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Journal of Anxiety Disorders 23 (2009) 563–574 Contents lists available at Journal of Anxiety Disorders The use of virtual reality in acrophobia research and treatment Carlos M. Coelho ,Allison M. Waters , Trevor J. Hine Guy Wallis a School of Human Movement Studies, University of Queensland, Level 5, Building 26, St. Lucia, QLD 4072, Australiab Griffith Institute of Health and Medical Research and School of Psychology, Griffith University, Brisbane, Australiac Queensland Brain Institute, University of Queensland, Brisbane, Australia Acrophobia, or fear of heights, is a widespread and debilitating anxiety disorder affecting perhaps 1 in 20 Received 11 August 2008 adults. Virtual reality (VR) technology has been used in the psychological treatment of acrophobia since Received in revised form 22 January 2009 1995, and has come to dominate the treatment of numerous anxiety disorders. It is now known that Accepted 29 January 2009 virtual reality exposure therapy (VRET) regimens are highly effective for acrophobia treatment. Thispaper reviews current theoretical understanding of acrophobia as well as the evolution of its common treatments from the traditional exposure therapies to the most recent virtually guided ones. In particular, the review focuses on recent innovations in the use of VR technology and discusses the benefits it may offer for examining the underlying causes of the disorder, allowing for the systematic assessment of interrelated factors such as the visual, vestibular and postural control systems.
ß 2009 Elsevier Ltd. All rights reserved.
Contributing factors to the development and maintenance of acrophobia . . . . . . . . . . . . . . . . . . . . . . . . . .
Self-locomotion and the role of the visual and vestibular systems . . . . . . . . . . . . . . . . . . . . . . . . . .
Fear of heights treatment models—an historical perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acrophobia therapy efficacy: early theory-based approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction and overview of acrophobia naturalistic type. Although long since recognized as a disorder,there is still some doubt about its typology due to its similarities to According to the Diagnostic and Statistical Manual of Mental both panic disorder and Disorders, Fourth Edition (DSM-IV) ), acrophobia, which agoraphobia Acrophobia is an extreme fear of heights, is considered a specific phobia of appears closely related to the fear of elevators and fear of flying,both of which belong to the specific phobia, situational type, asdefined in the DSM-IV ().
Acrophobic behavior typically involves the avoidance of a variety * Corresponding author.
E-mail address: (C.M. Coelho).
of height-related situations, including stairs, terraces, apartments 0887-6185/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
doi: C.M. Coelho et al. / Journal of Anxiety Disorders 23 (2009) 563–574 and offices located in high buildings, bridges, elevators and plane Of course, failure to remember any particular experience trips. Considering the striking breadth of aversive situations and related to the emergence of the disturbance might equally be stimuli, it is not surprising that individuals with acrophobia feel attributed to problems inherent in self-report and memory rather impaired and restricted in their movements, even in comparison to than a non-associative etiology (see sufferers of other specific phobias ). To overcome these methodological problems, Specific phobias have a high prevalence rate in epidemiological data, both in adolescents () and longitudinal study of a large cohort of approximately 1000 in adults (In the ECA Study, which was participants from birth. Corroborating the previous findings, the comprised of 20,000 participants, 4.7% of the participants fulfilled authors noticed that serious falls that resulted in fracture, the criteria for a diagnosis of acrophobia ). A study dislocation, laceration or intracranial injury had no positive by presented similar correlation with fear of heights in later ages. Contrary to what results with 6.3% of men and 8.6% of women presenting with was expected, falls resulting in serious injuries between the ages of acrophobia. Moreover, this disorder tends to evolve to chronicity five and nine occurred more frequently in participants who later largely due to the pervasive avoidance of a wide- exhibited no discernable fear of heights at the age of 18. Also, no range of height-related situations that form part of everyday living.
participant with fear of heights at 18 possessed a history of serious It is clear then, that due to its high prevalence, chronicity, and falls before nine years of age. In summary, ‘participants with less cumulative social impact, research into the underlying causes and fear of heights seem to be those who sustained more injuries due to treatment of acrophobia is particularly important. Additionally, by falling and the conditioning experiences do not seem to have studying underlying acrophobic fear mechanisms, uncertainties produced fear. .' p. 135). Similar results can be addressed relating to its typology as well as etiology and were reported by namely that treatment. This review makes particular reference to the use of participants without fear of heights reported a higher incidence virtual reality (VR) technology as a superior methodology in of fearful and painful experiences than those with fear, suggesting achieving improvements in both the treatment and our under- that those without fear are likely to pay for their innate lack of standing of the causes of this disorder.
Longitudinal studies of this type do offer some insight into the 2. Contributing factors to the development and maintenance of processes involved in fear of height acquisition but one might still arguer that failures in recall may lead to underreporting ofsignificant events. Several studies suggest that memory reliability This section reviews the available evidence on contributing decays in the space of weeks or months ( factors to the development and maintenance of acrophobia. To ; ). Perhaps more date, this literature has tended to focus either on factors gleaned significant, is the possibility that phobia emerges through from the broader literature on specific phobias or on the role of accumulation of subtle, non-traumatic experiences which are specific factors in isolation, rather than on the development of a not in themselves very memorable (e.g. comprehensive model of acrophobia fear acquisition per se.
; ) but that cannonetheless influence long-term behaviors ( 2.1. Non-associative accounts ; and give rise to fear ) that is difficult to associate with conditioning.
Because some individuals who fear heights have often been In general, all of these arguments undermine non-associative unable to report a clear height-related aversive experience as a theories of phobias but accord with acrophobia phenomenology. In primary etiological factor, some authors have proposed hereditary particular, a fear of heights is acquired before verbal acquisition, at or non-associative accounts in the disorder's development a time when children are learning to crawl and falls occur, educating children about the nature of surfaces compared fearful and non-fearful participants' responses to heights locomotion, balance and posture (). Whilst in an attempt to assess their acquisition of height-related fear.
