Can you really smell memories? How childhood scents get 'etched' onto the brain
By David Derbyshire for MailOnline
Updated: 08:08 GMT, 6 November 2009
Scientists have shown that 'odour memories' get 'etched' onto the brain
From the sudden whiff of school cabbage to the pungent smell of hospital disinfectant, nothing transports people back to their childhood more than an unexpected smell.
Now scientists think they have discovered how scents from the past make such a lasting impression.
Using brain scans, they have shown that new 'odour memories' - such as the association of a perfume with a person - really do get 'etched' onto the brain.
The 'signature' of the memory is different from other types of memories, they found.
Dr Yaara Yeshurun, who led the study at the Weizmann Institute of Science in Israel said early smells had a 'privileged' status in our memories.
Scientists have long known that smells are one of the best ways to evoke the past.
Past studies have shown that memories triggered by smells are more vivid and more emotional than those triggered by sounds, pictures or words.
The new study, reported in the journal Current Biology, tried to mimic the creation of childhood memories of smells in 16 adult volunteers.
In a laboratory, the volunteers were shown a picture of an object as they were exposed to a whiff of either a pear or fungus.
Ninety minutes later they were shown the same picture with the other smell.
A week later, the scientists tested which of the associations was remembered more strongly by exposing the volunteers to the same smells.
All the tests were carried out while the volunteers were inside a functional Magnetic Resonance Imaging (fMRI) scanner which monitored brain activity.
Overall, the volunteers found it easier to remember unpleasant smells rather than pleasant ones.
But the MRA scans also showed that part of the hippocampus region of the brain 'lit up' in a peculiar way when the volunteers were exposed to the first smell they had been exposed to the week before.
But their brains did not respond in the same way when the volunteers sniffed the second smell.
The experiment was repeated using sounds rather than smells to see if they had the same impact on memory.
'We found that the first pairing or association between an object and a small had a distinct signature in the brain,' said Dr Yaara.
'This "etching" of initial odour memories in the brain was equal for good and bad smells, yet was unique to odour.'
The researchers also found that they could predict what a person what remember later based on the activity in their brains on the first day.
Dr Yeshurun said it makes good evolutionary sense for people to remember unpleasant memories.
But the findings show that there is something 'particularly special' about early memories of smells, he added.
Smells may trigger such strong memories because our ancestors were more dependent on their noses to avoid poisonous plants, rotten food or enemies than modern people.
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The toy cupboard at my grandmother's house had a particular smell. I cannot tell you what it was, but sometimes now, as an adult, I will catch a whiff of it. The smell brings with it memories I thought were lost, memories of visits to my grandparents' house, of my grandmother, and of playing with the toys from the toy cupboard. But why do smells have this power to unlock forgotten memories?
Neuroscience is a lot like a detective story – we have to look for clues to reveal the cause. But before we examine the clues, what background information do we have about the case?
What we know is that smell is the oldest sense, having its origins in the rudimentary senses for chemicals in air and water – senses that even bacteria have. Before sight or hearing, before even touch, creatures evolved to respond to chemicals around them.
Sight relies on four kinds of light sensors in the human eye, cells known as receptors, which convert light into the electrochemical language of our brain, and touch relies on different receptor types for pressure (at least four of these), for heat, for cold and for pain, but this pales into comparison for what is required for detecting smell. There are at least 1,000 different smell receptor types, which regenerate throughout your lifetime, and change according to what you are used to smelling. The result of this complexity is that we are able discriminate many, many different kinds of smells.
We do not, however, have names for all the smells we can differentiate. Smell is perhaps the sense we are least used to talking about. We are good at describing how things look, or telling how things sounded, but with smells we are reduced to labelling them according to things they are associated with ("smells like summer meadows" or "smells like wet dog", for instance). An example of this “hard-to-talk-about-ness” is that while we have names for colours which mean nothing but the colour, such as “red”, we generally only have names for smells which mean the thing that produces that smell, such as “cedar”, “coconut” or “fresh bread”.
So now we have the background information, what are the important clues? Well, first, the part of the brain that is responsible for processing smells – the “olfactory bulb” – is next to a part of the brain called the hippocampus. This name means “seahorse”, and the hippocampus is so-called because it is curled up like a seahorse, nested deep within the brain, a convergence point for information arriving from all over the rest of the cortex. Neuroscientists have identified the hippocampus as crucial for creating new memories for events. People with damage to the hippocampus have trouble remembering what has happened to them.
Although they can learn new skills, like riding a bike, and new facts, like what someone is called, they do not create memories of doing these things or having the experiences. This “episodic memory” is precisely the kind of memory I have when I recall visits to my grandmother. And the olfactory bulb, seat of smell in the brain, is conveniently placed just next to the hippocampus, the primary brain nucleus for these memories.
Now, admittedly, this evidence is powerful, but circumstantial. We have the suspect (smell) placed at the scene of the crime (next to the hippocampus). But we are going to need more than circumstantial evidence if the case is going to stand up the scientific court. I hope my next piece of the evidence, a second clue from neuroscience, will convince you as to why smells are so powerful in unlocking memories.
Smell is unique among the senses in that it enters directly deep into the brain. If we look at the major pathways travelled by the other senses, such as hearing and vision, they start at the sense organs – that is, the eyes or the ears – and move to a relay station called the thalamus, before passing on to the rest of the brain.
With smell the situation is different. Rather than visiting the thalamic relay station on its journey into the brain, smell information travels directly to the major site of processing – the olfactory bulb – with nothing in between. We do not know what stopping off at the thalamus does for the other senses, but it certainly means that signals generated in the other senses are somehow “further away” from the nexus of processing done in the brain.
Could this be part of the reason why smells are both hard to put into words, but also able to trigger deeply hidden memories? Memory research has shown that describing things in words can aid memory, but it also reduces the emotion we feel about the subject. When we come up with a story about our memories, we start remembering the story as much as the raw experience.
So with my grandmother’s toy cupboard, that particular, unique, smell was picked up by the complex smell receptors in my young nose. The smell experience of the cupboard, which I have never found a name for, travelled directly into my brain, lodging next to the part specialised for encoding experiences. There it got entangled with the other memories of the cupboard, untouched by language, difficult to think about on purpose, but still lodged in my memory. Now, years later, the smell is not only enough to relive that experience but it is also enough to pull out the rest of the memories along with it.
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