Are eruptions how the brain makes things happen?
When we look at neuronal activity we see a lot of it…all over the place, all the time, and on really fast timescales. How does that fast, widespread activity trigger the slow and discrete events we experience: thoughts, perceptions, decisions, actions? There aren’t many hypotheses out there. We have one and we call it the eruption.
We discovered the eruption by studying mating flies. It’s a good system for looking at the competition between drives: when danger arises, should the flies stop mating to escape, or stick it out and endure the consequences? How long they’ve been mating is a key variable (longer matings are more likely to be successful) and we had previously identified a network of neurons in males that measures time during mating. They use a protein called CaMKII, whose activity declines over several minutes during mating. But each of the neurons in the network has a CaMKII timer, and the timers are noisy, and the flies need a yes/no decision…NOW! How do they decide?
They compare notes.
Each cell in the network sends the others a signal that increases as its private estimation of elapsed time increases. The network stores and accumulates this information in the form of cyclic AMP. Accumulated cAMP excites the neurons, further increasing network activity and cAMP production. This positive feedback loop is restrained by neurons whose own estimate of time is slower than the average, but once enough time has elapsed the population activity becomes unrestrained causing an eruption (!) of network activity. The eruption is a large, sustained influx of calcium that broadcasts the network’s decision to the rest of the nervous system, telling the flies that the mating has gone on long enough that the flies can separate and escape if things get bad.
We’re excited about the eruption because we think it explains more than just an obscure insect behavior: it’s a hypothesis for how the brain makes things happen.
Eruptions have a lot of similarities with action potentials, but they act at the network level. Their reliance on molecules like cAMP and CaMKII remind us that neurons are cells, not transistors. Cells have all sorts of computational methods—that’s how they create the diversity and sophistication of the biological world. And that makes most of the events that lead up to the eruption in this system invisible to the electrode- and calcium-based tools on which most neuroscientific studies are based. So eruptions may have been overlooked in other systems.
Or they may only exist in flies…we doubt it.
Here’s how first author Stephen Thornquist summarized the results on Twitter
And here’s a cool blog post about timing and the eruption from Hessam Akhlaghpour