Gamma - 25-100 Hz (40 Hz typical)
- Result of fast-spiking interneurons. These reach firing rates of about 40Hz.
- These neurons are all coupled to each other through electrical synapses. These electrical synapses synchronize all of the action potentials across the whole population.
- FS neurons target the soma/axon of pyramidal cells. They cause inhibition, and act multiplicatively. This results in gain control of pyramidal cell activity (normalization of representation).
- The oscillatory inhibition only allows pyramidal cells to fire during the phase where they are least inhibited. This synchronizes the action-potentials of pyramidal cells and allows for the use of a temporal code.
- This is the oscillation frequency of local, cortical computation.
Alpha - 8-12 Hz
- Likely the result of thalamocortical feedback loop.
- Seen during awake and wakeful sleep states.
- Sleep is probably where the brain does inverse learning, and this means activating the thalamocortical loops.
- Long-range synchronization of cortical areas and thalamus. Can route information from different cortical areas through thalamus in this manner.
Theta - 4-8 Hz
- Primarily in the hippocampus. Not sure of the biophysical mechanism.
- Hippocampus has a map of your location that is encoded with place cells (neurons that fire when you are in a particular location). Across all of hippocampus there is one full cycle of the theta oscillation.
- Standing wave that covers hippocampus.
- Keeps the location information available at all times
- Across hippocampus place cells change their spatial resolution
- Place cells will fire in order throughout the theta cycle. The neurons whose receptive fields are activated most strongly will fire first, and those with weaker receptive fields will fire later.
- This sets up a temporal coding mechanism.
- Something like STDP would be able to learn the sequences.
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