Adolph noted that learning strategies do not transfer from crawling Results showed that 21 participants were categorized as developing to walking, the avoidance of heights does seem to be maintained their fears through non-associative pathways. Additionally, the non- across developmental postures fearful group also had participants who had experienced direct ). It is therefore plausible that some of these conditioning relevant events but did not develop a fear of heights.
skills act as latent inhibition, thus preventing more skilful children These data were seen as consistent with a non-associative account from developing a fear of heights.
for the acquisition of the fear of heights, in which ontogeneticlearning is not required, at least for the fear of heights.
2.2. Role of cognitive processes A few years later, were able to extend these findings using a much larger clinically defined group Combining cognitive and biological factors, of acrophobics (n = 148) who were age and sex-matched to a non- argued that fear of heights may develop in ways similar to panic phobic control group. The phobic group met the DSM-III-R criteria disorder. That is, cognitive biases may develop leading individuals for simple phobia ) and had a mean age of 40.3 years.
to increasingly interpret bodily sensations that are tied to The results of this study questioned once again the significance of movement in height-related situations as threatening. These associative-learning events in the acquisition of fear of heights, researchers assessed 100 students for bodily sensations, acropho- since the categories consistent with the ethological theory bia, spider phobia, cognitive bias, trait anxiety, and depression accounted for 83 participants. Moreover, the authors did not find using a suite of questionnaires. Analysis revealed a correlation significant differences between groups with respect to the between measures of acrophobia and a bias to interpret and report proportion of participants who exhibited other height-related internal bodily sensations of anxiety as threatening. There was no fears, or who experienced relevant associative-learning events. The such bias evident for external stimuli or social stimuli and the bias ages at which these events had occurred were also not significantly was not found with measures of spider phobia. The authors different. All this evidence led the authors to rule out a latent concluded that ‘in the case of the development of acrophobia, inhibition hypothesis (e.g. individuals may come to interpret some bodily sensations as C.M. Coelho et al. / Journal of Anxiety Disorders 23 (2009) 563–574 indicative of dizziness and nausea, and these may occur in ) as well as inherited factors, such as preparedness situational circumstances which might lead the individual to (e.g. ). If the majority of humans have an associate these signs with the possibility of an imminent inherited disposition to avoid heights and this avoidance can later catastrophic fall (e.g. if they are looking down a circular stairway, develop into a phobia, it is equally possible that people with low or standing on a chair)' p. 1000). Thus, levels of fear develop an absence of fear through habituation that conclusion highlights the possibility that develops from frequent exposure opportunities. Conversely, those cognitive and biological vulnerabilities can underlie the develop- born with a particular fear can fail to overcome it due to lack of safe ment of height fear in humans and that a catastrophic, condition- exposure in early life (see ing event may not be necessary.
The importance of each vulnerability and learning factor 2.3. Contemporary learning models remains a matter of ongoing debate. What is clear from studiesin children (e.g. ; Davey et al.'s integrated approach is echoed in the work of is that a fear of heights accompanies the emergence of who emphasized the need to recognize self locomotion (crawling) and that habituation processes can aid the concept of preparedness, that is that certain stimuli are in overcoming a fear of heights.
evolutionally predisposed to evoke fear responses (This, they argued, could help answer some of the questions 2.4. Self-locomotion and the role of the visual and vestibular systems that classical conditioning accounts could not. In particular, theyargued that it was necessary to provide an explanation for the fact One of the earliest laboratory-based investigations of fear of that not all fearful participants have a history of failed learning heights was based on studies of the ‘visual cliff'—a transparent experiences, while at the same time some participants without fear walkway spanning a drop. These studies were conducted with a have been through traumatic experiences ). From variety of animals (e.g. and human their perspective, and in contrast to non-associative accounts (see a diathetic-stress model offers the most attractive framework for describing acrophobia, since it ). The animals and humans considers learning histories (such as vicarious and modeling showed various reactions to the cliff, each of which was learning) within the context of inherited ‘prepared' liabilities, such characteristic of the particular species studied. For example, in as the personality trait of neuroticism, for example. Together, these human infants, the studies suggested a developmental shift vulnerabilities make certain individuals more or less susceptible to corresponding to a marked increase in a fear of heights after the developing anxiety disorders ( onset of crawling ). Although some initial degree of fear of heights is a normative experience of development, What has yet to emerge from this more holistic, multifaceted most individuals overcome this fear through processes inherent to approach is a description of how the various factors interact.
development itself – practice, exposure, development of mastery, Combining learning and biological factors appears to suggest that etc. – to eventually retain a degree of healthy wariness about the development of acrophobic depends on a combination of heights that is functional and adaptive to survival. At present, it is specific and general factors including, for example, cognitive unclear if acrophobia is in someway related to a failure to achieve biases, the extent of prior experience with heights, and possibly, this developmental milestone.
the development of a sense of control over the height environment.
recently tested eight participants Control might therefore be achieved in a learning process similar to diagnosed with acrophobia in a virtual reality environment and the one used by in the treatment of the found that participants experienced elevated anxiety not only to fear of dogs. They argued that participants who had previous increases in height, but also when required to move laterally at a experience with dogs have more knowledge of how dogs behave, fixed height. These anxiety levels were significantly higher than and that they may view a dog's behavior as predictable and those elicited by viewing the fear-evoking scene without move- controllable, and hence less threatening ment. Coelho et al.'s results suggested a potential link between As it will be argued later, participants with acrophobia might tend movement and acrophobia in line with earlier developmental to interpret the discrepancies present in elevated studies of infant crawling, such as the ones of environments as sign of a possible fall. Through extended . As children learn to detect threats to balance experience of visuo-vestibular interactions in elevated locations, and find compensatory strategies to regain posture when in the link between this sign and an imminent fall becomes disequilibrium acrophobic participants might diminished, and with it, the associated fear.
