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Interaction with objects in the environment typically requires integrating information concerning the object location with the position and size of body parts. The former information is coded in a multisensory representation of the space around the body, a representation of peripersonal space (PPS), whereas the latter is enabled by an online, constantly updated, action-orientated multisensory representation of the body (BR). Using a tool to act upon relatively distant objects extends PPS representation. This effect has been interpreted as indicating that tools can be incorporated into BR. However, empirical data showing that tool-use simultaneously affects PPS representation and BR are lacking. To study this issue, we assessed the extent of PPS representation by means of an audio-tactile interaction task and BR by means of a tactile distance perception task and a body-landmarks localisation task, before and after using a 1-m-long tool to reach far objects. Tool-use extended the representation of PPS along the tool axis and concurrently shaped BR; after tool-use, subjects perceived their forearm narrower and longer compared to before tool-use, a shape more similar to the one of the tool. Tool-use was necessary to induce these effects, since a pointing task did not affect PPS and BR. These results show that a brief training with a tool induces plastic changes both to the perceived dimensions of the body part acting upon the tool and to the space around it, suggesting a strong overlap between peripersonal space and body representation

My thesis aims at demonstrating the existence of two parallel networks that are involved in stimulus-dependent and rule-dependent visuomotor associations. In the first experiment we demonstrated a biphasic time-course of motor cortical excitability during a counter-imitative task: a stimulus-dependent automatic simulation at 150 ms from the stimulus onset and a rule-dependent voluntary motor preparation at 300 ms. Moreover we identified two regions involved in this biphasic pattern by means of the repetitive transcranial magnetic stimulation (rTMS). The double-dissociation between the effects of offline rTMS to the lateral prefrontal and parietal cortices, on the early and late components, allowed us to hypothesize the presence of two different anatomo-functional pathways: a parieto-(premotor) network mediating early stimulus-dependent responses and a (temporo)-prefrontal network producing late rule-dependent responses. Thus, the first experiment adds important information on the mechanisms by which the brain is both tuned to produce imitative responses in a fast automatic way but is also capable of overriding them by means of a parallel, more flexible, visuomotor coupling that follows arbitrary visuomotor associations. The fast imitative tendencies seemingly persist also during the online performance of a counter-imitative task. The two processes access the motor output by two partially independent neural substrates. We took a step forward in the second experiment where we investigated, by means of the combined rTMS and fMRI techniques, the functional connectivity in the rule-dependent executive pathway. We used the condition-and-map approach to highlight the brain regions where this pathway passes through to give a counter-imitative response. We hypothesised that this region could be the premotor cortex, and probably the dorsal part of it. Our data showed that exactly in the dorsal premotor cortex there is an interaction between the TMS stimulations and the tasks.

Interaction with objects in the environment typically requires integrating information concerning the object location with the position and size of body parts. The former information is coded in a multisensory representation of the space around the body, a representation of peripersonal space (PPS), whereas the latter is enabled by an online, constantly updated, action-orientated multisensory representation of the body (BR). Using a tool to act upon relatively distant objects extends PPS representation. This effect has been interpreted as indicating that tools can be incorporated into BR. However, empirical data showing that tool-use simultaneously affects PPS representation and BR are lacking. To study this issue, we assessed the extent of PPS representation by means of an audio-tactile interaction task and BR by means of a tactile distance perception task and a body-landmarks localisation task, before and after using a 1-m-long tool to reach far objects. Tool-use extended the representation of PPS along the tool axis and concurrently shaped BR; after tool-use, subjects perceived their forearm narrower and longer compared to before tool-use, a shape more similar to the one of the tool. Tool-use was necessary to induce these effects, since a pointing task did not affect PPS and BR. These results show that a brief training with a tool induces plastic changes both to the perceived dimensions of the body part acting upon the tool and to the space around it, suggesting a strong overlap between peripersonal space and body representation