|Home | About | Journals | Submit | Contact Us | Français|
The current study tested whether the effectiveness of distraction using virtual reality (VR) technology in reducing cold pressor pain would maintain over the course of eight weekly exposures. Twenty-eight adults, 18 to 23 years of age, underwent one baseline cold pressor trial and one VR distraction trial in randomized order each week. VR distraction led to significant increases in pain threshold and pain tolerance, and significant decreases in pain intensity, time spent thinking about pain, and self-reported anxiety, relative to baseline. Repeated exposure did not appear to affect the benefits of VR. Implications for the long-term use of VR distraction as a non-pharmacological analgesic are discussed.
The use of Virtual Reality (VR) technology to enhance distraction is a relatively new approach to acute pain management. Although definitions of VR distraction in the literature vary considerably, most VR interventions involve a human-computer interface in which the user interacts with a responsive, multi-sensory virtual environment. VR applications typically utilize a head-mounted wide-angle display and headphones to present the virtual environment. The user interacts with the virtual environment by manipulating a joystick or mouse, or via head- or hand-motion-sensitive tracking devices, resulting in a sense of being more or less “present” in the virtual environment17,19. VR distraction is presumed to affect pain perception by competing for finite attentional resources and by blocking external stimulation associated with the real environment and the painful stimulus20.
In their 2005 review, Wismeijer and Vingerhoots identified 10 studies of VR distraction for analgesia involving a total of 108 participants. Although they documented encouraging analgesic effects for VR distraction for a wide range of medical procedures and laboratory pain, the findings were limited by the small samples (6 of the studies were case studies; two involved 7–12 subjects) and methodological weaknesses20. More recent, larger scale studies provide additional support for the effectiveness of VR analgesia for cold pressor pain in children3 and burn patients ages 9–4012.
Despite evidence of the efficacy of VR- distraction, uncertainty about the real-world value of VR distraction remains due to the concern that the distractive properties of VR conferring it such success may attenuate with repeated exposure. Because distraction interventions are thought to work by competing for attention otherwise directed towards the painful stimulus, the analgesic effect observed may diminish with repeated exposure as the novelty of the VR decreases10,15. Infants, for example, will show a decrement in orientation toward a distractor after repeated exposure to the stimulus13.
This process, known as habituation, has been understudied in the VR literature. Only a few investigators have exposed participants to more than one VR distraction trial. Hoffman et al. implemented VR distraction in seven burn patients during three daily physical therapy sessions11. Pain ratings were significantly lower in the VR condition relative to baseline. More importantly, a within-subjects analysis of this effect showed that pain reductions did not diminish over the three trials. In a case study of a 32-year-old male burn patient, Hoffman et al. showed significant reductions in patient ratings of time spent thinking about pain, pain bothersomeness and unpleasantness, worst pain, and average pain during a VR condition relative to baseline, which were maintained over five daily physical therapy sessions10. In a second case study, Hoffman et al. showed similar reductions in pain and anxiety during a VR condition relative to another active distraction condition in two male youth9. Repeated VR exposure was only reported for one patient, whose pain ratings remained lower during the VR condition, but to a lesser degree than in the first exposure. However, these findings are difficult to interpret because the VR and non-VR conditions were administered during different clinical procedures each day. To our knowledge, there are no published laboratory studies of repeated exposure to VR distraction.
While the findings of Hoffman and colleagues are significant, the case study nature and limited sample size of these studies substantially limits the generalizability of the findings. Thus it is important that the hypotheses and results of the aforementioned studies be tested in a larger-scaled, controlled experimental design. The present study examined the effects of distraction via VR technology over eight weekly trials with a larger sample size than has been reported in the literature, and in a controlled laboratory environment.
