The functional significance of the alpha rhythm is widely debated. It has been proposed that alpha reflects sensory inhibition and/or a temporal sampling or "parsing" mechanism. There is also continuing disagreement over the more fundamental questions of which cortical layers generate alpha rhythms and whether the generation of alpha is equivalent across sensory systems. To address these latter questions, we analyzed laminar profiles of local field potentials (LFPs) and concomitant multiunit activity (MUA) from macaque V1, S1, and A1 during both spontaneous activity and sensory stimulation. Current source density (CSD) analysis of laminar LFP profiles revealed alpha current generators in the supragranular, granular, and infragranular layers. MUA phase-locked to local current source/sink configurations confirmed that alpha rhythms index local neuronal excitability fluctuations. CSD-defined alpha generators were strongest in the supragranular layers, whereas LFP alpha power was greatest in the infragranular layers, consistent with some of the previous reports. The discrepancy between LFP and CSD findings appears to be attributable to contamination of the infragranular LFP signal by activity that is volume-conducted from the stronger supragranular alpha generators. The presence of alpha generators across cortical depth in V1, S1, and A1 suggests the involvement of alpha in feedforward as well as feedback processes and is consistent with the view that alpha rhythms, perhaps in addition to a role in sensory inhibition, may parse sensory input streams in a way that facilitates communication across cortical areas. SIGNIFICANCE STATEMENT: The alpha rhythm is thought to reflect sensory inhibition and/or a temporal parsing mechanism. Here, we address two outstanding issues: (1) whether alpha is a general mechanism across sensory systems and (2) which cortical layers generate alpha oscillations. Using intracranial recordings from macaque V1, S1, and A1, we show alpha band activity with a similar spectral and laminar profile in each of these sensory areas. Furthermore, alpha generators were present in each of the cortical layers, with a strong source in superficial layers. We argue that previous findings, locating alpha generators exclusively in the deeper layers, were biased because of use of less locally specific local field potential measurements. The laminar distribution of alpha band activity appears more complex than generally assumed.
School of Medicine