How the Brain Creates a Timeline of the Past | Quanta Magazine

Marc Howard, a cognitive neuroscientist now at Boston University, and Karthik Shankar, who was then one of his postdoctoral students, wanted to figure out a mathematical model of time processing: a neurologically computable function for representing the past, like a mental canvas onto which the brain could paint memories and perceptions. The brain can then map the intermediate representation back into other activity for a temporal experience — an inverse Laplace transform — to reconstruct a compressed record of what happened when.Get Quanta Magazine delivered to your inboxThe cognitive neuroscientists Marc Howard (at left) and Karthik Shankar, now at Boston University, have devoted the better part of the past decade to developing a general mathematical framework for how the brain builds a temporal context for episodic memories.Cydney Scott for Boston University Photography (Howard); Courtesy of Karthik ShankcarJust a few months after Howard and Shankar started to flesh out their theory, other scientists independently uncovered neurons, dubbed “time cells,” that were “as close as we can possibly get to having that explicit record of the past,” Howard said. “I thought, oh my god, this stuff on the blackboard, this could be the real thing,” Howard said.“It was then I knew the brain was going to cooperate,” he added.Invigorated by empirical support for their theory, he and his colleagues have been working on a broader framework, which they hope to use to unify the brain’s wildly different types of memory, and more: If their equations are implemented by neurons, they could be used to describe not just the encoding of time but also a slew of other properties —  even thought itself.But that’s a big if. Hundreds of neurons seemed to be working together to keep track of the order of the trials, and the length of each one.“You get activity patterns that are not simply bridging delays to hold on to information but are parsing the episodic structure of experiences,” said Matthew Shapiro, a neuroscientist at Albany Medical College in New York who was not involved in the study.The rats seemed to be using these “events” — changes in context — to get a sense of how much time had gone by. “It looked like a Laplace transform of time,” Howard said — the piece of his and Shankar’s model that had been missing from empirical work.“It was sort of weird,” Howard said. Even though the neural mechanisms that allow us to remember an event like our first day of school are different than those that allow us to remember a fact like a phone number or a skill like how to ride a bike, they might rely on this common foundation.The discovery of time cells in those brain regions (“When you go looking for them, you see them everywhere,” according to Howard) seems to support the idea. And that might very well help us make sense of timekeeping as it’s involved in the prediction of events to come (something that itself is based on knowledge obtained from past experiences).Howard has also started to show that the same equations that the brain could use to represent time could also be applied to space, numerosity (our sense of numbers) and decision-making based on collected evidence — really, to any variable that can be put into the language of these equations.