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AS I prepare the room, it feels as if I鈥檓 getting ready for a seance. I close the curtains to block out most of the light and place two chairs about a metre apart. I prop up a large mirror on one and sit in the other so that I can just see my reflection in the near darkness. Then I set a timer for 10 minutes and wait patiently for the faces to appear.

When they do, it is startling. At first the distortions in the mirror are small: a lifted eyebrow, a twitch of the mouth. But after seven minutes my reflection suddenly looks fake, like a waxwork. And then it is no longer my face. For a few seconds an old man with a thickly wrinkled brow and down-turned mouth stares back at me.

The first time saw the faces, he was also haunted by an old man. 鈥淚 remember very well that I was frightened by the extremely serious expression of the old man gazing at me,鈥� he says. A psychologist at the University of Urbino in Italy, Caputo has now replicated the illusion many times, in many people 鈥� and he thinks it is far more than a spooky Halloween party trick.

By breaking down the recognition of our own face, the illusion could tell us much about the way we construct our sense of self and identity. The unsettling visions also seem to trigger the same sense of dissociation seen in people with post-traumatic stress disorder (PTSD) and schizophrenia, which could help us understand the processes that create their terrifying symptoms.

Although Caputo was the first to report the illusion from a scientific perspective, it may have been known in folklore for thousands of years. 鈥淭here is a long tradition of people using reflective surfaces to bring about hallucinations,鈥� says , an experimental psychologist at the University of Oxford. Ancient Greek depictions of Dionysian rituals, for instance, show initiates gazing into a mirror, perhaps in an attempt to see spirits from the underworld. Then there are the games in which people would chant 鈥淏loody Mary鈥� into a mirror in the hope of conjuring her ghost. According to legend, this spirit could reveal the future.

Caputo first came across the illusion by accident four years ago. As part of an experiment looking at self-identity, he had set up a space entirely enclosed by mirrors in which volunteers could confront multiple reflections of themselves. Usually the experiments were conducted in normal lighting, but early one morning he decided to sit in front of the mirrors in semi-darkness. When the strange faces came he feared he was hallucinating. But curiosity got the better of him. Eventually, he tried the illusion out on other people and found they saw faces, too.

Since his chance discovery, Caputo has found that most people perceive some degree of eerie distortion to their face if they stare into a mirror in low light for at least 10 minutes. 鈥淯sually, after about 1 minute of mirror-gazing, the eyes start to move or shine, the mouth opens, or the nose becomes very large,鈥� he says. 鈥淚f you continue to gaze there are very big changes, until completely new faces appear.鈥� And it鈥檚 not just human faces that are seen 鈥� some report .

鈥淪ome see animals and others see fantastical or monstrous beings staring back at them鈥�

At first, he suspected the apparitions might be caused by the Troxler effect, in which objects in the periphery of your vision begin to disappear as you stare at a scene, which is thought to occur when neurons stop responding to the unchanging input. But although such distortions might explain why some parts of the face in the mirror begin to morph and change, it is unclear why new features would pop up in their place.

This aspect makes it similar to another illusion that occurs when you stare at a point beyond the edge of a portrait photo, so the face is in your peripheral vision. Like the reflections in the mirror illusion, .

No longer you

One possible explanation for this illusion is that the incomplete view of the image disrupts the way the brain binds together features like the eyes, nose and mouth into a recognisable face. As the brain struggles to make sense of what it is seeing, it might pull scraps from our memory to make up for our poor perception 鈥� perhaps patching together a 鈥減hoto fit鈥� of different features so that it begins to look like another person. In Caputo鈥檚 mirror illusion, similar disruptions might occur thanks to the poor lighting; the apparitions vanish when you turn the lights up.

But what makes Caputo鈥檚 illusion so eerie is that it transforms the viewer鈥檚 own face staring back from the mirror. 鈥淵ou are suddenly conscious that there is another person behind the mirror,鈥� says Caputo.

Recognising our reflection is thought to be a key sign of our sense of self. It usually involves subtle interactions between our motor processes, perception and internal sensing of movement, as the brain links what it sees in the mirror to the sensations felt by the body to create its sense of identity. But when our reflection starts to change, that sense of self 鈥� the border between ourselves and others 鈥� becomes distorted too, perhaps explaining why the experience is so much more unsettling than other visual illusions.

