The Nature of Smell

Lyall Watson. Jacobson's Organ and the Remarkable Nature of Smell. New York: Norton, 2000. Pp. xv + 255. $24.95.

Lyall Watson's book, Jacobson's Organ and the Remarkable Nature of Smell, mixes fact with fiction and literature with science in an endearing way. The very first sentence, "Smell is a forgotten sense," awakens the reader's curiosity, while the author's subsequent reminder that "the all-pervading influence of odor on our lives and memories" provokes our appetite for further words of wisdom.These words are easy to read, and the author's style is imaginatively creative and expresses the kind of sentiments about olfaction to which I adhere, namely that we tend to underestimate our olfactory capacity and under-appreciate its sensory value.This is hardly surprising, given that there are few agreed descriptors for odors, and most of those in use are borrowed from other sensory systems. Even the expert perfumers talk of "base tones" and "top notes," and the great classifier of all things natural, Linnaeus, used associative animal, floral, and vegetable terms, together with terms taken from human sickness like "nauseous" and "foul." Unlike other senses, olfaction does not have a dedicated language.

Why, we might ask, has olfaction received such low sensory regard in human culture? As Watson points out, there are huge differences in odor evaluation across cultures, with few of the absolute qualities that apply to other senses. We are told of studies where Germans rated certain Japanese smells as being most unpleasant, while the smells Japanese disliked most were all German. Even within a culture there are different likes and dislikes, and a range of olfactory sensitivities that span a thousandfold differences. One important correlate of living socially has been the need for our senses to concur through language on how the real world is represented, and the human olfactory sense falls short on this account. With achievement of the upright bipedal posture and the evolution of language, olfaction languished in human heritage.

The ancient olfactory nervous system dominated the brain of our nocturnal mammalian ancestors, linking primarily with the trilaminar paleocortex (ancient cortex).The human olfactory nervous system is, relative to the rest of the brain, so small it rarely gets a mention in most anatomical textbooks, while complete books are dedicated to vision, hearing, and tactile senses, all of which have their primary cortical links with the six-layer neocortex. Hence, it is easier for humans to cross-reference among these primary senses, and most synesthesias are in the domains of color, touch, and sound. Moreover, it is difficult for us to imagine smells as readily as we do auditory or visual images. We can all see an orange in our mind's eye, and recreate a popular tune or Beethoven symphony in the silence of our imagination and without any auditory prompt from music. Olfaction is different. We cannot easily imagine the smell of an orange, although we can recognize an orange smell.

This type of recognition memory is, and has been, precisely what is required of the sense of olfaction in most mammals: recognition of personal territory, of intruders, of mates, of what is good to eat, and of kin and offspring. Most of these complex biological odors have more components in common than there are differences. Focusing on the differences is achieved more readily with a vast repertoire of receptor types, while following a concentration gradient (trail) or determining the age of similar odors requires exquisite sensitivity. But human olfaction falls short on both these accounts.The human genome project has revealed that 76 percent of the 750 main olfactory receptor genes are nonfunctional or pseudogenes (Mombaerts 1999). Moreover, the sensitivity and definition that is required for olfactory recognition memories in non-primate mammals does not require an imagination--indeed, the kind of misinformation that imagination potentially contains could subterfuge the real olfactory world in which most animals live. So rich is the human imagination...