Tales of ghosts, wraiths, and other apparitions have been reported in virtually all cultures. The strange sensation that somebody is nearby when no one is actually present and cannot be seen (feeling of a presence, FoP) is a fascinating feat of the human mind, and this apparition is often covered in the literature of divinity, occultism, and fiction. Although it is described by neurological and psychiatric patients and healthy individuals in different situations, it is not yet understood how the phenomenon is triggered by the brain.
I will describe several cases of the FoP in neurological patients, suggesting that the FoP is caused by abnormal processing of sensorimotor signals. Based on these clinical data and recent experimental advances of multisensory own-body illusions, we designed a master-slave robotic system to generate specific sensorimotor conflicts at the trunk enabling us to induce the FoP and related illusory own-body perceptions experimentally in normal participants. Further lesion analysis in neurological FoP patients, supported by an analysis of associated neurological deficits show that the FoP is an illusory own-body perception with well-defined characteristics and associated with sensorimotor loss and caused by lesions in three distinct brain regions: temporo-parietal, insular, and especially frontoparietal cortex. These data show that the illusion of feeling another person nearby is caused by misperceiving the source and identity of sensorimotor (tactile, proprioceptive, and motor) signals of one’s own body.
In the last part of my talk I will describe two recent studies in different groups of patients suffering from schizophrenia (robotic psychiatry), where we investigated the effects of sensorimotor robotic stimulation on auditory verbal perception. I will conclude by discussing the subtle balance of sensorimotor brain mechanisms that generate the experience of ‘‘self’’ and ‘‘other,’’ and argue that robotic psychiatry in association with neuroimaging may significantly advance efforts for the diagnosis and also the treatment of positive symptoms in schizophrenia.