Physiological Monitoring as an Objective Tool inVirtual Reality Therapy

BRENDAK. WIEDERHOLD,1 Ph.D., M.B.A., BCIA, DONG P. JANG, M.S.,2SUN I. KIM, Ph.D., 2 and MARK D. WIEDERHOLD,1 M.D., Ph.D., FACP

ABSTRACTThe goals of this study were twofold: (1) to investigate nonphobics’ and phobics’ physio-

logical response in virtual environments, and (2) to analyze the trend of phobics’ physiologyduring virtual reality (VR) treatment. As a measure of physiology, heart rate, skin resistance,and skin temperature were acquired. The data for two groups of participants were analyzed:22 nonphobic participants (mean = 32 ± 9.4 years) and 36 subjects with fear of flying (mean =40 ± 12.1 years) who met the DSM-IV criteria for fear of flying. As a result, skin resistanceshowed significant differences between nonphobics and phobics, T(56) = 2.978 and p < 0.01,respectively. The physiological response of 33 phobic participants, who were able to fly with-out medicine after VR treatment, showed a gradual trend toward the nonphobics’ physiolog-ical responses as therapy sessions progressed. In this study, physiological monitoring, inparticular skin resistance, appeared to be useful both in understanding the physiologicalstate of phobic individuals and in evaluating the results of treatment in VR psychotherapy.

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CYBERPSYCHOLOGY & BEHAVIOR

Volume 5, Number 1, 2002© Mary Ann Liebert, Inc.

INTRODUCTION

VIRTUAL REALITY (VR) technology has re-cently attracted much attention in clinical

medicine. Given the new opportunities offeredby this technology, several studies have beensuccessfully conducted using VR for graded-exposure therapy, especially in the treatmentof phobias.1–8 Most of these studies relied onthe individual’s subjective feeling of distressfor evaluating anxiety level by using subjec-tive units of distress (SUDs) or self-reportquestionnaires. While analyses of SUDs orself-report questionnaires may elucidate thephenomenology of experiences, they remainsubjective, and posttest measures are depen-dent on the memory of an event. According toLang’s 1985 proposal, anxiety assessment

should include subjective and objective mea-sures. He also stated that the motor programof fear (as evidenced by physiological arousal)must be activated in order to change the per-son’s fear structure and have resulting behav-ioral change.9 A few researchers, therefore,have tried to objectively measure anxiety andstress responses in real time by using physio-logical response such as heart rate, respirationrate, skin resistance, skin temperature, and pe-ripheral brain wave EEG activity in virtual en-vironments.10–13 Wiederhold et al. founddifferences between the phobic’s physiologicalresponses and nonphobics’ responses whenplaced in a virtual flying environment relatedto their phobia.10 Meehan found a high andsignificant correlation between presence andskin conductance level with 10 nonphobic

1Virtual Reality Medical Center, San Diego, California.2Department of Biomedical Engineering, Hanyang University, Seoul, Korea.

participants in virtual room environments.11Stoermer et al. showed heart rate variabilitywas a powerful and easy-to-use instrument formonitoring the user’s stress level.12

While these studies all showed the necessityof monitoring user’s psychophysiological statesin VR psychotherapy, they did not find a sys-tematic relationship between nonphobics’ andphobics’ physiological response to virtual envi-ronments. The purposes of the study, therefore,were twofold. One was to investigate the differ-ences between nonphobics’ and phobics’ physi-ological responses in virtual environments.And the other was to analyze the changes thatoccur in phobics’ physiology as VR treatmentsessions progress and desensitization occurs.

MATERIALS AND METHODS

Participants

Twenty-two nonphobic participants (mean =32 ± 9.4 years) were recruited through localnewspaper advertisements. Thirty-six partici-pants with fear of flying (mean = 40 ±12.1 years) who came to the Virtual RealityMedical Center (VRMC) for treatment were re-cruited for the study. Phobic participants metthe DSM-IV criteria for a specific phobia.Table 1 shows demographic characteristics ofparticipants.

