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Tamar R. Makin, Nicholas P. Holmes, Claudio Brozzoli, Yves Rossetti, Alessandro Farnè; Hand-centered visual representation of space: TMS evidence for early modulation of motor cortex excitability. Journal of Vision 2008;8(6):375. doi: https://doi.org/10.1167/8.6.375.
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While avoidance movements might be rapid and relatively automatic (like Indiana Jones rolling under a closing barrier), target directed movements require planning (Jones grasping his hat at the last second). A wealth of studies has been devoted to reveal the neural basis of visuo-motor transformation in target-directed movements. However, less is known about the way in which rapidly approaching stimuli may be encoded and evoke reactive movements. Electrophysiological studies in the macaque premotor cortex have revealed visual receptive fields selective for 3D objects approaching the hand. In the present study, we explored hand-centered modulation of visual space in the human motor cortex. Subjects were engaged in a simple response task to a central go signal, and simultaneously presented with a (task-irrelevant) 3D ball, rapidly approaching a location either near to or far from their hand. Between 40–120ms after distractor ball appearance, a single TMS pulse was applied to the primary motor cortex contralateral to the responding finger, eliciting a motor evoked potential (MEP). At 80ms following ball appearance, the mean peak-to-peak MEP amplitude was significantly supressed when the ball approached the responding hand, as compared to far from it, regardless of hand position in space. Additional experiments showed that this hand-centered modulation of MEP amplitude was maintained irrespective of whether the subject's eyes and overt endogenous attention were oriented towards or away from the location of the ball. Furthermore, manipulations of subjects' covert exogenous visual attention had independent effects from the above hand-centered MEP modulations. Finally, when the distractor balls were replaced with stationary visual stimuli, no significant hand-centered modulation of MEP amplitude was observed. By demonstrating both early and selective modulation of motor cortex excitability, these findings constitute the first direct evidence that the human motor cortex represents visual peripersonal space in a hand-centered reference frame.
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