Tuesday, March 18, 2008

More about oscillations- Part I: Where do brain oscillations come from?

In reference to Oscillatory Matters:
If the reader goes back through all the posts linked in the post I've linked to here, he or she will learn that György Buzsáki has studied the hippocampus and brain rhythms all his research life. I completed an entire transcript of the Buzsáki interview by Ginger Campbell of Brain Science Podcast and sent it to her to do with as she wishes. I'm sure she will find good use for it. My main objective in writing a transcript was to understand the material in greater depth.

I plan to bring a series of notes here, based on the interview. This is Part I - Where do brain oscillations come from?

His opening statement in the interview is:
"Synchrony is an important mechanism to bring elements of a system together in time - it's critical - then I think everybody agrees that it's a pretty important mechanism. The only thing we can add to it is that oscillations do a fantastic job of doing that and they do it almost for free."

That "almost for free" aspect is important given that the nervous system is an expensive system to maintain metabolically.

What are oscillations?
*Nature is full of cyclic phenomena - movement of heavenly bodies, the calendar, the life cycles in biology, respiration, heart beat, menstrual cycles.. these are all rhythmic phenomena.

*Many man made objects are also based on rhythms or oscillations, such as clocks, radios, TVs, computers, cell phones, even electrical line transmission

*What is the use of all this repetitive phenomena? Artifact (side effect of how things are put together), or essential parts of the mechanism?

*A one or five megahertz clock could be built using random timing, but it would be incredibly difficult.

Oscillations synchronize events

*Imagine a car designed to not move smoothly; it speeds up and slows down irregularly. The average speed of this car and a regular car may be identical. On a smooth terrain both would take about the same length of time to get to the goal. When the terrain is difficult the car would be harder to drive.

Types of oscillators

1. "Harmonic" oscillators: predictability

* e.g. pendulum clock with motion almost perfectly sinusoid
* e.g. orbit of the moon around the earth
* "predictability" is that one need only look at motion of the moon for a short period of time to extract the phase information then one can predict the phase of the moon tomorrow or a million years from now
* harmonic oscillators are very useful for predicting events in time
* there are many examples in the brain of harmonic oscillators, exploited for exactly this purpose

2. "Relaxation" or "pause" : easy perturbation

* discrete events repeat with long or less predictable intervals
* e.g. clapping of hands: a discrete event is followed by silent period which may vary in length
* e.g. a dripping faucet: drips (discrete events) repeat at certain intervals; if observation period is short, within two drops, one cannot predict the occurrence of next event
* no timing precision, but they can be perturbed easily - tap on the faucet will produce a premature drop, determined exactly by the timing of the perturbation

* very effective to synchronize in large numbers if they are identical:

Imagine hundreds or thousands of faucets, simultaneously perturbed: phase reset means to start them all with the same phase (tap on the tap): if they are identical this need be done only once - they will drip simultaneously thereafter, demonstrating synchrony and phase reset-ability.

So where do oscillations come from? The short answer Buzsáki provides is that they seem to come from nature itself.

This next thought doesn't come from the interview, but I want to point out that even a glass of water sitting unperturbed will contain "oscillation" in the form of Brownian movement of the water molecules - and life contains a lot of water molecules.

This inherent atomic motion is unlikely to be completely stilled by being bound in a cell. In fact those who study motility in single-cell organisms have to separate out Brownian movement as a potential confounding factor.

What are neurons? They seem to be many things to many fields, but first of all they are long skinny single cells containing water, very easily perturbed, whose job seems primarily to be to signal - to chatter continuously. This entire interview, and the book Rhythms of the Brain, explains how this chatter appears to be organized into patterns.

1. Stephen Strogatz: known for his discovery of “small world” architecture. Sync: The emerging science of spontaneous order (2003) is for a general audience

2. Nancy Kopell: mathematician
We got Rhythm: Dynamical Systems of the Nervous System (pdf). N Am Math Soc 47: 6-16 (2000). (Buzsáki recommends her review of the analytical approaches to neuronal oscillators)

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