This is the second of my post on basic concepts in brain science.

Synchrony is to do with oscillations in the brain. I posted about what oscillations are here. To recap, a population of neurons that repeatedly, fire together (burst) and then go quiet and then fire together again, are said to oscillate. The speed of the oscillation is called the frequency and is measured in Hertz (Hz). When two distinct populations oscillate at different frequencies they are desynchronised, but when they both oscillate at the same frequency they are said to be synchronous.

Synchrony is a different concept to resonance. Resonance is where one thing is oscillating and a second thing is not, but then the second starts to oscillate at the same frequency as the first. Resonance is therefore when one thing oscillates in sympathy with another. The reason the second oscillates in sympathy is due to some connection between the two. For example, one object oscillating may be causally linked to another by the gas in our atmosphere and these vibrations my effect the second so it too starts oscillating.

Synchrony is also caused by a connection between two objects. Unlike resonance, where one object is originally oscillating and one is not, with synchrony both objects are originally oscillating. The key is that the frequencies at which each is originally oscillating are different. When they synchronise they may synchronise to a frequency that is different from either of the original frequencies. So for example, you may have two pendulums connected together by the beam they are both hung upon. One may be swinging at 20 Hz and the other at 40 Hz. The beam connecting the two creates a causal interaction. After a while and much interaction both may end up oscillating at 30 Hz. Both are synchronised to the same frequency, but at a different frequency than either was at originally. The reason they may have ended up at a different frequency is that the causal interaction is going both ways. The oscillation from one is effecting the oscillation of the other, and vice versa. With resonance the causal effect is one way, hence the second object oscillating in sympathy at the frequency of the first.

Now back to synchrony in the brain. In the brain you may have one population of neurons oscillating at one frequency and another population oscillating at another frequency. The neurons in one population are causally connected to the other by synapses, and vice versa. Over time the oscillation in each synchronises so that the bursts of firing in each population are at the same frequency. This can be likened two two people hitting a drum at the same tempo. Not only that but also they may have same phase. The phase refers to when the beats happen. Imagine two people drumming at the same tempo. Even though each is at the same tempo one person may hit the drum when the other person is quiet, and vice versa. If this is so they are said to be completely out of phase. If they hit the drum at the same time and therefore quiet moments also happen at the same time they are said to be in phase. Similarly if the two neural populations are synchronous and the burst of firing and moments of quiet occur at the same time then they are in phase.

Gamma-band oscillations (a population of neurons firing together at the rate of 30-80 Hz) can emerge in a population of excitatory and inhibitory neurons. The inhibition causes the moments of quiet in the oscillation. This provides windows for interaction at the moment inhibition wears off and there is a burst of firing. Excitatory signals from a different oscillating population can then take advantage of this because gamma band oscillations are sufficiently regular to allow prediction of the next burst. As long as the travelling time from the sending to the receiving group is also reliable, their communication windows for input and output are open at the same times (i.e. when the bursts occur). Packages of spiking signals from one population of neurons can therefore arrive at the other neuronal group in precise synchronization and enhance their impact. In short, synchronisation between two populations allows two populations to work together and provides the optimal conditions for transferring information. Pascal Fries discusses the mechanistic consequences of neuronal oscillations and calls this hypothesis ‘communication through coherence’. You can read a more technical report by him here.

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