Participants were recruited from a university campus using flyers and online posts. Thirty-five adults initially agreed to participate in the study; however one participant did not keep the initial appointment and three participants stopped attending sessions in the first three weeks. Three participants were dropped from the study because their baseline pain tolerance scores for the first four sessions reached the 4-minute safety ceiling, thus making it impossible to detect any improvements in pain tolerance due to VR distraction. An additional three participants demonstrated pain tolerance ceiling effects (i.e., tolerance scores greater than 240 sec.) in three or more of the later baseline sessions. These participants were retained in the study, but their data were excluded from the pain tolerance analyses. One subject's pain threshold data were excluded from analyses because of similar ceiling effects during baseline trials. Comparable rates of baseline ceiling effects, ranging from 10–19 percent of the total sample, have been reported by other researchers using similar cold pressor paradigms and the standard 4-minute maximum immersion time6,14.
The final sample consisted of 28 participants, 17 (60.7%) of which were male. Participant ages ranged from 18 to 23 years of age, with a mean of 18.86 years (SD = 1.27). No race or ethnicity information was collected.
A within-subjects design was used, eliminating inter-subject variability from week to week, thus increasing statistical power. Each study session consisted of two conditions, a baseline (control) condition in which the participant immersed his/her hand in the cold-pressor without distraction, and a VR distraction condition in which the participant interacted with the VR environment while immersing his/her hand in the cold-pressor. During the VR distraction condition, participants used a Playstation-2® controller (Model SCPH-10010U, Sony, Oradell, NJ) anchored to a table to play the videogame, which was viewed through a virtual reality head mounted display with integrated headphones. Condition order was randomized to control for order effects. Randomization was performed separately for each participant.
A Thermo NESLAB (Newington, NH) RTE-10 Digital One Refrigerated Bath was used as the cold-pressor apparatus. The RTE-10 was chosen due to its ability to circulate the water, which is necessary to maintain a constant water temperature and to avoid local pockets of heat near the hand of the participant. As noted in Mitchell, MacDonald, & Brodie, circulation is also needed to ensure comparable and reliable pain induction across multiple uses16. Also, because the RTE-10 is a digital apparatus, temperature can be set to a tenth of a degree and maintained automatically by the machine. The water temperature was maintained at 1 °C. This temperature was selected to limit ceiling effects which can occur at higher temperatures, as well as to ensure an intense pain stimulus which mimics as closely as possible those incurred in the clinical environment.
A stopwatch was used to measure pain threshold and pain tolerance times to a tenth of a second.
A Thermal Feedback System, manufactured by Bio-Feedback Systems, Inc. of Boulder, Colorado (Model DT-100; Power ID-91), was used to measure hand temperature before and after each trial to a tenth of a degree.
The Sony Playstation 2® Finding Nemo® “Catch Dory” game was chosen for this study. The game is played from a third-person perspective as the participant controls “Marlin,” chasing “Dory” as she rapidly swims away. This game was chosen because the time for completion of the game was greater than four minutes (i.e. the maximum allowed immersion in the cold pressor), it required active interaction by the participant throughout the trial, and it allowed for one-handed game play, freeing the other hand to be immersed in the cold pressor. The game is suitable for ages six and above, as determined by the Entertainment Software Rating Board (ESRB) and has been used with children aged 5–13 in similar laboratory pain studies without difficulty3. The Sony Playstation 2® controller was mounted to a base and anchored to a table to allow participants to manipulate the controller with one hand while the other hand was in the cold water. The equipment was connected to a portable Ground Fault Circuit Interrupter (GFCI) at the receptacle outlet, as recommended by OSHA for construction sites using high voltage power tools. The GFCI device was tested each day prior to use. All cords were secured to prevent tripping and were inspected daily for damage or fraying.
In the VR distraction condition a 5DT 800 HMD Virtual Reality Helmet adjustable head-mounted display with integrated headphones was used. The head mounted display (HMD), made by Fifth Dimension Technologies (Irvine, CA), was connected to the Sony Playstation 2® through a Sony RDR-6×7 DVD player/recorder and Impact Acoustics (Dayton, Ohio) S-Video to VGA converter (product number K0227).
This study was approved by the university Institutional Review Board (IRB). At the time of enlistment in the study, participants selected a convenient half-hour timeslot during which they attended a weekly study session. Informed consent was obtained from the participant during the first study session. All sessions were conducted in the same 4.9 m × 3.7 m carpeted laboratory room maintained between 22 and 23 °C. The same procedure was followed in all study sessions.
