Looking at a virtual hand reduces the pain in your real hand

Next time a nurse sticks a needle into your arm, don’t look away: it’ll be less painful. A new study shows that we feel less pain when we are looking at our body – and that this effect works with virtual reality too.

In 2009, Patrick Haggard and Matthew Longo of University College London showed that looking at your own body has an analgesic effect. The researchers shone infrared laser light on the skin of volunteers. Those who were looking at their body rather than at a neutral object said that they felt less pain. Scalp electrodes revealed that this analgesic effect was linked to weaker activity in parts of the brain’s cortex that process pain – although why this happens is unclear.

Since then, two different teams have tested the effect using the rubber hand illusion – in which a rubber hand is placed next to a person’s real hand, which is hidden from view. Stroking both the real and rubber hands with paint brushes convinces them that the rubber hand is their own. Extending this illusion, the teams wanted to know: can looking at a rubber hand that feels like one’s own alleviate pain in your real hand?

The studies were contradictory: one study showed an analgesic effect, but the other did not. Maria Sanchez-Vives at the University of Barcelona, Spain, and her colleagues argue that differences in the position of the rubber hand and real hand may have led to the differing results.

To test the effect of the rubber hand’s position, her team used virtual reality to induce the illusion. Instead of seeing a real rubber hand, participants were shown one via a VR headset instead. The researchers could change the location of this virtual rubber hand, relative to where a participant’s hand really was. Thermal electrodes attached to the volunteers’ hands made them feel heat. The virtual hand appeared to be attached to virtual electrodes too.

The team then stepped up the heat in small increments, asking volunteers to tell them when the heat started to feel painful – a measure of their thermal pain threshold. They found that when the virtual hand was located close to where a participant’s real hand was, they had a higher pain threshold. “If you look at a virtual body, it is analgesic, but it’s important that it’s co-located,” says Sanchez-Vives. If there is a displacement between the real body and the virtual body, this effect is reduced, she says.

The researchers think that virtual reality can be used as a therapeutic tool. “Classically, virtual reality has been used to deal with pain, mostly as a distractor,” says Sanchez-Vives. The new technique is not about distraction, but about using VR to use an existing mechanism in the brain to increase a person’s pain threshold. In theory, during a painful procedure, VR could show a person’s body in ways that could reduce pain. In an earlier study, Sanchez-Vives’ team had shown, for example, that looking at a virtual arm that’s reddish in colour lowers your pain threshold, while a bluish arm increases it.

Thomas Metzinger, a cognitive philosopher at the Johannes Gutenberg University in Mainz, Germany, agrees. “This research is very useful and quite promising because it opens a route to how we could therapeutically dampen down pain using virtual reality,” he says.

Both Sanchez-Vives and Metzinger say that the effect might be down to the brain trying to predict the location and intensity of the pain. If a nurse is about to stick a needle into your arm and you look away, the brain is unable to localise the pain without visual input. As a result, it lowers the pain threshold and pays attention to a larger area— which may explain why the needle causes more pain when you look away.