VTHand-centred space · Visuo-tactile cortical mechanims for a hand-centred spatial representation in humans

Visuo-tactile cortical mechanims for a hand-centred spatial representation in humans

When interacting with objects the brain needs to locate the limbs and the relevant visual information surrounding them. Studies on monkeys showed that information from different sensory modalities converges at the individual neuron level within a multisensory fronto-parietal network. This multisensory integration occurs within the space surrounding the body (peripersonal space) and has been linked to the sensory guidance of reaching and defensive movements, and localization of limbs in space. Before the implementation of this project, only indirect behavioral evidence existed of a similar representation in humans. This project has provide the first direct evidence for the existence of a multisensory mechanism in the human brain for the representation of the near space. We have identified the areas which provide a hand-centered representation of the near space by combining visual information about the location of the object and visuo-proprioceptive information concerning the position of the hand. To this aim, we used an innovative multimodal strategy involving complementary behavioural, functional magnetic resonance imaging (fMRI) and transcranial stimulation (TMS) approaches. More in detail, we have developed an experimental protocol based on BOLD-adaptation to identify neuronal responses reflecting the multisensory integrative mechanisms of the peri-hand space representation. This method allowed us to circumvent the limits of traditional fMRI analysis and go beyond the voxel-level resolution. We have demonstrated the plasticity of the peri-hand space when prosthetic limbs are experienced as one’s own body part. We were also able to show that a similar representation exists for the space surrounding other persons’s hand. Finally, we employed a TMS protocol to reveal the link between the the peri-hand space representation, the sense of position of the hand and the motor system. During the first year of the project, I worked on the optimization of the BOLD-adaptation protocol I have developed in the Brain, Body and Self Lab (Brozzoli et al., J. Neurosci., 2011). In order to achieve the first objective of the project, I have ran several pilot experiments for testing the possibility to study the hand-centered nature of peri-hand space representation in the MRI environment. After having defined the details of the most appropriate visual stimulation in terms of timing and duration, I have designed, set up and analyzed a first experiment. Aim of the experiment was to demonstrate that the visual location of an object is represented in a space centered on the hand, that is depending on visual and proprioceptive information concerning the hand position. The results provided the first direct evidence of a hand-centered representation of the near space arising through the activity of parietal and premotor areas (published, Brozzoli et al., J Neurosci., 2012). A second experiment with the same protocol has been acquired in order to investigate the second main question of the project: Is the peri-hand parieto-frontal network involved in the rapid plasticity of the space representation induced by prosthesis-use? To this aim, we employed a robust perceptual illusion known as Rubber Hand Illusion, as defined in the project, to show the following novel results: a link exists between the sense of ownership experienced over the prosthetic hand and the peri-hand space representation in the ventral premotor cortex; a link exists between the sense of position of the hand and the representation of the space around it in the posterior parietal cortex (published, Brozzoli et al., J. Neurosci., 2012). During the first year of the project, I also started to define the most appropriate parameters for the neurophysiological acquisition with TMS. As well, in collaboration with Giovanni Gentile, we conceived an experiment for studying the link between the multisensory representation of the hand and the sense of ownership experienced over it. During the second year of the project, through the collaboration with Giovanni Gentile, we were able to show that the sense of ownership experienced over the hand depends on the coherence between the sensory information arising in the space around the hand (published, Gentile et al., J Neurosci., 2013). I moved on to show the possible link existing between the peri-hand space representation and the motor behavior. I have spent time into developing an experimental set-up that could link the visual stimulation and the TMS trigger with high timing precision. I acquired data in a first experiment to show how the position of the hand can affect the cortico-spinal excitability per se. In a second experiment I presented an object falling near or far from the hand to study how the cortico-spinal excitability is modulated by the proximity between the visual information and the hand. The first study has been already reviewed by excellent scientific journals and it is now ready for a further submission. I’m currently working on the redaction of the second study. In parallel with the development of the TMS protocol, I have also demonstrated the possible link between peri-hand space and motor interaction with other people. By employing the BOLD-adaptation protocol developed during my post-doctoral period, I was able to show that the human ventral premotor cortex contains a representation of the space surrounding the hand of other people. This open to the interesting possibility that the peri-hand space representation might be useful for motor interactions with other individuals (published, Brozzoli et al., Curr. Biol., 2013).

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