The first author, three graduate students and eight undergraduate students served as experimenters. To minimize experimenter bias, each participant underwent trials conducted by several different experimenters.
Upon entering the laboratory room, the participant was asked to sit, with the cold-pressor on his/her non-dominant side, the Playstation 2® controller on his/her dominant side, and the HMD and audio-visual equipment in front of him/her. At this time, the baseline hand temperature of the participant was measured by attaching the temperature sensor to the tip of the index finger of the non-dominant hand.
Before each trial, the participant was instructed to immerse his/her non-dominant hand in the cold-pressor bath up to the wrist, palm-side down, and to leave his/her hand open (non-fisted). This was done to avoid the formation of heat pockets around the fingertips which can influence pain stimulus. The participant was told to report when they began to feel pain by saying “Pain now” and to remove their hand from the cold-pressor when the pain became unbearable. Finally, the participant was asked if they understood the directions.
In each trial, the participant immersed his/her hand in the cold-pressor up to the wrist. Timing began once the hand was immersed and ended when the hand was removed. The time read when the participant said “Pain now” was recorded as pain threshold, and the time read when the participant removed their hand from the water was recorded as pain tolerance. Following the participant's removal of their hand from the water, the hand was quickly dried with a towel and the hand temperature was measured as in the baseline measurement. During this time, several 100 mm visual analog scales were issued, as described below. Next, the participant was asked to place his/her hand in a warm water bath (35 °C) for approximately five minutes, or until the participant felt his/her hand had returned to baseline. The hand temperature was again measured, ensuring it was within 1 °C of the first baseline temperature taken upon entering the laboratory.
The experimenter read the directions above to the participant, and subsequently asked the participant to immerse his/her hand in the cold-pressor bath. The participant's pain threshold and pain tolerance times were recorded to a tenth of a second.
Before commencing the VR distraction condition, the participant was shown how to play the game by one of the experimenters. Approximately 3–5 minutes were devoted to teaching game play to the participant. The HMD was placed on the participant's head and adjusted to ensure a comfortable and secure fit. The participant's non-dominant hand was placed above the cold-pressor bath, and the dominant hand above the Playstation 2® controller. The participant was read the above instructions. Next, the game was commenced and the participant immersed his/her hand in the cold-pressor bath, as instructed. The participant's pain threshold and pain tolerance times were recorded to a tenth of a second as described above.
As compensation for participation in any number of study sessions, extra credit was issued in undergraduate psychology courses, where applicable. Also, as compensation for completing all eight study sessions, participants were given a ticket for a raffle of an Apple (Cupertino, CA) iPod® and several smaller prizes. The latter compensation was intended to limit participant attrition over the course of the eight weekly sessions.
Pain threshold was defined as the number of seconds of immersion in the cold-pressor bath until the participant reported pain. Pain tolerance was defined as the number of seconds of immersion in the cold-pressor bath until the participant removed his/her hand from the water.
Visual analog scales were used to measure the intensity of the participant's pain during immersion, the participant's anxiety during immersion, the amount of time the participant spent thinking about his/her pain during immersion, and the participant's level of enjoyment of the VR game (if applicable). Each scale consisted of a 100 mm line, with the left terminus labeled as the total absence of the variable (i.e. no pain, no anxiety, no time spent thinking about pain), and the right terminus labeled as the complete presence of the variable (i.e. worst possible pain, very anxious, 100% of time spent thinking about pain). These scales were scored by measuring the distance in millimeters from the left terminus of the scale to the mark made by the participant, indicating the degree to which the participant experienced each variable. Visual analog scales yield reliable and precise measurements of several aspects of the pain experience, and have been applied throughout the existing VR distraction literature9,10. Visual analog scales have also been shown to be a valid way to estimate state anxiety1.
