How Tool-Use Shapes Body Metric Representation: Evidence From Motor Training With and Without Robotic Assistance

Abstract

Previous evidence has shown that tool-use can reshape one’s own body schema, extending peripersonal space and modulating the representation of related body parts. Here, we investigated the role of tool action in shaping the body metric representation, by contrasting two different views. According to a first view, the shaping would rely on the mere execution of tool action, while the second view suggests that the shaping induced by tool action on body representation would primarily depend on the representation of the action goals to be accomplished. To this aim, we contrasted a condition in which participants voluntarily accomplish the movement by representing the program and goal of a tool action (i.e., active tool-use training) with a condition in which the tool-use training was produced without any prior goal representation (i.e., passive tool-use training by means of robotic assistance). If the body metric representation primarily depends on the coexistence between goal representation and bodily movements, we would expect an increase of the perceived forearm length in the post- with respect to the pre-training phase after the active training phase only. Healthy participants were asked to estimate the midpoint of their right forearm before and after 20 min of tool-use training. In the active condition, subjects performed “enfold-and-push” movements using a rake to prolong their arm. In the passive condition, subjects were asked to be completely relaxed while the movements were performed with robotic assistance. Results showed a significant increase in the perceived arm length in the post- with respect to the pre-training phase only in the active task. Interestingly, only in the post-training phase, a significant difference was found between active and passive conditions, with a higher perceived arm length in the former than in the latter. From a theoretical perspective, these findings suggest that tool-use may shape body metric representation only when action programs are motorically represented and not merely produced. From a clinical perspective, these results support the use of robots for the rehabilitation of brain-damaged hemiplegic patients, provided that robot assistance during the exercises is present only “as-needed” and that patients’ motor representation is actively involved.

Online proprioception feeds plasticity of arm representation following tool-use in healthy aging

“…The present results allow to step further and reveal the correlation existing between online proprioceptive acuity and the kinematics signature of the tool incorporation: the better proprioceptive acuity, the larger the extent of kinematic changes following tool-use. While these findings are in keeping with the role of proprioception in updating body representation 37 , 42 , 43 , they concur in indicating this body representation feeds internal models of motor control 1 , 13 , 14 . Our findings refine current models by suggesting that the state estimation of the body relies heavily on online proprioceptive inputs to monitor and update the represented metric of effectors.…”

Same action in different spatial locations induces selective modulation of body metric representation

“…Interestingly, compared to the baseline, participants exhibited a significant increase in the perceived arm length after the active training session, while no modulation was observed after the passive one. These results clearly suggest that the simple and passive reproduction of tool action is not enough to impacts on subjective body metrics; the effect occurs insofar as the action is actively enforced by the agent (Bruno et al 2019). Romano et al (2019) also showed that a training requiring different goal-oriented actions using the same tool induces a proximal shift of the perceived midpoint of the arm when the training mostly involved proximal movements (i.e.…”

“…Notably, the majority of studies on tool-use have investigated the effect of tools in modulating body representation without explicitly addressing the active motor component required to operate the tool and perform the appropriate action. A recent study by Bruno et al (2019) addressed this point by evaluating the effect of active and passive use of tools on body metrics. To do so, the authors developed a paradigm in which participants carried out two different training sessions.…”

Phenomenal transparency, cognitive extension, and predictive processing

“…A large number of neuropsychological findings (e.g. Iriki et al 1996;Berti and Frassinetti 2000;Maravita and Iriki 2004;Cardinali et al 2009a;Canzoneri et al 2013;Bruno et al 2019) suggests that, at least in primates, tools are incorporated in the body schema after a short period of usage. When this happens, the tool is used, in the same 15 automatic manner in which bodily parts are "used": just as when 15 Yet, it is nor very clear what this incorporation amounts to.…”

“…Cardinali, et al 2009a) and suggesting that the prior representation of the body is modified so as to "fagocitate" the tool (e.g. Bruno et al 2019). Notice that the two claims are not equivalent at least when it comes to what the body schema represents: in the rst case, it also represents an external object, whereas in the second case it doesn't.…”