Report Neurological and Robot-Controlled Induction of an Apparition

Department of Neurology, Hadassah Hebrew UniversityMedical Center, P.O. Box 12000, 91120 Jerusalem, IsraelSummaryTales of ghosts, wraiths, and other apparitions have beenreported in virtually all cultures. The strange sensation thatsomebody is nearby when no one is actually present andcannot be seen (feeling of a presence, FoP) is a fascinatingfeat of the human mind, and this apparition is often coveredin the literature of divinity, occultism, and fiction. Although itis described by neurological and psychiatric patients [ 1, 2]andhealthyindividuals in differentsituations [1, 3, 4], it is notyetunderstoodhowthephenomenonistriggeredbythebrain.Here, we performed lesion analysis in neurological FoP pa-tients, supported by an analysis of associated neurologicaldeficits. Our data show that the FoP is an illusory own-bodyperceptionwithwell-definedcharacteristicsthatisassociatedwithsensorimotorlossandcausedbylesionsinthreedistinctbrain regions: temporoparietal, insular, and especially fron-toparietal cortex. Based on these data and recent experi-mental advances of multisensory own-body illusions [5–9],we designed a master-slave robotic system that generatedspecific sensorimotor conflicts and enabled us to induce theFoPandrelatedillusoryown-bodyperceptionsexperimentallyin normal participants. These data show that the illusion offeeling another person nearby is caused by misperceivingthe source and identity of sensorimotor (tactile, propriocep-tive,andmotor)signalsofone’sownbody.OurfindingsrevealtheneuralmechanismsoftheFoP,highlightthesubtlebalanceof brain mechanisms that generate the experience of ‘‘self’’and ‘‘other,’’ and advance the understanding of the brainmechanisms responsible for hallucinationsin schizophrenia.Results and DiscussionDescendingwithhisbrotherfromthesummitofNangaParbat,one of the ten highest mountains in the world, ReinholdMessner felt a third climber ‘‘descending with us, keeping aregular distance, a little to my right and a few steps awayfrom me, just outside my field of vision’’ [10]. Messner ‘‘couldnotseethefigure’’but‘‘wascertaintherewassomeonethere,’’sensing‘‘hispresence’’[10].Thisapparition,thesensationthatsomebody is nearby when no one is actually present, is calledthe feeling of a presence (FoP) and has been described duringperiods of physical exhaustion [1, 3, 4, 11, 12] and has influ-enced occult literature and fiction [13]. Although people donot see the ‘‘presence,’’ they may describe its spatial locationand frequently turn around or offer food to the invisible pres-ence [14, 15]. Although the FoP has been described in psychi-atric [1, 2, 15, 16] and neurological patients [2, 16], its neuralorigin is unknown. A single case report showed that electricalstimulation in temporoparietal cortex induces the FoP, sug-gesting that disturbed sensorimotor processing (tactile, pro-prioceptive, and motor cues) is important [17]. However, thishas not been confirmed in other patients, and the significanceof these findings for the FoP in healthy subjects is unclear.Neurology and the FoPWe performed lesion analysis and analyzed the associatedhallucinations and neurological symptoms in 12 FoP patients(Table 1; Figure S1 available online). The presence was felt inall cases in close proximity to and behind the patient’s body(p < 0.01). The presence was lateralized (p < 0.01) in contrale-sional space (p < 0.01) and equally often in right or left hemi-space (not significant, n.s.; Table 1). Sensorimotor deficits(p < 0.01) and the experience of illusory movements of thepresenceduringmovementsofthepatient(n.s.)werefrequentsymptoms (Supplemental Experimental Procedures). Forlesion analysis, we used a multimodal imaging approach,relying on combined functional and structural neuroimagingdata to determine anatomical regions of maximal lesion over-lap [18–20]. This approach, which combined functional andstructural lesion data, was necessary because many patientssuffered from epilepsy, and in several patients, FoP wasinduced by electrical stimulation, and because the FoP israre. Projecting all lesions onto the left hemisphere, lesionoverlapanalysis highlightedthreecorticalregions: insularcor-tex,frontoparietal cortex,andthetemporoparietalcortex(Fig-ure 1A). We next compared lesion extent within these threecortical regions between FoP patients and control patientsmatched for complex hallucinations, etiology, and sensori-motor deficits (Figure 1B; Supplemental Experimental Proce-dures): lesion extent did not differ between both groups inBrodmann area 22 (p = 0.18) and 48 (p = 0.68), whereas FoPpatients had significantly larger lesions in Brodmann area 7(p = 0.01). These results show that although FoP is associatedwith insular, temporoparietal, and frontoparietal lesions, onlyfrontoparietal lesions (Brodmann area 7) were specificallyassociated with the FoP.Robotically Induced Bodily IllusionsIn order to study the FoP in healthy subjects, we designed amaster-slave robotic system [21] and investigated sensori-motor signals and their role in inducing FoP experimentallybyintegrating ourfindingswithprinciplesfromother bodyillu-