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Entrainment of neocortical neurons and gamma oscillations by the hippocampal theta rhythm

(Erika Nyhus) Can you briefly describe the significance of theta and gamma oscillations in the elevated maze task and REM sleep?

(Gyorgy Buzsaki) Oscillations are prevalent and evolutionarily maintained across species. Although there are few prominent oscillation frequencies they can serve many cognitive operations. This suggests that there are similar processing modes across the brain but the content depends on which areas are active. Faster oscillations (e.g. gamma) process local information whereas slower oscillations (e.g. theta) can integrate across larger brain areas. Slower oscillations (theta) can entrain faster oscillations (gamma) so the two types can be bound together.

Theta from the hippocampus is found during the elevated maze task and during REM sleep. It was originally thought that that theta was about movement. Now it is thought that theta relates to cognition: Spatial navigation and episodic memory. But theta does not have good temporal resolution, so you have to look at gamma, which is related to cortical input to the hippocampus.

Would the model described here for the transfer of information between the cortex and the hippocampus work in opposite directions during the elevated maze task and REM sleep?

The activity during REM sleep is similar to wake. The two-way theory suggests that during wake and during slow wave sleep information transfers from the cortex to the hippocampus and back again for encoding and consolidation of memories.

In the paper you state that, “The hippocampal theta rhythm might provide a mechanism for “gating” of sensory information and temporally biasing movement initiation by the hippocampal theta rhythm.” Can you please explain this statement.

Oscillations are continuous changes in the brain. You can predict what people remember based on their current brain state. This is because input at different oscillatory phases affects how well the input will be processed or encoded.

For memory, gamma and theta interact for effective encoding. Sensory input triggers local processing by gamma. Theta oscillations probe these local processors and allow for information to be encoded at specific phases.

Is the gamma range found in this article related to the 40 Hz oscillations often found in scalp recorded EEG in humans?”

The rhythms we are aware of (sleep spindle, gamma, alpha etc.) are exactly the same frequency in the tree shrew up to humans despite the scaling of brain size. The temporal dynamics are not compromised, so the brain develops to maintain those dynamics. Gamma is ubiquitous across the brain and across species – anywhere there are GABAa inhibitory interneurons there is a gamma time constant.