreed upon definition for presence. In part, this is due to the fact that presence has multiple aspects. The different definitions of presence reveal some of these different aspects, including automatic aspects (tested by involuntary responses), environmental aspects (as for "objective" immersion), and subjective aspects reported using cognitive recognition of subjective state in self-reports. We believe that emotions are a significant component of how presence is perceived. Emotions also have multiple aspects and form a significant part of all of our subjective judgements and automatic responses. Emotional responses mediate increases in heart rate and respiration. They consciously and unconsciously influence our learning and how we understand, describe and react to the world. A brief review of some of the research and methods for studying emotions will illustrate some of the parallels and interactions they share with the study of presence.

The literature on emotions is vast and derived from many sources. Darwin suggested that emotions are evolved behaviors that exist in parallels across species [10]. James and later Lange suggested emotions are interpretations of physical and visceral sensations [11, 12]. Lazarus expresses more recent thoughts on these interactions, proposing that emotions work through a set of interdependent systems including processes for cognitive appraisal, physical interaction between person and environment, coping, and emotional response itself (see figure 1) [13]. The appraisal process can assess an interaction from the person-environment process and pass its assessment to the emotional response or coping processes. It can also assess personality characteristics, goals, and emotional and coping responses to drive new actions in the person-environment process.

Emotions or coping responses also interact directly with the person-environment process without intervening cognitions.

Such theories of emotions have generated a long list of experiments and methods for exploring emotions. The physiologic theories drove the experimental measurement of galvanic skin response (response of sweat glands), pupil response, quantity of salivation, stomach pressure and stomach acidity [14]. Cardiovascular parameters such as blood flow, blood pressure, pulse, cardiac output and electrophysiologic parameters such as the electroencephalogram or event related potentials are other measures used in studies of emotion [15, 16]. Ethological methods are used to measure interactions by standardizing observing situations and specifying methods for scoring physical actions or expressions. For example, there have been at least fourteen standardized methods for scoring facial expressions from video/film recordings in order to measure emotional state [17]. Subjective methods of self assessment through questions have evaluated emotional states along category judgements (fear, anger, disgust, sadness, surprise, joy, interest) or along dimensions such as positive and negative affect [18]. Subjective questions have also been used to measure not only emotional response itself, but also related aspects of emotional function such as mood awareness [19], unconscious mimicry of other's emotions [20], emotional changes with attempts at emotional regulation [21], etc.
 

Conclusions

Our overview of the literature on emotions suggests aspects of presence that should be further explored, as well as possible techniques for such exploration. Emotions affect all behaviors, all cognitions, and all conscious and unconscious interactions between the individual and the environment. They have an impact on presence for all the definitions and assessment methods that we described above and therefore need to be considered in any complete assessment of presence. If we ignore emotions, we may ignore reporting biases in how people want to please an experimenter, how they "feel" about the benefits of technology, or even unconscious fears of the ubiquity of technology. More importantly, we ignore an essential part of human experience. People do not often think about their level of presence in the real world. They feel it. Similarly, we need to look at how people feel about presence in virtual worlds. We need to study the emotions and perhaps even the unconscious processing that occurs in using virtual environments. To paraphrase Freud, virtual environments may be a new "royal road to the unconscious" by providing a tool to investigate all of an individual’s internal reactions to the environment.

Looking at the complex theories around emotions encourages us to also look for similar levels of complexity in presence. Like emotions, presence is continuously changing and dynamic, influenced by physiological self perceptions, cognitive self descriptions, and recursive interactions of these variables with the environment. Both presence and emotions need to be studied by evaluation of physiology, behavior, and cognition as the three states do not necessarily correlate to one another for any particular environmental stimulus. People can show no fearful behaviors and have no fearful cognitions but still experience fear. There may be similar disjunctions for presence, emphasizing the importance of measuring it along multiple dimensions. We need to continue current efforts to measure physiologic variables like EEG in examining presence [22]. We need to use behavioral measures, perhaps standardized paradigms like Ainsworth’s Strange Situation which examines the types of attachments that children form with their parents [23]. Testing of different personality styles and the tendency to immersion will further expand our understanding of presence [7].

A more complete understanding of behavioral, cognitive, and emotional aspects of presence and reactions to virtual environments will help us in virtual environment design [24]. We will be able to create more effective experiences for learning, whether medical training or cognitive retraining [25]. We will be able to adapt virtual environment design to take into account the mental status of the virtual environment user [26]. Virtual environments have already been used to treat people with mental disorders and have several potential applications in this area [27]. Effective design of such treatments will require that we know how to optimize the emotional impact of these virtual therapeutic experiences. Continued experiments to understand more about presence will improve our ability to tailor environments for emotional learning [28]. Through exploring all the potential facets of presence whether emotional, cognitive, or behavioral, we can open up virtual environments to all the potential ways they can be applied.

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