The Science Of How Smells Work - Harold McGee

The episode opens with a definition, and the definition does most of the work for everything that follows. A smell, McGee says, is a perception generated in the brain but stimulated by molecules in the world — “little bits of the things around us.” This makes smell unlike sight, which trades in reflected light waves, and unlike hearing, which trades in pressure waves in air. Those senses report on energy; smell reports on substance. On his account it is “the most direct contact we have with the world,” because the signal carries information about the particular object it came from.

The mechanism he sketches is chemically literal. Small molecules escape an object, travel through air, are inhaled, and bind to a receptor in the nose. The receptor then reports to the brain that something has arrived from outside. The brain does not stop there: it converts the report into a perception using concurrent information and a stored database of experience, and returns an interpretation of what has been encountered — “not just based on that one thing alone.”

That two-stage structure explains why the same molecule can read differently to different people, and why the interview keeps returning to learning and context rather than to fixed chemistry. It also sets up the strangeness the host probes next: if touch is felt where the body meets an object, and taste is felt on the tongue, where exactly is a smell felt?

The answer is that the brain relocates the sensation. We detect odours in the nose, but we do not experience them there; we ascribe them to the object that produced them. The clearest demonstration is eating. Flavour is dominated by smell — aroma, in his preferred term for food — and the route is retronasal: with food in the mouth, exhaled air passes from the mouth up through the nose and out. That path is what tells us what is in our mouth.

Because the food is in the mouth and the smelling is happening above it, the brain performs a small act of translation and assigns the aroma to the place where the action is. This is why we believe flavour lives on the tongue. It is also why the model has testable consequences: if aroma is delivered by airflow, then breathing technique should change what we taste. McGee agrees immediately, and the variables he names are distance, depth of inhalation and frequency.

“Sniffing is a way of picking something up” — a repeated intake aimed at giving receptors and brain another chance to identify an ambiguous source. The host's dog, working a proffered hand with rapid in-out pulses, is doing the same thing. The interesting question is why short repeated samples beat one long draw, and the answer is adaptation: receptors do not have time to reset during a continuous stream, and the brain downgrades a constant signal to background — “the wallpaper of your experience.”

Alternating inhalation and exhalation turns the signal on and off, which restores contrast and lets attention lock on. McGee adds that animal sniffing happens in fractions of a second, a rate he has tried and failed to match himself. The practical implication is almost embarrassingly simple: smell is a sense that can be exercised in an ordinary conversation, and almost never is. His example is a laptop — nose to the keys, and the circuit boards announce themselves through the molecules they give off.

The host offers the neat formulation that smell is like touch where the thing touched turns volatile and goes inside you. McGee accepts it and sharpens it: the receptor must physically bind the molecule, grab onto it, in order to report. For a moment the thing smelled is part of you, which is “really intimate” and also explains why a bad smell makes people cover their noses — the intuition is that something is getting too close to the inside.