It smells like Christmas in the countries that celebrate: In Germany, for example, scents of fresh pine, sugar cookies, mulled wine waft through the air. And they are likely to trigger emotions or memories in many people's minds.

"When you smell [this familiar] scent again, you're back in the memory — in grandma's kitchen baking Christmas cookies," said Olaf Conrad, a doctor of otorhinolaryngology at Erlangen University Hospital. 

Otorhinolaryngology is a branch of medicine that specializes in the head and neck, often called ear, nose and throat medicine.

Scents don't only trigger Christmas memories, of course. My grandmother used to come by once a week when I was growing up to make pancakes for me after school. She has been dead for years, but whenever I smell pancakes, I immediately think of her. And the memory is stronger than what I feel when looking at old photos. I have often wondered why that's the case.

Scents — a powerful catalyst for memories

Smells are processed in the oldest part of our brain, the paleocortex. Olfactory processes happen close to the amygdala, which is in charge of our emotional responses, and the hippocampus, which is crucial for forming memories, Conrad explained. No wonder, then, that scents and memories are so closely connected!

"Tests have shown that our sense of smell is the strongest sensory impression when it comes to triggering memories," Conrad told DW. "And that's true for negative memories as well. People returning from war or children who've grown up in a warzone are triggered by the smell of gunpowder, for example."

The reason why the connection between scents and formative memories is so intense is that important, stand-out memories are "saved in the brain without a time-stamp," Conrad said.

In other words: When a smell connected to a formative memory hits us, our brain doesn't supply the crucial information that what we're experiencing is something from the past. That goes for traumatizing experiences as well as Christmas traditions or memories of a loved one.

A highly sensitive sense of smell can be painful

Most people will experience the connection between a smell and a memory or an emotion at some point. But a Highly Sensitive Person (HSP) experiences sensory stimuli on a much greater level than the average person.

For many Highly Sensitive Persons, that means they smell things people without the condition don't even register — or that smells are much more intense to them. Scents can make HSPs feel physically ill, throw them into emotional turmoil, or prevent them from concentrating.

Daphne, a young entrepreneur from Germany, is an HSP. For her, the scent of the perfume her mother used to wear is a strong emotional trigger.

"I have this problem with Chanel No. 5," Daphne told DW. "When I smell it, I am thrown right back into the pain I felt with the [death of] my mom."

She said the difference between HSP and people with a regular level of sensory sensitivity is that once the scent throws her back into the memory of her mother's passing, she would stay in the memory for long stretches of time, unless she didn't "make an active effort to bring myself out of it."

"The world fades in a matter of seconds, and you feel the exact same thing you did back then," Daphne said.

How smells are used in medicine

The strong link between smells and emotions can be put to good use.

"Professionals in palliative care have recognized how much calm aromatherapy can provide," Conrad said.

While smells can't help with the treatment of Parkinson's disease, the sense is an indicator of the neurodegenerative illness. Parkinson's makes patients lose their sense of smell.

"There's no one in the early stages of Parkinson's who can still smell well," said Conrad.

As for the connection between smells and Christmas memories, Conrad told DW about his personal experience with it.

"The big moment in our house was when we put up the tree, that was always too big for the small room it was in," he said. "The smell of fresh pine always put me in a Christmas mood."

Edited by: Zulfikar Abbany