There are minutes and hours of our lives in which nothing happens, and these don't seem on the surface to be very challenging for our memories. At least, they make for succinct stories: "I waited 20 minutes for the doctor to come in." "I tossed and turned for hours last night." But how do we know it's been hours? How do we represent these chunks of lost time in our memories, accounting for all the empty minutes without actually losing them? Researchers at Boston University think they've found the answer. Buried in the brain's memory center, "time cells" tick away the moments like the second hand on a clock.
Deep inside the brain, the hippocampus helps us to remember sequences of events and form new memories. Howard Eichenbaum and his colleagues implanted electrodes into the hippocampi of four rats. They wanted to observe which neurons were active at different points during a task that involved a delay and challenged the rats' sequential memories.
The rats were trained to complete several steps: First, they entered a corridor and saw (and sniffed) an object, either a green wooden block or half a green rubber ball. Then a door was opened, releasing the rats into the next part of the corridor. When the door shut behind them, the rats were trapped in the blank hallway for 10 seconds. After the delay, another door opened, leading the rats to a flowerpot filled with sand. The rats sniffed at the sand, which had been mixed with either cinnamon or basil. In training, the rats had learned to match each smell with one of the two green objects. If the smell was the correct match for the object they'd seen 10 seconds earlier, the rats could dig in the flowerpot to get a reward (a third of a Froot Loop, in case you wondered). If the smell didn't match, the rats could earn their reward by leaving the flowerpot undisturbed and going around the corner.
In trials, the rats repeated this mini-maze 100 or so times in a row. In order to succeed, they had to keep the order of recent events straight in their memories. (Did I see the green ball before the most recent delay, or before I saw the last flowerpot?) The researchers recorded the activity of a few hundred hippocampal neurons during the whole trial. About half of the neurons they looked at fired during the 10-second delay.
What was interesting about these cells was that they fired one after another throughout the delay. With their successive firings, the neurons covered the whole empty time from start to finish, like a team of runners in a very short relay race.
The researchers dubbed these neurons "time cells" because they seem to keep track of time. Similarly, "place cells" are neurons that are known to fire when a rat is in a specific place. They keep up their activity as a rat moves through an open space, pacing off an otherwise unremarkable landscape just as time cells appear to keep track of empty time.
When the researchers redid the experiment and doubled the delay in one block of trials, they saw some of the time cells adjusting their firing frequency, while other teams of cells kept up their regular tick-tick-tick. They think this means time cells can monitor relative time as well as absolute time.
Could the firing of similar time cells in human brains define how we understand time? Some research has suggested that we experience life in chunks of about three seconds. Each of those chunks makes up a single moment, the theory goes, separating "right now" from everything before and after.
It's an appealing idea: Even when nothing of note is happening, our brains are steadily observing and recording so that we can sort out events in recollection. Maybe other areas of the brain measure time in their own ways. Or maybe the only timekeepers are in the hippocampus--the memory center--meaning that to our brains, time is only important in its passing.
MacDonald, C., Lepage, K., Eden, U., & Eichenbaum, H. (2011). Hippocampal “Time Cells” Bridge the Gap in Memory for Discontiguous Events Neuron, 71 (4), 737-749 DOI: 10.1016/j.neuron.2011.07.012