Neural Correlates

We discuss here separately psychophysiological techniques aimed at studying brain processes. Such measures seem promising, because they yield “hard”, objective data which are in no way influenced by interpretation on the part of the participant. However, the interpretation of such data would be extremely difficult , because so little is known about the neural processes that are involved in the complex experience of presence. IJsselsteijn (2004) discusses several paradigms which could contribute towards an unambiguous operationalisation of presence in order to meaningfully interpret brain patterns. In addition to these problems of interpretation, brain imaging equipment is generally expensive and extremely intrusive. Body movements are restrained, and especially fMRI equipment tends to produce a lot of noise.

Electroencephalogram (EEG)

Description

An electroencephalograph is a device which amplifies and records electrical activity from the scalp using a number of small electrodes which are temporarily adhered to the scalp. EEG has been suggested to study presence from a cognitive (Pugnetti, Mendozzi, Barberi, Rose, & Attree, 1996) or emotional (Huang et al., 1999) perspective. Schlögl, Slater, & Pfurtscheller (2002) discuss properties, advantages and disadvantages of EEG. Advantages are that it is non-invasive, has a high time-resolution and can be used in almost any environment. Disadvantages are the poor signal to noise ration and inter- en intra-trial variability. They suggest that Adaptive autoregressive (AAR) parameters can be used to continuously classify the EEG spectrum. This might be useful in presence research. More specifically, they suggest that EEG could be used to enhance the “breaks in presence” technique.

Research

Pugnetti et al. (1996) have used EEG and auditory evoked potentials (EP) to study cognitive tasks carried out in a VE (n=10). An inverse relationship was found between EEG alpha amplitude and performance measures (time, errors).

Strickland & Chartier (1997) investigated whether it is possible to obtain valid brain activity readings when subjects are wearing a headset, and whether there are differences in brain activity in similar virtual and real environments. Participants (n=14, within-subjects design) performed a set of tasks both with and without a HMD. Tasks were sitting with eyes closed, sitting with eyes open looking straight ahead, tracking the movement of a hand, and looking around. No artefact differences were found in Alpha, Theta, or Beta1 frequencies. Beta2 showed muscle artefacts caused by the weight of the helmet. Delta frequencies also showed artefacts, possible due to poor headset visual quality. Authors conclude that there was no interference of the helmet. Results show that there are indeed differences between real and virtual image processing, but variations are more affected by subject and task than by brain area.

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Functional Magnetic Resonance Imaging (fMRI)

Description

Functional magnetic resonance imaging (fMRI) is a technique which detects changes in the blood flow to the brain, using magnetic fields to provide images of the areas of the brain that are activated during a cognitive task. Thus, brain activity patterns associated with various types of mental activities can be studied.

Research

Studies using fMRI as a presence measure have not yet been reported.
Hoffman, Richards, Coda, Richards, & Sharar (2003) have investigated whether participants can experience presence during an fMRI scan. In their study, participants (n=7, within-subjects design) engaged in a VE depicting a winter landscape in which they could throw snowballs. During this experience they were in an fMRI scan. There were two conditions: a high-presence condition in which view was unobstructed, and a low-presence condition in which a white cross obstructed part of the view. Subjective presence was measured by one question. All subjects reported higher presence in the high-presence condition (mean rating was 7.0 in high presence condition, 4.1 in low-presence condition, 10 point scale). fMRI results are not reported in the study, because the authors fear misinterpretation.

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