Neural Synchronization Followup

Two posts ago I asked a question about neural synchronization – if a group of neurons are firing synchronously, don’t we lose information?  In other words, if many neurons are all saying the same thing, do we really gain anything by having them all say it?  Additionally, if a group of neurons is “busy” firing synchronously, wouldn’t that preclude them from doing anything else or being responsive to other inputs?  Synchronization appears to be an important property of the brain, so to try to get a better understanding of how it might work, I read several articles (Fries, 2005; Schnitzler & Gross, 2005; Singer, 1993), along with Gyorgy Buzsaki’s book ”Rhythms of the Brain” (2006) and I now have a better idea of how neural synchronization might work, and the purposes it might serve.

The most critical piece I was missing is that neural synchronization does not necessarily mean that a group of neurons are all firing in sychrony (i.e. every neuron in the group firing in every cycle of the oscillation).  Instead, it could be that the membrane potential of the synchronized neurons are oscillating together.  This would effectively result in a synchronized group varying in how excitable (likely to fire) they were, together - giving windows when the neurons were all easily excitable, and periods where they were not very excitable at all.  This would allow neurons to be synchronized and still have the information that gets transmitted be interesting – a spike could still mean something beyond “we are all synchronized”.

Another mistaken idea I thought I had heard was that “synchronization allows neurons to communicate / transmit information.”  This didn’t make sense to me because it doesn’t seem like you need synchronization to communicate – a single neuron can fire and send out an action potential to other neurons regardless of what other neurons are doing.  The missing piece was that synchronization isn’t needed for communication per se, but rather synchronization might allow for selective communication.  There is a vast anatomical connectivity between neurons, and it seems likely that only certain subsets of neurons need to be communicating at a given time.  If a “sending” and “receiving” group are oscillating together, then there will be periodic “windows” when the receiving neurons are excitable enough to receive information.  At other points in the cycle, the receiving neurons would likely be unresponsive to input because they would be at a “trough” in their excitability (they would be hyperpolarized).  Thus, only neurons which were synchronized with the receiving neurons would be able to send to them.

From the above sources, a few of the possible functions of synchronization might be:

• “Binding” disparate information together.  If neurons in separated areas representing related information are synchronized, and if another group of neurons which is “interested” in this information is also synchronized with them, then the “interested” area can receive information only from the related, relevant areas, and not receive information from other currently unrelated neurons which have anatomical connections to it.
• Selective communication.  As mentioned above (and in some ways similar to the first point), synchronization could generally allow groups of neurons to selectively communicate and filter out the “noise” of unrelated but anatomically connected other neurons.
• Greater impact.  Since in many cases it takes many post-synaptic potentials to cause a neuron to fire, the effect of one or a few neurons firing an action potential at a target neuron may have little effect on the target.  However, if many neurons are synchronized and fire at the same time, they can have a much greater chance of pushing the target over firing threshold.
• Facilitating synaptic changes / plasticity.  There is evidence to suggest that some / many aspects of synaptic plasticity require many incoming post-synaptic potentials in a very short time window in order to occur.  Many “upstream” synchronized neurons firing at the same time would likely have a much greater chance of effecting synaptic changes than more spread out, unsynchronized firing.

A few other interesting points I noticed:

• Relativley high frequency oscillations seem to be used for small groups of synchronized neurons, and lower frequency / slower oscillations may be used for larger synchronized groups.  This may be due to the mechanics of synchronization such as longer axonal transmission times over longer distances, more “links in the chain”, etc.
• One of the potential challenges to synchronization is that, in order to stay “in phase”, the transmission time from each sending neuron to each receiving neuron needs to be very close to the same.  Amazingly, some evidence suggests that some networks seem to be tuned so that longer-distance axons are more heavily myelinated, resulting in faster conduction speeds for longer distances, and thus very close latency between closer and further connections (such as from the thalamus to the cortex) (Salami et al., 2003).
• Magnetoencephalography is one useful tool for measuring synchronization, since it has the time resolution needed to detect the rapid voltage changes.

References

Buzsaki, G. (2006). Rhythms of the brain. New York: Oxford University Press.

Fries, P. (2005). A mechanism for cognitive dynamics: Neuronal communication through neuronal coherence. TRENDS in Cognitive Sciences, 9, 474-480.

Salami, M. et al. (2003). Change of conduction velocity by regional myelination yields constant latency irrespective of distance between thalamus and cortex. Proceedings of the National Academy of Sciences of the United States of America, 100, 6174-6179.

Schnitzler, A., & Gross, J. (2005). Normal and pathological oscillatory communication in the brain. Nature Reviews Neuroscience, 6, 285-296.

Singer, W. (1993). Synchronizatoin of cortical activity and its putative role in information processing and learning. Annual Review of Physiology, 55, 349-374.