Fear centre of the brain protects against illusions

Researchers from the University of Bonn have shown that impairment of the brain’s amygdala — which plays a key role in processing of emotions, particularly fear – makes people more susceptible to deception, allowing illusory perceptions to arise much faster and more pronounced. Their work has been published in The Journal of Neuroscience.

The ‘rubber hand illusion’ is a classic sensory illusion in which the subject places both hands on a table, one of the hands is covered and a deceptively realistic rubber hand is placed next to it. Then the real hand and the rubber hand are stroked rhythmically with a brush at the same time. After some time, the vast majority of subjects feel that the artificial hand is part of their own body.

A team led by Prof Dr René Hurlemann from the University Hospital Bonn (UKB) conducted this experiment on identical twins who are both suffering from Urbach-Wiethe syndrome. In this rare disease, the amygdalae in both temporal lobes of the brain are defective.

“As a result, the twins’ body perception is prone to interference,” said Prof Dr Hurlemann. Indeed, the rubber hand illusion experienced by the twin sisters was particularly rapid and very pronounced — and remained that way when the researchers repeated the experiment on the twins one year later. The scientists therefore suspected that the amygdala plays an important role in protecting against body perception disorders.

The researchers pursued this lead, repeating the experiment on a control group of 20 healthy women. This showed that the rubber hand illusion took much longer to arise in healthy subjects than in the twins with defective amygdalae. Furthermore, a standardised questionnaire showed that women with intact amygdalae had a much weaker sensory delusion than the twins.

“Until now there had been little association between the amygdala and the rubber hand illusion,” said lead author Dr Franny Spengler, who spent several years working in Prof Hurlemann’s team.

In the next step, the research team used a brain scanner to measure the structural volume of the amygdalae in 57 subjects (36 women and 21 men). Additionally, the rubber hand experiment was carried out again and the time until the rubber hand illusion occurred was measured. The smaller the amygdala, the faster the illusion set in.

The scientists then administered a spray of the hormone oxytocin — which inhibits the activity of the amygdala — as well as a placebo into the nose of the volunteers at two consecutive appointments. The oxytocin increased the rubber hand illusion: the effect occurred much faster and was significantly stronger than after placebo administration.

For Prof Dr Hurlemann, these results indicate that the amygdala generally has a protective function against impaired body perception. He noted, “In the literature, the amygdala has often been described as an alarm system generating fear responses to external hazard stimuli. What is new is that this brain structure also plays a major role in body perception.”

This protective structure apparently prevailed early on in evolution: as hunters and gatherers threatened by wild animals and enemy clans, humans would probably have had little chance of survival if they suffered from body illusions in the event of danger. The scientists are now wondering whether the amygdala may also play a role in diseases associated with a disturbed body schema.

“We are at the beginning of an important scientific trail that may also be relevant to mental illness,” Prof Dr Hurlemann said.

Image credit: ©stock.adobe.com/au/Nuno Monteiro

Please follow us and share on Twitter and Facebook. You can also subscribe for FREE to our weekly newsletters and bimonthly magazine.