FORUM Presence and The Sixth Sense Abstract 2 Presence and Hypothesis Selection This paper discusses the notion that presence may be An important current in perceptual theory, exem- considered as a selection mechanism that organizes the plified by the work of Richard Gregory, is that percep- stream of sensory data into an environmental gestalt or tion involves the act of selection between hypotheses. perceptual hypothesis about current environment. A par- This is illustrated by the famous Kanisza triangle ticular environmental gestalt results in scan-sensing of the (figure 1). Downloaded from http://direct.mit.edu/pvar/article-pdf/11/4/435/1623799/105474602760204327.pdf by guest on 28 September 2021 world in a particular pattern reminiscent of saccades and It is far more likely that a white triangle is overlaying fixations in eye scan paths. The environment hypothesis the three black circles rather than the (physically cor- is continually reverified or else a break in presence oc- rect) hypothesis that three circles have had parts cut out curs. Presence is therefore compared to visual hypothe- of them and then arranged perfectly to create a triangu- sis selection in the work of Richard Gregory and Law- lar shape. The hypothesis is typically accepted so strongly that triangle edges between the circles are usu- rence Stark. The implications for measurement are ally seen, although there are no such edges, of course. discussed, and it is concluded that physiological measures The scan path theory of Lawrence Stark postulates a indicating breaks in presence are worthy of study, and top-down cognitive model of visual perception. A visual that the study of presence is also the study of what hypothesis results in a repeated pattern of eye move- maintains an environmental gestalt. ments consisting of alternate saccades and fixations. There is a way to look at the world when the hypothesis about what we are seeing is one thing rather than an- 1 Introduction other. The recurring fixations are as if the observer is repeatedly verifying the hypothesis: this is a kitchen Presence is often thought of as the sense of rather than a bedroom, a cafe´ rather than a bathroom, ‘being there’ in the virtual environment. Draper, an indoor experience rather than an outdoor one, a Kabur, and Usher (1998) and IJsselsteijn, Freeman, duck rather than a rabbit. (See figure 2.) Notice how and de Ridder (2001) provide reviews of this area. In your pattern of eye movements changes according to this paper, we outline another way of thinking about how you decide to see this figure. There is a way to look presence, one that does not rely on a characterization at a duck that is different from how to look at a rabbit. of subjective experience. Presence is considered as a Of course, the physical shading that defines the figure is perceptual mechanism for selection between alterna- invariant. tive hypotheses: “I am in this place” and “I am in A top-down cognitive mechanism in conjunction with that place” (“I am confused”). This idea is based on the “stuff ” of the image determines your eye move- the approaches to perception of Richard Gregory ments, rather than eye movements and perception being (1998) and Lawrence Stark et al. (2001). This view determined by what is out there. In other words, we see of presence was first discussed by Slater and Steed (2000). In this paper, we further reflect on this idea and its consequences for measurement. Mel Slater [email protected] Presence, Vol. 11, No. 4, August 2002, 435–439 Department of Computer Science © 2002 by the Massachusetts Institute of Technology University College London Slater 435 436 PRESENCE: VOLUME 11, NUMBER 4 Downloaded from http://direct.mit.edu/pvar/article-pdf/11/4/435/1623799/105474602760204327.pdf by guest on 28 September 2021 Figure 2. A duck or a rabbit? Figure 1. The Kanisza Triangle. what is already in our mind’s eye. In the movie The Sixth Sense (directed by M. Night Shyamalan), dead people were said to “see what they want to see.” It seems that the same may be true for live people. At any time, but especially when we are immersed in a VE, we may be receiving competing signals from several different environments. The issue of presence is interest- ing only when there are competing signals from at least two environments. This is fundamental to the concept of presence in this paper. If (somehow) an actor were receiving signals from only one environment, then by definition that actor is present in that environment. Of course, there are all sorts of other things that may be studied: the degree to which the actor is interested in, Figure 3. The pit room (UNC, UCL experiment; © ACM 1999). paying attention to, involved in, that environment. But these are not presence. pice rather than the