Apparatus

The virtual environment system for this studyconsisted of a head-mounted display (MRG4,Liquid Image Inc., Toronto, Ontario, Canada),electromagnetic head tracker (INSIDETRAK,

Polhemus Inc., Toronto, Ontario, Canada) andflight seat with subwoofer that delivered vibra-tion to the subjects. It was designed by Drs.Hodges and Rothbaum of Virtually Better, Inc.(Atlanta, GA) who previously performed VRtreatment for acrophobia and fear of flying.1,5

Physiological measures

Skin resistance (SR) was measured to deter-mine changes in sweat gland activity. SR gen-erally decreases as sweat gland activityincreases. SR was monitored with two silver/silver chloride electrodes placed on the ringand index fingers of the left hand. For heartrate (HR), a small amount of electrode gel wasplaced on each disposable electrode attachedto the participant’s right and left wrist. Tem-perature was measured by a thermistor at-tached to the middle finger of the participant’sright hand with cloth tape. An I-330 C-2 com-puterized biofeedback system manufacturedby J&J Engineering (Poulsbo, WA) was used tocollect physiological data. For phobic partici-pants, the Subjective Units of Distress Scale(SUDS) was administered. It is used to rateanxiety on a scale from 0 to 100, with 0 beingno anxiety and 100 being the most anxiety theyhave ever felt.

Procedures

Nonphobic. After signing an informed con-sent, a 5-min eyes-open baseline was taken.The participant then was placed in the MRG4head-mounted display. The participant was al-lowed to look around the virtual plane to be-come oriented for a short while before the20-min exposure began. The participant worea head-mounted display and viewed a three-dimensional computer-generated image of thefollowing flying scenes: sitting in the passen-ger cabin of a plane with the engines on, taxi-ing, taking off, flying in good weather, flyingin bad weather, and landing.

Phobic participants. Phobic participants wereall given the following protocol:

� Session 1: All treatment procedures and theirrationale were explained, and informed

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TABLE 1. DEMOGRAPHIC CHARACTERISTICS OF PARTICIPANTS

Nonphobic Phobic

Totaln 22 36

GenderMale/female 10/12 14/22% 45/55 39/61

Age (years)Mean 32.0 40SD 9.4 12.1Range 18–51 20–73

consent for treatment was obtained. Thepatient was given an intake to assess forseizure history, heart problems, and med-ication usage.

� Session 2: All participants were taught relax-ation and diaphragmatic breathing skills,with the use of visual feedback of heartrate and respiration rate, prior to begin-ning the VR exposure therapy.

� Sessions 3–8: Participants were placed in aVR environment after an initial 5-minbaseline of diaphragmatic breathing. TheVR world was progressed to more andmore difficult scenarios as the patient wasable to maintain physiological control. Atevery step, the therapist could see andhear what the client was experiencing inthe virtual plane. If the participant’s levelof anxiety became overwhelming, partici-pants returned to a less stressful level oftreatment, or simply removed the head-mounted display and exited the virtualaircraft.

Analysis

The percentage change from baseline wasused for analyses rather than absolute valuesbecause physiology levels often vary widelyby individual and environment. Therefore, be-fore comparing physiology with presencemeasures, percentage change of heart rate(DHR) was calculated as follows:

DHR = (MeanVR 2 MeanBaseline)/MeanBaseline

where MeanVR is the mean of heart rate dur-ing experiencing VR and MeanBaseline is themean of Heart Rate during baseline.

