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.
Highlights
Acting with tools is a familiar aspect of everyday life
If the body metric representation primarily depends on whether, during tool-use, the action programs and goals are motorically represented rather than merely produced, we would expect to find a significant increase of the perceived forearm length in the postwith respect to the pre-training phase after the active training phase only
According to the first view, the actual execution of tool action would be enough for the shaping to occur, while the second view postulates that a coexistence between action goals to be accomplished and bodily movements is necessary; i.e., it is not enough that the bodily movements are merely executed, the action programs and goals have to be motorically represented
Summary
Acting with tools is a familiar aspect of everyday life. People use tools for eating cakes, moving logs, picking up leaves, and writing papers. A seminal study by Iriki et al (1996) on non-human primates showed that a repeatedly used small rake expanded the receptive fields of parietal visuo-tactile neurons to encompass the space around both the hand and the rake. Studies on healthy subjects showed that tool-use might increase the impact of far visual distracters on tactile discrimination (Maravita et al, 2002; Holmes et al, 2004) as well as the sensitivity to the affording features of out-of-reach objects (Costantini et al, 2011). Studies on patients with visuotactile extinction indicated that the severity of their extinction could be modified by using tools, which extend the reach of hand actions (Farnè and Làdavas, 2000; Maravita et al, 2001; Farnè et al, 2005). Patients with neglect only for the hemispace close to their body have been found to worsen their performance in a line bisection task in the far space when using a tool like a long stick (Berti and Frassinetti, 2000; Neppi-Mòdona et al, 2007)
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