For most of us, standing at the edge of a skyscraper would induce toe-tingling, stomach-swooshing terror. But for some, this is true even of standing at the top of a tall escalator, or simply walking down the stairs in their own homes, shooting a cold current of fear through everyday life.
The NHS hasn't traditionally offered counselling for acrophobia, extreme fear of heights, but a startup spun out of Oxford University thinks that it might have the answer - in the form of therapy delivered within immersive virtual worlds. Oxford VR has carried out a successful trial of a therapeutic programme to treat a fear of heights that is delivered through a VR headset.
The trial examined over 100 people who had suffered with acrophobia for an average of 30 years. Each underwent a number of immersive experiences in the virtual world and the results were remarkable. Patients reported on average reduction of 68 percent in their fear of heights, compared to when they first began the therapy.
Following this success, Oxford VR recently announced that it has raised £3.2 million from investors including Oxford Sciences Innovation, University of Oxford, Force Over Mass, RT Capital and GT Healthcare Capital Partners.
But how does VR therapy work? The startup's fear of heights programme involves visiting a number of different floors in a shopping mall, and slowly bumping up the fear factor. After easing themselves in, participants are asked to attempt feats many of us would find terrifying in the real world - stepping out on a narrow plank suspended several floors above a shopping mall atrium to rescue a cat, for example.
And although the graphics don't yet quite approximate reality, the therapy is designed to trigger the same psychological and physiological responses as the real situation would.
"Because they know it's not real, they put themselves in harm's way so as to speak, knowing that no harm can actually happen," says Barnaby Perks, CEO of Oxford VR. Herein lies the magic of therapeutic VR.
The situation is designed to feel as real as possible by incorporating state of the art VR and gaming technology. The 12 people working on development of the VR all have strong pedigrees in virtual reality.
"They're all ex-industry - VR games," says Perks. "You have a mix of character artists, environment artists, animators and VR programmers."
And this is a crucial feature of the therapy. "The reason we acquire people from the games industry is that gamification is really important for making the therapeutic intervention engaging," says Perks.
Right now, fear is currently measured by participant self-reporting via questionnaires, as is the de facto measurement in these forms of clinical trials. But in the foreseeable future, these measurements could be gathered by physiological sensors measuring participants' fear responses.
Although for some patients, this is not necessary. Perks brings up the example of one particular patient, who after only a morning session was not only able to step onto an escalator she previously couldn't stand at the top of, she lifted her arms off halfway down in a show of her newly relaxed state.
VR has been employed in clinical and therapeutic programmes before, in the context of exposure therapy. However, Oxford VR is pioneering a slightly different approach, dispensing with a real-life therapist altogether and instead incorporating a virtual therapist that delivers cognitive behavioural therapy alongside the VR experience.
This virtual therapist will guide the patient through their treatment, asking them questions about their thoughts and feelings, and asking them how they'd like to progress through the activity.
Perks says VR treatment could be applicable for any condition that has proved treatable in response to cognitive behavioural therapy, one of the most strongly evidence-based psychological therapies around today. This form of therapy takes as its target the maladaptive thoughts people suffering mental health problems suffer. For example, in the context of depression, these thoughts may concern the individual's feelings of worthlessness.
CBT involves patients being asked to confront and dissect these thoughts as well as trying out new behavioural responses to situations - for example asking someone with social anxiety to engage a work colleague in conversation. The patient is generally asked to record information about these experiences and then report back to the therapist.
Perks argues that this is a perfect use case for VR therapy, because in addition to going away and doing exercises for homework, patients can undertake simulations of the experiences that scare them in a safe setting, with real time guidance from an automated therapist.
Of course, many both inside and outside of the world of clinical psychology will object to the idea of removing the element of human interaction from therapy. Indeed, since Freud - who was an advocate of the idea of transference between patient and therapist - there are schools of thought that stress the empathic bond as essential for a patient's recovery.
However, Perks disputes this in regard to the delivery of CBT, because it's a form of therapy that can be easily formalised into discrete stages.
"The very best therapists are those who diagnose effectively, assign the right protocols, and then stick to those protocols," he says. "The therapists who don't get very good outcomes - they tend to drift around and be too idiosyncratic in their approach."
However, he does not propose the complete automation of all therapies, saying that there will likely be some that will always require a human element, and others that can be automated to varying degrees.
In some cases, these are activities you couldn't easily replicate in the real world. As Perks points out, some of the fear of heights activities are far too dangerous to recreate in real life. Others are simply less feasible - gathering together a large crowd willing to hear an impromptu speech from someone with a fear of public speaking might be difficult to pull off.
Many would argue that the extreme lack of available therapists to deal with what has been termed the 'mental health crisis' currently afflicting the UK reason enough to push ahead with alternatives. The number of people seeking treatment for mental health conditions increased from 1.2 million in 2010 to 2 million in 2015, while at the same time, the number of mental health nurses fell 15 percent - and between 2016-17, 5,876 patients were referred to a different health trust for treatment.
"The great scandal of mental health is a very small portion of people with mental health conditions are treated," says Perks. Alternative therapies that don't require human input, and thereby lessen the pressure on the already over-stretched mental health services could offer a welcome reprieve and create greater availability for those most in need.
The team is now working on similar clinical prototypes for social anxiety and, perhaps more surprisingly, psychosis.
Psychosis is something that is generally considered to require medical intervention, but Perks says that a large part of the difficulty comes from fear of engaging in certain interactions, leading patients to hole up in their homes and sometimes develop concurrent conditions like agoraphobia.
"The thing we're focusing on here is getting people around social situations, so getting them to be comfortable with doing things that they ordinarily just wouldn't do and withdraw from," says Perks.