Percentage change of skin temperature(DST) and percentage change of skin resistance(DSR) were also calculated using the samemethod. Data was analyzed using the conven-tional Student t test.14

RESULTS

The results of the conventional Student t testbetween nonphobics and phobics physiologi-cal responses in the virtual environment are

shown in Table 2. Session data from the firstexposure session for both phobics and non-phobics are used. At this point, phobics hadlearned diaphragmatic breathing, which non-phobic participants were not taught prior toexposure. The percentage change of skin resis-tance (DSR) showed significant differences be-tween nonphobics and phobics, T(56) = 2.978,p < 0.01. Before the experiment, it was pre-dicted that percentage change of heart ratewould also show significant difference be-tween the two groups, however, this differencedid not occur. It is felt that more sensitive mea-sures, such as analysis of heart rate variability,might show differences.

After completion of treatment, 33 of the36 phobic participants were able to fly withoutmedication, indicating a success rate of 91.6%.Figure 1 illustrates the trend of DSR of non-phobic participants and “treatment-responder” phobic participants. Nonphobicsshowed physiological arousal at the beginningpart of the virtual exposure, but by the end ofthe 20-min flight, physiology had actually sta-bilized and showed less arousal than at base-line, prior to the beginning of the virtualexperience. According to previous research,repetition of a stimulus that is novel because ofits unexpectedness reduces the information inthe stimulus and, thus, the reaction is rapidlyhabituated.15–17 Contrary to nonphobic partici-pants, phobic participants still remain arousedthroughout the first 20-min VR session. Thephysiological responses of the 33 phobic treat-ment responders, who successfully flew with-out medicine after VR treatment, showed agradual trend toward the nonphobics’ physio-logical responses as therapy sessions contin-ued. This implies that desensitization, atreatment based on gradually and systemati-cally exposing the phobic person to the feared

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TABLE 2. STUDENT T TEST BETWEEN PHOBIC ANDNONPHOBIC PARTICIPANTS

Significance t (two-tailed)

DHeart rate 20.906 0.369DSkin temperature 21.042 0.302DSkin resistance 2.978 0.004

object or situation, and having them calmthemselves in the anxious situation, was effec-tive in the treatment of fear of flying using avirtual environment. It also proved the asser-tion of Foa and Kozak that, as treatment con-tinues and habituation occurs, there should bea lessening of arousal.18

The physiological responses of the threetreatment nonresponders who failed to flyafter VR therapy are illustrated in Figure 2.The patterns appeared to be irregular and dur-

ing the last session did not show any similarityto the nonphobic response. A comparison be-tween the phobic responders and nonrespon-ders is shown in Figure 3. The average SUDSand the average change of SR showed a de-crease over treatment in the treatment respon-ders, whereas those who were nonrespondersshowed less arousal physiologically for all ses-sions, but more SUDS as treatment progressed,indicating more subjective arousal. The factthat they responded less physiologically might

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FIG. 1. The average change in skin resistance for nonphobic participants and phobics’ 1st, 3rd, and last session whotook a flight without medication following completion of treatment.

FIG. 2. The average change in skin resistance in nonphobic participants and phobics’ 1st, 3rd, and last session whodid not fly after treatment.

also indicate a lack of immersion, which led toa lack of desensitization.

CONCLUSION

In future studies, more systematic researchshould be performed to include more sensitivephysiological measures, such as blood pres-sure and heart rate variability. It appears how-ever that SR is clearly a sensitive and usefulmeasure of treatment response and that physi-ological monitoring is a useful part of a phobictreatment protocol. Phobics do show a signifi-cantly different response than nonphobicswhen placed in the virtual world, and this dif-ference tends to decrease over time as desensi-tization and habituation occur. Physiologyappears to be a useful tool both in understand-ing the physiological state of phobic and non-phobic individuals and in evaluating theresults of treatment in VR psychotherapy.

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FIG. 3. The comparison between phobic treatment responders and nonresponders: average Subjective Units of Dis-comfort Scale (SUDS) and the average change in skin resistance (SR).

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Address reprint requests to:Brenda K. Wiederhold, Ph.D., M.B.A, BCIA

Virtual Reality Medical Center6160 Cornerstone Court East, Ste. 155

San Diego, CA 92121

E-mail: bwiederhold@alliant.edu

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