Bad memories can be difficult to endure
Low doses of a commonly-used anaesthetic could prevent the formation of painful memories, say researchers.
The University of California scientists found that sevoflurane gas stopped patients remembering "emotive" images, New Scientist magazine reported.
Scans showed it interfered with signals between two key areas of the brain.
It is hoped the work could eventually help eradicate rare instances of anaesthetised patients remembering the full horrors of their surgery.
While anaesthetic drugs are mainly used to make patients fall unconscious before operations, their effects on the body are frequently far more complex.
The Californian researchers, writing in the journal Proceedings of the National Academy of Sciences, were investigating the outcome of much lower doses of the gas than those used prior to surgery.
They treated their volunteers either with the anaesthetic, or a placebo gas, and then showed them a series of photographs.
Some of these had everyday content, such as a cup of coffee, while others had images designed to provoke a far more powerful emotional response, such as a bloody severed human hand.
One week later, the volunteers were asked to recall as many of the images as they could.
Those given the dummy gas remembered approximately 29% of the powerful images, and 12% of the others.
However, those who had received sevoflurane could remember just 5% of the "emotive" images and 10% of the others.
Brain scans revealed that the gas appeared to interfere with impulses between the amygdala and hippocampus, areas of the brain known for their involvement in the processing of emotion and memory.
"This study reports the discovery of an agent and method for blocking human emotional memory," the researchers wrote.
They added that understanding how drugs could stop this happening might provide clues to "intraoperative awareness" - rare instances in which the memory-disrupting qualities of anaesthetic drugs fail and patients can recall the experience of undergoing surgery.
Awake during operations
While this suggested that the gas could prevent the acquisition of new memories following painful events, it does not point to any effect on pre-existing memories, good or bad.
Dr Anthony Absalom, from Cambridge University, said that other anaesthetic drugs had been found to interfere with memory formation.
"If a patient is having an uncomfortable or distressing procedure but not a general anaesthetic, sedative drugs not only make them more relaxed, but help them not to remember it afterwards.
"The same is true in intensive care settings, where patients can spend long periods with tubes into their lungs."
He said that it was unlikely that anaesthetic drugs could interfere with memories that had already been formed.
However, he agreed that it could improve understanding of what happens when patients claim to remember operations even though they have been fully asleep.
"Approximately one in 5,000 patients reports remembering details of operations, and it's a struggle to understand why - but this kind of research might help," he said.