Layer I of primary sensory neocortex: where top-down converges upon bottom-up.

A fundamental paradigm shift is underway in the behavioural neurosciences which promotes renewed discussion of neural correlates of conscious sensation. A growing body of evidence that the primary sensory areas are central to conscious processing is inconsistent with conventional bottom-up models of passive sensory transformation. The new paradigm emphasizes the interaction of top-down corticocortical influences with bottom-up sensory feedback. This review develops the new interactive paradigm with respect to the behaviourally relevant N1 component of the somatosensory evoked response in primary somatosensory neocortex which is only observed during conscious states and predicts touch discrimination behaviour. N1 is generated by layer I excitation apparently in response to backward projections from higher order cortical areas. The backward corticocortical projections mediating top-down influences are anatomically concentrated in layer I where they strongly excite the subpopulation of pyramidal neurons with extensive distal apical dendrites. This subpopulation includes the forward-projecting pyramids subserving corticocortical reentrance as well as the corticobulbar projections mediating cortical control over fine sensory-oriented movements. This interactive paradigm views conscious sensation as an active behaviour with the optimal spatial resolution of primary areas serving as the sensory-motor interface at the major convergence point between reentrant bottom-up and top-down pathways. A device is proposed for the reconstruction of cortical sensory processing based upon the central importance of the primary area as the focus of convergent top-down cortical projections.