Scientists implant false memories into mice - and cast doubt on human memories

Our memories play vital roles in defining who we are. As you recall your childhood, you might smile now at the minor tragedy of dropping an ice cream cone, remember the determination as you started cycling for the first time, and perhaps still feel the fear of being lost in a shopping mall when you were only five - all in contrast to the thrill of a ride in a hot air balloon.

Though it's difficult to calculate the brain's storage capacity, Paul Reber, professor of psychology at Northwestern University, US, estimated it is about 2.5 petabytes, enough to hold sufficient TV shows on a digital video recorder to watch continuously for more than 300 years.

But as you'll already be aware, your brain does not function like a video recorder, and you can't simply "watch" memories continuously. Sensory information - things you see or hear - is stored for fractions of a second, perhaps moving into short-term memory, where items might be retained for half a minute. Then, long-term memories form through encoding information in a process that may be heightened by your emotions: if you're scared by someone pointing a gun at you, you might remember the gun but not his or her face.

Several science fiction tales are based on the mysteries of memory. In the movie Inception a team of corporate spies infiltrate people's dreams to discover information and plant false memories. Last month, scientists reported they had indeed introduced a false memory - albeit in mice.

The animals were genetically engineered, so that when forming memories in cells within the hippocampus they would create a light-sensitive protein. They had also been implanted with optic fibres that could deliver pulses of light to their brains, which could cause this protein to reactivate the memories.

The mice were placed in a safe chamber with distinctive features, where they behaved normally. Then, they were moved to another chamber with different features, where their feet were subject to a mild electric shock. As the shock was applied, light was shone on their brains, to activate the memory of the safe chamber.