February 18, 2019

Neurons and the Nature of Time

“In June of 2007, Albert Tsao, a nineteen-year-old native of Silver Spring, Maryland, was working in Trondheim, Norway, at the Kavli Institute for Systems Neuroscience. Tsao was a summer intern in the lab of May-Britt and Edvard Moser, married researchers who were well known in neurobiology circles for discovering “grid cells”—neurons that, by tracking our position, create a navigational map in the brain. Grid cells are located in an area of the brain called the medial entorhinal cortex. Tsao was curious about the relatively uncharted region next door—the lateral entorhinal cortex, or L.E.C. After implanting tiny electrodes in the L.E.C.s of some rats, he set them foraging for bits of chocolate cereal in a series of boxes, some black, some white. He recorded the electrical spikes buzzing from individual neurons, hoping to spot a pattern. When no clear signal emerged, he put the data aside.

Time passed. Tsao became a Ph.D. student in the Trondheim lab; in 2014, the Mosers shared half a Nobel Prize. In 2015, just before he graduated, Tsao decided to take another look at his L.E.C. data. Eight years earlier, when he had examined the neurons’ individual behaviors, they had seemed to fire in unstable patterns that made no sense. This time, he conducted a different kind of statistical analysis, focussing on their combined firing patterns. When he looked at that data, he developed a hunch that the neurons were involved in representing the passage of time.

To test this idea, Tsao’s colleagues in the Moser lab conducted more experiments, putting the rats through twelve sessions in the black-or-white boxes. By the time that the new experiments were complete, Tsao had moved back to the United States, and, from there, he ran the statistical analysis again. Surveying the L.E.C’s collective-firing activity in the course of all the foraging expeditions, it seemed as though the brain region had kept track of each excursion separately. Tsao then analyzed another data set, this one from an experiment in which the rats ran a continuous loop around a figure-eight-shaped maze. In that case, the L.E.C. didn’t separate the different runs—the activations were piled on top of each other. Within each individual trial, however, the neurons seemed to mark the animal’s progress at different points along the loop.”

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