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.
Next, the mice were placed back in the safe chamber. They froze in fear, showing they had a false memory of receiving an electric shock there. In their paper on the results, researchers mentioned "engram-bearing cells", referring to the neural networks that may hold individual memories. The research builds on evidence that engrams are physically real, and though we are far from being able to use them to split up business empires there are some parallels to Inception.
"If you want to grab a specific memory you have to get down into the cell level," Dr Xu Liu, a lead author of the study on mice, said in a BBC report.
"Every time we think we remember something, we could also be making changes to that memory - sometimes we realise, sometimes we don't. Our memory changes every single time it's being 'recorded'. That's why we can incorporate new information into old memories and this is how a false memory can form without us realising it."
Other researchers have shown that there's no need for brain implants and electric shocks to create false memories. Instead, they can be formed in people with little more than stories and photographs.
Professor Elizabeth Loftus, of the University of California, US, is among the leading researchers on false memory, and was recently honoured by the American Psychological Foundation.
Loftus started by examining eyewitness testimony and found that the nature of questioning affected the "facts" that people recalled. Later, prompted by a case involving a woman who apparently remembered repressed memories of her father raping and murdering a childhood friend, she began investigating whether people could "remember" entire events that never happened.
In one experiment, Loftus asked 24 students about four childhood experiences that apparently occurred with a loved one. Three were real and one was false - involving them being lost on a shopping trip. Seven of the 24 students remembered being lost, with some even supplying extra details.
Photographs can also play a role in inducing false memories. Researchers at Victoria University of Wellington, New Zealand, showed individuals a doctored photo in which they appeared to be taking a hot-air balloon ride during childhood. Asked to remember details of the trip, 50 per cent of participants developed at least partial memories. A later experiment found that stories of the trip were more effective than photos in creating false memories.
There is also evidence that people are more likely to remember a newspaper story if it is accompanied by a photo.
Sometimes, we might wish to forget things. And evidence is emerging that this might be more readily accomplished than anyone expected. In the late 1990s, Karim Nader, now a professor in the Department of Psychology at McGill University, Canada, found that rats given a drug that suppressed protein synthesis - the creation of proteins by cells - before recalling a scary memory did not re-store the memory. This has led to tests with people recalling horrific incidents whilst being given medication to reduce emotions, and then finding the memories less stressful. Though some experts, including his supervisor, thought Nader's early experiment unlikely to succeed, he won a Nobel Prize for his research into memory. "The fact that memory turns out to be far from permanent is a positive thing for human survival," he told CBC News.
Earlier this year a study found people formed fuzzy long-term memories of a video, if within hours they were given misinformation.
But mostly, of course, we want to boost memory, not impede it. And here, a study reported this month might gladden your heart, for there's no need to take strange medicines. Instead, for better memory, just drink two cups of cocoa a day.
Martin Williams is a Hong Kong-based writer specialising in conservation and the environment, with a PhD in physical chemistry from Cambridge University.