similarly learn to use the visual, vestibular and somatosensory Both the non-associative and the diathesis-stress models systems to learn how to move in height-related situations.
accord with account that experience and Moreover, similar to motion sickness, fear of heights might be habituation with the environment can work towards eliminating due to a conflict between vision, somatosensory and vestibular biologically relevant fears. In the particular case of heights, senses This mismatch can occur because of the habituation is observed in individuals who go through repeated image flow associated with head movement i.e. motion parallax.
exposure to heights such as ‘steeplejacks, roof-workers, and tight- Motion parallax is the perspective transformations of the retinal rope artists, who achieve a remarkable degree of postural balance image, produced by either the movement of an observer or the with seeming insensitivity to height' ( movement of objects in the visual world ().
, p. 513). The absence of fear of heights in certain These transformations are greatly reduced for objects viewed at individuals can be taken as evidence for both learning processes large distances ). This issue is addressed later in such as habituation, latent inhibition and immunization (e.g. The evidence of the abovementioned research studies suggests that it is too soon to defend a behavioral, diathesis-stress or non- associative model for the etiology of fear of heights. It seems that The conflict exists when the prevailing inputs from the visual and vestibular some models are in fact quite complementary. Some of the liability systems are at variance with stored patterns derived from previous interactionswith the spatial environment (see ).
for fear of heights might be inherited, however it is also likely that C.M. Coelho et al. / Journal of Anxiety Disorders 23 (2009) 563–574 such a fear might only manifest if other factors: vicarious, exposure itself. This model gives emphasis to the quality and informational or behavioral, are part of the individual story.
quantity of information as the main factors to increase the self-efficacy that participants acquire and the manner in which they 3. Fear of heights treatment models—an historical perspective incorporate that information when assessing their own capacities().
proposed one of the first theoretical accounts of acrophobia to be used as a basis for treatment. The two-factor 4. Acrophobia therapy efficacy: early theory-based approaches theory of phobia treatment is still in use today and has remainedpractically unchanged. Fundamentally, this theory states that The earliest research on acrophobia treatment followed these phobic anxiety is a conditioned response, triggered by a condi- early theoretical approaches. For example, exam- tioned stimulus (the phobic situation). However, it also adds that ined a ‘‘contact desensitization'' treatment (CD) involving the use situational avoidance develops and remains due to the reinforce- of contact with the therapist's hand and arm while engaging in ment of reduced anxiety (negative reinforcement). This successful approach responses to the desired behavior. The physical avoidance of the phobic situation assures the perpetuation of the assistance was then gradually eliminated with eventual indepen- anxiety. Currently, it is widely accepted that because avoidance dent response rehearsal by the participant. Ritter considered the decreases anxiety in the short-term, it is a major contributor to fear therapeutically effective components of CD to be: (a) information regarding the avoided object or situation; (b) observation of ). However, by avoiding model(s) performing target behaviors with no adverse conse- and/or escaping their fears quickly, phobic individuals do not quences; (c) contact with the phobic object; (d) contact with the experience their distress diminishing in anxiety-provoking situa- therapist; (e) skill obtained in performing the desired behavior through rehearsal; (f) hierarchical exposure to the avoided object Following on from Mower's early work, devel- or situation and to the desired responses. For testing these oped a technique called ‘systematic desensitization' that was a components, the therapist worked with 12 participants (10 milestone in the development of effective phobia treatment.
women and two men) who were extremely fearful of heights.
Wolpe had participants ‘‘avoid'' avoidance, that is, remain Participants were exposed to one of the following three conditions: steadfast in the presence of the feared stimulus. (1) treatment with CD; (2) treatment which involved all elements explains that this procedure consists of imaginarily linking feared of CD except for physical contact, or (3) no treatment. The situations with muscle relaxation, which works as an anxiety treatment consisted of successive approaches (in groups) to inhibiting response. Central to the development of this procedure progressively higher landings of a staircase with nine landings, was the notion that deep muscular relaxation and the imagination 15.8 m high in total, until two minutes exposure was achieved at of relaxing scenes was an anxiety antagonist. Wolpe suggested that the highest landing. The contact between the therapist and the relaxation could be capable of suppressing the anxiogenic participant involved placing the therapist's (woman's) arm around properties of the stimulus, breaking the link between them. A the waist of each participant and leading him/her to the testing series of later studies supported the fact that systematic site. Gradually the therapist removed the arm while the participant desensitization was effective in eliminating fears and after Wolpe, was still at the lowest landing, and gradually the therapist numerous studies have tried to uncover the active therapeutic extended the period of time the participant stayed at the testing site without contact. The exposure was performed in groups and part of Later, self-efficacy theory ) identified the the treatment involved the participants linking arms and walking to participants' judgment about their ability to perform specific the edge of the stairway. Results showed significantly better results actions as a major cognitive determinant of human behavior.
in the group with physical contact. In another study, Although self-efficacy judgments and anticipated outcomes jointly compared CD with the same treatment without contact (demon- determine behavior, Bandura claimed that phobic behavior is stration-plus-participation – DP) and modeling. The study involved influenced more by self-efficacy judgments than by outcome 13 women and two men with severe height phobia. Results revealed expectations. When people have confidence that they can that CD treatment was similar to DP and modeling, producing successfully execute various courses of action, then their clinically reliable benefits. In a related study, expectations regarding consequences should tend to be the major did not observe differences in acrophobia treatment with CD determinants of performance variation. However, when people as a function of ‘‘warm'' versus ‘‘cold'' types of therapist behavior.
have strong incentives to perform a given action their perceptions The warm therapist maintained eye contact, used voice inflection of self-efficacy will tend to exercise the greater influence over and facial expression of friendliness and support. The cold therapist whether they attempt it and how well they succeed ( avoided eye contact and spoke ‘‘nonchalantly'' while maintaining a blank facial expression.
