Support with firm conviction that one's left arm belongs to someone else: it seems paradoxical but can actually happen, and not because of a psychiatric disorder. A brain injury - for example, a stroke - more frequently in the right hemisphere, can compromise the body representation, the cognitive "map" of our body that the brain uses to make us interact with the space around us.
Thus one can no longer recognize a limb - usually the left arm or leg - as one's own. This feeling of exclusion (a neurological disorder called somatoparafrenia ) is so rooted that the "repudiated" limb no longer responds to the threats of painful stimuli: this is what emerges from an Italian research published in Brain.

I approach … but I don't bite you. The study, conducted by Angelo Maravita and Daniele Romano of the Department of Psychology of the Milan-Bicocca University, in collaboration with the University of Pavia and the Niguarda Hospital "Ca 'Granda" in Milan, involved three groups of 5 patients affected from various pathologies that involve loss of sensitivity or movement of the limbs: somatoparafrenia, hemiplegia and anosognosia.
"Our aim - the authors of the study explain - was to study whether the loss of the sense of belonging of a part of the body was accompanied by a loss of the ability to anticipate the arrival of a painful stimulus . For this reason we have studied somatoparaphrenic patients: the hypothesis was that if the patient believes that the hand is not his, then he will not even be alerted by the arrival of a threatening stimulus directed towards that hand ».
"To check the specificity of this effect, we also studied patients who, with the same brain lesion located in the right hemisphere, are not affected by disorders of belonging of the parts of the body, namely: patients with sensory anosognosia, who are not aware of a deficiency of the sense of touch, and patients with left hemiparesis, that is affected by a motor deficit, but not accompanied by disorders of belonging or body awareness ».

It's not mine, you sting well . A potential source of pain (a needle) was approached to the limbs of all patients, and the physiological response to this "threat" was measured by skin conductance, "a device that measures the electrical conduction capacity of the skin and detects the activation of the autonomic nervous system in the presence of conflicting stimuli (such as in the "truth machine") or with emotional value: in our case, a potentially harmful stimulus that approaches our body ".
Unlike the other patients, the somatoparafrenici showed an absence of cutaneous response on the "forgotten" limb at the approach of the needle: although seeing that the left arm was going to be stung, they did not show any physiological reaction. As if their brains were "uninterested" in the imminent danger. "The process of loss of self-consciousness - the scientists clarify - is so profound that one cannot even perceive the threats and does not activate any defense reaction, not even reflected".

Cognitively free (apart from the stroke) . All of this in patients who are perfectly capable of understanding and wanting: «Historically, somatoparafrenia was confused with a schizophrenic-type psychiatric disorder . But a series of observations, such as the fact that it occurs more frequently for injuries to the right hemisphere or that it appears in people with no history of psychiatric illnesses have now convinced the scientific community that it is a neuropsychological disorder, or a disorder of higher cognitive functions and non-psychiatric. In this type of study, Italian research has always been in the vanguard and is still extremely active in our University ».

A complex bond . The research sheds new light on the relationship between brain, body and space: «The representation of our body is a necessary function to interact with the external world. Through it we evaluate the sensory consequences of an approaching stimulus and prepare the appropriate answers . A disorder such as somatoparaphrenia affects not only the representation of the body, but also the relationship of the patient with the surrounding space, hindering the effect of rehabilitation treatments ".
"This same paradigm for the study of pain anticipation has also been used in our laboratory to study the representation of the body in healthy people, in situations where the body representation is distorted through perceptive illusions or manipulations of the visual characteristics of our body. These studies see us actively engaged in the search for how the brain represents the body of each of us, creating the basis for consciousness and the relationship with the world around us ».
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