The horrors of war, the torment of childhood trauma, the distress of being victimized by violent crime — it may be possible to erase these recollections from our brains.

Scientists have taken another step toward being able to edit out bad memories while leaving good ones intact — something that could one day be used to treat people suffering from post-traumatic stress disorder (PTSD) or anxiety disorders linked to painful memories.

You might be able to erase different aspects of the memory. — Wayne Sossin , Montreal Neurological Institute

Previous research has found that memories are not as stable as once thought. One chemical process recalls a memory, and a different one stores it, but that process can be blocked, and the memory lost.

"Depending on how you remind the person, you might be able to erase different aspects of the memory," said Wayne Sossin of the Montreal Neurological Institute and Hospital at McGill University, whose lab collaborated with researchers at the Columbia University Medical Center in New York.

It didn't work out so well in Eternal Sunshine of the Spotless Mind, a movie in which two aggrieved lovers undergo a procedure to wipe one another from memory, but traces remain and they find each other again.

The new research doesn't go as far as picking specific memories — or people — to erase, but it shows that different types of memories can be discerned and targeted, even when they are stored within the same neuron.

"One might have thought there was a unified process where memories were maintained — that you couldn't erase one without erasing them all," Sossin told CBC News.

'Could be promising'

Sossin and his colleagues set out to reverse the storage of two kinds of memories:

associative — a link learned from experience, such as being mugged in a dark alley.

non-associative — links arising from conditions peripherally related to the experience, such as being in a city at nighttime, where one can expect there to be dark alleys.

They speculated that both the strength and the retention of different types of memories were governed by variants of an enzyme called PKM.

The researchers induced, with electrical stimulation, both types of memories in a mollusc and found they could erase memories separately by targeting the PKM variants. Their paper was published June 22 in the journal Current Biology.

"We do it by interfering with different molecules," Sossin said. "We identified molecular targets that were specific for associative and non-associative memory."

The two types of memory can be distinguished on a molecular level; though it would be much more difficult to pinpoint an exact recollection — say, running through a meadow versus running through a war zone.

But if it can be done, one mental health expert sees potential for this technique.

"Where I think it could be promising is in these instances of more acute trauma — if someone was involved in a robbery or other violent crime of some kind, for example," said Mark Henick, national director of strategic initiatives at the Canadian Mental Health Association.

He said there is evidence that intervening in the immediate aftermath of a traumatic event can reduce the likelihood of developing PTSD later, "if you can effect the memory before it becomes ingrained."

U.S. soldiers take part in a suicide prevention class in Fort Riley, Kan., in 2009. (Chris Hondros/Getty Images)

However, Henick said, psychological trauma is, in many cases, not the result of one isolated event, as in the case of a child who's abused for years.

"How do you erase a lifetime? A relationship? How do you erase someone's father from their lives?" Henick said. "Or you look at intergenerational trauma in Aboriginal communities — how do you erase someone's ancestry? We know that stress and trauma can be passed biologically."

And trauma is often more complex, involving "overgeneralizations" beyond the event itself. Anxiety rooted in the memory of a gunshot can be triggered by the sound of fireworks, or a car backfiring, for example.

A motor neuron is injected with a molecule that blocks the activity of the PKM enzyme, which is linked to the strength and retention of memories. (Schacher Lab/Columbia University Medical Center)

Negative memories stronger

That's where the ability to target associative and non-associative memories could be useful.

"In general, negative memories are stronger than positive, and they are more apt to bring along additional unrelated [non-associative] memories that wouldn't normally get associated with the event or the fear," Sossin said.

But one doctor who specializes in treating people with PTSD worries about what her patients might be losing besides a memory.

"A lot of people can experience post-trauma growth," said Dr. Vivien Lee, a clinical psychologist at the Centre for Addiction and Mental Health in Toronto. "They work through it and grow as a person, they become a better version of themselves. Adversity is what makes us grow as individuals."

She also foresees the potential for a slippery slope.

"My concern would be that people would start doing that for any kind of negative [memory]," she said.

Sossin also said he would have some reservations about removing memories rather than letting them evolve, as they inherently would.

"New experiences changes your view of something," he said. "You want memories to be able to be updated."

Besides, he says, science is "a long way" from using electrical manipulations this way on people. For now, it's only in movies.