A series of (2 × 8) within-subjects analyses of variance (ANOVA) were conducted to ascertain the effects of condition (i.e., VR distraction vs. baseline) and week on pain threshold, pain tolerance, pain intensity, anxiety, and time spent thinking about pain over the eight weeks of the study. The ANOVA results are presented in Table 1. Median values for pain threshold and pain tolerance in both experimental conditions are shown in Table 2. As seen in Figures 1 and and22 respectively, VR distraction led to significant increases in pain threshold and pain tolerance relative to baseline. VR distraction also led to significant decreases in pain intensity, anxiety, and time spent thinking about pain (see Table 1). Power analyses conducted using G*Power (version 3.0)5 indicated that adequate power (greater than .80, according to Cohen, 1988)2 was obtained for the pain tolerance, pain intensity, anxiety, and time spent thinking about pain analyses (See Table 1). However, lower power (.53 to .57) was obtained for the pain threshold and level of enjoyment of VR analyses. Therefore, these analyses should be interpreted with caution. Pain tolerance and pain threshold were significantly correlated at each time point, with correlations ranging from .53 to .77 (ps < .01).
Analyses of condition by week effects were not significant for any of the dependent variables. As shown in Figure 3, the standard mean difference (SMD, calculated by taking the difference between the mean of pain tolerance during intervention and baseline and dividing this by the square root of the pooled standard deviation) effect sizes for the primary dependent variables (pain tolerance, pain threshold, and pain intensity) across the 8 weeks of the study demonstrate some scatter, but only slightly negative slopes (slopes = −0.0063, −0.0011, and 0.0133 for pain tolerance, pain threshold, and pain intensity, respectively). Effect sizes for pain tolerance fell within the moderate range, effect sizes for pain threshold fell in the small to moderate range, and effect sizes for pain intensity fell in the small range, according to Cohen's (1988) standards2.
The virtual reality distraction intervention appeared to be very effective for this sample of 18- to 23-year-old adults. Participants demonstrated increases in pain tolerance and pain threshold during the VR condition relative to their baseline, showing the largest and most stable effects for pain tolerance. Moreover, VR distraction led to significant decreases in participant ratings of pain intensity, anxiety, and time spent thinking about pain. The overall findings of this study are consistent with studies currently in the literature7,9,10,11. However, the magnitude of effects in the current study exceeds those of other studies. To the best of our knowledge, this study is the only of its kind to find significant improvements due to VR distraction in all of the aforementioned indices of the pain experience within a single sample. Furthermore, none of the above effects diminished significantly over the course of the eight weekly exposures to VR, as indicated by the absence of condition by week effects in all of the variables and the absence of meaningful degradation in effect sizes over time. This suggests that participants did not habituate to the distracting properties of VR.
The finding that participants did not habituate to the distracting properties of VR is in agreement with the limited existing literature, but with a larger sample size and more extended timeline of exposures than previous studies in the literature10,11. This finding is of special importance because it addresses the concern noted by Hoffman et al. that if the novelty of VR wears off with repeated exposure, the efficacy of VR distraction from pain would decrease10. Moreover, this finding suggests that VR distraction interventions may have the potential to be clinically effective over an extended course of exposures.
Although the observed effects of VR distraction in this study were impressive, laboratory pain differs from clinical pain in many ways. First, the participants knew they were in complete control of the pain experience, meaning they could remove their hands from the water at any time. Secondly, the duration of the pain experience was limited to a maximum of four minutes. Also, because many patient populations may require the use of central nervous system depressants, such as opioids, a patient's ability to interact with a complex VR environment may be decreased significantly. This may limit the efficacy of VR intervention during these periods of a patient's treatment. Finally, the participants experienced pain in a more subdued atmosphere than that of a hospital, which may have important implications on the clinical use of VR distraction in highly anxious patients. However, we chose to study experimental pain for the present study in order to avoid the many uncontrolled factors influencing pain in the clinical setting. This allowed for a standardized pain stimulus to be delivered to all participants over the course of the eight weekly trials. Had clinical pain been studied instead, variations in the same clinical procedure among trials would have varied the intensity of the pain stimulus from week to week, thus adding considerable error variance. As noted by Edens & Gil, studies utilizing experimental pain in the laboratory setting allow for closer control of the experimental environment than in the clinic4. Many experimental pain stimuli are also safe to implement repeatedly within a single, short laboratory visit, thereby allowing for within-subject experimental design that avoids problems of inter-subject variability in pain tolerance and threshold. Thus, the use of experimental pain stimuli allows for the careful study of clinically-related phenomena and processes in a more controlled and constant environment1,4.