Several treatment models emerged from this early work, in compared two types of particular, ‘‘reinforced practice'' and ‘‘self-efficacy'' treatments.
treatments for acrophobia: systematic desensitization applied by Reinforced practice is a combination of ‘‘therapeutic ingredients'' the therapist versus the same intervention applied by the combined in a therapeutic model (and assumes participant with the help of a tape recorder replaying desensitiza- that improvements in cognitive and physiological components of tion instructions. The study, which involved 30 acrophobic anxiety will occur after modeling approach responses and participants (nine men and 21 women), revealed that the two continuous practice of these responses interventions were equally effective. Interestingly, the only difference emerged in a later follow-up test after eight months, The therapeutic ingredients which revealed a greater improvement for participants who used included exposure to stimuli that trigger fear, therapeutic the tape recorder. The results suggest that desensitization is an instructions, client progress control, feedback on performance, effective treatment even with reduced therapeutic contact. The and conditional performance reinforcement authors concluded that some participants functioned better in a ). In contrast, self-efficacy treatment places more therapeutic situation that offered greater autonomy and control emphasis on the phobic individuals' self-efficacy regarding how over the procedures used. Additionally, the ‘‘tape recorder'' well they can execute the requisite behavior rather than on the participants might have taken on a longer-term responsibility of C.M. Coelho et al. / Journal of Anxiety Disorders 23 (2009) 563–574 self-help after the official end of treatment since this version required a higher personal responsibility from the participants.
In contrast to study, found acrophobic thoughts in a clinical found the therapist's role in treating acrophobic group (N = 58) to be related to faulty thinking statements acrophobia to be important. The authors compared the exposure about themselves, but only when accompanied by reports of model with the self-efficacy model in 32 participants with severe discomfort or stress. Moreover, in comparing social anxiety with fear of heights or driving. The self-efficacy model was significantly acrophobia, the authors found that negative self-statements played more effective than the exposure model regarding actual behavior, a greater role in situation-specific (e.g. acrophobia) compared with sense of self-efficacy, anticipatory anxiety and anxiety during more pervasive anxiety (e.g. social phobia).
performance. However, unlike the reinforced practice model of also found perceived danger to be an the emphasis of the self-efficacy model is not important feature in fear of heights. Marshall and colleagues found upon simple exposure to the feared stimuli. The latter model that in a sample of 50 participants (29 fearless and 21 fearful of emphasizes the quality and quantity of information as the main heights), many of those with acrophobia had irrational beliefs factors in the efficacy of the treatment in addition to the use of such about catastrophic consequences that might occur when they information when assessing their own capacities. This and other entered a high location. They reported that height fearful minor treatment differences lead to further studies in which participants were more likely to believe, and repeat to themselves, acrophobia represented the major symptom.
that the structure will collapse, a strong wind will suddenly blow also tested the effectiveness of them off the building, or that they will accidentally fall from the systematic desensitization, in a study with negative practice and balcony of a theatre.
relaxation. Negative practice helps clients learn to control their Conversely, in a study with a smaller number of participants anxiety symptoms by voluntarily practicing the symptoms (N = 15), found self-efficacy (mea- associated with anxiety, and then by attempting to make the sured as the participants' self-assessed ability to ascend, stand and outcomes of this practice as close as possible to the involuntary look down, in each of the balconies of a 10-story building) to be anxiety response. Participants (n = 58) participated in six weekly more accurate in predicting acrophobic behavior than perceived treatment sessions and it was found that both negative practice danger and anxiety arousal. In fact, four of the 15 participants gave and desensitization produced a substantial reduction in the very low or zero perceived danger ratings for all height tasks at pre- test. These are nevertheless unusual findings since more recent also compared the systematic studies ) confirm danger expec- desensitization and self-efficacy models in participants with tancies related to heights as good prediction factors.
severe acrophobia. The authors again concluded that the self- examined differences between efficacy model was more effective than desensitization when participants with acrophobia (n = 59) and without acrophobia treating acrophobic participants in respect to anxious behaviors (n = 59) regarding their danger expectancies before and during associated with heights, with improved perceptions of self-efficacy exposure to heights. Before exposure, participants with acrophobia and reduced anticipatory anxiety, and danger-related thoughts.
estimated a higher probability of falling, worse consequences from Once again, this treatment relied on the mastery of subtasks the fall, and also considered their anxiety levels as more including reaching intermediate goals, physical support (holding appropriate to the situation than the control group's anxiety the client's arm), modeling (demonstration), elimination of levels. The estimates of falling increased when the participants defensive maneuvers (e.g. rigidity or standing still), and various started to climb a ladder, whereas the control group's estimates did other performances.
not. The authors argued that insight in acrophobia was diminished In a similar vein, performed two studies of in the clinical group, contrary to the usual finding that individuals participants with acrophobia. In the first study, the total duration with phobias have considerable insight into the inappropriateness of the exposure was ignored and the participants were exposed to and excessiveness of their distress (e.g. ).
stimuli only until their anxiety came down to a baseline. The study Later studies by and also assessed the effect of the use of self-statements as a coping appeared to contradict the strategy. In the second study, the total exposure time remained findings of in so far as they concluded constant, varying only the exposure frequency, sometimes that perceived danger and anxious arousal did not significantly intermittently for brief periods, sometimes continuously until predict acrophobic behavior. In fact, until its fourth edition in 1994, the baseline was reached. The brief exposures resulted in little or the DSM included in the diagnostic criteria the requirement that no change, while the other procedures reduced fear. The inefficacy the phobic individual understood fear as irrational and dispropor- of brief exposures is consistent with other theories, such as the tionate to the danger imposed by the object or situation. But in incubation theory of which suggested that brief 1998, proposed a sub-category of exposures may even cause an increase of fear. In this respect, poor insight similar to the one used to specify obsessive excessive brief exposure can have similar effects to those of compulsive disorders. This suggestion was made after their data avoidance, thereby perpetuating the fear.
