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At the Heart of It All:
The Concept of Presence
Department of Broadcasting, Telecommunications, & Mass Media
Table of Contents
Conceptualizations in the Literature
Causes and Effects of Presence
Causes of Presence as Invisible Medium
Causes of Presence as Transformed Medium
The Effects of Presence
Future Presence Research
About the Authors
A number of emerging technologies including virtual reality, simulation rides, video conferencing, home theater, and high definition television are designed to provide media users with an illusion that a mediated experience is not mediated, a perception defined here as presence. Traditional media such as the telephone, radio, television, film, and many others offer a lesser degree of presence as well. This article examines the key concept of presence. It begins by noting practical and theoretical reasons for studying this concept. Six conceptualizations of presence found in a diverse set of literatures are identified and a detailed explication of the concept that incorporates these conceptualizations is presented. Existing research and speculation about the factors that encourage or discourage a sense of presence in media users as well as the physiological and psychological effects of presence are then outlined. Finally, suggestions concerning future systematic research about presence are presented.
Virtual reality. Simulation rides. Home theater. 3-D IMAX films. State-of-the-art video conferencing. Computers that "talk." Although these emerging technologies are different in a number of ways, each of them (and many others) is designed to give the user a type of mediated experience that has never been possible before: one that seems truly "natural," "immediate," "direct," and "real," a mediated experience that seems very much like it is not mediated; a mediated experience that creates for the user a strong sense of presence. Meanwhile, traditional media including the telephone, radio, film, and television continue to offer us a lesser sense of presence as well. This paper is about the concept of presence: what it is, what is known about how it is generated and the effects it has on media users, and how it might be studied.
Why examine the concept of presence? There are compelling practical and theoretical reasons. An enhanced sense of presence is central to the use, and therefore the usefulness and profitability, of the new technologies mentioned above and others such as the video telephone, high definition television (HDTV), home and arcade video games, the World Wide Web (WWW), and more. These technologies either are now changing or are expected soon to change many of the ways we work, play, and live. In the business world video conferencing has already begun to replace physical travel [(Muhlbach, Bocker, & Prussog, 1995)]. Related systems are used in telemedicine [(Crump & Pfiel, 1995; Hamit, 1995)] and telepsychiatry [(Abkarian, King, & Krappes, 1987; Dongier, Tempier, Lalinec-Michaud, & Meunier, 1986; Jerome, 1986)], in distance learning [(Chu & Schramm, 1967; Hackman & Walker, 1990)], and for legal testimony from remote locations [(Miller, 1991)]. Virtual reality may have begun with military training and flight simulators [(Karr, Reece, & Franceschini, 1997; Rheingold, 1991)], but it is now or soon will be used for everything from arcade games [(Cook, 1992; Martin, 1997; Robertson, 1994)] to architectural and interior design [Yan & Ouhyoung, 1994)] to new kinds of exercise equipment (e.g., the VRbike [(Tectrix, 1995)]), to virtual sex [(teledildonics (Harvey, 1995))] to underwater exploration [(Stoker, Barch, Hine, & Barry, 1995)] to the training and assessment of surgical skills [(Ota, Loftin, Saito, Lea, & Keller, 1995)] and much more [(see "Information Group," 1997)]. Developing technologies in cinematography and film presentation have transformed the movies, providing us with life-like encounters with "Jurassic Park" dinosaurs, extraterrestrial "Aliens," and terrifying "Twisters" (the film "brings screen fiction unnervingly close to virtual reality" [(Ryan, 1996)]). Simulator rides (or "motion-based movies") add hydraulic movement to sequences that typically feature rapid-point-of-view movement, so "viewers" can take a Sub Oceanic Shuttle from San Francisco to Tokyo [("Take undersea tube," 1992)] or move through the environment of a music video [(Moon, 1993)] [(see Mahoney, 1996)]. Future generations of human-computer interfaces, with "intelligent agents" that have their own personalities and follow users' bidding, will surely further transform our use of the modern computer [(see Coughlin, 1996)]. But despite what may be the beginning of a new trend in which companies add social scientists and other "interaction specialists" to their design teams [(Aldersey-Williams, 1996)], most design decisions concerning all of these technologies are made by trial and error, lore, and "seat of the pants" exploration [(see Huston-Stein & Wright, 1979)]. A better understanding of what presence is, what encourages and discourages it in users, and its effects, should save valuable time and money and improve the end-product in the design of new and the redesign of current media technologies.
On the theoretical side, scholars in communication, psychology, and other fields want to better understand psychological and physiological processes as they occur in nonmediated settings; how humans organize and interpret information in their environment, store and retrieve memories, make decisions, etc. To accomplish this, researchers often use mediated stimuli as a substitute for the nonmediated stimuli of interest (for convenience as well as control) and assume that their findings will apply in both contexts. A few of many examples that could be cited are studies of person perception [(Ekman, 1982; Feshbach & Cohen, 1988; Kleck & Mendolia, 1990; Provine, 1989)], how people estimated time to collision in auto accidents [(Hoffmann & Mortimer, 1994; McLeod & Ross, 1983)], the causes and effects of motion sickness [(Alexander & Barrett, 1975; Parker, 1964, 1971)], and the treatment of phobias [(McNally, 1987)]. Our current understanding of these processes is based on studies in which it has been assumed that mediated (i.e., presence-inducing) stimuli are exactly the same as nonmediated stimuli; if that assumption is wrong, we need to know (work by [Gale, Golledge, Pellegrino, and Doherty (1990)] suggests it may be wrong, at least in some learning contexts). So a better understanding of presence will allow us to refine and improve our psychological theories.
Media scholars are interested in particular in how people are influenced by media presentations; an understanding of presence can enhance our theories here too. For example, [Shapiro and Lang (1991)] used a model of memory developed by [Johnson (1983)] to explain how people incorporate information from television into their judgments about the "real" world. They suggest that mediated experiences that closely mimic nonmediated ones cause difficulties for the reality-monitoring process so that when memories are retrieved, mediated and nonmediated experiences are confused. [Ditton (1997)] found tentative empirical support for this idea. Media scholars have begun to consider the role of presence in theories concerning the negative impact of violent portrayals, the positive impact of prosocial portrayals, the perceived credibility of news and information, the persuasiveness of advertising, and more.
