WE studied neural interactions by cross correlation analysis during representational plasticity induced by intracortical microstimulation (ICMS). Neuron pairs were simultaneously recorded in area 3b in adult New World monkeys, and in cortical field SI in adult rats. In normal animals, the degree of correlated spontaneous activity corresponded to the extent of receptive field overlap. After several hours of ICMS, the spatial extents of cortex over which correlated activity could be recorded was enlarged several-fold. Mapping experiments revealed that increased correlated activity was only recorded within that cortical sector that was representationally reorganized, indicating a close relationship between cortical reorganization and cooperative processes. Results support the hypothesis that discharge coincidence is crucial for the formation of functionally coupled neural groups, and implicate dynamically maintained groups in the genesis of postontogenetic plasticity.