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@@@ "The theory is that learning involves processing in the cortex, and the hippocampus reproduces this pattern of activity during retrieval, allowing you to re-experience the event," said Brian Wiltgen of UC Davis.

The researchers tested this idea for the first time in mice using the novel technique. The rodents were genetically modified so that when the nerve cells were activated they turned a fluorescent green and were able to be switched off by light.

The mice were put in a cage and given a mild electric shock. Most mice will explore when placed in a new environment, but when put in a cage in which they had previously received a shock the mice will freeze in place in a "fear response." The team found they could isolate the cells involved in making the memory and demonstrate they were reactivated during memory recall. They were then able to switch off nerve cells in the hippocampus, and show the mice lost their memory of the shock. The team found that turning off other cells in the hippocampus did not affect the memory.

"The cortex can't do it alone, it needs input from the hippocampus," said Wiltgen. "This has been a fundamental assumption in our field for a long time and Kazu's data provides the first direct evidence that it is true."

The findings were published in the journal Neuron.

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