Three sensory pathways. Two follow the same rules: report to the thalamus first, then up to cortex, laying down a map along the way. The third follows none of them — it walks straight into the territory of emotion and memory.
Sensory signals entering the brain usually follow a set routine: converted to electrical signals out in the periphery, changing trains a few times in the brainstem, into the thalamus — the central reception desk — and only then released to the cortex. → visual pathway takes exactly this route. So do hearing and touch. Smell is the one exception:
The critical part inside the ear is the cochlea — a snail-shaped spiral tube containing a thin membrane (the basilar membrane). The clever thing about that membrane is that it is stiff at one end and floppy at the other: high frequencies can only shake the stiff end near the entrance, while low frequencies have to travel deep in to reach the floppy end. So "sound," which arrives as a single mixed vibration, is spread out along a line by pitch in the very first step — highs at this end, lows at that one.
This "position stands for frequency" arrangement is called tonotopy, and it is preserved all the way up to the cortex: neighbouring neurons in A1 prefer neighbouring pitches. It's the same design as vision's "neighbouring positions are neighbouring on the cortex."
Something else worth knowing: hearing changes trains far more often in the brainstem than vision does, because it has to work out something urgent on the spot — where the sound came from. The superior olive in the brainstem compares the arrival time of the same sound at the two ears, and humans can resolve differences down to about ten millionths of a second. That has to be finished at the lowest level, because any higher up would be too late.
One consequence: from the brainstem onward, the two sides' auditory signals mix extensively. So losing one auditory cortex rarely deafens that ear — the information was already backed up on both sides.
"Feeling something" isn't one sense; it's several crammed into one word: light touch, vibration, joint position (proprioception), pain, temperature and itch. They do not travel the same road through the spinal cord at all:
Both routes end up in the thalamus and are then sent to S1, the strip behind the central sulcus. S1 carries a map too, but it is a severely distorted one: hands, lips and tongue take up enormous area, while the back and thighs are squeezed into a narrow band. The allocation isn't by how big the body part is, but by how many receptors are there and how fine a resolution is needed. You can test it directly: a fingertip can tell apart two points two or three millimetres apart; on your back they have to be four centimetres apart before you can tell there are two.
That famous "cortical homunculus" cartoon is a convenient simplification. Finer recent measurements find that S1 and motor cortex don't hold one clean ordering of the body but rather body-part regions interleaved with another kind of region involved in whole-body coordination — the map is more complicated than the cartoon.
At the roof of the nasal cavity is a small patch of olfactory epithelium, whose neurons each express just one odorant receptor. Humans have around 400 working receptor genes (Linda Buck and Richard Axel won the 2004 Nobel for discovering this system).
How can 400 receptors cover a whole world of smells? By combination. One odour molecule stirs several receptors at once, and one receptor responds to several molecules — so a smell corresponds not to "which single receptor lit up" but to which group of receptors lit up, and how strongly. Like spelling unlimited words from a limited alphabet.
Then come the two things that make this route genuinely unusual:
One: it doesn't go to the thalamus. Fibres leaving the olfactory bulb enter piriform cortex directly, and at the same time hand copies straight to the amygdala and the entorhinal cortex — the doorways of emotion and of memory respectively. Every other sense has to queue at the thalamus, be gated and filtered; smell walks right in. (Strictly, there is also a route via the mediodorsal thalamus to orbitofrontal cortex, involved in consciously identifying and categorizing odours; but the main road really does bypass the thalamus.)
Two: it has no map. Hearing is laid out by frequency, touch by body part, vision by position — smell can't be, because odours have no natural ordering. There is no "smell in between" lemon and petrol; you cannot spread odours along a line. So in piriform cortex, one odour activates a set of neurons scattered all over, and position itself carries no meaning.
A 2014 paper estimated humans can discriminate over a trillion odours, and it was reported everywhere. It was then sharply criticized on mathematical grounds by several researchers: the figure is extrapolated from very few experimental points, and a differently but equally reasonable set of assumptions moves the answer by orders of magnitude. What can safely be said today is only that it's "far more than the ten thousand people used to assume, but nobody knows how many." When you see a startling round number, it's worth asking how it was arrived at.
That experience of catching a scent and being yanked back a decade arrives more suddenly, and with less control, than a photograph or a tune. It isn't mysticism, it's wiring: vision and hearing have to go through the thalamus, then their own primary cortex, then layer after layer of processing before eventually reaching the amygdala and hippocampus. Smell leaves the olfactory bulb and, one or two synapses later, is already in piriform cortex, amygdala and entorhinal cortex at once — and entorhinal cortex is the hippocampus's front door.
Put differently: the other senses go "recognize first, then generate feeling"; smell delivers feeling and recognition almost simultaneously, with the recognizing route actually the slower of the two. Which is why a smell so often hits you first, and only seconds later do you work out what it was. → amygdala → hippocampus & entorhinal → thalamus
Topic 1 Perception as Inference · Topic 2 Attention · Topic 7 The Language Brain · Topic 8 The Construction of Emotion · Topic 35 Neural Coding
Tonotopy · von Békésy · the travelling wave (Nobel) · Dorsal column–medial lemniscus pathway · Cortical homunculus · Buck & Axel · the odorant receptor family (Nobel) · Piriform cortex