BEYOND MAPS - A DYNAMIC VIEW OF THE SOMATOSENSORY SYSTEM

Current theories on how tactile information is processed by the mammal ian somatosensory system are based primarily on data obtained in studi es in which the physiological properties of single neurons were charac terized, one at a time, in behaving or anesthetized animals. Yet, the central nervous system relies on the concurrent activation of large po pulations of neurons to process the variety of sensory stimuli that co ntribute to normal tactile perception. The recent introduction of elec trophysiological methods for chronic and simultaneous recordings of th e extracellular activity of large numbers of single neurons per animal has allowed us to investigate, for the first time, how populations of neurons, located at multiple processing stages of the somatosensory s ystem, interact following passive and active tactile stimulation, The rat trigeminal somatosensory system was used as a model for this inves tigation. Our results revealed the existence of highly dynamic and dis tributed representations of tactile information, not only in the somat osensory cortex, but also in the thalamus and even in the brainstem. I n these structures, we identified broadly tuned neurons with multiwhis ker receptive fields (RFs). In the thalamus, a large percentage of neu rons exhibited shifts in the spatial domain of their RFs as a function of post-stimulus time. During these shifts, the center of the neuron' s RF moved across the whisker pad from caudal to rostral whiskers, but not in the opposite direction, suggesting that these spatiotemporal R Fs may encode directional information. Further studies revealed that s omatosensory representations were maintained by dynamic interactions b etween multiple convergent afferents, since they could be altered in a matter of seconds by reversible sensory deprivations, Overall, these results suggest that the rat somatosensory system relies on both spati al and temporal interactions between populations of cortical and subco rtical neurons to process multiple attributes of tactile stimuli.