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enter the body before they can be consciously identified. With every inhalation, molecules travel through the thin craggy pathways that begin at the nostrils and head toward the brain. They speed past the olfactory cleft, a narrow opening toward the top of the nose. They hit the olfactory receptors, which are housed on the hairlike tips of the millions of neurons that peek through a gold-hued mucous membrane called the olfactory epithelium.
    Every human has around 350 different types of these receptors, which are unique proteins on both the left and the right side of the upper nostrils. These receptors are the gateway to the complex dance of perception. They connect to the smell molecules upon arrival and then transfer signals toward the brain by chemical impulse. Every human has between six and eight million neurons in the nose to do just that. These signals are fired rapidly, by many neurons at a time, forming a pattern not unlike a line of musical notes, or the HTML coding of a webpage. When combined, the brain interprets the signals as a smell, an “odor image.”
    These patterns are both complicated and minute. Scientists have found that if the chemical structure of two smells are identical except for just one carbon atom, the patterns sent in response are nonetheless distinguishably altered. Nonanoic acid, for example, is a nine-carbon chain that yields the salty smell of cheese. Decanoic acid, with only one carbon atom added to its structure, however, smells rancid, like sweat.
    These patterned signals travel on pathways made by neurons, which snake from the nose through a thin sheet of bone called the cribiform plate, and are deposited in the olfactory bulb, which lies toward the bottom of the brain. The bulb takes these patterns, like reading the score of a piano concerto or lyrics to a lullaby, and sends them farther on to the olfactory cortex. The cortex, in turn, relays an interpretation to other parts of the brain like the thalamus, which deals in conscious perception, and the limbic areas, for emotional response.
    Scent molecules had been entering my nose and traveling up to my brain unhampered for twenty-two years before the accident. When I breathed in the scent of chicken stock while working at the Craigie Street Bistrot, those rich poultry particles hit my olfactory receptors and spurred a slew of signals to my brain. I would stop, sniff, and think: I smell chicken stock . I never thought about the process. I never thought about how I could tell the difference between the scent of chicken and veal stock, between lard and butter. It was a movement too complicated, too minuscule, and entirely too invisible for me to notice, let alone to care about.
    But it’s a process that is of intense interest in the scientific community today. How is the chemosensory world represented in the brain?
    Each step in the path of a single smell from the nose to the brain is known. What is not clear, however, is exactly how the body begins with these molecules and ends with the conscious thought I smell chicken stock . The process of recognizing and identifying smell, from the first neuronal firing to the higher workings of the brain, is still quite mysterious, even to the most advanced researchers and experts.
    We know that it begins with just one molecule. Just one molecule of the scent of a rose, a wet dog, an old book on the top shelf at the library enters the nose. It travels up the nostril to the olfactory receptors. And that’s where the first puzzle takes place.
    Theories about how, exactly, the olfactory receptors recognize and connect with odor molecules have been debated for decades. Researchers have found that each of the 350 different types of receptors expressed in the nose can pick up a specific number of odor molecules. Some can recognize a handful, while others are attuned only to one type. Similarly, some odor molecules can react with just one receptor; others, many more. Today, it is widely believed that the