On the relationship between LFP beta oscillation amplitude and firing rate of individual neurons in monkey motor cortex

Beta oscillations are prominent in motor cortical local field potentials (LFPs), and their relationship to the local neuronal spiking activity has been extensively studied. Many studies have shown that in motor cortex, spikes of individual neurons tend to lock to the phase of LFP beta oscillations. However, there are contradictory results concerning whether there is also an intrinsic relationship between the amplitude of LFP beta oscillations and the firing rate of individual neurons. To resolve this controversial issue, we correlated the LFP beta oscillation amplitude recorded in macaque motor cortex with spike counts of individual neurons during visuomotor behavior, in two different manners. First, in an analysis termed task-related correlation, we included data obtained across all behavioral task epochs. These task-related correlations were frequently significant, and of either negative or positive sign. Second, in an analysis termed trial-by-trial correlation, we included only data from a well-defined steady-state pre-cue epoch, and calculated the correlations across trials. Such trial-by-trial correlations were weak and rarely significant. We conclude that there is no intrinsic relationship between the spike count of individual neurons and LFP beta oscillation amplitude in macaque motor cortex, beyond each of these signals being modulated by external factors such as the behavioral task.