The instant you catch a whiff of coffee or smoke, your body is already making moves your mind has not yet noticed. The question that keeps scientists busy is which judgment lands first in the brain: what the odor is, or whether it is good or bad.
Walk into a kitchen at 7 a.m. One burner warms a pan of butter and onions, another hides a scrap of plastic melting near a coil. Your head tilts, your nose adjusts its sniff, and your hand drifts toward a window latch. Only then does the narration arrive: caramelizing, and something burning. In that split second, your brain has performed a high-speed triage.
The shortest path in sensory biology
Smell runs on a shortcut. When odor molecules reach receptors in the nose, signals travel to the olfactory bulb and from there spread to the piriform cortex, the amygdala, and other regions that handle memory and decision making. Unlike vision or hearing, there is no obligatory relay in the thalamus on the way to cortex. That anatomy helps explain why smells can feel urgent and deeply tied to memory.
It is also a source of recurring confusion. The hippocampus is a memory hub, not a standard relay for the senses. Most senses pass through the thalamus; olfaction is the outlier, sending early messages directly to areas involved in emotion and value, including parts of the amygdala and orbitofrontal cortex. This unusual wiring has long fueled the idea that the affective punch of odors might register before we are able to identify what we are smelling.
Identity versus valence is not a simple race
Researchers have spent the past two decades trying to pin down the sequence. The answer, so far, depends on what you mean by first and by know.
In human brain recording studies, signals that distinguish one odor from another often appear in cortical patterns within a few hundred milliseconds. Work led by scientists such as Jay A. Gottfried and colleagues has shown that the piriform cortex carries rapid, distributed representations that can separate odor identities even before a person is ready to name them. Machine learning methods trained on these neural patterns can decode identity while a participant is still searching for the word.
At the same time, other teams have documented lightning-fast bodily responses to threat-laden smells. Groups working with Noam Sobel at the Weizmann Institute, Johan N. Lundström at the Monell Chemical Senses Center, and Jonas K. Olofsson in Stockholm have reported early adjustments in sniffing, subtle head movements, and autonomic changes after exposure to aversive odors. These shifts can happen within the first breath and can precede conscious recognition. In animal work, direct pathways from the olfactory bulb to amygdala and hypothalamic regions support rapid avoidance or approach.
Put these strands together and the apparent contradiction softens. Elements of identity and elements of value emerge in parallel, at different speeds, in different readouts. If you measure a reflexive change in sniff strength or heart rate, a coarse version of valence can show up very quickly. If you measure a neural signature that distinguishes coffee from gasoline, identity-specific information is also available early. If you ask a person to press a button for like or dislike, or to name the odor, behavior lags behind both.
What headlines miss and what they capture
This nuance is often flattened into one of two claims: that the brain decides what an odor is before it decides whether it is pleasant, or the exact opposite. The real story sits between those poles. Odor identity and valence are not single switches; they are layers of processing that unfold on overlapping timelines.
There are reasons for that layering. Identifying the source of an odor is a pattern recognition problem that the piriform cortex is built to solve. It receives a mosaic of inputs from the olfactory bulb and can rapidly assemble an odor object. Valence, meanwhile, is both a property of the stimulus and a property of the person. Rotten food tends to be aversive, but learned associations, hunger, cultural experience, and context reshape how pleasant or threatening a smell feels. Orbitofrontal and limbic circuits integrate these ingredients over time. Early signals bias you toward approach or withdrawal, later signals refine that feeling as the brain settles on what the odor likely is and what it means for you right now.
When studies rely on conscious reports of liking, valence appears later than identity in the timeline. When studies rely on reflexes and autonomic measures, a rudimentary valence response can precede reliable identification. Both observations can be true at once because they tap different stages of the system.
A timeline you can feel, if not see
Think again about that kitchen. In the first sniff, receptors fire and the olfactory bulb begins to pattern the input. Within a few hundred milliseconds, the piriform cortex holds a sketch that could, with the right analysis, be read as onions or as something synthetic. Almost in parallel, circuits tied to the amygdala and brainstem tune your breathing and posture. You lean away before you know why. Another breath sharpens the picture. Orbitofrontal regions integrate identity with experience and state, and your evaluation crystallizes: appetizing, but check under the pan for trouble.
None of this is slow. The system trades in tens to hundreds of milliseconds. Yet the feeling of knowing arrives as a single moment because perception stitches discrete steps into a continuous experience.
Why it matters beyond curiosity
Understanding the sequence is not just academic. Food and fragrance industries design products that exploit early biases and later judgments, nudging us toward approach before we can articulate why. Clinically, changes in smell are among the earliest signs of neurodegenerative disease. In Parkinson’s and Alzheimer’s, both odor identification and hedonic ratings can degrade, though not always in lockstep. People recovering from viral infections sometimes report parosmia, a distortion that can flip the valence of familiar smells; coffee may suddenly smell foul even when it is correctly recognized as coffee. Mapping how identity and valence travel through the brain could help explain these experiences and point to therapies that separate safety signals from distorted perception.
There is also a public safety angle. Rapid, low-level valence signals help us avoid hazards, from gas leaks to spoiled food. Training and experience can sharpen identity recognition so that reflexes point us in the right direction rather than merely away. Firefighters and chefs alike rely on the marriage of the two.
The takeaways, with caveats
- Smell has a privileged route to brain areas that handle value and memory, skipping the thalamic detour that other senses use.
- Early neural patterns can encode what an odor is, while early reflexes can encode whether it is likely good or bad. These processes run in parallel.
- Conscious judgments of like or dislike and accurate naming take longer, so the apparent order depends on how you measure and define knowing.