indicated that some people actually have poor insight as to the real All of these studies were important steps in the process of danger of the feared situations, even when distant from them. In validating different treatment programs for acrophobia. Never- their study, 64 participants (34 women and 30 men) with theless, the studies were essentially designed to explore and acrophobia were placed on the same ladder of 27 steps used in compare acrophobia treatment models, and were not specifically an earlier study Participants were then designed to understand the trigger features associated with fear of asked to predict their probability of falling. On the basis of this, two heights, as the visual-cliff studies had done with human infants groups were created: a poor insight group composed of those and animals.
predicting more than a 50% chance of falling, and a good insightgroup composed of those predicting less than 50% chance of falling.
5. Acrophobia therapy efficacy: the role of cognitive processes Interestingly, each group's average estimate for the probability offalling was very different. On the last step, the good insight Other early treatment studies of acrophobia focused on the acrophobic group estimated a 9.84% chance of falling and the poor cognitive processes that occur during exposure to heights ( insight group estimated a probability of 88.6%.
C.M. Coelho et al. / Journal of Anxiety Disorders 23 (2009) 563–574 These findings suggest that it is possible to develop a fear of entertainment industries (for review see heights via a number of distinct pathways. Some participants have ; When applied to the health fear despite being sure that they will not fall, and others feel a real sciences, the military origins of VR mean it is perhaps most apposite danger from this situation. This would also explain why some for the treatment of the fear of flying (aerophobia) and the fear of researchers find danger expectations to be important as predictors heights (acrophobia). In fact, during testing of a VR device in 1992, of fear () and others do not found that it produced emotional and One feature universal to all studies is the presence physical symptoms in the participant that seemed to be phobic of irrationality about the specific height situation. This might be a behaviors, similar to those of a person reacting to a feared situation.
signal that a trigger, exclusively related to height environments, is The device was a ‘‘flying carpet'' that flew in a particular direction causing some people to ‘overreact'. Overall, these studies suggest depending on which area was stepped upon. The observers then the absence of an underlying ‘‘general anxiousness'' in participants realized that the symptoms were not due to motion sickness but to with acrophobia. Instead, there appears to be a specific cata- real fear (see The first clinical VR application for strophic cognitively based association with height situations, in acrophobia was described by while particular during exposure to height stimuli.
approached the same question used VR for aerophobia.
by combining self-report with a physiological measure (heart rate).
Currently, VRET is seen as a viable alternative to exposure in Their study group was comprised of 19 participants (nine men and vivo since it can elicit fear and anxiety ) thereby 10 women) for whom acrophobia imposed marked and distressing functioning as an alternative mode of inducing exposure ( impairment on their everyday life. Since the study was designed to Additionally, when using a VR assess the influence of cognitive processes, half of the participants system, the therapist and patient do not need to leave the were instructed in adaptive thinking during an exposure in vivo consulting room. This implies a saving of time and money (e.g.
task and the other half underwent exposure alone. In contrast to the studies described above, the authors concluded that cognition but also brings the was unimportant in the process of habituation to heights, since benefit of not risking possible public embarrassment for the changes in participant anxiety during treatment did not precede patient as well as preserving their confidentiality changes in heart rate (HR). The authors also found that although ). VR can, in this way, be only the group instructed in adaptive thinking changed in the an intermediate step towards live exposure, i.e. a step between a cognitive measures, participants improved significantly on HR and completely protected consulting room and the ‘threatening real the behavioral measure in both conditions.
world'. However, VR may also represent an end in itself since it The role of cognitive factors in contributing to the etiology and offers the ability to render height-related situations that are not maintenance of acrophobia remains unresolved. Many of the more easily accessible to the therapist and patient, e.g. an airplane, a recent attempts to understand their role have made use of virtual high-rise building, etc. ().
reality (VR) technology. Exposed to modern, high-fidelity systems, VRET may also be useful as a form of therapy for clients who participants can become thoroughly immersed in the task, in have difficulty with imaginal exposure The objects, entities and event perception ( VR-based treatment can also be more precisely tailored to each This so-called ‘presence' in the virtual environment (see client, offering greater control over stimuli and even the possibility then becomes a tool for of creating stimuli of greater magnitude than one could experience exploring insight in people with acrophobia. Knowing that an in the real world Equally, by creating stimuli of augmented sense of presence is related to an increased amount of lesser magnitude than is experienced in the real world, partici- anxiety in individuals with a specific phobia ( pants may start treatment even if they are too anxious and fearful ), it is possible to evaluate thoughts of acrophobics with poor to be treated via real exposure insight, in a realistic VR environment. Here participants would have a similar visuo-vestibular input to a real situation, but The flexibility and confidentiality offered by VR brings with it without any physical danger. Conversely, acrophobics with good another important advantage in that it can encourage more people insight might be susceptible to less cognitive bias and hence more to seek treatment. Indeed, one assessment of students with a fear prone to a visuo-vestibular susceptibility. Given its potential to of spiders showed that almost 90% would prefer VR exposure over provide these answers, this review will now evaluate what has in vivo exposure therapy been done in the area of VR and acrophobia research.
This is particularly important for participants with phobiassince they do not seek help readily (According to 6. The potential utility of VR in treating acrophobia , help-seeking is higher among individuals withagoraphobia (fear of open spaces) and social phobia, than for other Since 1995, there has been a significant lack of studies on the specific phobias.
treatment of acrophobia outside the field of virtual environments.