Beyond these reasons for examining presence, the fact that there is so much intense popular as well as academic interest in technology that creates a sense of presence beckons study. Why do we want these experiences that (at least in some sense) aren't "real"? What does presence offer us? Aside from its practical uses, what needs does it fulfill? How do these gratifications compare to those offered by the other media and nonmedia activities?
Despite the centrality and importance of presence, it has not yet been carefully explicated, operationalized, or studied. The work that has been done is fragmentary and unsystematic, in part because the people interested in presence come from many different academic fields (including communication, psychology, cognitive science, computer science, engineering, philosophy, and the arts). Further, research conducted for or by private industry and government has typically remained proprietary. This article provides a unifying explication of the presence concept and brings together much of what is known or has been suggested about presence in the hope that it can serve as a starting point for systematic research and theory on this topic.
In the remainder of this paper we (a) review several conceptualizations of presence in the literature and provide a unifying explication of the concept, (b) review some of what is known about the causes and effects of presence, and (c) recommend attributes of a program of research concerning this concept.
Conceptualizations in the Literature
As suggested above, a diverse group of people are interested in presence, how to create it, how to use it effectively, and how it mediates or generates a variety of responses. A review of several relevant literatures finds six interrelated but distinct conceptualizations of presence. Each of these conceptualizations, and where possible corresponding operational definitions, is presented here; this is followed by a detailed explication that encompasses all six conceptualizations.
1. Presence as social richness
To some scholars, primarily those who study communication in organizations, presence is the extent to which a medium is perceived as sociable, warm, sensitive, personal or intimate when it is used to interact with other people. Social presence theory [(Short, Williams, & Christie, 1976)] and media richness theory [(Rice, 1992)] were developed to better match communication media and organizational tasks to maximize efficiency and satisfaction. This is necessary because communication media are said to differ in the extent to which they "(a) can overcome various communication constraints of time, location permanence, distribution, and distance, (b) transmit the social, symbolic, and nonverbal cues of human communication; and (c) convey equivocal information" [(Rice, 1992, p. 452)].
To measure social presence subjects perform various tasks with different media and evaluate each medium via a series of bipolar, seven-point semantic differential items including impersonal-personal, unsociable-sociable, insensitive-sensitive, and cold-warm [(Perse, Buton, Kovner, Lears, & Sen, 1992; Short, Williams, & Christie, 1976)]. The social presence ranking thus depends on the interaction of the medium and the task at hand and is based on the subjective judgment of the user. Media richness or information richness is measured less subjectively by examining a medium's "capacity for immediate feedback, the number of senses involved, personalization and language variety" [(Rice, 1992, p. 4)].
Presence as social richness is related to two important concepts originally applied to nonmediated interpersonal communication: intimacy and immediacy. [Argyle and Dean (1965)] suggested that interactants vary physical proximity, eye-contact, intimacy of conversation topic, amount of smiling, and other behaviors to establish an equilibrium between conflicting approach and avoidance forces and thereby optimize an overall level of intimacy. Other scholars have expanded the list of intimacy behaviors to include posture and arm position, trunk and body orientation, gestures, facial expressions, body relaxation, touching, laughter, speech duration, voice quality, laughter, olfactory cues, and others [(Cappella, 1981; Hall, 1966; Mehrabian, 1969; Patterson, 1973)]. A medium high in presence as social richness allows interactants to adjust more of these variables and therefore more precisely adjust the overall level of intimacy.
[Weiner and Mehrabian (1968)] outlined how choices of language can help create a sense of psychological closeness or immediacy. Others have suggested that intimacy behaviors [(e.g., Hackman & Walker, 1990)] and even the choice of a medium for interaction [(e.g., Heilbronn & Libby, 1973)] also influence this sense of immediacy. Although language and therefore immediacy can be varied within any medium that can transmit language, it seems logical to expect immediacy and presence as social richness to be correlated [(see Short, Williams, & Christie, 1976)].
2. Presence as realism
A second conceptualization of presence concerns the degree to which a medium can produce seemingly accurate representations of objects, events, and people -- representations that look, sound, and/or feel like the "real" thing. This conceptualization is typically used by human factors engineers to assess consumers' responses to variations in the characteristics of a medium. For example, in a study of television, [Hatada, Sakata, and Kusaka (1980)] manipulated viewing angle, display area, viewing distance, and other variables and then asked subjects to report their subjective evaluation of the "sensation of reality" they experienced. [Neuman (1990)] varied the resolution and screen size of high definition television systems and measured viewers' evaluation of a "sensation of realism effect." Heeter (1995) asked users of consumer virtual reality entertainment systems, "How real did the overall experience feel?" This conceptualization of presence is often used in a vague manner that fails to distinguish between two key types of "realism," which are here termed "social realism" and "perceptual realism." Social realism is the extent to which a media portrayal is plausible or "true to life" in that it reflects events that do or could occur in the nonmediated world (this is analogous to what [Potter (1988)] labels the semantic component of the "magic window" dimension of perceived reality). While presence as realism may include this type of social realism it also includes a perceptual element that is separate: a scene from a science fiction program may be low in social realism but high in perceptual realism because although the events portrayed are unlikely, the objects and people in the program look and sound as one would expect if they did in fact exist. On the other hand, the people and events in an animated presentation may be high in social realism but because they are not "photorealistic," they are low in perceptual realism.
3. Presence as transportation
Another conceptual definition of presence involves the idea of transportation. Three distinct types of transportation can be identified: "You are there," in which the user is transported to another place; "It is here," in which another place and the objects within it are transported to the user; and "We are together," in which two (or more) communicators are transported together to a place that they share.
"You are There"
This is perhaps the oldest version of presence. The oral tradition of early humans involved the telling of tales that transported each generation of listeners to a different time and place where the events occurred [(Biocca & Levy, 1995)]. Written narrative can have the same effect [(Gerrig, 1993; Radway, 1991)]. Recent AT&T advertisements told us that the telephone was "the next best thing to being there." Borrowing from the [1976 Jerzy Kozinski novel] and 1979 film, [Reeves (1991)] used the term "Being There" to describe how viewers experience the environment they encounter on television. Again with regard to television, [Kim (1996)] defines presence as a "feeling of being a part of the phenomenal environment created by television and not being a part of the physical environment surrounding the viewer and the television set" (p. 27). (The phrase often spoken by television hosts following a commercial break, "Welcome back," is consistent with the idea that viewers are "transported" during viewing.) The "you are there" concept is often used in discussions of virtual reality, which takes users to a virtual environment and leads to the "suspension of dis-belief that they are in a world other than where their real bodies are located" [(Slater & Usoh, 1993, p. 222)]. [Sheridan (1992)] discusses teleoperation (human manipulation of elements of a remote environment) and, following [Minsky (1980)], defines telepresence as "feeling like you are actually 'there' at the remote site of operation," while virtual presence is "feeling like you are present in the environment generated by the computer" (p. 120). (See also [Rheingold, 1991], who calls telepresence a "form of out-of-the-body experience" (p. 256), and [(Biocca & Levy, 1995; Heeter, 1992; Held & Durlach 1992; and Steuer, 1995)] for similar definitions). The concept of transporting users to remote physical places can also be found in "virtual tours" of art exhibits, museums, and tourist destinations on the world wide web [(WWW Virtual Tours, 1997)].
