Neurons Unexpectedly Encode Information in the Timing of Their Firing | Quanta Magazine

It’s a relationship between the continuous rhythm of a brain wave — the overall ebb and flow of electrical signaling in an area of the brain — and the specific moments that neurons in that brain area activate. In this way, brain waves act like a clock.

Researchers began noticing phase precession decades ago among the neurons in rat brains that encode information about spatial position. Human brains and rat brains both contain place cells, each of which is tuned to a specific region or “place field.” Our brains seem to scale these place fields to cover our current surroundings. As you leave one place field and enter another, the firing of the first place cell peters out, while that of the second picks up.

What about timing? As the rat passes through a place field, the associated place cell fires earlier and earlier with respect to the cycle of the background theta wave. As the rat crosses from one place field into another, the very early firing of the first place cell occurs close in time with the late firing of the next place cell. Their near-coincident firings cause the synapse between them to strengthen, and this coupling of the place cells ingrains the rat’s trajectory into the brain (strengthening of synapses occurs only when two neurons fire within tens of milliseconds of each other.)

Pulling out signals from human brains required sophisticated statistical analysis, because humans exhibit a more complicated pattern of overlapping brain waves than rodents do — and because less of our neural activity is devoted to navigation. But the researchers could say definitively that phase precession is there.

Other research suggests that phase precession may be crucial beyond navigation. In animals, the phenomenon has been tied to non-spatial perceptions, including processing sounds and smell.

Phase precession organizes the timing so that learning happens more often than it could otherwise. It arranges for neurons activated by related information to fire in quick-enough succession for the synapse between them to strengthen. “It would point to this notion that the brain is basically computing faster than you would imagine from rate coding alone,” Diba said.