Academics at Northumbria University have made the discovery

Lemon balm, a shrub that resembles a small nettle, improved "secondary memory" in tested volunteers according to two academics at Northumbria University.

In the laboratory it also had a biochemical effect known to be important to memory.

Dr Andrew Scholey and Dr David Kennedy, based in Newcastle, made capsules of dried lemon balm and gave them to a group of 20 volunteers.

Their memory was then tested one, three, and six hours later.

The findings, presented on Thursday at the British Psychological Society's annual conference in Bournemouth, showed no effect on working memory - the constant recall of continual events during the day.

'Ability to learn'

Dr Scholey said: "It did affect secondary memory, which is where a memory is stored, lost from consciousness, and then retrieved again.

"The ability to learn, store and retrieve information was improved."

Lemon balm improved the mood of the volunteers by enhancing calmness.

In the laboratory, lemon balm increased the activity of acetylcholene, an important chemical messenger linked to memory that is reduced in people with Alzheimer's disease.

Dr Scholey said it was too early to say whether lemon balm might help Alzheimer's sufferers, but said: "It may help conditions where the memory is fragile."

He said in the 16th century the herbalist John Gerard gave lemon balm to students to "quicken the senses".