A number of closed-ended questionnaire items have been used to measure this type of presence. After watching television, subjects in a study by [Kim (1996)] were asked to report how often they had had the following perceptions: "I felt I was in the world the television created," "the television-generated world seemed to me to be more like 'somewhere that I visited' rather than 'something that I saw'," and "my body was in this room, but my mind was inside the world created by the television." Following [Gerrig (1993)], Kim tentatively identified two factors that emerged in subject responses as "departure" (from the nonmediated environment) and "arrival" (in the mediated environment). In another study of responses to television, [Ditton (1997)] asked subjects, "How much of a sense of participation in the scene did you feel?" and "How much of a sense of involvement in the scene did you feel?". [Slater and Usoh (1993)] asked virtual reality users, "To what extent did you experience a sense of being 'really there' inside the virtual environment?" (p. 227). In a study of consumer responses to a second person virtual reality entertainment system (one in which users stay in front of a camera and watch a television monitor that displays images of themselves acting inside a virtual environment), [Heeter (1995)] asked subjects, "Which felt like the real you -- the being on the screen or the one the camera was pointed at?" All of these measures assess the degree to which media users are transported into a distinct mediated environment.
"It is Here"
Instead of transporting the user to a different place, a sense of presence may bring the objects and people from another place to the media user's environment. According to [Millerson (1969)], "Watching a television programme, we feel not so much that we are being taken out into the world, as that the world is being brought to us" (pp. 201-202). Extending this idea, [Flavell, Flavell, Green, and Korfmacher (1990)] examined whether 3 and 4 year old children believed that "an object seen on videotape could be touched or could come out if the top of the set were removed, whether it would spill out of the open container it was in if the set were turned upside down, and whether a person seen on videotape could see, hear, and know about the experimenter's ongoing actions" (p. 402). The youngest children seemed to fail to make what the authors termed this image-referent distinction.
[Reeves (1991)] suggested that even though adults don't express the beliefs that young children do, they may also fail to distinguish fully between images and referents (adults' sophistication may be the result of experience rather than development: for example, some theater-goers at the beginning of the film era are said to have panicked and run for the exits when a black and white film of an oncoming locomotive was shown [(Schoen, 1976)]). [Lombard (1995)] argued that when media users fail to distinguish between image and referent, they respond directly to what they see and hear in a mediated experience, as if what they see and hear was physically present in their viewing environment, rather than respond indirectly by decoding something they perceive only as a symbolic or representational message. A general measure of these direct responses used by [Lombard, Reich, Grabe, Campanella, and Ditton (1995)] was, "How much did you feel like it was happening to you?"
"We are Together" (Shared Space)
A third form of presence as transportation is found in literature concerning video conferencing as well as virtual reality. For example, in a study of video conferencing [Muhlbach, Bocker, and Prussog (1995) defined "telepresence in video communications" as "the degree to which participants of a telemeeting get the impression of sharing space with interlocutors who are at a remote physical site" (p. 301). This was measured by asking participants to report the degree to which they agreed or disagreed with statements such as "[It felt] as if we were all in the same room" and "[It felt] like a real face-to-face meeting" (p. 301). Some of the pioneers of virtual reality have suggested that its greatest potential is as a virtual gathering place in which people from around the block or around the world will be able to gather in a shared virtual space that is different from any of the individuals' "real" environments [(Lanier & Biocca, 1992)]. Precursors of these Distributed Virtual Environments (DVEs) include the popular "chat rooms" of today's Internet [(see Braham & Comerford, 1997; Rockwell, 1997; and Waters & Barrus, 1997 for detailed discussions)].
4. Presence as immersion
A fourth conceptualization of presence emphasizes the idea of perceptual and psychological immersion. [Biocca and Levy (1995)] note that in the most compelling virtual reality experiences, the senses are immersed in the virtual world; the body is entrusted to a reality engine. The eyes are covered by a head-mounted display; the real world is invisible. The ears are covered by headphones; ambient sound is muffled. The hands are covered by gloves or props: 'touch only the virtual bodies.' Virtual reality may share common elements with reading a book in a quiet corner, but this book has stretched in all directions and wrapped itself around the senses of the reader -- the reader is swallowed by the story. (p. 135)
Perceptual immersion, "the degree to which a virtual environment submerges the perceptual system of the user" [(Biocca & Delaney, 1995, p. 57)], can be objectively measured by counting the number of the users' senses that are provided with input and the degree to which inputs from the physical environment are "shut out" (see [Kim, 1996]). Not only immersive virtual reality systems but also simulation rides, IMAX theaters, and even standard movie theaters can be said to immerse the senses of media users.
Presence as immersion also includes a psychological component. When users feel immersive presence they are involved [(Palmer, 1995)], absorbed [(Quarrick, 1989)], engaged, engrossed. This psychological state typically is best measured via subject self-report (although observation of involved media users might also be a useful indicator). For example, a factor analysis of responses to items used by [Heeter (1995)] in a study of user reactions to consumer virtual reality systems resulted in an "involvement" factor containing the items "intense," "fun," "competitive," "addictive," and "exciting"; scores on this factor were the highest of all factors (8.7 out of 10).
5. Presence as social actor within medium
In a classic 1956 article, [Horton and Wohl] suggested that even though the relationship between a television personality and a television viewer is one-sided, with no possibility of real time interaction, skilled personalities use direct address camera views (in which the personality seems to be looking at the viewer), informal speech patterns, sincerity, and simplicity to generate a "simulacrum of conversational give and take [that] may be called para-social interaction" (p. 215). In a parasocial interaction media users respond to social cues presented by persons they encounter within a medium even though it is illogical and even inappropriate to do so. Studies have shown that people respond to interpersonal distance cues in [(Lombard, 1995), and even talk to [(Lemish, 1982)], the pictures of people on the television screen. The mediated nature of the "interaction" is ignored and the media personality is incorrectly perceived as a social actor.
