Figure 5. Experimental findings identifying deep cortical laminae sources of alpha oscillations.

(A) Attentional modulation of spike–field coherence in areas V1 and V4, adapted from (35). Non-human primates were cued to attend to a moving grating either inside or outside the recorded neuron’s receptive field. Red and blue traces show spike–field coherence in each area when attention was directed into or out of the receptive field, respectively. Only gamma-band activity was detected. (B) In area V4, gamma coherence was strongest in superficial layers and increased with attention. (C) Alpha coherence was localised to deep layers, where gamma coherence was minimal. (D) In deep layers of V4, alpha coherence decreased with attention, whereas gamma coherence was not observed. (E) Laminar recordings in V1 of non-human primates in period of wakeful rest. The layers specific activity time-locked to alpha troughs: average voltage traces phase-locked to troughs identified in the superficial layers (reference electrode marked “(ref)”). (F) Current-source density maps corresponding to the traces in (E), showing alpha-frequency sink/source alternations in deep layers around −100 μm and −400 μm. Adapted from (36). See also (110) for similar findings. The deep layer generators of the alpha activity might explain why oscillations are seen so strong in EEG and MEG recordings as the deep layer activity would be associated with currents in the long dendrites of layer 5 and 6 pyramidal neurons.