Rhythms of Memory

Our neurons learn best when they’re working on the same wavelength – literally, says a new study.

What are these neurons doing, you ask? Well, um...it's grown-up neuron stuff, OK?

For every synapse – every signaling junction between neurons – there’s a certain firing frequency that increases signal strength the most. In short, neurons work like tiny analog antennas – tuning into incoming signals and passing along the clearest, strongest ones as electrochemical messages.

This represents a huge breakthrough in our understanding of how our brains work. Neuroscientists have known for decades that we (and other animals) learn by strengthening connections between certain neurons – i.e., the more a pair of neurons communicate with each other, the more receptive and sensitive they become to each others’ electrochemical signals. But one central mystery remained: what makes some neurons more likely to “hear” signals from certain other neurons in the first place?

Now we know that every synapse has a frequency “sweet spot” – a certain frequency to which it’s most responsive. The farther away from a neuron’s nucleus a certain synapse is, the higher the frequency of its sweet spot. Thus, different parts of a neuron are tuned to different signal wavelengths.

As the journal Frontiers in Computational Neuroscience reports, a team led by UCLA’s Arvind Kumar and Mayank Mehta set out to expand on previous studies, which had found that stimulating neurons with high-frequency electrical pulses (or “spikes“) – around 100 per second – tended to strengthen synaptic connections, while stimulating them with low-frequency pulses – around one per second – tended to weaken those connections. (Click here for a straightforward explanation of how electrical pulses help transmit signals between neurons.)

In the real world, however, neurons typically fire much quicker bursts – only about ten spikes at a time – at a rate of around 50 spikes per second. “Spike frequency refers to how fast the spikes come,” Mehta explains. “Ten spikes could be delivered at a frequency of 100 spikes a second or at a frequency of one spike per second.”

Until now, neuroscientists lacked the technology to model the results of such brief firing bursts with much accuracy – but Mehta and Kumar designed a sophisticated computer model to test spike frequencies and burst lengths closer to real-world levels:

We computed the influence of these variables on the plasticity induced at a single NMDAR containing synapse using a reduced model that was analytically tractable, and these findings were confirmed using detailed, multi-compartment model.

In other words, they created a simulation of an NMDAR, a certain type of receptor for the neurotransmitter glutamate. These receptors are crucial for strengthening synapses – and thus, for memory formation and learning.

Using their model, the researchers made four major new discoveries:

1) The more distant from a neuron’s soma (main body) a synapse is, the higher the spiking frequency to which it’s “tuned.”  In fact, the same frequency can strengthen or weaken a synaptic connection, depending on where on the neuron that particular synapse is located.

2) Regular, rhythmic pulses cause greater changes in synaptic strength (i.e., greater synaptic plasticity) than irregular bursts do.

3) Short bursts of spikes significantly raise the timing dependence of synaptic plasticity – in other words, the shorter the burst, the more important it is to get the frequency spot-on.

4) Once a synapse learns to communicate with another synapse, its optimal frequency changes – say, from 30 spikes per second to 24 spikes per second.

The internet is now abuzz with chatter about the implications of these results. One intriguing idea is that gradual “detuning” among synapses could underlie the process of forgetting – and that future drugs or electrical therapies could help “retune” some of these rhythms.

The research also raises the question of how incoming rhythms from our senses – especially light and sound – might directly impact these firing frequencies. We’ve known for years that some types of epileptic siezures can be triggered by light flashing at certain frequencies, and that music playing at certain rhythms can pump us up or calm us down. Is it so far-fetched to suggest that rhythms like these might also help shape our thoughts and memories?

On that note, I’m off to stare at some strobe lights and listen to four-on-the-floor dance music…for Science!


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