Phases of VRET session: The phases are depicted in Table 2

During this phase, the participants will see no display (Black screen) through the HMD (In OFF mode) for 10 minutes. Concurrently, the VR simulator software will record the HR. The purpose will be to record the resting HR as per HR wrist band manufacturer’s instructions.

The HMD is turned ON once during the entire VRET session at this phase. In general, participants may show a higher physiological response when exposed to see something novel, this is referred to as orientation effects [43]. To overcome this, the participants in this phase will first view a surrounding 3D (3 Dimensional) stereoscopic scene of simulated dental environment through the HMD and only take a passive role by watching the interactive scene for 2 minutes. The participants are encouraged to turn his head and look around the virtual dentist’s office by way of the built-in motion tracking present in the HMD. Thus, the test subject will be lying on a real dentist chair while he will see its virtual counterpart inside the HMD as he looks around, turning his head. To allow the test subject to feel immersed in the virtual environment, we will include a generic 3D model of a person lying on the dentist chair such that when looking down, the subject will see what feels like his own body. The virtual environment seen by the patient will be displayed to the researcher on the computer screen and the HR will also be recorded.

In this phase, participants are exposed to five different VR scenarios (Idle, Mirror, Syringe, Drill with no sound, Drill with sound). The duration of each exposure will be 35 seconds. The interactive part of the simulated dental environment will be controlled by the researcher using the tool selection option on the UI window of the computer. The researcher will be able to control the playback of the VR scenarios.

VRET is conducted using a pre-determined hierarchy as follows:

Scenario #1 (Idle). This shows a dental operatory with various instruments surrounding the patient’s chair and a virtual dentist sitting next to the patient’s right hand inside the 3D scene.

Scenario #2 (Mirror). This shows the virtual dentist inspecting the oral cavity by picking the dentist’s mirror from the tray and approaching towards the patient’s oral cavity.

Scenario #3 (Syringe). This shows the virtual dentist performing injection by picking the syringe from the tray and approaching towards the patient’s oral cavity.

Scenario #4 (Drill with no sound). This shows the virtual dentist picking the drill and approaching towards the patient’s oral cavity with no sound of the drill.

Scenario #5 (Drill with sound). This shows the virtual dentist picking the drill. The drill makes the characteristic dental drill sound when approaching towards the patient’s oral cavity.

Participants will be exposed to the virtual dental scenarios in a gradual manner. To give the participants a gradual and optimal VRET, participants will be encouraged to rate their anxiety every 35 seconds following immersion with each VR scenario exposure by means of SUDS [39]. The general rule applied during exposure therapy is that the exposure is continued until the subjective score of the patient is reduced to less than 50 % [44].The VRET which we are planning to use in the current experiment has only a limited set of stimuli. Accordingly, we have decided to have a safe cut off of ≤2 SUDS scores before moving to the next VR scenario. This criterion was earlier applied in a similar study [17]. The exposures with scenario 1, 2, 3, 4 and 5 are repeated p,q,r,s and t times respectively until SUDS score of ≤2 is achieved. For instance, when participant reaches ≤2 SUDS with VR scenario of sitting idle on the dental chair then the next VR scenario of inspecting the oral cavity with a dental mirror is introduced. In subsequent sessions, the intensity of VR experiences will be progressively increased from a less threatening scenario to a more threatening scenario to make the simulation more realistic while focusing on areas of particular stress. For example, if a dental phobic participant report a fear of drills, the exposure is progressively increased from Scenario-1 (less fear evoking –‘Sitting in the dental chair’) to Scenario-5 (more fear evoking –‘Drill with sound’). To this end, situational cues that are simulated with VR scenarios are presented by the therapist in a well-controlled manner. During the simulated dental procedure participants will be asked to keep their mouth open similar to real dental experience from Scenario #2 (Examination with mirrors) through Scenario #5 (Drill with sound) and are advised to follow the instructions of the researcher, for example ‘to open their mouth really wide’. The added stimulus such as keeping the mouth open will be standardized and applied to all the participants. Participants will receive the therapy in one long session for 150 minutes with a 10-min break/s to avoid simulator sickness. If the participant’s SUDS rating has not decreased to 2 with any of the VR scenarios within 30 minutes of exposure the researcher will continue with the next VR scenario. The session will be video recorded for future validations and analysis of the subject’s body response during the VRET exposure.

Only verbal guidance and exposure techniques will be used on the participants during VRET. There will be no relaxation or other CBT-based interventions during therapy. Participants will be instructed to become as involved as possible and focus on their most frightening stimuli of the particular part of the virtual environment (describing the situation, any strange sensations and their feelings). This will be done to avoid dissociation from the VR experience. During the simulated treatment, real-time recording of the psychophysiological parameters (HR) will be determined to obtain more ‘objective’ measures of the physiological state of the participants during therapy. The HR responses obtained with different cues (Idle, Mirror, Syringe, Drill with no sound and Drill with sound) in this phase will be compared with the baseline phase (No display) to study the HR variations. The subjective measures of presence, realism and nausea (cybersickness) are recorded following the 1^st exposure of each VR scenario as shown in Table 2.

In this phase, the HR is recorded for 10 minutes and the participants are asked to fill out a series of self-reported measures as displayed in Table 2 and undergo the Behavioral Avoidance Test. The total number of VR exposures with each VR scenario will be summed up and recorded by the researcher. Also, the participants fill out the questionnaire on over-all presence, realism and nausea (cybersickness). After the VRET session, participants will remain in the waiting room for 15 min before leaving. This will be done in order to make sure that are no negative side effect of VRET a posteriori.

One week after treatment and at intervals of 3 and 6 months, the study participants will receive questionnaires that are used to assess the participants’ current dental anxiety scores and avoidance tendency (see Table 1) by email.