SUMMARY Attending to a stimulus enhances its neuronal representation, even at the level of primary sensory cortex. Cross-modal modulation can similarly enhance a neuronal representation, and this process can also operate at the primary cortical level. Phase reset of ongoing neuronal oscillatory activity has been shown to be an important element of the underlying modulation of local cortical excitability in both cases. We investigated the influence of attention on oscillatory phase reset in primary auditory and visual cortices of macaques performing an intermodal selective attention task. In addition to responses “driven” by preferred modality stimuli, we noted that both preferred and non-preferred modality stimuli could “modulate” local cortical excitability by phase reset of ongoing oscillatory activity, and that this effect was linked to their being attended. These findings outline a supramodal mechanism by which attention can control neurophysiological context, thus determining the representation of specific sensory content in primary sensory cortex.

The functional significance of the ␣ rhythm is widely debated. It has been proposed that ␣ 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 ␣ rhythms and whether the generation of ␣ 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 ␣ current generators in the supragranular, granular, and infragranular layers. MUA phase-locked to local current source/sink configurations confirmed that ␣ rhythms index local neuronal excitability fluctuations. CSD-defined ␣ generators were strongest in the supragranular layers, whereas LFP ␣ 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 ␣ generators. The presence of ␣ generators across cortical depth in V1, S1, and A1 suggests the involvement of ␣ in feedforward as well as feedback processes and is consistent with the view that ␣ rhythms, perhaps in addition to a role in sensory inhibition, may parse sensory input streams in a way that facilitates communication across cortical areas.

Previous research demonstrated that while selectively attending to relevant aspects of the external world, the brain extracts pertinent information by aligning its neuronal oscillations to key time points of stimuli or their sampling by sensory organs. This alignment mechanism is termed oscillatory entrainment. We investigated the global, long-timescale dynamics of this mechanism in the primary auditory cortex of nonhuman primates, and hypothesized that lapses of entrainment would correspond to lapses of attention. By examining electrophysiological and behavioral measures we observed that besides the lack of entrainment by external stimuli, attentional lapses were characterized by high amplitude alpha oscillations, with alpha frequency structuring of neuronal ensemble and single unit operations. Strikingly, entrainment and alpha oscillation dominated periods were strongly anti-correlated and fluctuated rhythmically at an ultra-slow rate. Our results indicate that these two distinct brain states represent externally versus internally oriented computational resources engaged by large-scale task-positive and task-negative functional networks.