This illogical treatment of mediated entities as social actors is not limited to television. "Virtual actors" are created with digitized data from sensors attached to a real person and computer voice synthesis; the data give a computer character human gestures, facial movements, and voice (e.g., Mario, a sports mascot seen on stadium screens [(Takiff, 1993)] or "Dev," the computerized bartender/news anchor on MSNBC's "The Site"). The Microsoft personal computer software product titled "Bob" features 14 on-screen characters that guide the novice user through his or her computer's functions; Cliff Nass and Byron Reeves of Stanford University call it a "social interface" [(see Coughlin, 1996)]. Intelligent computer agents of the future will have avatars (an incarnation in human form) with which users interact [(Boyd, 1996)], making the interaction more like interacting with another human. In the software product Dogz: Your Computer Pet [(Dogz, 1995)], users adopt one of several puppies which they then teach tricks, play games with, feed, groom, pet, and discipline as their "desktop companion" grows from puppy to adult dog (a version for cats is available as well). "The Tamagotchi" ("cute little bird") is a "cyber pet" that appears on a three-quarter-inch computer screen attached to a key chain and needs constant "food, exercise, play, medicine, etc."; it is hugely popular in Japan, the Far East, and now America [(Boccella, 1997)]. And in a software product popular in Japan called Princess Maker, the user controls a female animated character:
She's your little princess. You name her, wring your hands when she's sick, fret over her schooling. Like any caring dad you keep steady watch over her hobbies, clothes and manners. But if after all your lavish attention, she becomes a bar hostess strutting around in fishnet stockings or a club-swinging street tough -- no problem. Just reboot your computer and start again. [(Coleman, 1996, p. D3)]
The virtual characters in similar games have their own fan clubs in Japan; a magazine, Virtual Idol, "deals not with game-playing strategy but with the hobbies, life experiences and physical measurements of [these] people who do not exist" [(Pollack, 1996)].
In all of these examples users' perceptions and the resulting psychological processes lead them to illogically overlook the mediated or even artificial nature of an entity within a medium and attempt to interact with it; this phenomenon represents a fifth type of presence.
6. Presence as medium as social actor
The final conceptualization of presence involves social responses of media users not to entities (people or computer characters) within a medium, but to cues provided by the medium itself. Debate about the potential of modern computers to mimic humans officially began in 1950 with [Alan Turing's] "Turing Test" and continues today. While computers, robots, and androids in science-fiction often evoke social responses from other characters (and many audience members) because they seem so "human" (e.g., Data in Star Trek, C3P0 and R2D2 in Star Wars, Hal in 2001: A Space Odyssey, the Terminator in the Terminator films, the Replicants in Blade Runner, etc.), the phenomenon seems to exist even with today's less sophisticated computers. Nass and his colleagues at the Center for the Study of Language and Information at Stanford University have demonstrated in a series of studies [(Nass, Lombard, Henriksen, & Steuer, 1995; Nass & Moon, 1996a, 1996b; Nass, Moon, Fogg, Reeves, & Dryer, 1995; Nass & Steuer, 1994; Nass, Steuer, Henriksen, & Dryer, 1994; Nass, Steuer, Tauber, & Reeder, 1993)] that because computers use natural language, interact in real time, and fill traditionally social roles (e.g., bank teller and teacher), even experienced computer users tend to respond to them as social entities. In most of these studies a social psychology finding concerning human-human interaction is replicated in the context of human-computer interaction. For example, in human-human interaction we follow the rule "praise from others is more valid than praise from self" [(Jones, 1990; Joshi & Rai, 1987; Meyer, Mittag, & Engler, 1986; Wilson & Chambers, 1989)]. In a study by [Nass, Steuer, Henriksen, and Dryer (1994)] subjects evaluated a computer's performance in a tutoring task more favorably when the tutor computer was praised by a different computer than when it praised its own performance. These results were found despite the fact that the subjects consistently said that such responses to computers are illogical and inappropriate. Computer users also follow social rules concerning politeness and gender stereotypes. [Nass and Moon (1996a) demonstrated that these social responses are to the entity of the computer and not the person who programmed the computer.
[Nass, Reeves, and Leshner (1996)] found an even more surprising social response to a communication technology: just as individuals consider the work of specialists in a field to be of higher quality than the work of generalists, subjects in an experiment reported that the quality of the news or entertainment programs presented on different ("specialist") television sets was higher than when the same programs were presented on just one ("generalist") set. Another example of social responses to television comes from [Lemish (1982)], who observed people watching television in public places:
Certain viewer mannerisms suggested that television [not the people on television] was perceived as a communicative partner and not merely as a physical object. For example, viewers would rarely leave the viewing area in the middle of a segment. While it could be argued simply that viewers were involved in the program or that they were showing respect for other viewers, this observer could not avoid the impression that viewers acted as if leaving in the middle was rude and inconsiderate. (pp. 755-756)
In these social responses to computers and televisions users again ignore, in a counter-logical way, the mediated nature of a communication experience. Basic social cues exhibited by the medium lead users to treat the medium as a social entity.
Although the conceptualizations discussed above vary considerably, they share a central idea. Each represents one or more aspects of what we define here formally as presence: the perceptual illusion of nonmediation. The term "perceptual" indicates that this phenomenon involves continuous (real time) responses of the human sensory, cognitive, and affective processing systems to objects and entities in a person's environment. An "illusion of nonmediation" occurs when a person fails to perceive or acknowledge the existence of a medium in his/her communication environment and responds as he/she would if the medium were not there. Although in one sense all of our experiences are mediated by our intrapersonal sensory and perceptual systems, "nonmediated" here is defined as experienced without human-made technology (note that under this definition even hearing aids and eyeglasses are media that "come between" our environment and our perceptual system).
The illusion of nonmediation can occur in two distinct ways: (a) the medium can appear to be invisible or transparent and function as would a large open window, with the medium user and the medium content (objects and entities) sharing the same physical environment; and (b) the medium can appear to be transformed into something other than a medium, a social entity.
Presence in this view can not occur unless a person is using a medium. It does not occur in degrees but either does or does not occur at any instant during media use; the subjective feeling that a medium or media-use experience produces a greater or lesser sense of presence is attributable to there being a greater or lesser number of instants during the experience in which the illusion of nonmediation occurs.
