The recently discovered neurons that keep track of time – so-called time cells – are gaining traction in the memory community. Audience members found out why at a session for the CNS meeting Monday, as several researchers presented the latest on how these cells encode memories over time in rats, humans, and other primates.
Just as place cells encode information about location, time cells are neurons in the hippocampus that encode information about the passage of time. They “bridge the temporal gaps between associated memories,” said Howard Eichenbaum of Boston University.
Eichenbaum and his colleagues measured brain activity in rats when tasked with finding their way through a maze. The researchers kept track of where and when certain neurons fired based on the rat’s location in the maze, which included a tiny treadmill place in the middle. On the treadmill, the rat kept moving but obviously remained in the same spatial location. The subset of neurons that fired while the rat was in one place, they deduced, were the time cells.
Eichenbaum’s team is currently working out how time and location data are integrated to present a more complete picture. “Those two [parameters] form a kind of scaffolding, onto which we link on the various pieces of memories,” he explained.
Because it’s challenging to keep monkeys moving in a predictable way, establishing that primates have place cells has been difficult, said Wendy Suzuki of New York University. But her group’s findings do show the existence of time cells in the monkey hippocampus, providing “one of the strongest links between the rodent hippocampal literature and the monkey hippocampal literature.”
Suzuki and her colleagues ran monkeys through a series of tests, some of which required them to focus on time and some of which did not. While they found that the hippocampus was highly activated in situations where the time element was critical, they also found that hippocampal neurons were gathering this information regardless of whether it was required for the current task. Suzuki predicts that time is just one of the parameters for which neurons gather information to build the context around what ultimately becomes a memory.
Based on a computational model and a series of memory experiments with humans that tested identification, sequences, and word associations, Marc Howard of Boston University and his team have found that memories change in a predictable way over time. And this supposition seems to hold regardless of the memory system involved – what they call “scale invariance.” That means that, over time, the details about when and how long ago an event occurred that are held in a memory fade on a predictable timescale.
Lila Davachi of New York University centered her talk on the involvement of the hippocampus in forming memories. In a series of experiments, Davachi and her colleagues flashed two different sets of pictures. The pictures in the first set were largely similar to each other, while markedly different images were sprinkled throughout the second set. By measuring the activity in the brain during the tests, they discovered a spike in hippocampal involvement in the first set compared to the second. They interpreted this jolt of activity to mean that the hippocampus is more important in differentiating closely related patterns and not as much in more distantly related ones.