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The brain represents perceptual and motor information in several reference frames (for example body-centered, object-centered, or retinal-centered reference frames). In a simple sensory-motor program such as looking at and taking an object, at least three fundamental processes must be carried out by the cerebral cortex; (1) in order to recognize the target object, the cortex has to transform the pattern of excitation on the retina from a retinotopic coordinate system to a coordinate system centered on the object itself; (2) in order to bring a hand to the desired position in space, the cortex must transform the visual information related to the target location (relative to the hand) into an appropriate motor command of the reaching hand; (3) in order to guide coherent behavioral actions, more complex sensory-motor programs (for example, conditional reaching of a target) are constructed from time-dependent relations between these basic transformations. The cortex correlates sensory and motor events and learns to prepare responses to forthcoming events. Neurophysiological data on the motor area of the monkey allowed us to model the coordinate transformations from body-centered to arm-centered reference frames involved in the command of arm reaching movements in 3-D space. Anatomical and neuropsychological data suggest similar coordinate transformations along the visual pathway to relate retinal-centered to object-centered reference frames and we have thus extended the model to this coordinate transformation. Time integration seems to proceed differently since internal representations of programs are dynamically constructed. Available physiological and anatomical data on frontal areas (and particularly prefrontal cortex) help to predict specific learning mechanisms for time processing and then construct a model for learning sensory-motor sequences.
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