The combined benefits of VR approaches suggest that it holds This seems primarily due to the advent of virtual reality exposure great promise as a therapeutic tool for enhancing acrophobia therapy (VRET) in 1996, to which this review now turns. Although VR treatment outcomes. Since the technology in this area is is only a recent field of technological application in research and developing rapidly, there is good reason to believe that this same clinical practice, many of its core concepts have been explored and technology will soon be economically accessible to private developed over the past 50 years. Its roots go back to the therapists. Currently, there is ongoing VR work on posttraumatic development of the flight simulator in the American aerospace stress disorder, eating disorders, sexual dysfunction, schizophrenic and defense industries during the Second World War hallucinations, and addictions (for review see ). From the Army's first digital computer (the ENIAC) in the 1940s, to the Air Force's research into visualization helmets (VCASS)in the 1980s, the American armed forces have always been the main 7. Specific acrophobia treatments in virtual environments source of funding for the most important innovations in computertechnology. Besides the military, advances in computer graphics According to and technology in general have also been driven by the film and can be credited with the first use of VR in the treatment of C.M. Coelho et al. / Journal of Anxiety Disorders 23 (2009) 563–574 the acrophobia, although one might better describe it as an altered, A subsequent study by took a more rather than virtual, reality. In that work, Schneider used binoculars structured approach than any previous studies. The authors began with inverted lenses to alter the perception of depth, so as to by reproducing the real-world exposure site in VR. The authors magnify the sensation of height during a process of exposure in a then applied three 1-h sessions to 33 acrophobic participants (16 real context. This procedure was published as a case study. The exposures in vivo and 17 with VR) and noted that the VR treatment participant, a 40-year-old man who lived in a city filled with was as effective as exposure in vivo in combating anxiety and skyscrapers, looked out from an eighth floor with the binoculars.
avoidance. The measures used were the Acrophobia Questionnaire The effect of the binoculars was to give the impression of standing (AQ) (the Attitude Towards Heights Questionnaire on the 56th floor, and the removal of the binoculars at the end of (ATHQ) ) and a Behavioral Avoidance Test each viewing trial produced an immediate and dramatic decrease (BAT). This method, in contrast to the previous studies, showed in apparent height, thus helping to overcome the participant's improvements not only in the self-report but also in the BAT. The results remained the same in a six month follow-up with 29 of the By the early to mid-1990s, computer-based VR systems became participants. The authors claimed that: ‘‘Given the chronic nature widely available and clinicians began to consider their use in of the acrophobia (mean duration of 31.5 years) in our patients, it is treatments. In the first reported study, exposed 30 highly unlikely that the results are due to spontaneous recovery'' acrophobic participants to simulated height situations. After one week of treatment, 90% of the participants were capable of coping Taken as a whole, there is now a considerable body of evidence with real situations. A follow-up at 30 months revealed that 27 of that VR exposure is an effective means of treating acrophobia the participants were quite capable of using a glass elevator whilst looking outside. This study seemed to indicate that the treatment was beneficial in both the short and long term. Unfortunately, this study lacked any kind of rigorous analysis and it is possibly the encouraging fact, more recent studies have moved on to use VR to reason why a study by is help explore acrophobic triggers and features of treatment. For more often cited as the first clinical application of a VR system to acrophobic disorders. In their paper they presented a single case tested 26 participants with acrophobia in VRET sessions and study involving the assessment and treatment of a 19-year-old coping cognitive self-statements. Results showed improvements regardless of the addition of coping self-statements. Despite this, The success of this study motivated the authors to continue the importance of cognitive factors surfaced in a study by their research, and in the same year they published a more who found that D-Cycloserine (DCS) (a cognitive elaborate study in which they assessed 478 university students enhancer) combined with exposure therapy in a VR environment and selected 20 that had presented with a substantial fear and resulted in significantly larger reductions in acrophobia symptoms avoidance of heights (). Of these 20, 12 were compared with placebo and exposure therapy. The treatment was selected to participate in VR therapy, while the remaining eight conducted in VR and 27 participants (11 men and 16 women) with acted as an untreated control group. The treatment consisted of acrophobia were assigned a placebo plus VRET (n = 10); 50 mg of seven weekly sessions of 35–45 min. Anxiety, avoidance and stress DCS plus VRET (n = 8), or 500 mg of DCS and VRET (n = 9). The were significantly diminished among the 12 students involved in difference between the 50 mg and 500 mg was not found to be VR therapy but not in the control group. Although a clear advance statistically significant. DCS and VRET showed maintained in the assessment of VR therapy, the study was still open to improvements at three months follow up reevaluation in 21 of criticism as it lacked a control group treated with real exposure (i.e.
the 27 participants.
in vivo), provided no follow-up tests, and included no formalassessment of phobic avoidance.
8. Examining the triggers of acrophobic behavior using VR A year later, reported a case study in which they employed eight VR sessions lasting between 15 and 8.1. Visuo-vestibular triggers 28 min each. The treatment was conducted using standardprotocols, beginning with the least threatening situation, as in Some of the earliest attempts to understand fear of heights the classic treatment of systematic desensitization ( independently of cognitive factors were made by and reinforced practice ). The Subjective Units of Discomfort (SUDS) questionnaire (was administered postulated a physiological mechanism for heights vertigo, separate every 2–5 min. One month after treatment, the results indicated from psychological factors. Similar to motion sickness, heights significant improvement, with a decrease in both anxiety vertigo was, in their opinion, due to a conflict between vision and symptoms and the avoidance of anxiety-producing situations.
