Phase coding & working memory

Q: How can you very simply implement sequence storage, readout, and comparison using firing phase (rather than firing rate) as your neural code?

Q: Why does working memory hold 7 +- 2 items?

Q: Why is possibly transient phase-locking observed between the hippocampus and the entorhinal cortex, the cingulate cortex, and the prefrontal cortex?

Q: Why do the phases of place cells in the hippocampus (relative to the theta rhythm) precess as a rat moves further into the cell’s place field?

Read on for all this and more!

This excellent paper explains how to implement a few basic operations (sequence storage, sequence readout, and comparison) using firing phase as a neural code.

It also reviews the evidence that a phase code might be used in the brain, particularly in the storage of working memory:

The answers to some of the working memory stuff isn’t in the above paper, but can be found in the abstracts of some of Jensen’s earlier papers.

If you can’t wait, here’s the short answers to the other questions above (this is only a theory, it isn’t proven yet, although there’s some evidence):

Q: Why does working memory hold 7 +- 2 items?
A: Brain regions involved in working memory oscillate at two rates at once; a low-frequency theta oscillation (5-8 Hz) is subdivided into about seven subcycles by high frequency gamma oscillations (20-60 Hz). Perhaps this is a multiplexed short-term memory buffer; each of the seven subcycles holds one memory. So, during each theta cycle, the brain runs through each of the strored working memories.

Q: Why is possibly transient frequency-locking observed between the hippocampus and the entorhinal cortex, the cingulate cortex, and the prefrontal cortex?
A: Phase coding means transmitting information in the phase of individual spikes, as compared to an underlying oscillation. If there is a phase-coded signal being transferred between the hippocampus and one of these other areas, then both sender and receiver must be frequency-locked in order to be able to compare the phases of the spikes to the same underlying oscillation.

Q: Why do the phases of place cells in the hippocampus (relative to the theta rhythm) precess as a rat moves further into the cell’s place field?
A: The rat is replying a sequence of nearby places into it’s short-term memory. At first, as it is at location A, the “C” place cell is activated (because it’s nearby C). Perhaps it remembers sequence A,B,C,D,E. Then, it moves to location B. The “C” place cell now replays sequence “BCDE “. etc (actually, I’m not clear if it keeps replaying A, too, I think so; but ignore that for now).

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