Activity of somatosensory-responsive neurons in high subdivisions of SI cortex during locomotion

Responses of neurons in the primary somatosensory cortex during movements are poorly understood, even during such simple tasks as walking on a flat surface. In this study, we analyzed spike discharges of neurons in the rostral bank of the ansate sulcus (areas 1-2) in 2 cats while the cats walked on a flat surface or on a horizontal ladder, a complex task requiring accurate stepping. All neurons (n = 82) that had receptive fields (RFs) on the contralateral forelimb exhibited frequency modulation of their activity that was phase locked to the stride cycle during simple locomotion. Neurons with proximal RFs (upper arm/shoulder) and pyramidal tract-projecting neurons (PTNs) with fast-conducting axons tended to fire at peak rates in the middle of the swing phase, whereas neurons with RFs on the distal limb (wrist/paw) and slow-conducting PTNs typically showed peak firing at the transition between swing and stance phases. Eleven of 12 neurons with tactile RFs on the volar forepaw began firing toward the end of swing, with peak activity occurring at the moment of foot contact with floor, thereby preceding the evoked sensory volley from touch receptors. Requirement to step accurately on the ladder affected 91% of the neurons, suggesting their involvement in control of accuracy of stepping. During both tasks, neurons exhibited a wide variety of spike distributions within the stride cycle, suggesting that, during either simple or ladder locomotion, they represent the cycling somatosensory events in their activity both predictively before and reflectively after these events take place.