Visual-vestibular stimulation assists out-of-body illusion

People who've had an out-of-body experience (OBE) report the sensation of leaving their physical body and floating up above it. Many also say that their point of view shifts, so that they look down at their body for a period of time, before 're-entering' it.

While some take a more mystical approach to these experiences, researchers have linked them to problems with the vestibular system, a suite of organs in the inner ear that are sensitive to the direction of the pull of gravity and also to head movement, helping us sense when we're moving. Vestibular signals are also thought to be important for the feeling that our conscious self is located in our physical body, though senses such as vision contribute to this, too.

In a recent study in iScience, Hsin-Ping Wu and colleagues explore how OBEs might come about, reporting a technique which stimulates both vision and the vestibular system to create the illusion of an OBE in healthy people.

In the first of two experiments, participants wore a head-mounted display which initially put them into a virtual version of the lab room. In other words, the room around them, their body, and their movements all mimicked what they would have seen in real life. Then, they lay down on their back on a platform that the researchers could move up and down.

To start with, they were looking up at the virtual ceiling. Gradually, though, their viewpoint started rotate, until they were looking down at their virtual body. Their viewpoint then moved up and away from the platform, finishing at a point two metres above their body. (This was designed to mimic OBEs of floating above the body and looking down at it.)

This visual shift happened repeatedly. Sometimes, it wasn't accompanied by any physical movement. At other times, though, the participants were simultaneously lifted upwards by the platform. This vestibular stimulation didn't match the visual signals, which suggested that they were moving away from their body rather than 'forwards' towards it. In the final experimental condition, as their viewpoint shifted, the platform was lowered. This backwards motion 'matched' the visual signals that they were receiving — at least, to some extent. (Video showing how the set-up works is available here.)

At the end of each trial, the participant used a technique in which they imagined dropping a ball to report on how far away they felt they were from the ground, as well any feelings of motion, floating, or being separated from their physical body.

The team's analysis of this data revealed that even visual stimulation alone could induce the feelings that participants were actually moving upwards, away from their bodies. However, when — and only when — they were simultaneously moved down, so that the visual and vestibular data 'matched' the feeling their body was moving away from their supposedly-floating selves, this triggered an "OBE-like" illusion. In this condition, participants not only said they felt they were 'above' their body, but also reported sensations of lightness and a stronger feeling of disembodiment.

In the second study, the experimental set-up was similar to the first, but the background scene was eliminated, so the participants had more limited visual information. The team found that, without clear visual data showing movement, to decide where they were located in space, the participants relied on the vestibular information, when it was available — i.e. after they had been physically moved. The team also found that participants who'd been found in earlier tests to rely more heavily on visual versus vestibular cues reported stronger feelings of disembodiment when the platform moved down ('matching' the limited visual data).

The researchers write that this new work shows that vestibular signals likely contribute to OBEs. At least, they did in this lab-based study of healthy people, some of which reported 'OBE-like' sensations — which, it has to be said, were generally mild compared with those in classic OBE reports.

Exactly what triggers OBEs in the real world is still being explored. But patients with vestibular disorders are known to be more at risk, and it's been proposed that alterations to normal vestibular processing and/or the integration of vestibular signals with signals from other senses, including vision, cause the strange sensations.

This new study does support that idea. It's also the latest in senior author Olaf Blanke's ground-breaking research on bodily self-consciousness — work that has seen papers on everything from full body illusions to the creation of 'ghostly' presences.

As the authors of the new paper stress, more now needs to be done to explore OBEs. So this is unlikely to be the last from Blanke and his team on this particular deviation from typical perceptions of our conscious self as being located 'in' our physical body.

Read the paper in full.