Abstract

Pathological embodiment (E+) is a specific contralesional delusion of body ownership, observed following brain damage, in which patients embody someone else’s arm and its movements within their own body schema whenever the contralesional ‘alien’ arm is presented in a body-congruent position (i.e., 1st person perspective and aligned with the patient’s shoulder). This disorder is often associated with spatial neglect, a neurological syndrome in which patients are unaware of stimuli presented in the contralesional (often the left) space. Capitalizing on previous evidence demonstrating that prismatic adaptation of the ipsilesional arm to right-deviating prisms is effective in ameliorating neglect symptoms, here we investigated whether such amelioration also occurs in E+ patients with neglect when prismatic training is performed by the ‘alien’ embodied arm. Four left neglect patients (one with and three without pathological embodiment) underwent visuomotor prismatic training performed by an ‘alien’ arm. Specifically, while patients were wearing prismatic goggles shifting the visual field rightward, a co-experimenter’s left arm presented in a body-congruent perspective was repeatedly moved toward visual targets by another examiner. In a control condition, the co-experimenter’s arm was moved toward the targets from a body-incongruent position (i.e., 3rd person perspective). Neglect symptoms were assessed before and after training through paper-and-pencil tasks. In the E+ patient, neglect improved significantly more in 1st than in 3rd person perspective training, suggesting that prismatic adaptation of the ‘alien’ embodied arm is effective in modulating spatial representation. Conversely, for control E- patients (not embodying the ‘alien’ arm), we observed more limited improvements following training. These findings indicate that the ‘alien’ embodied arm is so deeply embedded in the patient body and motor schema that adaptation to prismatic lenses can affect multiple processing stages, from low level sensory-motor correspondences, to higher level body, motor and spatial maps, similarly as it occurs in normal subjects and neglect patients without pathological embodiment.

Highlights

  • We investigated whether visuomotor prismatic training with an ‘alien’ embodied arm is effective in ameliorating neglect symptoms to what is observed when the own real arm moves (Rossetti et al, 1998)

  • In order to answer this question, we compared the performance in a number of paper-and-pencil tasks pre- and post-training in a patient with (E+) and in a group of patients without (E−) pathological embodiment of an ‘alien’ arm (Experiment 1)

  • Our results showed that PT with the ‘alien’ arm ameliorated neglect symptoms more in the E+ patient than in control E− patients, but only when PT occurred with the alien arm in a body-congruent position

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Summary

INTRODUCTION

Pathological embodiment is a specific contralesional body ownership disorder, in which brain damaged patients ( on E+) embody someone else’s arm within their own body schema and are firmly convinced that it is their own arm, whenever it is located in a body-congruent position from a 1st person perspective (Garbarini and Pia, 2013; Pia et al, 2013a, 2016; Garbarini et al, 2013b, 2014, 2015, 2017; Berti et al, 2015; Fossataro et al, 2016, 2018). A similar degree of ovalization is observed in healthy subjects when performing the same bimanual task, with their own hands (Garbarini et al, 2013a) These results have been confirmed by further studies (Garbarini et al, 2015; Fossataro et al, 2016) and indicate that E+ patients do embody the movements of the alien arm, and the higher-order consequences of those movements. In two recent studies on healthy subjects and neglect patients (Ronga et al, 2017a,b), we showed that an oculomotor PT, only consisting in gaze shifts toward visual targets while wearing prismatic goggles, produces similar, weaker, after-effects in line bisection to those produced by PA induced through pointing movements (i.e., visuomotor PT). We reasoned that in presence of pathological embodiment, the effects of PA on neglect should be comparable to those observed following visuomotor PT (i.e., stronger), whereas in absence of the delusion, PA effects should be weaker

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