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For the first time, we have found 鈥渢ime cells鈥� in the human brain. These help us remember the sequence and timing of events, and they could be targets for treating memory loss caused by Alzheimer鈥檚 disease.

鈥淵ou鈥檒l remember that you saw Jennifer Aniston yesterday and not a few weeks ago because there鈥檒l be a cell that fired yesterday that didn鈥檛 fire on any other day,鈥� says Daniel Bush at University College London, who wasn鈥檛 involved in the discovery. 鈥淭hat鈥檚 a time cell.鈥�

These cells were first identified in rats based on their unique pattern of activity during the creation of episodic memories that help recall specific events. But they hadn鈥檛 been identified in humans until now.

Bradley Lega at the University of Texas Southwestern Medical Center and his colleagues found the signs of time cells in the brains of 27 people who were undergoing a procedure to remove part of their brain as treatment for their epilepsy.

For ethical reasons, it is difficult to study individual neurons in the brains of living people, says Steven Poulter at Durham University, UK. Recording from pre-surgical epileptic patients offers rare insight, he says.

Tiny electrodes inserted into the participants鈥� brains allowed Lega鈥檚 team to measure electrical activity from individual neurons in their hippocampus while the people completed a memory test. The test required participants to view a series of 12 to 15 words, which each appeared on a screen for 1.6 seconds, and then later recall as many of the words as they could.

Lega鈥檚 team found that the more consistently the time cells fired during memorisation, the more likely the participants were to remember the correct words. More consistent time cell activity across the task also increased the likelihood that the participants would recall the words more closely to the order in which they were presented. This is similar to the findings in rats.

In contrast, there was no association between time cell activity and answers that clustered the words by meaning, instead of recalling their order.

Bush says the activity of time cells is what enables the brain to make connections between particular events 鈥� such as seeing a specific word in a list 鈥� and when those events took place, which is crucial for the formation of memories.

We already knew how the brain encodes where something happened in space, thanks to the discovery of place cells and grid cells, recognised by the 2014 Nobel prize in physiology or medicine. 鈥淲e know that place cells provide the 鈥榳here鈥�, but this is the clearest evidence yet for the 鈥榳hen鈥� in the human hippocampus. It is feasible that these 鈥榳here-when鈥� signals are the scaffold for our episodic memories,鈥� says Poulter.

The time cells that Lega鈥檚 team found are located in the hippocampus, where memories are first formed. This is also one of the first parts of the brain to be affected by Alzheimer鈥檚, so memory cells in this area would be a good target for treatment, says Poulter.

In their study, Lega and his colleagues write that targeting time cells represents an intriguing strategy to treat memory deficits caused by other conditions too, including traumatic brain injury.

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