Psychophysical Measures
Psychophysical methods require an observer to give a subjective rating of the physical magnitude of a stimulus. This can be done by simply asking participants to assign a value to the degree of presence a stimulus induces (free-modulus magnitude estimation), by investigating the extent to which participants can discriminate between stimuli (paired comparison test), or asking participants to “translate” the intensity of their sensation of presence to a different modality (cross-modality matching).
Although only a limited number of studies in the field of presence have included psychophysical measures, at least two experiments have shown such measures to be sensitive to different levels of presence. In addition, these measures are relatively cheap, can be used unintrusively and are easy to use.
Like all subjective methods, psychophysical measures are prone to bias. Also, what these methods will measure is heavily dependent on the experimenter’s instructions, and on participant’s own interpretation of what they should rate. Schloerb’s method (see below), finally, seems to be more focused on realism than on presence.
Description
Concept: Physical presence.
Cross-modality matching (CMM) is a variation on magnitude estimation. It is based on the premise that a person can monotonically represent the experiences of one sensory modality through another modality by producing a “subjectively equal” representation using a measure of the second sensory modality. E.g., “make this light as bright as the strength of the presence you experienced in this virtual environment” (Welch, 1997). CMM is especially useful for the measurement of constructs that do not lend themselves easily to verbal scaling (IJsselsteijn, De Ridder, Freeman, & Avons, 2000).
Research
Not reported
Primary Source
- Welch, R. B. (1997). The presence of aftereffects. In G. Salvendy, M. J. Smith, & R. J. Koubek (Eds). Designs of computing systems: Cognitive considerations (pp. 271-276). Amsterdam: Elsevier.
Other Literature
- IJsselsteijn, W. A., De Ridder, H., Freeman, J., & Avons, S. E. (2000). Presence: Concept, determinants and measurement. Proceedings of the SPIE, 3959, 520-529.
- Sadowski, W., & Stanney, K. M. (2002). Measuring and managing presence in virtual environments. In K.M. Stanney (Ed.), Handbook of virtual environments technology. Mahwah, NJ: Lawrence Erlbaum Associates.
Description
Concept: Physical presence.
Free-modulus magnitude estimation is based on the method magnitude estimation as described by Stevens (1971, in Snow and Williges, 1998). Subjects are asked to assign any value to a first stimulus, and then assign successive numbers accordingly to following stimuli. The geometric mean of judgments of several subjects can then be taken as the psychological scale value, thus producing data on a ratio scale.
Research
A series of 3 experiments (n=36, mixed design) was conducted using this method, across which 11 independent variables were varied: scene update rate, visual display resolution, field of view, sound, textures, head tracking, stereopsis, virtual personal risk, interactions, presence of a second user, and object detail. Participants carried out 5 tasks in a VE, wearing a HMD. After each trial, participants provided a free-modulus magnitude estimate of their level of perceived presence during the trial.
Field of view, sound, and head-tracking showed the largest (positive) effects on perceived presence. Other significant (positive) effects were those of visual display resolution, texture-mapping, stereopsis, and the presence of a second user.
Sensitivity:the method was sensitive to the effect of several environmental parameters.
Reliability:not reported
Validity:factors which are hypothesized to enhance presence increased magnitude ratings.
Primary Source
- Snow, M. P., & Williges, R. C. (1998). Empirical models based on free-modulus magnitude estimation of perceived presence in virtual environments. Human Factors, 40, 386-402.
Description
Concept: Physical presence.
Welch, Blackmon, Liu, Mellers, & Stark (1996) approach presence as telepresence, or the experience of being in the same distant physical location as the devices you are controlling. The hypothesize that maximal presence occurs when the user:
- Feels immersed within the VE
- Feels capable of moving about in it and manipulating content
- Has an intense interest in the interactive task
To assess this, the method of paired comparison is used, which is well-established in psychological research. Subjects are exposed to pairs of VE’s, and are asked to indicate for each pair which one produces the greater amount of presence. The size of the perceived difference has to be estimated by a number between 1 and 100.
Research
The method was used in 2 experiments investigating the effects of interactivity, pictorial realism, and delay of visual feedback on presence. Participants (n=24, n=20, within-subjects design) were exposed to pairs of environments (24 pairs in each experiment) and asked to indicate their feeling of being physically located in the environments by paired comparison. The task consisted of driving a virtual car. Factors varied within-subjects were interactivity (driver or passenger), pictorial realism (high/low) and delay of visual feedback (short/long). Results show significant positive influences of all 3 factors.
Sensitivity:method discriminated between different conditions
Reliability:not reported
Validity:factors which are hypothesized to enhance presence increased the ratings.
Primary Source
- Welch, R. B., Blackmon, T. T., Liu, A., Mellers, B. A., & Stark, L. W. (1996). The effects of pictorial realism, delay of visual feedback, and observer interactivity on the subjective sense of presence. Presence: Teleoperators and Virtual Environments, 5, 263-273.
Description
Concept: Physical presence.
This is a variation on the paired comparison method, alike to the Turing test. It is based on signal detection theory. The idea is to set up a test in which the subject must make a series of observations under identical conditions, and determine his or her location based on the interaction. The subject is then asked whether s/he perceives that s/he is physically present in the specified environment. Presence is estimated by the ratio of correct identifications to incorrect identifications.
Schloerb (1995) suggests a set-up in which participants wear a head mounted display (HMD), which can either display a VE or filmed images from the real world. Participants are asked over many trials to indicate whether what they are seeing is real or virtual. The presence measure is based on the relative frequency of “yes” responses.
Research
Not reported
Primary Source
- Schloerb, D. W. (1995). A quantitative measure of telepresence. Presence: Teleoperators and Virtual Environments 4, 64-80.
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