Nearly everyone has experienced a moment when a faint fragrance brings a memory of a long-lost moment in time crashing back to the forefront of their minds.
Often we will have forgotten about the event completely, yet it transpires our unfathomable minds have filed it neatly in some unreachable corner of the brain, primed for instant retrieval.
It may be the perfume worn by a long-forgotten friend, the stench of petrol from a youth spent worshipping motorcycles or the haze of chlorine from summer months lazing by the pool.
It is amazing that a few simple airborne molecules can trigger such vivid recollections.
Dr Alan Hirsch is a US neurologist who specialises in the treatment of people who lose their sense of smell or taste.
But at the Smell and Taste Treatment and Research Foundation, which Hirsch runs, he is carrying out research into immediate recall of childhood memories by a particular odour, a phenomenon he refers to as olfactory-evoked recall.
Hirsch believes that the details evoked by nostalgic smells are not as important as the emotions they recall. But our minds reshape these memories, sending them through a rose-tinted filter that redefines them as "good times".
Experiences that may have seemed bad at the time can be reconstructed in our minds to seem better than they were, because they represent periods in our life that are now gone forever.
Childhood memories, for example, represent times when we were free from the responsibilities and anxieties of adulthood, so we may redefine them in an idealised way, even though many of the experiences we went through were difficult at the time.
In order to study the different odours that evoked nostalgia amongst the public, Hirsch and his staff canvassed around 1,000 people on the streets of Chicago and asked them which smells stimulated a childhood memory.
The results were interesting. Baked foods such as cakes and baking bread made up the largest category of nostalgic smells. Other cooking smells such as bacon, meatballs and spaghetti were the second largest category of reported smells.
But people born before 1930 tend to recall the odours associated with nature more than people born in later decades. This may reflect increasing urbanisation after the 30s.
What is not in doubt is that smell is a powerful sense. The olfactory system, the apparatus responsible for our sense of smell, has a pathway in the brain closely associated with the limbic system. The limbic system contains the amygdala and the hippocampus parts of the brain which are closely associated with emotion and memory respectively.
Human beings tend to emphasise vision over all other senses, but our sense of smell is important enough to evoke its own form of d�j� vu. Perhaps the foul and strange smells we experience today will be associated with fond memories in years to come.