Breaking the boundaries of brain plasticity: adaptive plasticity across the hand-face border

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One of the most prominent features of the primate somatosensory homuncular representation is the hand-face border. Reducing or eliminating somatosensory input from either the face or the hand results in cross-border reorganisation like that observed in amputees. Since this reorganisation is associated with gains/losses in cortical territory and is paralleled by negative perceptual consequences, it is generally assumed to be competitive, with cortical territory invading input-deprived neighbouring areas. It remains to be clarified, however, which mechanisms operate during positive plasticity, which occurs under conditions of enhanced afferent inputs and results in improved perception and sensorimotor performance.To address this question it is essential to systematically manipulate brain plasticity by manipulating afferent inputs. Recent research has shown that besides training and practicing, brain plasticity can be induced by training-independent repetitive sensory stimulation (RSS), which improves tactile performance at the stimulated finger and expands its somatosensory representation. In a preliminary study involving both the French and German teams we found that RSS on the finger altered tactile performance not only at this finger but also at the upper-lips. These results reveal the existence of a previously unknown perceptual learning phenomenon: the transfer of perceptual improvement across the hand-face border, and suggest that cross-border plasticity is not only competitive.This proposal builds upon this finding by systematically investigating the properties and mechanisms of cross-border plasticity in humans under conditions of intact afferent inflow. The proposed experiments investigate the properties of perceptual and sensorimotor learning, cross-border transfer, and the associated cortical reorganization. Specifically, we will investigate cortical reorganization in the somatosensory system following either finger or lip stimulation, the impact of finger stimulation on hand/mouth motor coordination, and the dependency of plastic changes on Hebbian learning rules. To investigate the impact of cross-border plasticity under ecological conditions (without RSS), we will study finger-lip performance in a selection of subpopulations with either enhanced or impaired sensory abilities.This project will increase our understanding of the mechanisms underlying the dynamic maintenance of brain organisation and the processes accompanying sensory learning in physiological and pathological conditions. The knowledge generated will make a substantial contribution to the domain of systems and behavioural neuroscience as it will allow us to identify and possibly act upon, the regulatory processes governing homeostasis of the human sensory system.