The paper
L. Reddy et al., “Human hippocampal neurons track moments in a sequence of events,” J Neurosci, doi:10.1523/jneurosci.3157-20.2021, 2021.

Neurons in the hippocampus of many species are known for encoding the “who, what, and where” of memories. Researchers had previously learned that some hippocampal neurons in mice also index memories sequentially, capturing the “when” as well. But whether this was the case in humans remained unknown until Leila Reddy had the opportunity to monitor neural activity in the hippocampus of epilepsy patients who already had electrodes implanted in this deep region of the brain. 

Reddy, a neurobiologist at the National Center for Scientific Research’s Brain and Cognition Research Center in France, and her colleagues asked those patients to memorize a sequence of pictures before replaying and randomly pausing the sequence and asking them which image would come next. Hippocampal neurons fired when participants saw each image and continued firing during 10-second gaps between sequences, the team found, suggesting the cells might be encoding more than just content. While 70 percent of the active neurons likely stored information on what was shown, the other 30 percent—which neuroscientists call “time cells”—sorted the images sequentially, she says. Subjects were able to correctly recall the next image in the sequence with greater than 90 percent accuracy. 

Reddy says it’s possible that time cells could record other types of information too. While the bulk of hippocampal neurons are focused on storing content, time cells leave “real estate available for other things to be encoded,” she says.

See “How Time is Encoded in Memories

Sandra Jurado, a neuroscientist at the Instituto de Neurociencias in Spain who did not participate in the research, says it was remarkable that this study used human subjects, as it’s rare to have access to the deep brain this way. The hippocampus, she adds, is a hub that receives many types of sensory input, so the fact that it might also track temporal information makes “perfect sense.”

Clarification (October 7): A former version of this story implied that the discovery of time coding in mice took place in 2020, but scientists have been studying the phenomenon since at least 2007. The piece also suggested that the high degree of recall accuracy was caused by the time cells, but the study did not explicitly test such a link, although other studies have. The Scientist regrets these errors.