The concept of presence, understood as the sensation of ”being there”, in a virtual environment, although being physically in a different place (for example, the experimental room), has been widely studied in the field of virtual reality to better understand the psychological mechanisms underlying virtual experiences.
In the literature, different techniques to measure presence have been proposed. Subjective techniques are mainly based on the use of questionnaires that allow the extraction of information about how the user has perceived the exposure to the virtual world. On the other hand, objective techniques are based on the analysis of user behavior during the exposure to the virtual environment and on the monitoring of physiological signals (such as electrocardiogram and skin conductance) to study possible changes in their parameters during the exposure.
One of the fields that is generating a growing interest in recent years is the analysis of brain activity to obtain information about the degree of presence. In this PhD Thesis, the use of Transcranial Doppler (TCD) is proposed to monitor brain activity during the exposure to virtual environments.
TCD is an ultrasound-based technique, which is non-invasive and has a high temporal resolution. It has not been used before in combination with virtual environments. It allows the measurement of blood flow velocity (BFV) in the main arteries of the brain. It has been widely used to monitor cerebral hemodynamics during cognitive tasks in psychophysiological research. These previous studies have shown that BFV measured by TCD increases when users are doing a cognitive task when compared with repose periods.
The global objective of this PhD Thesis is to analyze if the TCD technique can be a complementary tool to analyze brain activity during the exposure to virtual environments and if, consequently, it can be used as a tool to study presence from a neuroscientific point of view.
The initial hypotheses are the following:
1. TCD will be easily used in combination with virtual reality systems, because it is a non-invasive technique that has been previously used in psychophysiological research.
2. It will be possible to use the BFV data obtained using TCD to analyze changes of brain activity during the exposure to virtual environments, allowing the use of the technique to study presence from a neuroscientific perspective.
3. There will be differences in the observed BFV variations during the exposure to virtual reality systems with different levels of immersion and different navigation methods, which will be associated to different levels of presence in participants.
4. There will be correlations between the degree of presence in virtual environments measured by questionnaires and certain parameters of the BFV obtained during the exposure to the environments.
5. Each of the different individual factors that constitute a virtual experience (such as visual perception or motor tasks to navigate) will have an influence on the observed BFV variations.
To study the proposed hypotheses, four different experiments were conducted, during which the BFV was analyzed in different conditions:
1. Analysis of BFV in different navigation conditions. In this study, BFV was monitored using TCD during the exposure to a highly immersive virtual environment in different navigation conditions.
2. Analysis of BFV in different immersive conditions. In this study, BFV was monitored using TCD during the exposure to a less immersive virtual reality configuration, in order to compare the results with those obtained in the first study with the highly immersive configuration.
3. Analysis of BFV during a visual perception task. In this study, the influence on BFV of a single aspect of the virtual experience (simple visual stimulation) was analyzed. In this case, the signal was monitored while the user was exposed to changing illumination conditions.
4. Analysis of BFV during motor tasks. In this case, the signal was monitored while the participant performed simple motor tasks to control a joystick. That way, the influence of this factor on the observed BFV variations during the virtual reality experience could be analyzed.
During the first two studies, it was observed that there was an increase in BFV when the participants were exposed to a virtual environment. This increase could be generated by the complex interaction of different factors, such as visuospatial interaction tasks, attention tasks, the creation and execution of a motor plan, emotional changes and presence variations. In fact, observed BFV variations were different depending on the kind of navigation and the degree of immersion. Significant correlations between mean BFV in middle cerebral arteries during the exposure to the different virtual reality configurations and specific responses of the presence questionnaires were found.
The other studies that were included in this PhD Thesis were focused on the analysis of individual factors (visual and motor factors) that contribute to the virtual experience. Signal processing techniques that had not been applied previously for the study of BFV in psychophysiological experiments were applied, based on spectral analysis and on the calculus of non-lineal parameters of the BFV signal in the different experimental conditions. Results showed differences between the repose periods and the periods during which the tasks were performed. These results can have an important contribution for the analysis of the influence of individual factors such as visual perception and motor tasks on the observed BFV variations during the exposure to virtual environments.
Globally, the results from the different studies have shown that the TCD technique can be combined easily with virtual environments, even in the highly immersive ones, because the quality of the captured signals is not affected by the fact of being navigating in a virtual environment. Furthermore, the TCD signal monitoring does not affect to the capability of participants of focusing their attention on the virtual environment. On the basis of the studies conducted in the present PhD Thesis, future works with TCD could deepen in the study of presence generated by different kinds of virtual reality environments and configurations from a neuroscientific point of view.