the somatosensory and vestibular systems. They postulated that A few years later, directly compared such a discrepancy occurs when the vestibular and somatosensory the effectiveness of VR with a real-world exposure control. In receptors sense a body shift (the natural lateral head sway – 2 cm) comparison to the previous studies by which is not detected by the visual system, due to the large , the hardware used was less sophisticated, offering distance to the nearest viewed objects. ‘‘This conflict might be just 10 frames per second to define movement through the visual resolved by increasing the postural sway and thereby reactivating environment. All the participants received VR treatment in the first visual control'' p. 515). In other words, they two sessions and then kept receiving treatment in a real-world proposed that one reason for the increase in sway might be that environment. They discovered that the subsequent real exposure participants are attempting exploratory behavior aimed at did not lead to a significant improvement in the measurements of increasing visual balance feedback ( avoidance in the Acrophobia Questionnaire ) and of This make intuitive sense because the further a Attitudes Towards Heights ). Unfortu- viewer is from a visual target (e.g. height above a ground plane) the nately, all participants were exposed to the VR environment first smaller the motion parallax cue (provided by movement of the and this proved to be so effective that little or no room was left for head) becomes. This can be compensated by exaggerating head the later real-world treatment to produce measurable improve- movements and body sway. As point out; an alternative explanation for increased body sway is that it is simply C.M. Coelho et al. / Journal of Anxiety Disorders 23 (2009) 563–574 an indicator of poor control in the absence of strong visual cues, small sway area, despite a large sway locus length. Acrophobic rather than an attempt to drive the visual system more strongly.
participants' legs were observed to be shivering'' ( A growing body of literature shows that this conflict between p. 502). In a second group (n = 12) with two acrophobics, vestibular and visual information can lead not only to motion participants were blindfolded and guided to a roof without being sickness, but to anxiety as well told where they were going. In this condition, nonacrophobic participants lessened their body sway when the blindfold was A recent study from showed removed. In contrast, acrophobic participants' sway increased.
that fear of heights was probably not due to any vestibular These results seem to contradict those of malfunctioning, but rather possible cognitive bias in interpreting who found that fear of falling instigates a stiffened bodily sensations, thereby partially supporting posture in participants.
conclusions. Thus, it is possible that individuals There is a growing literature studying the link between anxiety with acrophobia tend to interpret visuo-vestibular conflict as an about falling and postural control (e.g. alerting sign of a possible fall.
Recently, corroborated the non-associative mode of fear acquisition in acrophobia by reporting evidence for These studies originally the existence of a physiological mechanism responsible of focused on the idea that a fear of falling in older adults was triggering the phobic behavior. Using a dual-task paradigm, they negatively impacting on their posture. To investigate this, the found lower scores in balance stability (on a mobile platform) in researchers studied individuals' standing posture at different participants with fear of heights. The authors also noted that the surface heights above the ground. It was hypothesized that the addition of a manual tracking task significantly impaired postural threat or risk of injury associated with a loss of balance would stability in acrophobics. The task consisted involved moving a increase when standing at a high surface height. This situation was joystick to keep a computer displayed circle inside a square. It was supposed to be similar to the one experienced by an individual who also found that individuals with acrophobia had lower scores than has a fear of falling. Even though they were not specifically controls in the manual tracking task even when the platform interested in acrophobia, the fact that these researchers system- remained stationary, suggesting abnormalities in the visual atically varied height in their studies makes their findings of perception of movement ).
relevance to the study of acrophobia as well.
In their review of VR technology as a research tool, What these authors generally reported was that the central describe how VR might be used to nervous system imposes modifications to postural control when investigate the role of vision in acrophobia. In particular the asked: fall anxiety increases (; ), ‘‘does pure visual information elicit acrophobia, or must appro- altering posture by reducing the amplitude and increasing the priate head orientation, signaled by neck proprioception and frequency of postural sway ( vestibular cues (e.g. looking down from a high bridge) accompany This is achieved by co-contraction of ankle joint agonist/ the visual information? To answer this, one can decouple the antagonist muscle pairs, resulting in a tighter regulation of participant's visual stimulation from his or her head and eye centre of mass (COM) This accommoda- movements, so that exactly the same visual information is tion may serve to reduce the permitted range of COM presented with the head facing downward (normal coupling) or displacement and minimize the probability of a loss of balance facing upward (decoupled)'' (, p. 561). These (). The increased frequency and decreased postural behaviors and visuo-vestibular cues can best be amplitude in elevated environments are usually accompanied by manipulated and measured in VR environments since they are an adoption of more posterior COM and centre of pressure extremely difficult to control in the real world ( position (COP) compared to the non-elevated environments Research along these lines will also allow the exploration of the These results are important because they show the possibility nature of fear of flying and its similarities and differences with fear that individuals with acrophobia have an extremely stiffened of heights. The two specific phobias are clustered together posture that impoverishes performance in a similar mechanism to suggesting that the situational and the natural environment that of Parkinson's Disease ( phobia type could share a common underlying theme In patients with Parkinson's disease, falls were Nonetheless it is also conceivable that fear of flying is found to be correlated with rigidity and not with tremor. In the composed of one or more other fears, being an expression of acrophobia case, a similar phenomenon might occur, but due to a several subtypes of fear and phobias such as fear of accidents, need postural strategy problem. It is plausible that height fearful of control, claustrophobia, acrophobia, social phobia, and symp- individuals' strategies to control balance become critically toms of panic attacks ( impaired and stiffened in the absence of visual cues or in the presence of unexpected visual environments. These studies on VR will assist in answering questions regarding the influence of the postural control reveal the need to further explore the influence of vestibular system in postural stability control and acrophobia.
postural threat on the control of upright stance.
