Waves in our brain make smells stick to our memories and inner maps, scientists say.

Norwegian researchers have now discovered the process behind this phenomenon.

The brain connects smells to memories through an associative process where neural networks are linked through synchronised brain waves of 20-40 Hz, researchers said.

"The signals from your nose translate and connect to memories in an orchestrated symphony of signals in your head," researchers said.

Each of these memories connects to a location, pinpointed on your inner map. So when you feel a wave of reminiscence triggered by a fragrance, think about how waves created this connection in the first place, they said.

"We all know that smell is connected to memories," said lead author, Kei Igarashi from the Norwegian University of Science and Technology (NTNU).

Researchers designed a maze for rats, where a rat would see a hole to poke its nose into. When poking into the hole, the rat was presented with one of two alternative smells.

One smell told the rat that food would be found in the left food cup behind the rat. The other smell told it that there was food in the right cup.

The rat would soon learn which smell would lead to a reward where. After three weeks of training, the rats chose correctly on more than 85 per cent of the trials.

In order to see what happened inside the brain during acquisition, 16 electrode pairs were inserted in the hippocampus and in different areas of the entorhinal cortex.

After the associations between smell and place were well established, the researchers could see a pattern of brain wave activity during retrieval.

"Immediately after the rat is exposed to the smell there is a burst in activity of 20 Hz waves in a specific connection between an area in the entorhinal cortex, lateral entorhinal cortex (LEC), and an area in the hippocampus, distal CA1 (dCA1), while a similar strong response was not observed in other connections," Igarashi said.

This coherence of 20 Hz activity in the LEC and dCA1 evolved in parallel with learning, with little coherence between these areas before training started.

By the time the learning period was over, cells were phase locked to the oscillation and a large portion of the cells responded specifically to one or the other of the smell-odour pairs.

The research was published in journal Nature.