One of the techniques used to monitor variations in presence during a virtual reality experience is the analysis of breaks in presence (BIPs). Previous studies have monitored peripheral physiological responses during BIPs in order to find a characteristic physiological response. In this work, blood flow velocity (BFV) in middle cerebral arteries (MCAs) has been monitored using transcranial Doppler ultrasound during the exposure to a virtual environment. Two BIPs of different intensity were forced during the virtual reality experience. Variations in BFV during each BIP and during the recovery periods that followed them have been analyzed. A decreasing trend was observed in BFV signal during the most intense BIP in most subjects. However, during the less intense BIP an oscillating behavior was observed. Significant differences have been found between the maximum percentage variations observed in each BIP. During the recovery periods, an increasing trend was observed. The mean response times (time elapsed since the beginning of the period until the maximum percentage variation in the period occured) ranged between 10.116 s and 12.774 s during the BIPs, and between 11.025 s and 13.345 during the recovery periods, depending on the vessel and on the kind of BIP.

Virtual reality enables people to behave and feel as if they were present in a virtual environment and therefore is a useful tool in many fields. In order to study the usefulness of virtual environments, the concept of presence is examined. Up to now, the most common method to measure presence has been to use subjective measures based on validated questionnaires about user experience. However, more objective measurements, such as physiological measurements, are now being considered. In this study, transcranial Doppler (TCD) sonography is presented as a brain activity measurement technique that can be used to study presence in virtual environments. Thirty-two subjects navigated in a virtual environment in different immersive conditions while TCD was monitored. The results show that there are changes in blood flow velocity in the subjects during moments associated with different levels of presence.