Background: Heautoscopy refers to a pathological experience of visual reduplication of one’s body with an ambiguous sense of self-location and a disturbing sensation of owning the illusory body. It has been recognized to occur in the course of strikingly diverse psychiatric and neurological disorders, such as schizophrenia, space-occupying lesions, frequently of the temporal or parietal lobes, migraine, epilepsy, and depression. The literature on the subject suffers from numerous conceptual inconsistencies, scarcity of clinical data, and a lack of theoretical integratory framework that could explain the uniqueness of these symptoms. Aims: In the study, we aimed to review all case reports on heautoscopy we could cull from the literature with an attempt to extract common factors and to foster a theoretical synthesis. Methods: All medical and psychological databases were rigorously searched, along with reference lists of the preselected articles. First-person reports were classified according to aspects of bodily self-consciousness primarily affected: body ownership, self-location, sense of agency and consequently, collated with their etiological backgrounds. Results: Out of over 140 case studies, a total of only 9 patients with heautoscopy were selected as satisfying functional criteria, carefully distinguishing heautoscopy from other typically conflated full-body anomalies: autoscopy, out-of-body experience, or feeling of presence. Numerous cases turned out to be mislabeling autoscopy or out-of-body experience as heautoscopy. In addition, several problems with existing neuroimaging experiments were identified. Conclusion: Phenomenological analysis revealed that from the patients’ perspective, heautoscopy resembles a somatesthetic-proprioceptive illusion, rather than a cognitive delusion, and occurs much less frequently than reported. A most peculiar symptom, described by some as a sense of “bilocation,” appears to stem from dynamic shifts in self-location and expanded body ownership, rather than an expanded first-person perspective. Although extremely rare in its pure form, heautoscopy gives a unique opportunity to explore the brain limits to the plasticity of bodily boundaries and the origin of the first-person spatial perspective.

© 2022 S. Karger AG, Basel

Corporeal self-consciousness has been recognized as consisting of two main components, i.e., sense of body ownership (“the body belongs to me”) and self-location (“my body has a particular location in space”) [1-4]. Although these have not been immediately differentiated from each other in the past theories of “bodily self,” they have recently been shown to be underpinned by separate neural systems whose selective lesions can cause very distinct symptoms.” (see asomatognosia [5, 6], somatoparaphrenia [7], anarchic hand syndrome [8], alien hand syndrome [9-11], phantom and supernumerary limbs [12], depersonalization disorder with disembodiment [13, 14]). Although in healthy conditions, the phenomenology of the corporeal self is rarely explicitly attended to (but see hyperreflexivity symptoms in schizophrenia [15, 16]) and it is typically experienced as fundamentally unified (the body localized matches the body owned), some conditions reveal its peculiar fragmentation.

Arguably, one of the most profound disturbances of this basic sense of selfhood, both at the level of global body ownership and self-location, is encountered in heautoscopy [17-19]. The phenomenon of heautoscopic hallucination is frequently collated with two other full-body hallucinations, i.e., out-of-body experience and autoscopy [17-22], but it remains crucial to notice how dramatically it differs from them and how its symptoms surpass a merely visual hallucination. While in the out-of-body experience or autoscopy, the patient usually preserves a singular first-person perspective (1PP), or ownership over a single body, such patients do not typically claim to be occupying two bodies at once [19-23]. In contrast, heautoscopy involves a highly disturbing illusory reduplication of one’s body with an accompanying sensation of unclear self-location and expanded body ownership (a sense of being in two bodies at once, or not knowing where the self is; see Figure 1 [19-25]. One of the few papers that attempted to classify these full-body phantom phenomena indicated that self-location and the origin of the 1PP in heautoscopy are either dynamically switching between the physical and illusory body or are at both positions at the same time [21]; nonetheless, it is crucial to note that these are nontrivially different states. To carefully distill which state the patients actually mean when describing their subjective experience, one should pay close attention to their first-person reports.

Typical symptoms of autoscopy (a), heautoscopy (b), out-of-body experience (c) (note: prototypical position depicted; out-of-body experience can occur in any position), feeling of presence (d) (note: prototypical position depicted, not necessarily the most common one). The red dot in each figure represents the body with which the patient identifies themselves (i.e., sense of body ownership and self-location), while the colored body represents the real physical body. Figure modified from Blanke and Metzinger [21].

