EARLY MOTOR INFLUENCES ON VISUOMOTOR TRANSFORMATIONS FOR REACHING - APOSITIVE IMAGE OF OPTIC ATAXIA

Coding of reaching in the cerebral cortex is based on the operation of distributed populations of parietal and frontal neurons, whose main f unctional characteristics reside in their combinatorial power, i.e., i n the capacity for combining different information related to the spat ial aspects of reaching. The tangential distribution of reach-related neurons endowed with different functional properties changes gradually in the cortex and defines, in the parieto-frontal network, trends of functional proper ties. These visual-to-somatic gradients imply the ex istence of cortical regions of functional overlaps, i.e., of combinato rial domains, where the integration of different reach-related signals occurs. Studies of early coding of reaching in the mesial parietal ar eas show how somatomotor information, such as that related to arm post ure and movement, influences neuronal activity in the very early stage s of the visuomotor transformation underlying the composition of the m otor command and is not added ''downstream'' in the frontal cortex. Th is influence is probably due to re-entrant signals traveling through f r onto-parietal-association connections. Together with the gradient ar chitecture of the network and the reciprocity of cortico-cortical conn ections, this implies that coding of reaching cannot be regarded as a top-down, serial sequence of coordinate transformation, each performed by a given cortical area, but as a recursive process, where different signals are progressively matched and further elaborated locally, due to intrinsic cortical connections. This model of reaching is also sup ported by psychophysical studies stressing the parallel processing of the different relevant parameters and the ''hybrid'' nature of the ref erence frame where they are combined. The theoretical frame presented here can also offer a background fur a new interpretation of a well-kn own visuomotor disorder, due to superior parietal lesions, i.e., optic ataxia. More than a disconnection syndrome, this can now be interpret ed as the consequence of the breakdown of the operations occurring in the combinatorial domains of the superior parietal segment of the pari eto-frontal network.