It should be noted that this illusion does not represent a perceptual or psychological malfunction or psychosis, in which the mediated experience is consciously confused with what is nonmediated or "real." Clearly when asked, users of any current or likely future medium can accurately report that they are using a medium (the "holodeck" in the "Star Trek" television series and films is a exception; see in particular the episode of "Star Trek: The Next Generation" titled "Ship in a Bottle").
This definition of presence can be applied to any medium and encompasses each of the six conceptualizations discussed above. A medium that becomes invisible and produces a perceptual illusion of nonmediation analogous to an open window can provide rich verbal and nonverbal information for social interaction (presence as social richness); objects and entities in such a medium should appear perceptually (if not socially) vivid and real (presence as realism); the illusion that there is no medium at work means there is no border between "this side" and "the other side" of the medium, so users can perceive that they have moved to the other side, that objects/entities from the other side have entered their immediate environment, or that they and other users are sharing a real or artificial environment (presence as transportation); the illusion of nonmediation will be more complete if the medium is perceptually and psychologically immersive (presence as immersion); and if we encounter people or entities within such a medium, even if there is no possibility of true social interaction with them, we are encouraged to respond to social cues they provide just as we would in nonmediated communication (presence as social actor within medium). Finally, when the medium itself presents us with social cues normally reserved for human-human interaction we are likely to perceive it not as a medium but as an independent social entity, a tranformed medium (presence as medium as social actor).
Because it is a perceptual illusion, presence is a property of a person. However it results from an interaction among formal and content characteristics of a medium and characteristics of the media user, and therefore it can and does vary across individuals and across time for the same individual. We turn next to the limited evidence, as well as the speculation, concerning which of these characteristics encourage and discourage a sense of presence in media users, and the effects of presence.
Causes and Effects of Presence
There has been relatively little research, and even less systematic research, conducted to investigate the factors that contribute to a sense of presence and the variety of consequences that it produces. "There is no scientific body of data and/or theory delineating the factors that underlie the phenomenon" [(Held & Durlach, 1992, p. 110)]. Previous discussions of presence have typically (a) been based on informed conjecture rather than research, and/or (b) focused only on specific media, and/or c) focused only on one or more specific conceptualization(s) of presence outlined above.
In this section we synthesize what is known about and what has been suggested about the causes and consequences of presence. The discussion can not be exhaustive, in either breadth or depth; where possible we refer the reader to detailed discussions of variables. Our goal is to identify key variables and groups of variables to provide a framework for a systematic program of research on the concept of presence. We begin with variables that encourage or discourage presence as an invisible medium and then discuss variables related to presence as a transformed medium. In each case these variables are divided into characteristics of media form, characteristics of media content, and characteristics of the media users. Finally, we consider a number of the physiological and psychological effects of presence.
Causes of Presence as Invisible Medium
As suggested above, presence is determined by formal and content features of a medium and by characteristics of the medium user.
The formal characteristics of media most often cited as important determinants of presence are those that involve sensory richness or vividness. [Steuer (1995) argues that a sense of presence is based in large part on a medium's vividness, which includes sensory breadth ("the number of sensory dimensions simultaneously presented") and sensory depth ("the resolution within each of these perceptual channels")(p. 42). Other writers have echoed these ideas: [Zeltzer (1992)] links presence to "a rough, lumped measure of the number and fidelity of available sensory input and output channels" provided by a medium (p. 128). [Heeter (1992)] points to the "range and intensity of stimuli human senses detect and interpret in perceiving the natural world" (p. 263). We discuss first the range of senses served by a medium and then characteristics related to the individual sense modalities.
Number and consistency of sensory outputs
Although it has rarely been examined by researchers, it is generally believed that the greater the number of human senses for which a medium provides stimulation (i.e., media sensory outputs), the greater the capability of the medium to produce a sense of presence [(Anderson & Casey, 1997; Barfield & Weghorst, 1993; Kim, 1996; Short, Willliams, & Christie, 1976; Steuer, 1995)]. For example, media that provide both audio and visual stimuli are said to produce a greater sense of presence than audio-only (or video-only) media. In one study [(Short, Willliams & Christie, 1976)], subjects reported greater social presence after an audio-visual task-based interaction than an audio-only one. Film and television should therefore more easily produce presence than radio. The addition of inputs for the senses of smell (as in Morton Heilig's Sensorama "experience theater" of the 1960s [(see Rheingold, 1991)] and a handful of films), body movement and equilibrium (as in simulation rides), and touch (in advanced virtual reality systems) each seem likely to contribute to a strong sense of presence. The importance of all sensory outputs are not equal, however. According to Christie (1974, as cited in Short, Williams, & Christie, 1976) research has found that visual media have more social presence than verbal (audio) media, which in turn have more social presence than written media. In general, our visual and aural senses dominate our perception and have been most often identified with presence.
Not only is the number of sensory output channels an important factor in generating a sense of presence, the consistency of information in the different modalities is key: "the information received through all channels should describe the same objective world" [(Held & Durlach, p. 110)]. Failure to meet this criterion emphasizes the artificial and thus the mediated nature of a media use experience (see [Stein and Meredith (1993)] for a detailed discussion of modality and information processing).
The number of channels of input accepted and processed by an interactive medium is also related to presence and is discussed below.
Visual display characteristics
Many characteristics of visual displays encourage a sense of presence, including image quality; image size and viewing distance, which together determine the proportion of a user's visual field occupied by an image; motion and color; variables related to the perception of dimensionality; and the use of a variety of camera techniques.
image quality. The perceived quality of an image depends on many characteristics, including resolution, color accuracy, convergence, sharpness, brightness, contrast, and the absence of "ghosts" or other noise. Very high resolution images (e.g., those containing 1125 or 3000 horizontal scan lines) have been shown to evoke more self-reported presence (no definition was given to subjects) than standard resolution images (200-525 lines) [(Neuman, 1990)]. [Reeves, Detenber, and Steuer (1993)] manipulated image quality by using multiple-generation copies of video stimuli and found that high quality images were regarded as more "realistic." [Bocker and Muhlbach (1993)] found that higher resolution images in a video conferencing system elicited reports of greater "communicative" presence (analogous to social richness). Images which are more photorealistic, for example a live-action scene or a photograph rather than an animated scene or a drawing, are likely to provoke a greater sense of presence as well [(Heeter, 1992; Zeltzer, 1992)] (one of the critiques of current virtual reality technology is that it has not yet achieved a [photo]realistic appearance).
image size. The formal feature that has received the greatest attention from researchers concerned with presence is probably the size of a visual image. Larger images have been shown to evoke a variety of more intense presence-related responses. For example, [Reeves, Detenber, and Steuer (1993)] showed subjects clips from action films. Subjects who watched on a 70 inch screen (measured diagonally) reported significantly greater agreement with the statement "I felt like I was a part of the action" than subjects who watched on a 35 inch screen. [Yuyama](1982, cited in Neuman, 1990) found that subjects reported a greater "sensation of reality" when they watched a 54 inch image rather than a 28 inch image.
