The effect of Mirror-Box Illusion on the Line Bisection Task

The body is ever-present to experience. Even when it is not the direct object of perception, it is embedded in first-person perspective. It is the material recipient for the sense of self, as well as the effector providing the sense of agency. Body representations show an impressive degree of plasticity, especially during finalized actions, where even external objects can be incorporated to carry out tasks. Such modifications have repercussions on various cognitive domains. This study aims to investigate how the embodiment of an external object can modulate spatial representation. An extensive corpus of works has examined the influence of body representations changes on peripersonal space. However, the understanding of the influence of the body on the representation of space is still limited. To deepen the understanding of this relation, 36 healthy participants were employed in a Mirror Box Illusion (MBI) paradigm. Participants were asked to tap at both ends of the Mirror Box to create the illusion of ownership towards the hand reflected in the mirror. Before and after this stimulation, participants were asked to complete an Embodiment Questionnaire and to carry out a Localization Task and a Line Bisection Task. Participants’ subjective reports portrayed the effect of the illusion, with items describing subcomponents of embodiment displaying higher scores after the synchronous stimulation. The localization task reported a systematic mislocalization of the biological limb and a shift of its perceived location towards the mirrored hand. This proprioceptive drift was present not only after synchronous stimulation but also after the asynchronous tapping when the right hand was visible. Subjective sense of embodiment and proprioceptive drift displayed a significant correlation, where the more participants reported the illusion as vivid, the more proprioceptive drift they displayed. Together, these results advocate for the efficacy of the MBI in inducing the embodiment of the reflected hand. Conversely, the line bisection task did not show any effect of the MBI stimulation. Despite the shift in the subjective body midline attested in the localization task, participants did not show any significant change in how they bisected the lines after the stimulation. Therefore, the modifications in body representations caused by the Mirror Box Illusion did not elicit any reported effect on the representation of space.