Brain scans show how sights and smells evoke the past.
Marcel Proust reflected that "the smell and taste of things remain poised a long time, ready to remind us... the immense edifice of memory". It's a familiar phenomenon: a single smell or sound has the power to conjure up entire scenes from the past. Now a British-led group of neuroscientists has come up with an explanation.
The key, the researchers claim, is that memories relating to an event are scattered across the brain's sensory centres but marshalled by a region called the hippocampus. If one of the senses is stimulated to evoke a memory, other memories featuring other senses are also triggered.
This explains why a familiar song or the smell of a former lover's perfume has the power to conjure up a detailed picture of past times, says Jay Gottfried of University College London's Department of Imaging Neuroscience, who led a recent study of memory retrieval.
"That's the beauty of our memory system," he says. "Imagine a nice day on the beach. The smell of sun lotion, the friends you were with, the beer you were drinking; any of these could trigger memories of the whole thing."
Gottfried's team made the discovery by presenting subjects with a series of pictures, each paired with an unrelated smell. The subjects were asked to form a mental link between the two: if, for example, a picture of a duck was accompanied by the smell of roses, the volunteers may have imagined a duck walking into a rose garden.
The participants were then shown the pictures again, this time without the smells and interspersed with novel pictures, while the researchers scanned their brain activity. Familiar pictures stimulated both the hippocampus and the piriform cortex, which deals with smell. New pictures had no such effect, the researchers report in the journal Neuron1.
The study shows that a visual stimulus can activate brain regions associated with a previously experienced smell. But does it work the other way around? After all, smell has long been hailed as the 'memory sense', the one most likely to provoke reminiscence.
“Odour memory seems to be the most resistant to forgetting Jay Gottfried , University College London”
The researchers did not address this question directly. "But odour memory seems to be the most resistant to forgetting," says Gottfried. Previous work has shown that memories of images begin to fade days or even hours after viewing, whereas recall of smells remains unimpaired for as much as a year2.
Gottfried suspects that odour-linked memories may persist even after the hippocampus has given up its orchestrating role. Patients with damage to their hippocampus can have amnesia stretching back several years, but still recall smells from their childhood.
But experts still do not really know why our noses have such a hold on our memories. How exactly do our brains make close associations, such that the smell of an aunt?s cake can transport one back to childhood? "If you could answer that you'd be looking at a Nobel Prize," says Gottfried.
Gottfried, J. A., Smith, A. P. R., Rugg, M. D.. & Dolan, R. J. Neuron, 42, 687 - 695, (2003).
Engen, T & Ross, B. M. Journal of Experimental Psychology, 100, 221 - 227, (1973).
University College London