You open a packet of biscuits, releasing a faint sweet, processed aroma you barely recognise and suddenly you are a six-year-old again in your great aunt's house. You haven't thought of her in years, but now the smell reminds you of her faded Persian carpet, her porcelain horses and even the polka dot dress she was wearing as you pinched custard cream biscuits off her best china plate.

The biscuit smell alone has been enough to take you back deep into your memories. Why?

According toDr Adam Osth, lecturer at the School of Psychological Sciences at the University of Melbourne, the reason smells are so effective in evoking memories may be because we aren't good at smelling.

"We have a lot of complex processing in our other senses where we make associations and recognise similarities, which helps us to navigate around the world. But it also means that it is easier to confuse things and that produces interference and forgetting," says Dr Osth, who is in the faculty of Medicine, Dentistry and Health Sciences.

"One possible reason why smell is very powerful at evoking memories may be because we simply aren't very good at it compared to our other senses. When we smell we can only process odours as being very distinctive from each other."

It means that because we don't have the processing power in our brains to notice subtle differences between smells in the way animals like dogs can, smells are more distinctive and are therefore possibly easier to remember.

Smell works

There is no doubt that smell is a potent source of memory. It has been confirmed in laboratory experiments where researchers test subject's response to certain cues. For example, when using words as cues a common response to "girl" is "boy".

"Studies have found that odours are extremely effective cues at promoting memory and they prompt much longer term memories than other types of cues," says Dr Osth.

The bigger question is why? We don't know the reasons for sure, but it may be to do with the way we use associations to understand the information we take in from our senses.

"Everything from your environment that you sense you associate together. So when you have a memory it is because something has reminded you of something else," says Dr Osth.

But while associations are important in helping us to interpret our world, the downside is that as more associations are stored up in our brains, the greater the possibility that different associations will overlap and be mixed up. Scientists call this interference and it is one of the explanations for why we forget.

"One possibility as to why smell is so potent for memories is that we don't have this interference as strongly as we do with our other senses.

"And one reason why that might be so, is because our sense of smell is more primitive with less brain processing power behind it than our other senses, such as sight and hearing that are more highly developed.

"Our other senses are processed in the brain very extensively, which enables a lot of abstraction and a lot of comparisons between senses. But the processing for smell is much simpler, which may explain why there would be less chance of associations getting overlapped when we process smells."

Complexity may cause forgetting

As an example, Dr Osth points out that an enormous amount of processing is needed to interpret what our eyes are telling us. When we turn our heads to look at something the brain is presented with a completely different set of inputs, but our brains are able to process that without losing the broader context of what we were seeing before.

It does this partly by making associations and recognising similarities. It means that when we shift our gaze we don't lose track of where we are or of the things that we are no longer looking at.

"The problem is that this complex processing can harm memory," says Dr Osth.

So is there a way we could better use smell to improve our memories?

Dr Osth says theoretically there is no reason why not. He speculates that we could develop scratch and sniff cards that give off particular smells that we could then associate with things we want to remember. But he warns there is also a major problem to using smell. Our range of recognised smells is just too narrow.

"Smell has a very limited palette to it.

"If you tried to pair smells to concepts you would start to run out of smells pretty quickly," he says.

Even if we were able to improve our range of smell through extensive practice, odours could inevitably become less effective as a source of memories because we would start to confuse the increasingly complex associations we are using smell for.

"So there may be a paradox here where the fact that we aren't very good at smell may be why smell can be so effective in triggering memories."

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