Time and the hippocampus

Emily Singer in Quantum:

Over the last few years, a handful of researchers have compiled growing evidence that the same cells that monitor an individual’s location in space also mark the passage of time. This suggests that two brain regions — the hippocampus and the entorhinal cortex, both famous for their role in memory and navigation — can also act as a sort of timer.

In research published in November, Howard Eichenbaum, a neuroscientist at Boston University, and collaborators showed that cells in rats that form the brain’s internal GPS system, known as grid cells, are more malleable than had been anticipated. Typically these cells act like a dead-reckoning system, with certain neurons firing when an animal is in a specific place. (The researchers who discovered this shared the Nobel Prize in 2014.) Eichenbaum found that when an animal is kept in place — such as when it runs on a treadmill — the cells keep track of both distance and time. The work suggests that the brain’s sense of space and time are intertwined.

The findings help to broaden our understanding of how the brain’s memory and navigation systems work. Perhaps both grid cells and other GPS-like cells aren’t tuned only to space but are capable of encoding any relevant property: time, smell or even taste. “It probably points to a broad thing the hippocampus does,” said Loren Frank, a neuroscientist at the University of California, San Francisco, who studies memory and the hippocampus. “It figures out the relevant axis for encoding experiences and then uses the cells to map those experiences.”

Learning intervals:

In Eichenbaum’s experiments, a rat runs on the treadmill for a set period — say, 15 seconds — and then gets a reward. As the animal repeats the cycle over and over, its brain learns to track that 15-second interval. Some neurons fire at one second, others at two seconds, and so forth, until the 15 seconds have elapsed. “Each cell will fire at a different moment in time until they fill out the entire time interval,” Eichenbaum said. The code is so accurate that researchers can predict how long an animal has been on the treadmill just by observing which cells are active. Eichenbaum’s team has also repeated the experiment, varying the treadmill’s speed, to make sure the cells aren’t simply marking distance. (Some of the cells do track distance, but some seem linked solely to time.)

Although these neurons, dubbed “time cells,” are clearly capable of marking time, it’s still not clear how they do it. The cells behave rather like a stopwatch — the same pattern of neural activity repeats every time you start the clock. But they are more adaptable than a stopwatch. When researchers change the conditions of the experiment, for instance by extending the running duration from 15 to 30 seconds, cells in the hippocampus create a new firing pattern to span the new interval. It’s like programming the stopwatch to follow a different time scale altogether.

What's going on?

György Buzsáki, a neuroscientist at New York University’s Neuroscience Institute whose lab did some of the first experiments exploring how the hippocampus tracks time, proposes that rather than monitoring time itself, these cells are doing something else — remembering a path through a maze or plotting the animal’s next move. Both memories and future plans unfold in time, so time cells may simply reflect this mental activity.

“That’s the number-one problem for me: Are there dedicated neurons in the brain doing nothing else but keeping track of time?” Buzsáki said. “Or do all neurons have functions that happen in sequential order, which for the experimenter can be translated into time?”

Buzsáki points out that it may not even make sense to think of hippocampal cells as independently coding for space or time. The human brain often considers time and distance interchangeably. “If one asks how far New York is from LA, the answers you get vary: 3,000 miles, six hours by flight,” he said. “In older language, distances were typically given by time — the days it takes to go from one valley to another — since it was not distance but the number of sunsets that was easy to calculate.”

I was exploring this general territory in my book on music, where I ended up looking at the brain's navigation system in connection with time. See Beethoven's Anvil (2001), pp. 135 ff. 

H/t 3QD.