Corticofugal Modulation of Sensory Information (Advances in Anatomy, Embryology and Cell Biology (187))

Sensory signals reach the cerebral cortex after having made synapses in different relay stations along the sensory pathway. The flow of sensory information in subcortical relay stations is controlled by the action of precise topographic connections from the neocortex. Several lines of research indicate that the massive corticifugal system improves ongoing subcortical sensory processing and reorganizes the receptive fields in visual, auditory and somatosensory systems. In all these sensory systems cortical neurons mediate both the highly focused positive feedback to subcortical neurons with overlapping receptive fields and a widespread inhibition to "non-matching neurons". This cortical feedback, which has been called "egocentric selection", can play a pivotal role in gating the sensory information that reaches the thalamus and cortex. Thus, corticofugal projections may contribute to selective attention since they enhance neuronal responses for attentionally relevant stimuli and by suppressing sensory responses of distractive stimuli. Also, corticofugal projections enhance oscillatory activity in order to synchronize neurons located in the same or in different relay stations in order to improve sensory processing. In conclusion, corticofugal pathways precisely control sensory transmission throughout the central nervous system. The processing of sensory information by the central nervous system is difficult to understand by the complex interconnections between subcortical and cortical areas. Anatomical studies have demonstrated the existence of precise corticofugal projections to subcortical relay stations (feedback connections). Although the majority of studies have concentrated on the ascending or feedforward system, the feedback connections are actually much more numerous than the ascending axons and may contribute to focusing information processing in the visual, auditory, and somatosensory pathways. We review anatomical and physiological data on the function of the corticofugal system in sensory processing and plasticity. We summarize anatomical and electrophysiological data that describe precise projections from sensory cortical areas to subcortical relay stations of the same sensory modality. Moreover, electrophysiological studies have demonstrated that the sensory cortex modulates neuronal responses at all levels of the sensory pathway. Special attention will be given to the primary somatosensory cortex modulation of tactile responses in the dorsal column nuclei, as an example of the corticofugal control of the ascending sensory information. Experimental results suggest that corticofugal projections act as filters that enhance relevant stimuli and reduce irrelevant stimuli according to the behavioral task in a precise moment. In conclusion, the corticofugal effects on neuronal synchronization, receptive field plasticity, and facilitation of relevant stimuli are examples of neuronal processes that may contribute to sensory processing and selective attention