In a study of motion sickness Parker (1971) showed subjects an eight minute video segment taken from the point of view of a driver of a car as it traveled a winding mountain road. Several subjects became nauseated and could not complete the session. In a follow-up study, [Alexander and Barrett (1975)] explained their subjects' less severe response to the same stimulus by noting that they presented it to the subjects on a smaller screen than Parker had used in his study. [Lombard, Reich, Grabe, Campanella, and Ditton (1995)] showed subjects 10 different short scenes featuring this same type of rapid point-of-view movement on a consumer-model television set with either a 46 inch or a 12 inch screen. Subjects who watched the larger images were more aroused (a skin conductance measure) and reported a greater "sense of movement," "enjoyment of this sense of movement," "sense of participation," and "involvement." [Ditton (1997)] had subjects view 15 scenes from films on either a 52-inch screen color television with surround sound audio or a 5-inch, black and white television with monophonic audio. Subjects who viewed in the large-screen condition reported a greater sense of "participation" and "involvement" in the scene. The difference reached significance when variance associated with the ability to screen irrelevant stimuli ([Mehrabian's (1976)] "screener" concept) was removed.
[Lombard (1995)] used a screen size manipulation to show that viewers respond to social cues they encounter in nonmediated communication, such as apparent interpersonal distance, in mediated experiences including television viewing. As predicted by Burgoon's Nonverbal Expectancy Violations Model [(Burgoon, 1978; Burgoon & Hale, 1988; Burgoon & Jones, 1976; Burgoon & Walther, 1990)], when subjects watched attractive and professional news anchors deliver stories on a large (42 inch) screen they reported more positive emotional responses to anchors and to the viewing environment, and then selected a viewing position that represented a smaller withdrawal from the encounter, than when the people appeared on smaller (26 inch or 10 inch) screens.
Because consumers are buying larger TV sets [(Pressler, 1996)] and because "bigger pictures are the essence of the coming HDTV format [(Thorpe, 1989)], image size is likely to be an increasingly important determinant of presence. (The combination of the larger size and improved quality of HDTV provides a picture that is "almost like looking out a window, almost feels three-dimensional," according to Joel Brinkley, author of "Defining Vision" [(Utley, 1997)]).
image size and viewing distance: Proportion of visual field. Along with larger images it seems logical to expect that when people are physically closer (but perhaps not excessively close) to an image, they feel a greater sense of being a part of the image and therefore a greater sense of presence. But these two variables also act together to determine the value of a third variable, the proportion of the user's visual field that the image occupies, also known as viewing angle [(Hatada, Sakata, & Kusaka, 1980; Nathan, Anderson, Field, & Collins, 1985)] and field of view [(Biocca & Delaney, 1995; Held & Durlach, 1992)]. A large image and a large viewing distance (e.g., in an IMAX theater) can result in the same proportion of visual field as a small image and a small viewing distance (e.g., in a virtual reality headmounted display). Studies by [Hatada, Sakata, and Kusaka (1980)] and [Yuyama (1982)] suggest that a "sensation of reality" is stronger in the former configuration, but more research is needed on the question.
motion and color. Despite a lack of research it seems reasonable to conclude that moving images that provide the illusion of continuous action can more easily evoke presence than still images [see (Anderson, 1993; Burr & Ross, 1986)]. Color images should evoke more presence than those in black and white.
dimensionality. There are several ways to make flat (two dimensional) images appear to contain the third dimension of depth. Principles of perspective have been used for centuries in drawing and painting. Near the end of the 17th century Andrea Pozzo painted "The Glorification of St. Ignatius" on the ceiling of the Church of Sant' Ignazio in Rome. Today spectators still look up to see "a three-dimensional panorama of arches supported by columns, windows, and sky, with human figures arranged in various positions throughout, some of them seemingly suspended in midair. . . . [I]t looks real, so real that it is virtually impossible to tell where the architecture of the church ends and the painting begins" [(Rock, 1984; see also Alberti, 1458/1966)]. The artist encourages illusions such as this by creating for the perceiver something very close to the pattern of light that would be etched on a viewer's retina if the person encountered the nonmediated scene. This involves "tricks" such as making distant objects smaller, making near objects block parts of objects "behind" them (interposition), and reducing detail and texture in distant objects. These and other techniques are increasingly applied by designers of virtual environments [(Heeter, 1992)], computer interfaces (e.g., Windows 95, VRML Internet sites), and television graphics [(Olds, 1990)] to create the illusion that mediated objects have depth. Film or video images with great depth of field [(Hatada, Sakata, & Kusaka, ] call this "pronounced perspective") are frequently combined with "matte" paintings of distant backgrounds to enhance a sense of three dimensional space. The moving film or video camera provides viewers with additional depth cues, as distant objects "move" more slowly than near objects as the camera pans a scene. (See [Gibson, 1979; Hagen, 1980; and Rock, 1984] for more detailed discussions of depth cues in pictures).
Stereoscopic images -- in which a slightly different view is presented to each eye, as in nonmediated perception -- also enhance the sense of presence. [Muhlbach, Bocker, and Prussog (1995)] found that participants in a videoconference that employed stereoscopic imaging were more likely to report that "I felt like I was face-to-face" and "I felt as if we were in the same room" (see [Patterson, 1992] for a review of human stereopsis as it relates to the design of stereoscopic displays). When several of these techniques to simulate dimensionality are combined the effect can be quite impressive, as in Sony's 3-D IMAX presentations.
camera techniques. The camera can be used in a variety of ways to create a sense of presence. Shot length, the distance from which objects or entities in an image are framed, appears to be important. Close-up views used in still images led subjects in a study by [Hatada, Sakata, and Kusaka (1980)] to report a greater sensation of realism. [Short, Williams, and Christie (1976)] report that Champness found the same result when subjects used a video conferencing system.
