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We have studied the origin and extent of axons within layer I of the primary somatosensory cortex (SI) of rats by using retrograde and anterograde tracers with an emphasis on reciprocal connections to layer I of SI from ipsilateral cortical areas that are the target of SI projections. Small crystals of 1,1',dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) labeled horizontal axons projecting in all directions within layer I, which extended for up to 4 mm with numerous terminal branches. Applications of horseradish peroxidase, Diamidino yellow, or fast blue to the pial surface of SI labeled a characteristic pattern of neurons below the application site that excluded neurons in layer IV of the barrel fields, unless the dye penetrated deeper than layer II. This provided a control for the effective depth of the layer I dye applications. Retrograde transport from layer I of SI was traced to the primary motor area, the lateral parietal areas, including the secondary somatosensory (SII) and agranular insular cortex ipsilaterally, as well as the homotopic areas of SI contralaterally. Injections of the anterograde tracer dextran amine at the same site as the SI surface application labeled dense fiber terminations in middle layers of these same secondary areas in the primary motor cortex (MI) or SII in the midst of cells labeled by retrograde transport from layer I of SI. Injections of dextran amine into these secondary cortical areas labeled fibers that coursed through deep layers to SI, where they ascended to layer I. These reciprocal corticocortical inputs to SI were concentrated in layer I, where they branched and extended horizontally across several SI barrels.

Studies of touch intensity discrimination in monkeys have identified a component of the somatosensory-evoked cortical potential, N1, generated within 50 ms of the stimulus, that predicts their behavioral performance. This study employed multiple-electrode arrays with relatively high spatial resolution (0.1 or 0.2 mm spacing) to record laminar profiles of somatosensory-evoked potentials (SEPs), multiple unit activity (MUA) and current source-densities (CSDs) at several sites across the postcentral gyri of two awake monkeys. This high resolution laminar analysis strongly supports our earlier hypothesis based upon low resolution data that N1 is generated by synaptic excitation targeted specifically at the most superficial cortical layers I/II: (1) The excitatory nature of N1 was indicated by elevated MUA which was maximal in layer III and extended down to subcortical white matter where fiber activity exceeded prestimulus levels; (2) In addition to CSD analysis, the observation that N1 was maximally negative within 0.10 mm of the border between layers I and II verified the superficial site of N1 synaptic excitation regardless of conductivity boundaries near the pial surface. A review of the anatomical literature finds that the most likely inputs responsible for N1 activation are the "backward" cortico-cortical projections from secondary somatosensory areas to SI which in area 1 are the major source of sensory-related input that specifically terminates in layers I/II. We suggest, therefore, that backward projections are involved in the conscious process of touch sensation as it is signaled by N1.