In recent years, in the treatment of phobias, exposure therapy in virtual reality is becoming more and more popular as an alternative for in-vivo exposure. Effectiveness of virtual reality exposure therapy (VRET) is comparable to in-vivo exposure therapy, though several characteristics of the VRET have an impact on the outcome of the therapy (e.g., immersion into the virtual environment (VE), familiarity with the VE). Additionally, the use of VRET varies from multiple exposure sessions to single-session VRET. Single-session therapy has an economic advantage and in in-vivo, post therapy outcomes show good results. In virtual reality, the assessment of outcome post therapy and in follow-up of single-session therapies is still needed for an evaluation of this approach. As an outcome measure, behavioral assessments are especially relevant for effectiveness studies as in fear of heights it is closer to the individual's life to know how high they voluntarily go up a building than to have hypothetical self-report questionnaire results.

Much research has been conducted on physiological correlates of the subjective experience of fear in exposure therapy as they are assumed to be a prerequisite for effective exposure treatment. Skin conductance level (SCL) and heart rate can be used for objective manipulation checks of exposure therapy. SCL is found to increase during fearful situations independent of setting while heart rate only increases during in-vivo exposure. Contrary to heart rate, heart rate variability (HRV) is not thoroughly studied in VRET yet. HRV is associated with the adaptability of an organism to new environments and cognitive functioning. High Frequency HRV is found to be reduced in individuals with mental disorders, and positive and negative mood inductions lead to differential HRV responses overall.

Respiration is a well-studied correlate of emotional experience and especially of the experience of fear and anxiety. In a series of experiments, it was found that sighing is tightly associated with relief in or after fearful or stressful situations and might become maladaptive when used disproportionally often. This study shows that respiration parameters have an impact on the handling of fearful situations in a reciprocal way. On the one hand, fear leads to an increased respiration rate and sigh rate while on the other hand, an altered sigh rate or respiration rate might have an impact on the experience of fear and be used as a defensive reaction to a fearful situation. As such, specific respiration patterns might act as emotion-driven behaviors (EDB). EDBs are responses to emotions that result in a short-term reduction of a negative state while in long-term support the maintenance of the phobia.

The aim of this study is to examine the effectiveness of a single-session VRET for acrophobia with a multimethod outcome design. Familiarity of the setting will be high with the use of a well-known tower in this area. Immersion into the VE will be assessed with a presence questionnaire. For a manipulation check, physiological data will be assessed, i.e., SCL, heart rate and HRV. Primary outcome measure will be a behavioral approach test (BAT) as behavioral assessment. Additionally, after four weeks, a follow-up assessment will investigate the stability of the effectiveness of the VRET in comparison to a waitlist control group. A second aim of this study is to investigate the impact of respiration as an EDB on the effectiveness of an exposure therapy. Therefore, the association between respiration and outcome of the VRET will be analyzed.

Hypothesis 1: Participants in the VRET condition show less height avoidance in the BAT after the intervention than participants in the control condition.

Hypothesis 2: Participants in the VRET condition show less height avoidance in the BAT in a four-week follow-up assessment than participants in the control condition.

Hypothesis 3: Participants in the VRET condition score significantly lower on the Acrophobia Questionnaire at follow-up than participants in the control condition.

Hypothesis 4: During the VRET, breath holding is used as EDB. Participants that hold their breath, profit less from the VRET than participants that do not hold their breath.

Hypothesis 5: During the VRET, sighing is used as EDB. Participants that sigh, profit less from the VRET than participants that do not sigh.

Inclusion Criteria:

• diagnosed acrophobia
• read, speak and write in German language

Exclusion Criteria:

• current panic disorder
• lifetime psychotic disorder
• current suicidality
• neurological disorder
• current substance use disorder

Equal gender distribution will be ensured because physiological processes and their impact on psychological processes can differ between men and women.

All collected de-identified individual participant data will be shared with other researchers.

All data will become available for six months with publication.

Data access requests will be reviewed by an external independent review panel. Requestors will be required to sign a data access agreement.

All roles are aware of the fact that the virtual reality exposure is the primary intervention. No information is given to the participant about the impact of the "video" in the waitlist control group.