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 1 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 with placebo, oxytocin treatment yielded enhanced RHI responses, again evident in accelerated illusion onset and increased vividness ratings. 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.