Considering the links between vestibular cortical processing and body ownership and bodily consciousness, it was thought that interference with peripheral vestibular signals might lead to consequences related to the neurobiology of bodily self-consciousness. Neuroimaging studies have reached conclusions showing that PIVC-associated brain networks are critical in the presence of VS. The aim of this study was to examine the contributions of artificial VSs on bodily self -consciousness along with their hemodynamic activities. For this purpose, after electrical stimulation of the participants' vestibular cortex at the level of the right temporoparietal junction and the level of the mastoid region, their performance in the VR environment was compared with fNIRS. According to the results of the study, we could verify our hypothesis that after artificial VS at the rTPJ and mastoid levels, the participants contributed to the formation of bodily self-consciousness in the VR environment.
Our results on brain hemodynamic activity showed cortical HbO activation in various cerebral regions during four different tasks within the VR environment. All of the tasks performed in the presence of without VS, VS on rTPJ level and VS on mastoid level sessions showed the strongest oxygenated brain hemodynamic activation in supramarginal gyrus-angular gyrus region connections on right and left lobes and secondary association sensorimotor cortex on left lobe. Here; it can be thought that the increase in the hemodynamic activity of these regions in the session without VS may be due to the encoding of spatial focus, visual-spatial processing and visual movement response information on the basis of tasks. As a result of VS on both rTPJ and the mastoid region, we observed hemodynamic activation in primary somatosensory cortex on right lobe and secondary sensorimotor cortex-ventral posterior cingulate cortex (PMC) junction. Electrophysiological studies in the field of body ownership have mainly focused on somatosensory processing and activity of different cortical areas using somatosensory evoked potentials, rTMS and EEG (47–49). However, few studies have investigated the effect of body ownership on sensorimotor integration and motor cortex excitability. Sensory conflict provided in a VR environment can alter sensorimotor integration, so that its corticomotor output can provide more insight into the neural basis of bodily ownership. Our results regarding strong activation in the supramarginal and angular gyrus in the inferior parietal lobe are in line with the findings of other studies that vestibular processing becomes stronger with performance of tasks (50–52).
After the VS provided at the rTPJ level, unlike the other sessions, increased hemodynamic activation was observed at primary somatosensory cortex-supramarginal gyrus in the left lobe and in the supramarginal gyrus in the right lobe. The neurocognitive body ownership model presented by Tsakiris proposed a neural network responsible for body ownership perception (53). According to this model, the conflict between somatosensory and visual inputs during bodily illusions interrupts multisensory integration and triggers the process of body ownership. The rTPJ, somatosensory cortex, posterior parietal cortex, and right posterior insula have been suggested as strategic areas of this process. It has also been suggested that the ventral premotor cortex contributes to a sense of body ownership and provides a link to the motor system (53–55). Consistent with these studies, we observed hemodynamic activation in the left part of the primary somatosensory cortex, which we did not encounter in other sessions, in addition to the supramarginal gyrus region with stimulation from the rTPJ level.
In the VS provided at the mastoid level, we observed an increase in hemodynamic activity in secondary somatosensory cortex - ventral posterior cingulate cortex (PMC) connection on left lobe, which we did not observe in other sessions. It has been suggested that the posterior medial cortex plays a role in generating subjectivity (56). In the literature, in a case of a patient with seizures originating from the posteromedial cortex and characterized by a sense of self, it has been reported that the patient has “depersonalisation”, that is, unreality, disconnection, or being an outside observer regarding the person's thoughts, feelings, sensations or body (10, 23). This report demonstrated the body's self-dissociation during seizure auras and after stimulation of the PMC. In our study, we achieved an increased hemodynamic response in the PMC region with VS on the mastoid region. Also, the fMRI study findings suggesting that the posterior insula region is activated only during galvanic stimulation applied at the mastoid level supports its vestibular function. This location is best associated with the PIVC region, where neurons in this region respond to the electrical stimulation of the peripheral vestibular nerve and vestibular, somatosensory, and optokinetic stimulation.
When we evaluate the hemodynamic activity results created by the sessions on different tasks of VR environment; unlike sessions with VS, in the session without VS, increased hemodynamic activation was observed in task of autoscopic hallucionation. Also, in both sessions with VS, significantly higher activation was observed in fNIRS channels in the case of autoscopic hallucination&room tilt illusion. It can be thought that the reason for this is that the autoscopic hallucination is observed only with a disintegration in the personal area and is not affected by a pathology in the processing of vestibular information. When there is no VS, because there is a conflict in the personal area, it can be thought that the stimuli from visual information in the VR environment may increase the hemodynamic activation in the relevant regions. However, due to modulated vestibular processing after both applied VSs, it requires the ability process vestibular information and encode extrapersonal space integration information available with the room tilt illusion association, in addition to the ability to encode personal space information in autoscopic hallucination. Therefore, more oxyhemoglobin concentration changes were observed in this case compared to the session without VS. Moreover, high hemodynamic activation was observed in all sessions in the heautoscopic illusion&room tilt illusion task. As this task requires more vestibular processing and personal and extrapersonal integration skills than any other task, we can claim that increased hemodynamic activity was observed in certain fNIRS channels in this task. It was hypothesized that the effect of the parieto insular vestibular cortex on the room tilt illusion differs from that in out-of-body experiences and heutoscopy because during the room tilt illusion, there is only abnormal processing of body position in the person's extrapersonal space, without pathologies of body and body ownership (22).
Although most studies have identified the contribution of visual signals in determining self-position, other studies have highlighted the role of vestibular signals in integrating bodily experience by combining signals from bodily, retinal, and geocentric spaces in multimodal spaces (22). Because of TPJ activity receives visual, tactile, proprioceptive, and vestibular signals about body orientation depending on the environment, it was considered essential for calculating self-positioning and visuospatial perspective (27, 57). The multisensory integration that takes place in this region seems necessary for self-positioning and consistent perspective experience of the form. Therefore, the experience of self-positioning appears to be a necessary condition for self-consciousness and also for awareness of external events (58).