The neural basis of the behaviorally relevant N1 component of the somatosensory-evoked potential in SI cortex of awake monkeys: evidence that backward cortical projections signal conscious touch sensation.

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.