The literature on this subject, both in the field of psychiatry and neurology, consists mainly of case studies or small case series, and a systematic phenomenological investigation with their theoretical integration is still lacking (but see some theoretical considerations in [18, 19, 26-30]). Notably, multiple psychopathological factors can lead to a heautoscopic report, and it is not clear to what extent these symptoms are driven by anomalies regarding self-location, ownership, sense of agency, or mere spatial attention [22]. One should also seek to explain whether heautoscopic hallucination has a delusional character, or whether it resembles primarily a genuinely abnormal corporeal awareness. Here we intend to provide a comprehensive review of all published case studies of patients in heautoscopic state we could identify, with a particular emphasis on their first-person subjective experience and with a critical discussion of their strikingly diverse etiological background. The primary aim was to reevaluate the usefulness of this clinical concept (occasionally questioned, see [25]), help differentiate heautoscopy in medical practice, identify trends in contemporary research on the unity of bodily self, and suggest potential future directions. Finally, it is argued that heautoscopy might constitute an incredibly valuable and informative condition for research on self-consciousness, but in order to move it forward, a terminological and conceptual rigor is indispensable.

Most relevant databases, i.e., PsycINFO, Medline, Web of Science, Google Scholar were searched to identify case studies for inclusion in the systematic review. We utilized reference lists of preselected articles to thoroughly scan for further potential information. We focused primarily on English sources but also consulted a few critical early works from French and German literature. Key search words were: heautoscopy, autoscopy, autoscopic, double + hallucination, doppelganger illusion, the “double” illusion (as named in older literature), double + hallucination, mirror hallucination, second “self.” Importantly, we only selected those case studies which satisfied the criteria clearly distinguishing heautoscopy from autoscopy or out-of-body experience, i.e., self-location should not only be incorrectly shifted, but it also needs to feel expanded, confusing, or ambiguous, and it is associated with a sense of duplicated body ownership (as per Blanke’s definition [20-22]). In other words, the mere visual appearance of “the double” was not sufficient to classify a psychopathological state as heautoscopy.

In total, out of over 140 reviewed case studies, we included only 9 which satisfied the functional criteria of heautoscopy (see Table 1 for collated information on etiology, symptoms, and extensive first-person reports). Numerous articles were identified as mislabeling much more common autoscopy or out-of-body experience as heautoscopy and were therefore discarded from the review [25, 31-41, 49-55] (i.e., no abnormal, ambiguous self-location and no expanded body ownership was reported). Although both heautoscopy and autoscopy overlap over a striking symptom of hallucinating “the double,” it is only the patients with heautoscopy that report the actual changes in body ownership or self-location (an example from a purely autoscopic case reported by Lippman’s, when the clinician asked, “Well, then, are there two of ‘you’?”, the patient responded, “Yes, but I deduce that there are two. I don’t feel two.” [56]).

Phenomenology of heautoscopic experience (9 cases selected out of 140 case studies)

A surprisingly small number of compelling cases indicates that heautoscopy in its pure form occurs much more rarely than often claimed. The identified group of genuine cases was highly heterogeneous, with etiological backgrounds varying from schizophrenia, through epilepsy or migraines, to mood disorders. Cases involving epileptic seizures mostly demonstrated an ictal focus in the right temporal regions [23, 31, 46] (see also [57]). We identified some “borderline” cases that did not reveal any alterations of full-body ownership or self-location, and thus could not be classified as a full-fledged heautoscopy, nonetheless involved somewhat similar sensations of partial reduplication of certain bodily parts (such as an experience of owning two heads on one body [58, 59]). This is aligned with a view that heautoscopy is an extreme form on the whole spectrum of anomalies affecting the “unity” of bodily “self.” It remains concordant with some other studies, which demonstrate that it is sometimes possible for patients to incorporate illusory objects or supernumerary limbs into their body boundaries (a pertinent example could be the case of a 64-year-old female with right parasagittal parietal meningioma who insisted on owning 4 legs, 2 physical, 2 invisible, but all felt equally real to her [60]).

