Toronto: It may soon be possible to erase bad memories from the human brain.

Canadian scientists at the University of Toronto and the local Hospital for Sick Children have found a link between a given memory and specific neurons - the cells in the brain that transmit information - that store it.

The human brain has over 100 billion neurons, but memories are stored in only small number of them. Scientists have been trying to identify these precise neurons that encode a given memory.

Now in their experimental study on mice (which has 100 million neurons), the Toronto research team has succeeded in identifying precise neurons that carry a particular memory.

Unlike in the past when scientists had deleted an entire brain region in mice to try and erase a memory in the hopes of finding out about how memories are normally stored, the Toronto team has succeeded in removing only the small portion of neurons that stored a specific memory.

"Though previous studies have provided important evidence suggesting that specific neurons are involved in a memory, we believe this (study) paper is the first to establish causal links," a university statement quoted study leader and physiology professor Sheena Josselyn as saying.

In their previous experiments, the same research team had found evidence that in mice, fear memories are stored in specific neurons within a brain structure known as the lateral amygdala (LA) that have a high amount of a specific protein (CREB).

This means that CREB levels helps dictate which neurons are involved in storing a memory.

Now in their latest study, the research team destroyed only these LA neurons with high levels of CREB and found that mice no longer remembered the fearful event. The research team also showed that random removal of any LA neurons does not erase the fear memory. You have to remove only specific set of neurons that store a memory.

"Our experiences, both good and bad, teach us things," said study leader Josselyn.

"If we did not remember that the last time we touched a hot stove we got burned, we would be more likely to do it again. So in this sense, even memories of bad or frightening experiences are useful.

"However, there are some cases in which fearful memories become maladaptive, such as with post-traumatic stress disorder or severe phobia. Selectively erasing these intrusive memories may improve the lives of afflicted individuals," she said.

"Our studies suggest that one strategy would be to target interventions to that small subset of neurons actually involved in storing a memory, rather than the entire brain. It sounds like a futuristic film, but our results in mice do provide proof-of-principle that this may one day be possible in humans," said co-researcher Paul Frankland.

The study is published in the March 13 issue of the journal Science. IANS