Saturday, March 22, 2008

Oscillations Part IV: Complex Systems

(This is a continuation of notes taken from Ginger Campbell's podcast #31 with György Buzsáki, and a continuation from Part III:)

Complex systems

* science progresses when it understands ideas from other disciplines and finds the commonalities
* neuroscience has progressed in one direction and physics and statistics and certain parts of engineering progressed almost independently and made large movements forward in an area that is called today complex systems.
"The "complex system" offered a very rich toolkit for neuroscience to think about interactions in the brain in a new way."
"..ideas and principles that have a common thread across different disciplines are substrate free. But whenever we want to understand the mechanisms we have to translate these interesting principles into mechanisms on a given substrate."

* mechanisms have to be understood and broken down into pieces.
* responsibility of neuroscientists lie in translating interesting explanations into neuronal mechanisms
"..the new field of neuronal computation and experimental neuroscience must work together to see how the ideas that spring out from your brain, from your head, can be really tied to reality rather than just express imagination."

The sound of many hands clapping:

* provides convincing example of synchronization
* clapping can slowly change from random noise - "clapclapclapclapclapclap" - to synchronized: "CLAP!.... CLAP!.... CLAP!"

* after a few seconds the clapping of the entire audience can become rhythmic
* what is the cause of the emergence of the rhythm? That's a very tough question, because there is no real "cause" as such
*"emergence" is the word being used - it starts with a random interaction of a few: some people just happen to clap their hands in synchrony, and with the same rhythmicity, and the surge of the sound of these individuals influence their neighbours, and eventually the rhythmic clapping becomes dominant


1. Bi-directionality:

* once the pattern or the rhythm emerges, it captures every single individual in the room - their degree of freedom decreases: i.e., their tendency is to stop doing their self-organized activity and become part of the synchronized clap sound formation

* Hermann Haken, laser physicist from Germany, talks about "bi-directional causation"
* the elements of the system emerge and produce, give rise to a new quality - we can call it an "order parameter", or the rhythm in this case
* the rhythm will then cause or effect the elements to do one thing, and not another

* the rhythm is a very fundamental property - it makes the questions of the skeptics so difficult to answer
* what many brain scientists ask is, if brain oscillations are important, remove them and see what happens, remove oscillations without affecting anything else and see how the brain will be affected
* it is not possible, because the reason why the oscillation emerges is because of the behaviour of the individual elements
* the two of them are tied in an interesting bi-directional manner

2. Cost effectiveness of single oscillators:

* the goal of hand-clapping is to generate as much impact with sound as possible
* if hand-clapping is rhythmic, the surge of the sound is several orders of magnitude stronger than.. at least it's louder ... than compared to the random clapping
* this increase of the output can be achieved at no cost - if you count the number of claps, or the rhythm of the clapping, during rhythmic clapping compared to random clapping, the frequency of the individuals is actually slower - individual investment and the energy that's being invested is less, yet the outcome is much stronger.

* with regard to systems of neurons, if the goal of computation is to not only compute something but to transfer that information to target neurons, the best way to do that is synchronize them
* synchrony through oscillations comes for free:
- e.g., during a particular type of epilepsy, called petit mal or generalized epilepsy, many neurons in the thalamus and the neocortex become entrained into 3 Hz spike and wave.
- this synchrony can be measured, and it's very obvious by recordings, but because it's epilepsy we thought for a long time that epileptic activity is extremely energy consuming
- it came as a surprise when the methods for measuring energy use with fMRI was applied to these epileptic episodes, many investigators in several laboratories almost simultaneously found that both signals became negative when the spike and wave epilepsy emerged, meaning that less energy was used.

* if you understand the mechanisms of how oscillations come about, and what of a single neuron invests into it, then it is not surprising at all - it is almost obvious.
* in 1992 we described a phenomenon, hippocampal ripples, at 200 Hz, very fast oscillation; we showed that we can get synchrony, very nice, tight synchrony, without any change of the firing rate.

Further reading:
1. Synergetics:
"The connection with chaos theory and catastrophe theory is in particular established by the concept of order parameters and the slaving principle, according to which close to instabilities the dynamics even of complex systems is governed by few variables only."

2. Self-organization of brain function
3. Zoltán Néda (Bebes-Bolyai University Romania): the spontaneous synchronization of hand clapping

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