Inducing a shadow in the mind

A new report in Nature demonstrates that electrical stimulation of the temporoparietal junction in the brain induces a sensation of the presence of an illusory “shadowy person”. One of the hallmarks of certain forms of schizophrenia is just this phenomenon: the eery feeling of someone’s presence. Now, it has been demonstrated in a study using electrical brain activation in a person without a history of psychiatric problems.

Basically, the study was performed on a subject that was at the preoperative stage of surgery for epilepsy. A normal procedure is to anaesthesize the patient before opening the skull, and then wake the patient up before stimulating the brain. The aim of such stimulation is to find the location of epilepsy onset, as well as to stimulate the areas surrounding this region, in order to map functionally important regions (e.g. language that are important in language). Such preoperative procedures are known to lead to a better surgical sensitivity, i.e. the ability to remove all of the abnormal tissue, and a higher surgical selectivity, i.e. avoding removal of normal tissue. In this way, neurosurgeons often evoke a number of sensations and behaviours in patients, including the disruption of speech, visual phosphenes, and memory deficits.

In the present case, the neurosurgeons found that electrical stimulation lead to a feeling of the presence of another person. Moreover, the patient reported that this figure was taking the same posture as herself, and even sometimes interfering with a task she was performing:

When stimulated (…) the patient had the impression that somebody was behind her. Further stimulation induced the same experience, with the patient describing the “person” as young and of indeterminate sex, a “shadow” who did not speak or move, and whose position beneath her back was identical to her own (“He is behind me, almost at my body, but I do not feel it”). (…) Further stimulations [other location] were applied while the seated patient performed a naming (language-testing) task using a card held in her right hand: she again reported the presence of the sitting “person”, this time displaced behind her to her right and attempting to interfere with the execution of her task (“He wants to take the card”; “He doesn’t want me to read”).

Stimulation of the temporoparietal junction (shown with an arrow in the image above) thus seems to distort some kind of body image, or maybe even efference copy (PDF) of self-actions. Both functions that are dramatically affected in abnormal brain states following certain kinds of delusional schizoprenia and brain injury.

The finding also nicely relates to the Swiss group’s earlier study combining EEG, TMS and the study of an epilepsy patient, where it was found that disruption of the temporoparietal junction function led to an “impaired mental transformation of one’s own body”. Here, the researchers concluded that:

the [temporoparietal junction, TPJ] is a crucial structure for the conscious experience of the normal self, mediating spatial unity of self and body, and also suggest that impaired processing at the TPJ may lead to pathological selves such as [out-of-body experience].

Here is the full story:

Induction of an illusory shadow person
By Arzy et al
Nature 443, 287

Stimulation of a site on the brain’s left hemisphere prompts the creepy feeling that somebody is close by.

The strange sensation that somebody is nearby when no one is actually present has been described by psychiatric and neurological patients, as well as by healthy subjects, but it is not understood how the illusion is triggered by the brain1, 2. Here we describe the repeated induction of this sensation in a patient who was undergoing presurgical evaluation for epilepsy treatment, as a result of focal electrical stimulation of the left temporoparietal junction: the illusory person closely ’shadowed’ changes in the patient’s body position and posture. These perceptions may have been due to a disturbance in the multisensory processing of body and self at the temporoparietal junction.
Top of page

1. Laboratory of Cognitive Neuroscience, École Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland
2. Presurgical Epilepsy Evaluation Unit, University Hospital, Geneva 1211, Switzerland
3. Department of Neurology, University Hospital, Geneva 1211, Switzerland
4. Center for Cognitive Neuroscience, Dartmouth College, Dartmouth, New Hampshire 03755, USA

-Thomas