Secondly, the age range of the participants used in this study was limited. Age differences in cognition and attention may affect the efficacy of VR over repeated exposures and/or the rate at which participants habituate. However, the existing literature on VR habituation employs several different age populations, all with converging results consistent with this study. Therefore, the effect of age may be relatively minor, given that age-appropriate VR equipment and software can be provided to patients of different ages. Replication of the current study in both younger and older populations would expand the generalizability of the findings.
It is possible that participants' expectancy effects may have influenced the results of this study. That is, although the participants were not directly told what the hypotheses of this study were, they might have guessed the hypotheses and subsequently tried harder to keep their hand in the water while playing the videogame.
Several participants were not included in the pain tolerance analyses because of repeated ceiling effects during baseline. Thus, these findings are not necessarily generalizable to individuals with exceptionally high pain threshold and/or pain tolerance. Also, because research assistants involved in running study sessions had prior knowledge of VR distraction theory and the hypothesis of this individual study, the potential for experimenter bias exists. However, the risk of bias was mitigated by ensuring that several different research assistants served as experimenters for each participant over the course of the eight week study.
Although pain threshold was correlated with pain tolerance, pain threshold proved to be a less sensitive, more erratic dependent variable in this study. However, this issue may be unavoidable. In order to reliably report the onset of pain, participants would need to direct their attention to pain sensations, which is incompatible with the goals of a distraction intervention. The same concern applies to pain intensity. Thus, it may be advisable to rely primarily on pain threshold as a dependent variable for laboratory studies of distraction-based pain management.
Finally, this study employed a third person rather than first person perspective (i.e., the participant saw the game character, Marlin, as he/she manipulated his movements in the virtual environment, rather than viewing the virtual environment as if through Marlin's eyes). As noted in Schubert et al. and Schuemie et al., a first person perspective is thought to provide a superior sense of presence relative to third person, although third person perspectives also can induce presence18,19. Thus, the VR software used in the present study may not have achieved the optimal level of presence, relative to more elaborate and expensive variations on this technology. It is possible that a virtual reality intervention with a first person perspective could be even more efficacious over repeated exposures than the third person perspective virtual reality stimuli used in the current study.
The maintained efficacy of VR distraction over repeated exposures observed in the current study indicates that VR distraction is a viable pain intervention, capable of sustaining its powerful analgesic effects over an extended period of time. To the best of our knowledge, the current study is the only study of VR habituation to use an extended period of exposures in a sample larger than seven participants11. However, further study is needed to evaluate the long-term effectiveness of VR distraction using repeated clinical, rather than laboratory-based pain stimuli, with a comparable or larger sample size. In addition, as recommended by Glantz et al., future studies of VR habituation in clinical populations should also address the cost effectiveness of VR distraction relative to other, less expensive distraction techniques8.
The authors thank the following undergraduate research assistants who aided in experimental procedures and data management: Victoria Grossi, Joseph Keller, Cyrus Mistry, Jessica Wentling, and Monica Jimeno. The authors would also like to thank several graduate students for their support and collaboration: Emily Kastelic, Claire Ackerman, Soumitri Sil, and Lindsay Dillinger. Finally, the authors would like to thank Dr. Lynanne McGuire, Ph.D. for her assistance in reviewing this manuscript in preparation for publication.
This study was supported in part by a University of Maryland, Baltimore County Undergraduate research award to the first author and by grant No. R01HD050385 from the National Institute for Child Health and Development, National Institutes of Health.
Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The authors are not aware of any potential conflicts of interest.