Subjective camera shots create a view through an actual or implied entity's eyes and so transform the viewer from an event-spectator into an event-participant [(Zettl, 1990)]. Ideally, this close association with the camera's point of view encourages the viewer to "participate in an event psychologically (feeling a part of the event) and occasionally also even kinesthetically (reacting physically to the screen event such as shouting approval, clapping, or moving one's arms when watching a boxing match)" [(Zettl, 1990, p. 221)].
Subjective camera shots are common in film and television and especially prevalent in computer video games (e.g., Doom, Hexen, Wolfenstein, and Duke Nukem). All of the "camera shots" in virtual reality are subjective views, and in many cases the illusion is enhanced because users can see "their" hands and feet as they navigate the environment. [(Bricken (1991)] notes that watching a dynamic representation of one's hands within a virtual world is "convincing evidence that you're There"; see also [Held & Durlach, 1992]).
The most common subjective technique is direct address, in which the person in the image (e.g., a news anchor or talk show host) speaks directly to the camera and therefore, apparently, the viewer. [Horton and Wohl (1956) suggested that the ability of television personalities to talk directly to viewers this way is an essential part of parasocial interaction. Producers sometimes have a character "break the fourth wall" (the viewing screen that separates the viewer from the people in the image) by suddenly turning to directly address the viewer (this technique was used effectively in the television series "Moonlighting").
A subjective camera technique that dominates video games and simulation rides, and is increasingly found in action-adventure films (e.g., Speed, The River Wild) and television commercials (e.g., for automobiles), is rapid point-of-view movement. Here a moving camera mimics for the viewer the non-mediated experience of a person or object moving quickly through an environment. Producers "mount a camera to a racing car, strap it to a ski racer's helmet, or run with it through a city street," all in an effort to allow the viewer to "experience what driving a race car, skiing down a steep slope, or jogging through a city street feels like" [(Zettl, 1990, p. 221)]; recent innovations such as miniature radio-controlled helicopters and cars (e.g., "Flying Cam," "[Innovision Optics]") have made the technique increasingly practical and affordable. Common responses to rapid point-of-view movement in IMAX films and virtual reality include queasiness, dizziness, and even nausea [(Azar, 1996)]. (In fact, psychologists often study motion sickness by using filmed stimuli that feature continuous rapid point-of-view movement [(Alexander & Barrett, 1975; Parker, 1964, 1971)].
One study has demonstrated the effect of rapid point-of-view movement on presence. [Lombard, Reich, Grabe, Campanella, and Ditton (1995)] showed subjects 10 short scenes from commercially available video tapes that featured rapid point-of-view movement. Viewers of both a small (12 inch) and large (46 inch) television reported an enjoyable sense of physical movement, excitement, involvement, and a sense of participation.
Other subjective camera shots include over-the-shoulder shots and shots that employ a "shaky" handheld camera. Again, at least the intended effect is to make the viewer experience a sense of presence by becoming part of the scene, seeing through the eyes of an implied or actual character.
Aural presentation characteristics
Although frequently undervalued [(Kramer, 1995)], mediated sounds clearly are important in generating presence. The two most frequently identified characteristics of aural presentations in discussions of presence are quality and dimensionality.
As with visual images, sound quality involves several variables, including frequency range, dynamic range (variations in loudness), and signal to noise ratio (which quantifies the degree of various forms of distortion in the sound reproduction process) [(see Alten, 1990; Everest, 1987)]. Although it seems logical to conclude that high quality audio is more likely to generate presence than low quality sounds, the scant available evidence is mixed. [Reeves, Detenber, and Steuer (1993)] showed subjects scenes from action films and varied the fidelity of the soundtracks by controlling both frequency range (with a graphic equalizer) and signal to noise ratio (via tape "hiss" added to the recordings). The presentations with high fidelity sound were judged more "realistic," but it was the low fidelity sounds that made subjects feel more "a part of the action."
We hear in "three dimensions," so the spatial characteristics of sound should be important for a sense of presence. [Biocca and Delaney (1995) note that
the aural realism of virtual spaces requires replicating the spatial characteristics of sounds like the changing intensity of a race car engine as it approaches a listener and screeches past (Doppler effect); or the tapping of footsteps as they echo in a dark, empty corridor; or the chatter of a conversation off in the corner of a room (cocktail party effect). (p. 81)
Spatialization, or 3-D sound, is an attempt to add these spatial characteristics to mediated sounds [(Kramer, 1995; see also Anderson & Casey, 1997; Blauert, 1983; Wenzel, 1992)]. Dimensional sound is created in stereo (2 channel), quadraphonic (4 channel), and especially in surround sound systems (in which the amplitude, phase, and frequency of sounds arriving at each ear are adjusted to create the illusion of dimensional space).
There is little empirical research available concerning the effect of aural dimensionality on the sense of presence. [Reeves, Detenber, and Steuer (1993)] found no differences concerning presence for monaural presentations and presentations for which the dimensionality of sound was enhanced via Dolby surround sound decoding. On the other hand, Christie (1973a, as cited in [Short, Williams, & Christie, 1976)] found that social presence was greater on self-report measures for a "multi-speaker audio system" than a single speaker system. In other studies (e.g., [Ditton, 1997]), dimensionality is confounded with other variables, making it impossible to draw specific conclusions. Despite the lack of hard evidence, it seems likely that dimensional audio, at least in many circumstances, evokes increased presence.
The volume (loudness) of mediated audio stimuli also may have an impact on presence, with particularly low and perhaps particularly high levels less effective than moderate ("realistic") levels (see [Everest, 1987]). [Anderson and Casey (1977)] suggest that the proper use of ambient sounds and music can "evoke an atmosphere or sense of place, thereby heightening the overall feeling of immersion in the virtual environment" (p. 47).
Stimuli for other senses
Visual and aural stimuli may be the most common sensory outputs available in mediated experiences, but there are at least four others, each of which is likely to enhance presence: olfactory output, body movement (vection), tactile stimuli, and force feedback.
Adding the smells of food, flowers, or the air at a beach or in a rain forest to the corresponding images and sounds seems likely to enhance a sense of presence for media users. In the 1960s arcade ride Sensorama, the simulation of a motorcycle ride included sending puffs of the aromas of exhaust, pizza, and flowers into users' nasal passages at appropriate times [(Hellig, 1993; Rheingold, 1991)]. In the John Waters film [Polyester (1981)], theater-goers were given special "scratch-and-sniff" cards "enabling [them] to fill [their] nostrils with whatever scent or stench that the film's heroine, Francine Fishpaw, was inhaling" [(Rickey, 1996)]. Olfactory stimuli, however, have been used only rarely in mediated presentations because they are difficult to control and deliver [(Biocca & Delaney, 1995)]; their effectiveness in generating a sense of presence has not been tested empirically.