8.2. Postural triggers 8.3. Visual dependence and motion found similar results It is well known that motor action and depth vision are linked to in which normal individuals exposed to () and that some participants rely more heights exhibited increased body sway. In a study with 30 on visual cues to control balance than others ( participants in which eight had acrophobia, Participants with an increased dependence noticed that the frequency and amplitude of on visual field information (FD) are characterized as less physically sway increased significantly (measured with a stabilometer) at stable and more reliant on visual cues (particularly motion visual 10.2 m (on a roof) but showed no change at 1 or 2 m. In participants cues) for controlling body stabilization. By contrast, the partici- with acrophobia (n = 6, since two were unable to stand on the roof) pants not dependent on this information (FI) are more physically the postural pattern was of: ‘‘small and frequent vibrations with a stable and rely less on vestibular and somatosensory cues ( C.M. Coelho et al. / Journal of Anxiety Disorders 23 (2009) 563–574 Moreover, FD participants (Much of the subsequent research has have a greater sensitivity to motion cues due to self-movements been focused on exploring the viability of this technology in than FI participants treatment. It is now known that both the real-world and VRET This increased reliance on visual cues may be a vulnerability regimens are highly effective for treating acrophobia. A recent factor that promotes disequilibrium during certain situations of meta-analysis ) presented estimates daily living that include complex environments, moving visual of effect size for VR treatments across a range of phobias and fields and heights (and thus may predispose anxiety disorder in comparison to in vivo exposure and control participants to the development of agoraphobia or acrophobia conditions and found VRET was ‘‘slightly, but significantly, more effective than exposure in vivo, the gold standard in the field'' It is possible that self-motion has an important role in triggering (p. 568). Because VR hardware is fear of heights. As previously noted (i.e., becoming more reasonably priced ), acrophobic participants experience high and VR systems can create a greater variety of stimuli and levels of anxiety when required to move laterally at a fixed height.
situations applicable to treatment, practitioners and researchers further suggested that oscillation phases of are still actively exploring the potential of this technology (for locomotion, involving monopedal stance (on one leg), could review see ). Overall, increase balance instability especially in field-dependent partici- VR offers some practical advantages over real-world exposure such pants. Visual dependence seems to increase with height, and as (1) better control of the situation by the therapist; (2) avoidance participants can be especially visually dependent when walking on of public embarrassment; (3) preservation of confidentiality; (4) a narrow support Although speculative at this maintenance of the protective environment of the therapist's office stage, it is possible that perceptual estimates of heights relate to (see also ).
cognitive and other fear-relevant biases when they are activated by Besides the utility of VR as a tool for treatment, it also offers a concerns about acting in the environment, as recently noticed by route into further investigating and understanding acrophobia.
Our own studies based on VR treatment suggest to us that motion researchers found that high fear of height participants (n = 35) combined with simulated height, rather than height per se, can estimated the vertical extent of a two-story balcony ledge to be trigger phobic responses ( higher compared to 36 participants with low acrophobic symp- In fact, between 1993 and 2001, the majority of toms. Additionally, given that perception informs us about the studies were related to the etiology of acrophobia and suggested a opportunities for action and the costs associated with these actions non-associative account. Nonetheless, both the non-associative proposed that geographical and the behaviorist account accept the important role of learning height perception is influenced by the risks of the inherent dangers to overcome the fear, whether it is learned or innate. The relative of behaving near the edge of a high drop-off. This suggests a magnitude of each vulnerability and learning factor in the possible interactive model in which cognitive factors (e.g. fear of diathesis remains a topic of current debate. At the moment, we falling; catastrophic interpretations), perceptual factors (e.g. visual would tend to agree with dependence), learning factors (e.g. prior exposure to heights), and suggestion to explore the possible mechanisms of diathesis- biological factors (e.g. heredity) can interact, provoking either stress interactions in the development of anxiety and fear. In our habituation or extreme fear in height-related environments.
particular case, this review suggests that the exploration of visuo-vestibular and motion mechanisms that might be present in 9. Summary and conclusion acrophobia is also important. It is clear from seminal studies inchildren (e.g. Campos, 1978; ; It emerges from the present review that numerous studies with ) that a fear of heights accompanies the emergence of acrophobic participants aimed to investigate the efficacies of self-locomotion (crawling) and that habituation processes can aid different treatment models and not necessarily to study acro- in overcoming a fear of heights, supporting the idea that the phobia itself. Between 1973 and 1992, research on participants etiology of acrophobia is intimately linked to body movement with acrophobia was essentially focused on comparisons between the exposure model and the self-efficacy model. Treatment within It seems plausible that an increased dependence on dynamic the self-efficacy model was usually more structured and had more visual cues for controlling body stabilization might be a key factor therapeutic components, such as the mastery of subtasks, in the predispositions to the development of acrophobia ( physical support, modeling and elimination of defensive man- euvers and this in turn lead to it producing the most effective between motion and fear still requires systematic examination. In clinical approach for the treatment of acrophobia. Although the coming years research investigating the links between perhaps not the main focus of their work, the self-efficacy acrophobia, motion triggers, and implications for acrophobia approach also provided important insights into the root causes treatment, will draw heavily on VR technology, which is proving and triggers of acrophobia. For example, noticed to be an invaluable tool for measuring physiological and that a person's judgment about his or her ability and confidence to behavioral indices in response to controlled visual and motion perform various specific actions (self-efficacy) is a major triggers. VR is probably the most useful tool to decouple visual determinant of human behavior. In a similar vein, we and other information, head orientation, proprioception and vestibular researchers have begun to recognize the crucial role of movement cues, and independently evaluate their role in acrophobia. As in generating acrophobic responses in sufferers, making it a key such, we look forward to VR ushering in a period of discovery that feature to address in treatment ( will greatly enhance our understanding of this debilitating and ). Movement has also widespread phobia.
emerged as an important fear trigger. As we have argued in thisreview, it seems likely that motion parallax and visuo-vestibular conflict play an important role in the development of acrophobiain some sufferers ( This work was funded by a grant from the Portuguese Science From its outset in 1995 as a treatment of acrophobia, VR has and Technology Foundation (ref. SFRH/BPD/26922/2006), awarded come to dominate the treatment of numerous anxiety disorders to the first author.
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