Scientists say they have discovered how a strong smell or a song can sometimes trigger a vivid memory.

Researchers in the US say they have pinpointed the precise region of the brain that sparks such recollections.

They say the discovery may explain why what appears to be a simple song or a random perfume can lead us to remember evenings with friends or holidays abroad, for instance.

The CA3 region of the hippocampus is essential for the phenomenon called 'pattern completion'

Professor Dan Johnston, Baylor College

According to scientists at Baylor College of Medicine in Houston, Texas, the CA3 region of the hippocampus is key to this process.

It has long been known that this part of the brain plays a key role in long term memory development.

It is essential for initial storage of memories before they are transferred for storage elsewhere. As a result, a person whose hippocampus is injured cannot form new memories.

Lab tests

The US scientists made their discovery by carrying out tests on mice, according to an article in the journal Science.

They genetically engineered a strain of mice in which a protein called the NMDA receptor was absent only in the CA3 region of the hippocampus. This receptor plays a key role in learning and memory.

The scientists trained the mice to perform a series of tasks within a specially constructed maze. They performed as well as those mice which had not been genetically engineered.

However, when key cues or prompts were removed from the maze, the mice were lost. They were unable to remember what they were supposed to do next. The mice that had not been genetically engineered did fine.

The scientists said this showed that the lack of a NMDA receptor in this region of the hippocampus prevented them from recreating memories.

Further research

That theory was backed up by tests on the brain activity of the mice which highlighted the role of the NHDA receptor in recollecting.

Dan Johnston, professor of neuroscience at Baylor College, said: "It appears that the CA3 region of the hippocampus is essential for the phenomenon called 'pattern completion' - that is the ability to recall memories from partial representations of the original."

The scientists are now working on plans to test their findings further by examining out how exactly the whole process works.

Professor Johnston said: "The next step is to determine how the neurons of the hippocampal CA3 region process the information to recreate a memory from only a portion of the original."

Scientists from the Massachusetts Institute of Technology and Hokkaido University School of Medicine in Sapporo in Japan were also involved in the study.