physical place of a CAVE. Of The critical issue is how will the actor act? To which course, the participant has abstract knowledge that “re- set of signals will the actor respond? ally” they are in a CAVE, but visual perception over- In the pit room (figure 3) (Usoh et al., 1999), the rides this knowledge and the bodily system reacts as if participant walks into a virtual room with an 18 m pre- they were in the pit room: heart rate rises, locomotion is cipice at its center. In fact, the person is standing in a carefully judged, and the subject reports symptoms of CAVE-like system or wearing a head-mounted display, anxiety (Meehan, Insko, Whitton, & Brooks, 2001). feeling the cables, temperature, and the physical signals Given competing signals from different environments, of that real place. But visual signals indicate that they at any moment action is chosen on the basis of selection are standing in a room with a dangerous precipice. Pres- between alternative hypotheses, alternative interpreta- ence in the virtual room at any moment results in choice tions. Some signals (such as the weight of the cables) of the hypothesis that indeed this is a room with a preci- will be in the background in one interpretation (“Iam Slater 437 in the pit room”), and come to the foreground (“Iam Behavioral measures such as the looming response in a CAVE”) in another. How the participant scan- require events in the environment that may have noth- senses the environment will depend upon which inter- ing to do with the application. Physiological measures pretation is dominant. The visual signals of the pit room (Meehan et al., 2001) appear to offer a promising way may be discounted and defocused when the interpreta- forward. However, physiological measures for anxiety tion “I am really in a CAVE” becomes the dominant (as in the pit room) have been used, but they are appro- reality. Meehan added static haptics representing the priate for only a very small minority of applications. edge of the precipice, and Basdogan, Ho, Srinivasan, What is the physiological response appropriate to and Slater (2000) added force feedback to a shared vir- seeing a virtual chair, or a room, or the deck of a ship? Downloaded from http://direct.mit.edu/pvar/article-pdf/11/4/435/1623799/105474602760204327.pdf by guest on 28 September 2021 tual manipulation task, both resulting in increased re- Basing physiological responses on specific events or ported presence. Perhaps the greater the number of objects within the VE does not offer a general consistent sensory signals belonging to an environment, solution. the greater the probability that the interpretation corre- A break in presence (BIP) is the moment of switch sponding to that environment will be selected. between responding to signals with source in environ- The presence selection mechanism is in answer to a ment X to those with source in environment Y. It is fundamental question: Where am I? A way to think of equivalent to the aha! moment in gestalt psychology: this is that the totality of signals may be organized into the switch between seeing the duck and the rabbit, for alternative gestalts, with each one corresponding to a example. If we take X as the VE and Y as the physical different interpretation and different set of actions world in which the experience is located, then one mea- (Slater & Steed, 2000). sure of presence is the number of transitions per unit Actions may be of several types. They may be invol- time from X to Y. It has been shown how this can be untary or voluntary. For example, there may be involun- modeled as a stochastic process (Slater & Steed, 2000). tary muscular tremors or heart racing when the pit is The big problem with this approach is that the partici- first seen. A voluntary action is a decision to walk pant has to somehow signal the moment of transition; around the ledges at the sides of the pit room rather in the experiment reported in the earlier paper, this was than just walk across the “empty space.” Actions may be done verbally. The problem is not that giving the signal conscious or unconscious. A participant may not be breaks presence in the VE (which has already been bro- aware of postural sway in response to an event, for ex- ample. Actions may be supported or denied by the par- ken) but that the count itself may be unreliable. If the ticipant. A London firefighter when once seeing an early count is zero, is this because there really were no transi- version of the pit room clearly jumped back in order to tions or because the subject forgot to report any? avoid “falling in.” Later, he denied that there was any A way forward with respect to measurement may be a such occurrence and said that he felt nothing when see- combination of physiological measures and BIPs.