The brain decade in debate: VI. Sensory and motor maps: dynamics and plasticity

This article is an edited transcription of a virtual symposium promoted by the Brazilian Society of Neuroscience and Behavior (SBNeC). Although the dynamics of sensory and motor representations have been one of the most studied features of the central nervous system, the actual mechanisms of brain plasticity that underlie the dynamic nature of sensory and motor maps are not entirely unraveled. Our discussion began with the notion that the processing of sensory information depends on many different cortical areas. Some of them are arranged topographically and others have non-topographic (analytical) properties. Besides a sensory component, every cortical area has an efferent output that can be mapped and can influence motor behavior. Although new behaviors might be related to modifications of the sensory or motor representations in a given cortical area, they can also be the result of the acquired ability to make new associations between specific sensory cues and certain movements, a type of learning known as conditioning motor learning. Many types of learning are directly related to the emotional or cognitive context in which a new behavior is acquired. This has been demonstrated by paradigms in which the receptive field properties of cortical neurons are modified when an animal is engaged in a given discrimination task or when a triggering feature is paired with an aversive stimulus. The role of the cholinergic input from the nucleus basalis to the neocortex was also highlighted as one important component of the circuits responsible for the context-dependent changes that can be induced in cortical maps

Beyond maps: a dynamic view of the somatosensory system

Current theories on how tactile information is processed by the mammalian somatosensory system are based primarily on data obtained in studies in which the physiological properties of single neurons were characterized, one at a time, in behaving or anesthetized animals. Yet, the central nervous system relies on the concurrent activation of large populations of neurons to process the variety of sensory stimuli that contribute to normal tactile perception. The recent introduction of electrophysiological methods for chronic and simultaneous recordings of the 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 system, interact following passive and active tactile stimulation. The rat trigeminal somatosensory system was used as a model for this investigation. Our results revealed the existence of highly dynamic and distributed representations of tactile information, not only in the somatosensory cortex, but also in the thalamus and even in the brainstem. In these structures, we identified broadly tuned neurons with multiwhisker receptive fields (RFs). In the thalamus, a large percentage of neurons 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 RFs may encode directional information. Further studies revealed that somatosensory representations were maintained by dynamic interactions between 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 spatial and temporal interactions between populations of cortical and subcortical neurons to process multiple attributes of tactile stimuli.