Summary: Researchers report inhibited activity of the amygdala makes people more susceptible to deception when it comes to illusory perception. The study suggests the amygdala might help to protect us against distortions in self-perception.
Source: University of Bonn
If functionality of the brain’s amygdala is impaired, illusory perceptions arise much faster and more pronounced. This was discovered by a team of researchers led by the University of Bonn, who studied identical twins in whom both amygdalae are damaged. Further experiments with volunteers showed that this brain structure, which is widely known for its eminent role in fear processing, apparently provides effective protection against body perception disorders. This insight from basic research may also enable a better understanding of mental illnesses. The results are published in advance online in the Journal of Neuroscience.
The “rubber hand illusion” is a classic sensory illusion based on experiments published in 1998 by Matthew Botvinick and Jonathan D. Cohen. 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 Dr. René Hurlemann from the Department of Psychiatry and Psychotherapy at 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,” reports Hurlemann. The rubber hand illusion experienced by the twin sisters was particularly rapid and very pronounced. One year later, the researchers repeated the experiment on the twins – with the same result. The scientists therefore suspected that the amygdala plays an important role in protecting against body perception disorders.
Deception is weaker with intact amygdala
The researchers pursued this lead with further experiments. They repeated 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 standardized 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,” reports lead author Dr. Franny Spengler, who spent several years working in Prof. Hurlemann’s team and then moved to the University of Freiburg.
In the next step, the research team at the University Hospital Bonn 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. Result: The smaller the amygdala, the faster the illusion set in.
Oxytocin promotes rubber hand illusion
The scientists then administered an oxytocin spray and a placebo spray into the nose of the volunteers at two consecutive appointments. “Studies show that the hormone oxytocin inhibits the activity of the amygdala,” said Dr. Markus Heinrichs from the Department of Psychology at the University of Freiburg, who was also involved in the study. The oxytocin increased the rubber hand illusion: The effect occurred much faster and was significantly stronger than after placebo administration. Heinrichs: “Apparently, an intact amygdala reduces susceptibility to the rubber hand illusion.”
For Hurlemann, these results indicate that the amygdala generally has a protective function against impaired body perception. “In the literature, the amygdala has often been described as an alarm system generating fear responses to external hazard stimuli,” reports the UKB scientist. “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. Hurlemann: “We are at the beginning of an important scientific trail that may also be relevant to mental illness.”
A protective mechanism against illusory perceptions is amygdala-dependent
Most people have a clear sense of body ownership, preserving them from physical harm. However, perceptual body illusions — famously the rubber hand illusion (RHI) — can be elicited experimentally in healthy individuals. We hypothesize that the amygdala, a core component of neural circuits of threat processing, is involved in protective mechanisms against disturbed body perceptions. To test this hypothesis, we started by investigating two monozygotic human twin sisters with focal bilateral amygdala damage due to Urbach-Wiethe disease. Relative to 20 healthy women, the twins exhibited, on two occasions one year apart, augmented RHI responses in form of faster illusion onset and increased vividness ratings. Following up on these findings, we conducted a volumetric brain morphometry study involving an independent, gender-mixed sample of 57 healthy human volunteers (36 female, 21 male). Our results revealed a positive correlation between amygdala volume and RHI onset, i.e., the smaller the amygdala, the less time it took the RHI to emerge. This raised the question of whether a similar phenotype would result from experimental amygdala inhibition. To dampen amygdala reactivity, we intranasally administered the peptide hormone oxytocin to the same 57 individuals in a randomized trial before conducting the RHI. Compared to placebo, oxytocin treatment yielded enhanced RHI responses, again evident in accelerated illusion onset and increased vividness ratings. Taken together, the present series of experiments provides converging evidence for the amygdala’s unprecedented role in reducing susceptibility to the RHI, thus protecting the organism from the potentially fatal threats of a distorted bodily self.
Compelling evidence indicates that the amygdala is of vital importance for danger detection and fear processing. However, lethal threats can arise not only from menacing external stimuli but also from distortions in bodily self-perception. Intriguingly, the amygdala’s modulatory role in such illusory body perceptions is still elusive. To probe the amygdala’s involvement in illusory body experiences, we conducted a multi-methodological series of experiments in a rare human amygdala lesion model, complemented by a morphological and pharmaco-modulatory experiment in healthy volunteers. Our findings convergently suggest that the amygdala’s integrity is indispensable for maintaining an unbiased, precise perception of our bodily self. Hence, the amygdala might shield us against distortions in self-perception and the resultant loss of behavioral control of our organism.