Alpha and gamma oscillations characterize feedback and feedforward processing in monkey visual cortex

Cognitive functions rely on the coordinated activity of neurons in many brain regions, but the interactions between cortical areas are not yet well understood. Here we investigated whether low-frequency (alpha) and high-frequency (gamma) oscillations characterize different directions of information flow in monkey visual cortex. We recorded from all layers of the primary visual cortex (V1) and found that gamma-waves are initiated in input layer 4 and propagate to the deep and superficial layers of cortex, whereas alpha-waves propagate in the opposite direction. Simultaneous recordings from V1 and downstream area V4 confirmed that gamma- and alpha-waves propagate in the feedforward and feedback direction, respectively. Microstimulation in V1 elicited gamma-oscillations in V4, whereas microstimulation in V4 elicited alpha-oscillations in V1, thus providing causal evidence for the opposite propagation of these rhythms. Furthermore, blocking NMDA receptors, thought to be involved in feedback processing, suppressed alpha while boosting gamma. These results provide new insights into the relation between brain rhythms and cognition.