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Briefly, what we smell are light organic molecules, tiny clusters of atoms bonded together. They are all organic compounds, compounds usually built around carbon, the ‘Lego’ of life: hydrogen, oxygen, nitrogen, sulphur and, to a lesser extent, phosphorous and bromine. The clusters of these atoms are tiny (only a small molecular structure can ‘float’ in a gaseous way) but highly complex. We breathe in and the contents of the air washes over two postage stamp sized patches of receptors at the top of the nose connected to several slender waxy rods protruding through the through holes in the skull. This physical chemistry is processed in the limbic lobe, one of the oldest parts of the brain and the seat of sexual and emotional impulses, which seems to govern much behaviour and motivation and memory. The patterns of molecules are decoded by the sensory neurones here and electrical impulses are sent to the ‘nose-brain’ of the cerebral cortex to be re-coded, turning patterns into sensory experience, rather as pixels make a picture on our screens. The patterns are myriad and the differences minute – operating on an atomic level, the position of single electron can totally alter the smell of something.

It is memory that everyone mentions when they talk about smell. Research on smell memory has now confirmed the everyday impression that these memories are laid down by experience right from the beginning, like a reference library, and that there are what are called ‘memory bumps’ – like bumper crops – in childhood and adolescence which remain stable over very long periods; the curve for forgetting them seemingly different from memories laid down by other senses. They lie in the limbic system which has nothing to do with conscious thought, and the sense of smell has the power to suppress the rational, critical left brain and to stimulate the creative, dreamy right brain. When we hallucinate, we temporarily lose the balance of sensory suppression between the two but interestingly we rarely dream about smell. In trials just over 20 per cent of women recorded smelling their dreams and this pleasure was accorded to only 2 per cent of men. When we sleep, we seem to dampen down all the senses. The necessity for smoke alarms is based on research that proved that 80 per cent of people will not smell the smoke while they are asleep; they will wake up only once they cannot breathe. The exaggerated perception of smell, hyperosmia, is very rare, and very disturbing. Dr Oliver Sacks related a case study where a patient found for some few days that his smell perception became inexplicably uninhibited; the raised intensity and dominance of his smell perception created turmoil, his understanding of the world was seriously compromised, and he found it almost impossible to reflect or reason in the abstract. Sacks concludes that some inhibitor, designed to keep the perception of smell in its place, must be part of the olfactory system. This profound, learnt, recall makes sense when one considers that evaluation of smell is one of the most important evaluations an organism can make. From the cognitive point of view the hedonic information, the perception of pleasantness and unpleasantness, is the most salient part of olfaction. Two perceptual pathways are used through the brain, one for processing averse responses and one for appetitive, and our responses are infinitesimally fast. The question of survival is at the root of everything.

Of all our five senses the sense of smell is the first to develop. Even before we are born, in utero, our sense of smell is already fully formed and functioning. At birth we appear to have almost no inborn preferences as regards smell, apart from an immediate draw to breast milk. The world of the newborn is like swimming in a multisensory soup; it is a synesthetic world distinguishing much less clearly

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