The olfactory cortex sits along the base of the brain, tucked into the ventral (bottom) surface of the temporal and frontal lobes. It is not a single neatly bounded structure but rather a chain of interconnected regions that receive direct input from the olfactory bulb, the first relay station for smell signals. What makes it unusual among sensory brain areas is both its ancient evolutionary origins and its wiring: smell is the only sense whose signals reach the cortex without first passing through the thalamus, the brain’s main sensory switchboard. That quirk has real consequences for how odors connect to memory, emotion, and behavior in ways other senses do not.
The Main Subregions and Where They Sit
Neuroscientists generally describe the primary olfactory cortex as a set of four core subregions. Working from front to back, the first is the anterior olfactory nucleus, which sits directly behind the olfactory bulb in a stalk-like structure called the olfactory peduncle. It serves as an early processing hub and is the main bridge connecting olfactory processing between the left and right hemispheres of the brain.1PubMed. The anterior olfactory nucleus revisited – An emerging role for neuropathological conditions? Just behind it lies the olfactory tubercle, a small mound on the brain’s underside. Then the piriform cortex, the largest of these regions, stretches along the junction of the frontal and temporal lobes. It is often subdivided into a frontal portion and a temporal portion based on their positions relative to the boundary between those two lobes.2eLife. Characterizing functional pathways of the human olfactory system
Neuroimaging work has confirmed that these four subdivisions can be reliably parcellated in living human brains, with each showing distinct patterns of connectivity. The medial-rostral portion corresponds to the anterior olfactory nucleus, the adjacent caudal portion to the olfactory tubercle, and the frontal and temporal piriform cortices occupy the lateral portions of this strip.2eLife. Characterizing functional pathways of the human olfactory system Beyond these four, two additional structures are often grouped under the olfactory cortex umbrella: the cortical amygdala and parts of the entorhinal cortex. Both receive direct projections from the olfactory bulb, though they also serve broader roles in emotion and memory.
Why Smell Bypasses the Thalamus
Vision, hearing, and touch all route through the thalamus before reaching the cortex. Smell does not follow this rule, at least not for its initial cortical processing. Odor signals travel from receptors in the nose to the olfactory bulb, and from there project straight to the piriform cortex and the other primary olfactory areas. Research has shown that slow- and fast-wave activity states in the olfactory cortex appear to gate the inflow of information underlying conscious smell perception, performing a filtering role that the thalamus handles for other senses.3PubMed Central. Perception without a thalamus how does olfaction do it?
That said, the thalamus is not entirely absent from the smell circuit. A nucleus called the mediodorsal thalamus is involved in higher-order olfactory behaviors, including odor discrimination, olfactory attention, and the hedonic perception of flavors, meaning how pleasant or unpleasant a food or drink tastes.4PubMed Central. The role of the mediodorsal thalamus in chemosensory preference and consummatory behavior in rats So the thalamus contributes to smell, but it enters the circuit at a later, more integrative stage rather than serving as a mandatory first gate.
What Each Subregion Does
The four core subregions are not just anatomical distinctions on a map. They have meaningfully different jobs.
The Piriform Cortex and Odor Identity
The piriform cortex is where the brain assembles the identity of a smell. Individual olfactory receptors in the nose each respond to a limited set of chemical features, but most real-world odors are complex blends. The piriform cortex combines these fragmented signals into a unified percept, essentially answering “what am I smelling?” Research using functional brain imaging has found that the piriform cortex can decode odor categories with above-chance accuracy, and that its internal associative connections are critical for telling apart similar smells within the same category. When those associative connections were dampened pharmacologically, the piriform cortex lost its ability to separate closely related odors, and participants’ actual ability to discriminate those odors declined in parallel.5eLife. The role of piriform associative connections in odor categorization
The piriform cortex also has a well-documented relationship with epilepsy. Its unique anatomy as primary olfactory cortex predisposes it to involvement in focal seizures, and the olfactory auras that some people with temporal lobe epilepsy experience, sudden phantom smells that precede a seizure, are a direct clinical marker of piriform cortex activity.6Frontiers in Neurology / PubMed Central. The piriform cortex and human focal epilepsy
The Olfactory Tubercle and Reward
The olfactory tubercle has long been underappreciated, partly because it is small and hard to isolate in imaging studies. But it plays a distinct role in linking smell to reward and motivation. In animal studies, activating the dopamine pathway that feeds into the medial olfactory tubercle was enough to generate preferences for specific odors and locations, while blocking dopamine receptors there prevented those preferences from forming. Inactivating the same pathway even erased previously learned odor preferences and impaired the ability to learn new odor-reward associations.7eLife. Activation of the dopaminergic pathway from VTA to the medial olfactory tubercle generates odor-preference and reward This helps explain why certain smells can feel instantly rewarding or aversive before you consciously decide how you feel about them.
