With the help of neural mapping, scientists have delved into the understanding of how we differentiate smells.
Although there are striking aromatic differences between coffee, peppermint, and pine, a new mapping of the nose's neural circuitry has revealed a haphazard patchwork of the receptors for such disparate scents.
It was also revealed that the seemingly random arrangement is faithfully preserved across individuals and even species, with the cells that process the same scent located in precisely the same location on the olfactory bulb, the brain's first processing station for odours.
Researchers suspect that the deliberate randomness in rodents' odour maps is likely also found in humans, which have only one-third as many receptors but are capable, in some extreme cases, of discerning tens of thousands of distinct smells.
And it was Harvard University neuroscientists who described the crazy-quilt map of odour-processing neurons on the front lines of the olfactory system.
"It had been thought that the layout of the olfactory bulb was variable from individual to individual, but followed a chemotopic order where cells handling similar odor responses are near each other. Here we show that the layout is actually very precise the same from animal to animal but doesn't appear to follow any chemotopic order whatsoever," Nature quoted Markus Meister, the Jeff C. Tarr Professor of Molecular and Cellular Biology in Harvard's Faculty of Arts and Sciences, as saying.
For the study, Meister and colleague Venkatesh N. Murthy, worked with mice and rats and recorded neural responses to several hundred distinct odours, including anise, beer, cloves, coffee, ginger, lemon, orange, peppermint, pine, rose, and even fox pheromones.
The neuroscientists found that across individuals and even across the two species, bundles of neurons from a given type of odour receptor known as glomeruli were found in almost exactly the same spot on the olfactory bulb, a sensory structure measuring some four to five millimeters across and located at the very front of the brain.
"Glomeruli from different receptors line the surface of the olfactory bulb like an array of close-packed marbles. Across individuals the location of a given glomerulus varies by only one array position. Compared to the size of the map, this represents a remarkable developmental precision of one part in 1,000," said Murthy.
Later, the researchers analysed whether nearby glomeruli detect similar odours, such as those with similar chemical structures.
In earlier studies, neuroscientists have hypothesized axes of similarities along which odours might be classified.
"One might expect that nearby glomeruli should have similar odour sensitivities, but we were surprised to find this was not the case. The odor response spectra of two neighboring glomeruli were as dissimilar as those of distant glomeruli," said Meister.
This seemingly haphazard layout of sensory properties stands in marked contrast to other brain maps, such as those governing vision, touch, and hearing.
Fro the above three cases, our brains represent the outside world using ordered maps such as when neighbouring points in visual space activate neighbouring points on the retina.
"That sort of arrangement makes sense, since most brain computation is local, relying on short connections between nearby cells. This is necessary because the connections between neurons occupy most of the volume available to the brain, and long-distance connections require more of this volume," said Murthy.
The study was published in the recent issue of the journal Nature Neuroscience.