Presence should be enhanced when our body is moved in physical space during a mediated experience. A handful of films, including [Earthquake (1974), Gunnm (1993), Midway (1976), and Rollercoaster (1976)], have been presented in Sensurround, in which theater seats vibrate to enhance the illusion that the viewer is experiencing, for example, an earthquake or an air and sea battle. Simulation theaters [(Showscan, 1991)], simulation rides (e.g., Disney's "Star Tours"), and sophisticated flight simulators all use hydraulic motion platforms to create illusions of acceleration, deceleration, and other gravitational and inertial forces. Evidence for the contribution of body movement to a sense of presence is only anecdotal; it is likely that some types of movement contribute more than others.
Two additional types of sensory media output likely to contribute to presence occur only in interactive media experiences (interactivity is discussed in the following section).
A virtual environment that truly "feels real" should be able to simulate the sensation of surface textures like sandpaper or velvet, the resistance of surfaces like rocks or pillows, and the sensation of physical resistance like moving an oar or stick through water, mud, oil, or rocks. [(Biocca & Levy, 1995, p. 84)
Although complex and currently expensive, it is possible to stimulate receptors in a media user's hands, muscles, and joints to reproduce the sensations we perceive when we touch and manipulate objects in nonmediated experience. Effective tactile or haptic sensory output produces the sensations of touching a surface; force feedback systems respond to user input with the sensations of physical resistance. It is generally assumed that each of these makes a substantial contribution to a sense of presence [(Biocca & Delaney, 1995; Heeter, 1992; see also Sutherland, 1965)]. While these types of media "displays" might seem exotic, force feedback can be created with simple "mediating objects" such as a joystick, steering wheel, or hand grip. The latest generation of computer joysticks for home computer games includes force feedback output and is receiving high marks from video gaming experts:
The combination of visual cues with physical ones makes the experience more truly immersive than any advance in graphics or processor technology Next Generation [a magazine for leading edge computer gaming] has seen... It should open the way for entirely new types of games, where feel replaces graphics as the premium experience delivery mechanism. [("Get a grip," 1996, p. 42)]
Artists can use a special pen and a pressure-sensitive digitizing tablet to obtain "virtual tactility" so that using greater force produces wider and darker lines: "Pressure sensitivity is an enhancement that adds a new level of sensitivity and realism to computer art" [(Steltzer, 1992, p. 14)]. Again, research is needed to determine the role of haptic and force feedback output in the encouragement of presence.
Most writers have either implicitly assumed or explicitly suggested that a major or even the primary cause of presence is the ability to interact with a mediated environment. The concept of interactivity is complex and multi-dimensional, but in this context an interactive medium is one in which the user can influence the form and/or content of the mediated presentation or experience as in [Steuer's (1995)] definition. The degree to which a medium can be said to be interactive depends on a number of subsidiary variables. Five primary ones will be discussed here.
The first variable is the number of inputs from the user that the medium accepts and to which it responds. [Biocca and Delaney (1995)] discuss a variety of user inputs, including voice/audio input (e.g., speech recognition systems that allow a computer to accept and respond to voice commands), haptic input (e.g., television knobs and buttons and computer mice, joysticks, wands, etc. that record user commands via object manipulation), body movement and orientation (kinetic) input (e.g., data gloves, body suits, and exoskeletons that translate body movements into electronic signals a computer can use to "fit" the user in a virtual environment), facial expressions and eye movements, and psychophysiological input (e.g., heart rate, blood pressure, muscle tension, skin resistance, and brain waves could be input to a computer for mood management or enhanced mediated interpersonal communication); see [Biocca and Delaney (1995)] for a complete discussion of these. The extent to which each of these media input channels contributes to interactivity and to presence has not been demonstrated.
The number (and type) of characteristics of the mediated presentation or experience that can be modified by the user also help determine the degree to which a medium can be called interactive. [Steuer (1995)] identifies the dimensions of temporal ordering (order of events within a presentation), spatial organization (placement of objects), intensity (of volume, brightness, color, etc.), and frequency characteristics (timbre, color). Others might include size, duration, and pace. [Heeter (1992)] suggests that a highly responsive virtual environment, one in which many user actions provoke even unnatural responses (e.g., entering a room produces verbal or musical greetings or rain!) could evoke a greater sense of presence than less responsive environments.
A third variable is the range or amount of change possible in each characteristic of the mediated presentation or experience. Interactivity, and perhaps therefore presence, is enhanced by expanding the degree to which users can control each attribute of the mediated experience. For example, in a highly interactive virtual environment the user can look out in any direction; move over large distances in each one; proceed at any pace and in any sequence desired; pick up, feel, and move many different objects each with different textures; and change the type and volume level of ambient sounds. In a different context, the larger the vocabulary of a computer speech recognition system (i.e., the more words it recognizes and to which it responds appropriately) the more interactive is the computer use experience.
A fourth variable important for interactivity and presence is the degree of correspondence between the type of user input and the type of medium response. [Steuer (1995)] suggests that the "mapping" between these two can vary from being arbitrary (e.g., pressing a sequence of keys on a keyboard to adjust a visual display) to natural (e.g., turning one's head in a virtual reality system to see the corresponding part of the environment). It is a "widely accepted working hypothesis" that "using our familiar sensorimotor skills to manipulate virtual objects directly by means of whole-hand input devices . . . contributes to our sense of presence much more than writing programs, twisting knobs, or pushing a mouse to accomplish the same task" [(Zeltzer, 1992, p. 129; see also Bricken, 1996; Held & Durlach, 1992; Sheridan, 1992)].
The final variable is the speed with which the medium responds to user inputs. The ideal interactive medium responds in "real time" to user input; the response or lag time is not noticeable. Although it accepts and responds to only audio input and uses only a limited frequency range, the telephone is highly interactive in terms of this criterion because interactions via telephone seem to occur in real time (except with calls over exceptionally long distances). On the other hand, the computational difficulty of processing inputs related to the user's position can cause even an advanced virtual reality system to present images an
Credo che ti possano tornare utili
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