The Cortical Amygdala and Instinctive Reactions
The cortical amygdala, particularly a subdivision called the posterolateral cortical amygdala, handles the instinctive behavioral responses to odors. When an animal encounters the smell of a predator, it freezes or flees; when it encounters the scent of a potential mate or a food source, it approaches. The posterolateral cortical amygdala mediates both types of response through distinct output pathways: neurons projecting to the medial amygdala drive innate aversion, while neurons projecting to the nucleus accumbens drive innate attraction.8eLife. Where Is the Olfactory Cortex? Location and Functions Optically activating these neurons in the cortical amygdala was sufficient to produce behaviors that mimicked actual responses to innate odors, confirming this region’s critical role in hardwired smell-driven behavior.9PubMed Central. The participation of cortical amygdala in innate, odour-driven behaviour
From Smell to Conscious Awareness
Recognizing that an odor is present and identifying what it is are not the same thing as being consciously aware of smelling it. That transformation seems to happen downstream of the primary olfactory cortex, in the orbitofrontal cortex (OFC), a region on the underside of the frontal lobes. Neuroimaging evidence points to the right orbitofrontal cortex specifically as a critical site for conscious olfactory perception, facilitating the transformation of upstream olfactory signals into a percept you actually experience.10PubMed Central. Right Orbitofrontal Cortex Mediates Conscious Olfactory Perception
The OFC also plays a key role in learning what odors mean. During odor discrimination tasks, neurons in the OFC rapidly adjust their responses as the associations between odors and outcomes change. When an odor that previously predicted a reward starts predicting something neutral, OFC neurons update accordingly, encoding new odor-outcome associations on the fly.11PubMed Central. Associative encoding in anterior piriform cortex versus orbitofrontal cortex during odor discrimination and reversal learning This is why the meaning of a smell can shift with context: the same cologne might feel comforting or nauseating depending on your history with the person who wore it.
How Smell Connects to Memory
The famous Proust effect, where a whiff of something transports you back to a vivid childhood scene, is not literary exaggeration. It reflects genuine anatomy. The olfactory cortex feeds directly into the entorhinal cortex, which is the main gateway to the hippocampus, the brain’s memory-formation hub. Physiological studies have demonstrated that stimulation of the olfactory bulb and piriform cortex elicits strong responses in the entorhinal cortex, with many entorhinal neurons responding to inputs from multiple olfactory areas.12SpringerLink. Inputs from the olfactory bulb and olfactory cortex to the entorhinal cortex in the cat. II. Physiological studies. This direct anatomical pipeline, with no thalamic detour, is thought to explain why odor-triggered memories tend to feel more emotional and more vivid than memories triggered by sights or sounds.
Smell, Taste, and the Experience of Flavor
Most of what people call “taste” is actually smell. When you chew food, volatile compounds travel up the back of your throat to the olfactory receptors, a route called retronasal olfaction. The brain merges this retronasal smell signal with taste information from the tongue to create the unified experience of flavor. This merging begins surprisingly early in the processing chain. Even within the olfactory cortex itself, interconnections between taste and smell pathways start to form, and the two signals become increasingly intertwined as they move toward higher areas.13Neuroenology. Retronasal Smell
The interaction is not merely additive. A congruent taste can actually speed up the brain’s processing of a retronasal odor. When participants received a sweet taste alongside retronasal vanilla, the brain processed the odor signal faster than when the same vanilla was paired with an incongruent sour taste.14PubMed. Influence of simultaneous gustatory stimuli on orthonasal and retronasal olfaction This means your brain is not simply receiving two independent channels and combining them; the taste you are experiencing actively shapes how quickly and how strongly you perceive the smell.
An Evolutionarily Ancient Architecture
The olfactory cortex is among the oldest parts of the mammalian brain. It is classified as paleocortex, a type of three-layered cortex that is evolutionarily conserved across vertebrates, in contrast to the six-layered neocortex that expanded dramatically in primates and that handles vision, language, and abstract reasoning. The olfactory cortex’s three-layered structure shares common characteristics with the general cortex of reptiles, and a similar organization is found in the hippocampus, both being derivatives of ancient pallial tissue.15PubMed Central. Development and Organization of the Evolutionarily Conserved Three-Layered Olfactory Cortex This ancient heritage helps explain why smell is so deeply entangled with emotion and memory: those circuits were built together long before the neocortex came along and took over the rest of sensory processing.