The rareness of the heautoscopic condition does not immediately entail that we should dismiss its existence, rather that we should pay closer attention if the clinical diagnosis does not involve false positives (is this symptom different enough from autoscopy? Does it really involve occupying two bodies at once or rather self-location switches dynamically?). Collected reports reveal that heautoscopy tends to resemble a somatesthetic-proprioceptive illusion rather than a cognitive delusion, as patients do not report any abnormal beliefs associated with their experience and even spontaneously notice the unreality of their sensations [24, 43] or are embarrassed of them [56]. According to patients’ experiences, a heautoscopic state is usually short lasting and does not primarily involve disturbances in sense of agency or spatial attention but rather affects self-location and body ownership [21, 40-48].

Most importantly, the phenomenological investigation shows that a peculiar sense of bilocation represents the patients’ derived ad hoc conclusion from the initially experienced rapid shifts in global self-location rather than a rudimental, primary symptom of occupying two bodies at once (or having a drastically expanded 1PP). On the other hand, the expanded sense of body ownership and self-identification with the phantom body occurs in almost all cases. It appears as a more consistent symptom that leads to ambiguous self-location (although, undoubtedly, more cases are needed to verify it). Even before sensations of shifts in self-location, patients experience a confusing sense of ambiguity, sometimes triggered by their purely visual hallucination. Patients’ quotes suggesting this temporal order of symptoms progression are emboldened in Table 1.

At the very interesting intersection of psychiatry and neurology, heautoscopy stands out as a fascinating phenomenon, and if one were to take the few first-person reports of patients seriously, it may be the only psychopathological condition that fragments the robust phenomenology of the unity of bodily self. Other paroxysmal disorders of the body image or somatognosia refer to various usually short-lasting bodily illusions, which shift self-location or body ownership incorrectly but never really fully disintegrate the origin of 1PP [41]. In other words, although malleability of global body representation is apparent in these conditions, they rarely (if ever) affect hypothetically one of the deepest, most solid brain predictions that there must be a singular subject and a singular 1PP (here the term prediction is used as in the predictive coding approach to the brain function [75]). In light of these considerations, heautoscopy constitutes an incredibly valuable and unique window to explore the limits and plasticity of the brain self-model.

The reviewed research so far has not gone far beyond the mere recognition that heautoscopy can occur (extremely rarely), and very little is known about its enabling conditions. Autoscopy and out-of-body experience, having been significantly more common in clinical practice, received much more attention in neurology, neuroscience, and experimental psychology than heautoscopy [22, 61, 62, 66-69]. The existing integratory studies typically point to the link of broadly understood corporeal awareness (an experience of having a body) with the activity of the temporo-parietal junction (TPJ)[63-65] and regions underlying the representation of peripersonal space [70, 71]. The exact mechanism of bodily consciousness has now been firmly associated with the concept of multisensory integration [71-74], which unfortunately frequently serves as an umbrella term, referring to rather various mechanisms, and it is not clear how it relates to subcomponents of the bodily self [70, 71]. A dominant theoretical framework points out that the neuroarchitecture of TPJ is exquisite for self-consciousness, as their trimodal neurons with large receptive fields are suitable to represent a match/evidence weighing between the relevant visual, tactile, and vestibular cues. Speculatively, these cell populations typically aim to diminish any arising incoherencies across modalities to avoid uncertainties (in line with the predictive coding account [75]), so that a potential discrepancy between proprioceptive and visual inputs regarding the body can be discarded as noise (and lead to the inhibition of an abnormal signal).

However, having reviewed the neuroimaging research in this area, we could not draw any certain conclusions on neural correlates of heautoscopy [70-74]. Arguably, due to the evident rareness of the condition and the scarcity of phenomenological data, there are very few articles addressing the neural underpinnings of heautoscopy per se. Another impediment to extracting some reliable neural trends was that after the thorough review of published neuroimaging studies, a number of conceptual and terminological contradictions have been identified. First, by Blanke’s own definition, heautoscopy entails that “the subject experiences seeing their body and the world in an alternating or simultaneous fashion both from an extracorporeal perspective and from their bodily visuospatial perspective” [20, 21]. In his further neuroimaging studies, he uses 2 allegedly heautoscopic cases to draw conclusions about important differences between heautoscopy and autoscopy, but the patients clearly do not meet the predefined criteria for heautoscopy. As explicitly stated about patient 1: “the patient did not experience abnormal self-location and always experienced perceiving the world and the autoscopic body from the normal 1PP,” and about patient 2: “the patient reported strong self-identification with the physical as well as the autoscopic body but did not experience any changes in self-location and the 1PP.” Clearly, these 2 cases should not be taken as clear heautoscopic examples and should not be incorporated to demarcate neurological differences between heautoscopy and autoscopy.

