Network oscillations and intrinsic spiking rhythmicity do not covary in monkey sensorimotor areas

We investigated the relationship between local field potential (LFP) oscillations and intrinsic spiking rhythmicity in the sensorimotor system, because intrinsic rhythmicity has the potential to enhance network oscillations. LFPs and 918 single units were recorded from primary motor cortex (M1), primary somatosensory cortex (S1, areas 3a and 2), posterior parietal cortex (area 5) and the deep cerebellar nuclei (DCN). Some cells were antidromically identified as pyramidal tract neurons (PTNs). In each area the power of approximately 20 Hz LFP oscillations was assessed during periods of steady holding, when such oscillations have previously been shown to be maximal in M1. Oscillations were strongest in area 5 and weakest in the DCN. Using a previously developed method, the postspike distance-to-threshold trajectory was determined from the interspike interval histogram for each cell. Many cells had significant peaks, suggesting an intrinsic tendency towards rhythmic firing. Surprisingly, trajectory peaks were most common for M1 PTNs (115/146 cells) and rarest for area 5 neurons (12/82 cells). The extent of intrinsic spiking rhythmicity is not therefore simply related to the strength of 20 Hz oscillations in the sensorimotor system. These results suggest that intrinsic rhythmicity is not required for the generation and maintenance of oscillatory activity.