The relationship between emotional components and the embodiment effect during the Rubber Hand Illusion: an EEG-fNIRS study

Repeatedly stimulating the hand of a person and a dummy hand in front of him it is possible to induce the inclusion of the object in the body schema (Rubber Hand Illusion, RHI). One of the key strengths of the RHI is the incongruence between what a person feels via the somatosensory pathways and what he sees. When the illusion has been induced, participants tend to provide irrational and supernatural explanations for sensory phenomena that do not fit. In view of this fact, stimuli presented on or near to the artificial hand generally elicit behavioural and neural responses that would normally occur if they had been disclosed on the participant’s own hand. However, previous investigations focused mainly on primary variables such as physical features of RHI. In order to investigate the contribution of some affective processes in the definition of the phenomenon, in the present research the classic paradigm was amended to include a phase of stimulation with emotional value, by using an emotional threatening condition. The illusion was induced in 16 participants, stimulating synchronously the dominant and the artificial hand for three minutes with two brushes. The study included three induction sessions, followed by the emotional stimulation or by a control condition. The emotional condition included the presence of a direct aversive stimulus toward the artificial limb. Simultaneously, cortical responses of the participants were measured using Electroencephalography (EEG) and functional Near-Infrared Spectroscopy (fNIRS). The analysis focused on data related to the subjective experience, the cortical hemodynamic responses and the modulation of the frequency bands components of EEG. All participants reported having experienced the illusion, with different reactions to emotional stimulation. fNIRS data suggested the contribution of the frontal and parietal regions to support this illusion, with oscillatory responses partially different as a function of the emotional stimulation condition. Therefore, where as previous study have shown some crucial factors for the onset of the RHI, such as spatial coherence between the position of the artificial hand and the coordinates of the body schema, the present results showed that even the emotional processes are able to modulate the appearance of the illusion and the related embodiment effect.