A closer look at another bigger case series by Blanke seeking neural correlates of these illusions also reveals inconsequence in diagnosing heautoscopy [22, 55]. In particular, revisiting the primary literature with the original first-person reports of the examined patients made apparent that subjects merely used the term “split personality” as a metaphor to refer to their purely autoscopic hallucination, and they did not claim to have accompanying somatosensory sensations or any abnormal sense of self-location. Therefore, the authors’ conclusion that the left TPJ damage specifically characterizes heautoscopy should not be taken as valid. Evidently, the most prominent review studies on heautoscopy are not based on its pure form (with an ambiguous self-location). There are two potential solutions from there: we either need to point to insufficiency of these studies and render them uninformative or completely modify the definition of heautoscopy as not requiring somatosensory anomalies regarding body ownership, 1PP, or self-location. The latter solution would entail blurring the conceptual boundary between heautoscopy and autoscopy and as a result, would question the usefulness of the concept of heautoscopy in the first place. All these lines of arguments show that research on heautoscopy is still in its very nascent, even prescientific stages.

In order to move forward, one should incorporate into the clinical analysis rigorous phenomenological interviews that have the potency to distill whether a symptom constitutes a real, primary sensation or rather an interpretation of a subtly different sensation. Most importantly, a clinician who is aware of these differences would not run at the risk of misinterpreting the experience of autoscopy with heautoscopy and would strive to disambiguate if the symptoms involve a rapid shift in self-location or rather a sense of occupying two positions at once. Heautoscopic cases tend to cluster within a collection of symptoms encompassing an ambiguous self-location and expanded body ownership, but it is still not clear what happens with the patients’ 1PP. Only when certainty in symptom classification is attained, one can successfully proceed in investigating the underlying disruptions in neural circuitry.

An additional remedy to the scarcity of clinical data is the development of functional virtual reality models of the heautoscopic illusion. Combining virtual reality techniques with a clever induction of multisensory conflicts has so far been highly successful in driving global and partial body illusions in healthy human subjects, i.e., out-of-body experience [76, 77], body swap illusion [78], arm extension illusion [79], giant or tiny body illusion [80], supernumerary limb illusion [81], rubber hand illusion [82, 83]. To establish an analogous body illusion model for heautoscopy, new paradigms should be devised [84-86]. Once such an illusion works in healthy participants, neuroimaging techniques should be employed to illuminate relevant brain network components. So far, functional magnetic resonance imaging studies showed that the TPJ activity reliably reflects experimentally triggered global changes in self-location in various multisensory conflict settings [87, 88]. That remains consistent with lesion studies on neurological and psychiatric patients with pathological sense of self-location, which consequently demonstrate focal damages to TPJ [21, 55]. However, to provide a full-fledged model of bodily self, it is not sufficient to point out that TPJ serves as a neural correlate; rather one should aim to provide a theoretical framework explaining what TPJ does in terms of causal mechanisms in the global network architecture [89]. To that end, a far-reaching goal would be to go beyond passive neuroimaging and explore the effects of active perturbation of individual network components with a high spatiotemporal precision (a precision that is not available to transcranial magnetic stimulation [89]).

Finally, studies on heautoscopy, however, valuable for psychiatry and neurology in their practice are also of not-to-be-underestimated relevance to philosophy and theoretical neuroscience of the self, the origins of 1PP, self-consciousness, and the unity of self [90-92]. Although it has long been recognized that the brain representation of body and its owned parts is malleable and susceptible to experimental manipulations, the possibility of splitting 1PP and fragmentation of self-location constitutes a qualitatively different, novel insight, which deserves to receive particular attention and motivate future empirical endeavors. In the end, what is at stake is the illumination of most fundamental mechanisms behind what it is like to be a unified conscious agent embodied in space and time.

The article is exempt from ethical approval as it is solely based on published literature.

The authors have no conflicts of interest to declare.

This work was supported by the Polish Ministry of Science and Higher Education Diamond Grant. The funder had no role in study design, data interpretation, analysis, or writing the review.

Joanna Szczotka and Michal Wierzchon reviewed the literature, Joanna Szczotka wrote the manuscript, and Michal Wierzchon provided critical review.

Data are not publicly available on legal or ethical grounds; the review was solely based on published literature with its independent data availability policies.

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