Why can smells unlock forgotten memories?

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?

 
Complex sense

 
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”. [continue]