Smell Loss as an Early Warning Sign of Neurodegeneration
Because the olfactory system’s anatomy places it at the intersection of so many brain circuits, damage to it often shows up earlier than damage elsewhere. In Parkinson’s disease, the characteristic protein clumps (alpha-synuclein aggregates) appear first in the olfactory bulb and the dorsal vagal nucleus before spreading to the motor regions that produce the tremors and stiffness people associate with the disease. Reduced sense of smell, called hyposmia, frequently appears years before any motor symptoms.16Brain Disorders. Olfactory Dysfunction: An early indicator of parkinson’s disease
A similar pattern occurs in Alzheimer’s disease. Amyloid-beta plaques and abnormally phosphorylated tau protein have been detected in the olfactory epithelium (the nasal lining) and the olfactory bulb of Alzheimer’s patients, and the degree of pathology in the olfactory epithelium correlates with the severity of cortical amyloid and tau ratings. Within the olfactory bulb’s glomeruli, these proteins show progressively greater expression as the overall stage of Alzheimer’s pathology advances.17BMB Reports. Olfactory neuropathology in Alzheimer’s disease: a sign of ongoing neurodegeneration Smell testing is increasingly recognized as a cheap, noninvasive screening tool that could flag people at risk of these diseases before the more devastating symptoms appear.
Training Can Physically Reshape the Olfactory Brain
The olfactory cortex is remarkably plastic, meaning it physically reorganizes in response to experience. Part of this plasticity comes from the olfactory bulb itself, which is one of the very few brain regions where new neurons are generated throughout adult life. These adult-born neurons, while still immature, enable structural and functional synaptic changes that shape how the circuit responds to experience.18PubMed Central. Olfactory neurogenesis plays different parts at successive stages of life, implications for mental health The output of the olfactory bulb’s projection neurons depends not only on what the nose is detecting but also on the activity of these continuously regenerated inhibitory interneurons.19PubMed Central. Adult Olfactory Bulb Neurogenesis
This plasticity has practical implications. Structured olfactory training, typically involving repeated sniffing of a set of distinct odors over weeks or months, has been shown to induce measurable neural reorganization, altering functional connectivity networks in people with smell loss.20PubMed Central. Recovery of olfactory function induces neuroplasticity effects in patients with smell loss Even in people with long-standing anosmia (complete smell loss), an olfactory training program significantly increased sensitivity to odors and produced modifications in functional connectivity across multiple brain networks.21PubMed Central. Olfactory training induces changes in regional functional connectivity in patients with long-term smell loss
Professional training pushes these changes even further. A longitudinal study of sommelier students found that their olfactory bulb volume increased over the course of training, alongside thickening of the right entorhinal cortex, while no such changes occurred in a control group.22PubMed Central. Olfactory bulb volume and cortical thickness evolve during sommelier training Professional perfumers show a similar pattern: increased gray matter volume in orbitofrontal areas surrounding the olfactory sulcus, with the volume in the anterior piriform cortex positively correlated with years of professional experience.23PubMed. Perfumers’ expertise induces structural reorganization in olfactory brain regions The olfactory brain is not a fixed piece of hardware. It grows and refines itself the more you use it.
Aging and the Olfactory Network
Smell declines with age, and this is reflected in measurable changes in the olfactory brain’s structural connectivity. A study spanning participants aged 25 to 95 found that the structural connectivity of the olfactory network predicted a person’s age with high accuracy and that specific connection pathways showed significant age-related changes.24PubMed Central. Structural connectivity of the human olfactory network and its relation to aging and olfactory function This matters beyond simple inconvenience. Declining smell affects appetite and nutrition in older adults, reduces the ability to detect dangers like gas leaks or spoiled food, and, as noted earlier, can signal the onset of neurodegenerative disease. Understanding the olfactory cortex’s anatomy and connectivity gives researchers a clearer target for interventions aimed at slowing or compensating for age-related smell loss.
Why the Olfactory Cortex Is Hard to Study
Compared to the visual or auditory cortex, the olfactory cortex has been mapped in far less detail, and part of the reason is purely practical. Standard functional brain imaging struggles with this region. The olfactory cortex sits near the base of the skull, close to air-filled sinuses, and the interface between brain tissue and air creates signal distortions that degrade image quality. Delivering precise odor stimuli inside a scanner is also more technically challenging than flashing a visual pattern or playing a tone. Ultra-high-field imaging, using magnets at 7 Tesla rather than the standard 3 Tesla, has improved the ability to measure activation in these areas, offering higher spatial resolution and sensitivity in exactly the regions where conventional scanners fall short.25PubMed. Assessment of olfactory information in the human brain using 7-Tesla functional magnetic resonance imaging As these technologies become more accessible, the map of the olfactory cortex is likely to get considerably more detailed in the coming years.