The neurophysiological correlates of illusory hand ownership

The rubber hand illusion has been established as one of the most important tools in the quest for understanding body ownership. Such understanding may be vital to neuro-rehabilitative and neurosurgical therapies that aim to modulate this phenomenon. Numerous brain imaging and TMS studies indicate that a wide ranging network of brain areas is associated with illusory hand ownership in the RHI. However, while we have a good idea of where neural activity related to the RHI occurs, the question of how these networks interact on the temporal basis is still rather unexplored as the few EEG studies that have investigated this question have relied on problematic stimulation methods or have failed to induce a strong sense of illusion in participants. Avoiding these limitations the experiments in this thesis provide insights into the temporal dynamics of body ownership in the brain. Experiment One (presented in Chapter Three) focussed on establishing that the purpose-built, automated setup induced the Rubber Hand Illusion reliably as measured by proprioceptive drift measurements and questionnaire ratings. The evoked visual and tactile responses elicited by the setup were identified and timing and intensity of illusory hand ownership were found to be comparable to the existing literature. The results of this experiment provided guidance regarding necessary adjustments to the RHI setup for the following experiments in order to avoid confounds induced by avoidable differences between conditions. Experiment Two (presented in Chapter Four) used a setup adjusted according to the findings of Experiment One and recorded evoked responses and oscillatory responses in participants who felt the rubber hand illusion. A combination of experimental conditions was applied to rule out confounds of attention and body-stimulus position. In addition two control conditions were applied to reveal the neural correlates of illusory hand ownership. The experiment revealed a reduction of alpha and beta power as well as an attenuation of evoked responses around 330 ms over central electrodes associated with illusory hand ownership. Also, the results indicate that body-stimulus processing and illusion processing as measured by evoked potentials might emanate from the same cortical network. Experiment Three (presented in Chapter Four) tested if the findings of the second experiment in regard to illusion effects were robust against changes in stimulus duration. The reduction in alpha and beta power and the attenuation of evoked responses at 330 ms were found to be robust against changes in stimulus duration. Together with the results from Experiment Two, these findings provide the first EEG marker of illusion related activity in the RHI induced by an automated setup with varying stimuli length. Experiment four (presented in Chapter Five) investigated if the neural correlates identified in the Experiment Two and Experiment Three were indeed related to the feeling of illusory hand ownership in the RHI and not to a mere remapping of visual receptive fields. To test this, evoked and oscillatory responses were recorded during the somatic rubber hand illusion, a non-visual variant of the RHI. The somatic rubber hand illusion was found to be associated with an attenuation around 330ms post-stimulus on central electrodes, similar to the classic RHI in Experiment Two and Three. This indicated that this illusion effect in evoked responses was not related to a remapping of visual receptive fields as a result of the RHI but to the neurophysiological processes of the RHI itself. To summarise, the results of the experiments presented in this thesis indicate that an attenuation at 330ms in evoked potentials is associated with illusory hand ownership in both, the classic RHI and the somatic RHI. Further, attenuation in alpha and beta band power is associated with illusory hand ownership in the classic RHI.