Mixed memories (Image: Monkey Business Images/Rex Features)

In the 1940s, neurosurgeon Wilder Penfield found his patients would recall seemingly random information – the smell of cookies for instance – when he stimulated different brain areas with electric shocks. Two studies have now found evidence to support the memory storage theory that Penfield stumbled across. The research, in mice, even demonstrates that it is possible to manipulate brain cells to create false memories.

Mark Mayford of the Scripps Research Institute in San Diego, California, and colleagues genetically engineered mice so that neurons that fired would fire again when the brain was injected with a drug.

The team put the mice individually into a box in which each was exposed to a colour and smell, which encouraged a group of neurons to form a memory of the conditions. Because these neurons fired during memory-making, they could be reactivated when the drug was injected, allowing the researchers to induce an involuntary memory of the box.


Then, the researchers put each mouse into a second box, with different colours and smells. They injected the drug, making the mouse remember the first box, and then gave it a small electric shock.

Normally the shock would encourage the mouse to fear its immediate environment – the second box. Not in this case, though: when the mouse was shocked, it was sitting in the second box while strongly remembering the first. Consequently, it developed a fear of a mixture of both boxes – in effect an imaginary environment. The only time the mouse would panic was when it was in the second box and the drug was injected.

Mayford says this “hybrid” false memory suggests that two different groups of neurons encode each memory and do not interfere with one another.

“It sounds like something my mother would say: if you want to remember something, go to the place you learned it,” says Sheena Josselyn of The Hospital for Sick Children in Toronto, Ontario, Canada. “The fact that you can introduce new bits of information into memory opens up a whole new universe of research.”

Susumu Tonegawa of the Massachusetts Institute of Technology reports a different way to create false sensations. Mice were genetically engineered so that when they formed a memory of a box, the neurons involved became responsive to light.

While the mouse was forming the memory of the box, Tonegawa’s team gave it an electric shock. Then they put it in a different box and delivered pulses of light through optic fibres implanted in the mouse’s brain. This activated the neurons associated with that memory and no matter where the mouse was it froze, terrified of a shock.

When the researchers looked at which neurons had been activated, they found that about 2 per cent of the cells in a specific area of the hippocampus – the brain’s memory centre – were involved. Tonegawa says the method could be used to induce different types of complex behaviour in mice based on their experiences. Next, his lab plans to see how parts of the brain beyond the memory centres connect to make the mouse freeze. This is the “ultimate experiment” for showing that memories are kept in discrete areas of the brain, he says.

Journal reference: Mayford’s study: Science, DOI: 10.1126/science.1214985. Tonegawa’s study: Nature, DOI: 10.1038/nature11028