What鈥檚 clear is that our facial processing and self-recognition abilities are far more fluid than we might imagine, although more work is needed to confirm the exact processes that contribute to the ghostly visions. But whatever its underlying cause, the illusion is beginning to attract wider attention, because its unsettling effects seem to create the symptoms of another widely known state of mind.

鈥淲hat Caputo has stumbled across is a way of inducing a form of dissociation,鈥� says at University College London. Dissociation is typified by disruptions to the conscious experience, such as the perception that time is speeding up or slowing down, and the feeling that everything seems unreal or mechanical. These sensations, which seem to come up during mirror gazing, are common during traumatic experiences, making the illusion interesting to clinical psychologists like Brewin. 鈥淭here is quite strong evidence that people who have more dissociative experiences while they are going through a traumatic event are at greater risk of developing PTSD,鈥� he says.

PTSD often involves the recurrence of vivid flashbacks, which Brewin thinks could be linked to the way that memories are formed during the dissociative experience. Working with neuroscientist and others at University College London, he has developed a model of memory formation in which memories get laid down in two parts: autobiographical memories that give context to an event, and 鈥渟napshots鈥� that capture a scene in a lot of detail but with little context.

According to Brewin鈥檚 theory, dissociation impairs the formation of the context-giving memories. Without that anchor, the snapshot memories are more likely to return as involuntary flashbacks. Along these lines, research has shown that people who have trouble constructing their autobiographical memory are also more at risk of PTSD, perhaps because the flashback scenes are untethered to the rest of their life story.

Reflective therapy

Brewin is now turning to Caputo鈥檚 illusion to test the idea. His team used a standard clinical questionnaire to assess people鈥檚 levels of dissociation after a period of mirror-gazing. It produced many of the same symptoms, including the sense of physical detachment and a distorted perception of the passing of time.

He also gave the volunteers a series of memory tasks, such as telling them a story straight after they had experienced the illusion and then asking them to recall details of it 10 minutes later. , which seems to tally with the context-encoding problems predicted by Brewin鈥檚 theory. People in a dissociative state sometimes experience flashbacks after seeing a scary film, so it might be interesting to see whether Caputo鈥檚 illusion, and the associated memory effects, make this more likely.

Terhune thinks the illusion is a useful tool. 鈥淭here鈥檚 a lot of value in inducing dissociative states,鈥� he says. He is using the illusion to try to understand the way our emotions influence the dissociative state, but he cautions that suggestion may play a role in the illusion, with participants primed to see things by researchers who for ethical reasons need to inform them beforehand of the possibility of visions.

Caputo鈥檚 interests lie in a different, but equally devastating, condition. He recently teamed up with researchers from the University of Milan and the University of Verona to look at the effect of the illusion on people with schizophrenia. In some cases, he thinks it could be used therapeutically.

The team found that the illusions were particularly powerful among people with schizophrenia 鈥� starting more quickly and involving multiple faces. At first Caputo was concerned that the illusion might be harmful, but several people reported that it was helpful to see the face of a personality they had internalised. For example, . 鈥淭he patients described becoming aware of a danger which was within themselves,鈥� says Caputo.

The idea would be to confront the visions during therapy. A similar approach encouraged people with schizophrenia to create computer avatars that embodied the voices they heard in their heads. After six sessions, in which a therapist helped them to interact with the avatar so that it became less threatening, most participants reported improvements in their symptoms. Caputo thinks the mirror illusion could have the same effect without using specialist software.

鈥淧eople with schizophrenia found it useful to confront the visions in the mirror鈥�

For the time being, Caputo is still exploring just how far he can stretch the illusion. Since his initial study, he has gone on to show that a similar effect occurs when subjects wear a Shinto mask. In this case, , gurning back at the participants. He has also found that some people experience when they stare at another person in dim lighting. Intriguingly, the sensations are more intense, perhaps because the subtle feedback from one face to another 鈥� as their expressions mimic small changes in the other 鈥� amplifies the feeling of distortion.

More powerful visions may be a step too far for me. The final apparition to visit the mirror is a grotesque, gargoyle-like creature. Luckily, my timer beeps soon after. The 10 minutes are up, and the spell is broken.

This article appeared in print under the headline 鈥淢irror, mirror鈥