An amazing discovery

the biggest thing in smell in three decades.

I’ve been a bit quiet recently because my attention has been captured by an amazing event: a group of young scientists and engineers in San Fancisco may have taken the first step towards what was hitherto considered impossible: a smell equivalent of the cochlear implant. Apologies if I’m a bit breathless.

A bit of Chat GPT will help here:

“A cochlear implant works by bypassing the damaged parts of the inner ear and directly stimulating the auditory nerve with electrical signals. In normal hearing, sound waves vibrate the cochlea’s hair cells, which convert those vibrations into nerve impulses the brain interprets as sound. A cochlear implant replaces their function: an external microphone captures sound, a sound processor analyzes and encodes it, and a transmitter sends the information to an internal electrode array surgically placed in the cochlea. These electrodes stimulate different regions of the auditory nerve corresponding to different frequencies, allowing the brain to perceive sound and enabling people with profound hearing loss to regain functional hearing.”

On a blog post published in the last few days a group of engineers: Lev Chizhov, Albert Yan-Huang, Thomas Ribeiro and Aayush Gupta have finally, as they put it themselves, Induced Smells With Ultrasound. Low intensity ultrasound can be focused on nerve tissue and excite the cells within it. This is a huge field of Neuroscience at the moment and ultrasound is the Great Hope of non-invasive brain-computer interfaces. I’ve always been wary of it because no one know how ultrasound works. Nevertheless these guys have focused ultrasound on the olfactory bulb and report reproducible, clear smell sensations. These percepts seem stable: stimulate a particular spot, get a particular smell. Move the focus a bit, get a different smell.

Why am I excited? First of all no one had ever elicited sensations reliebly. Electrical stum up the nose has failed, my laser fibers are better forgotten.etc. The only sensation came if somone punched you in the nose (metallic, blood). Now these people hav done it! Some of my readership may be aware that I am a proponent of a theory according to which the smell of a molecule is determined by its molecular vibrations. This finding is sensational because it suggests that the olfactory bulb, even without any odor molecules present, contains a spatial code for odor qualities. Something about the organization of the bulb allows a purely physical perturbation—mechanical vibrations from ultrasound—to unlock specific percepts.

This observation, just maaybe, lands squarely in the territory of the vibrational theory of olfaction. For decades, the idea that olfactory receptors detect molecular vibrations rather than shape alone has been considered controversial, but it refuses to die. The new ultrasound work essentially shows that regional stimulation alone is sufficient to evoke odor qualia, provided it is delivered to the correct micro-region of the olfactory bulb.

Could the olfactory bulb itself be organized according to molecular vibrational frequencies in the way the cochlea maps sound frequencies along its length?

To be taken with a huge pinch of salt: Left, a mapping to notes of the scale of diatomic streach vibrations. Right: what is these vibrations were mapped on the olfactory bulb and could be stimulated directly?.

1. The Idea in Outline

Imagine the olfactory bulb as a kind of smell keyboard. At one end, glomeruli respond to molecules with slow vibrations (like halogenated compounds around 600 cm⁻¹). At the other end, glomeruli respond to high, fast vibrations (like sulfur-containing thiols above 2400 cm⁻¹). Between the two ends lies a continuous spectrum—terpenoids, ethers, musks, aromatics, carbonyls, nitriles, isothiocyanates—each landing in its own “frequency register.”

This doesn’t replace shape-based mechanisms. Instead, it gives an orthogonal dimension: One axis for where on the bulb the vibration projects, another for what pocket the molecule fits into. Together, they create a 2D map from which the cortex reconstructs the percept.

2. Why Ultrasound Makes This Testable

Ultrasound interacts with biological structures through mechanical and electrical effects—micro-vibrations, membrane deformation, ion channel modulation.

If certain regions of the olfactory bulb correspond to particular vibrational “notes,” then stimulating that region with ultrasound could simply mimic the downstream effect normally caused by the right odorant.It would be like tapping a piano string directly instead of striking the key. In this view, ultrasound isn’t “pretending to be an odorant”—it’s triggering a pre-existing vibrational map in the bulb.

3. A Tonotopic Smell Spectrum

Below is my current best wild guess for a vibrational smell atlas, from low to high frequencies:

600 cm⁻¹ — Halogen Sweet Zone C-Cl . Sweet, chloroform-like.

800–1000 cm⁻¹ — Terpenoid Modes Skeletal ring deformations. Pine, citrus rind, alkanes.

1100 cm⁻¹ — Ether/O-C–O Region Clean, airy, sweet-ethereal.

1350 cm⁻¹ — Musks Macrocyclic and polycyclic CH₂ bends. Warm, diffusive, velvety.

1500–1600 cm⁻¹ — Aromatic/Nitro/Amide Middle Band

1700–1800 cm⁻¹ — Carbonyl Territory: Aldehydes, ketones, lactones. Acetone.

2000–2150 cm⁻¹ — Nitriles and Alkynes: metallic

2200–2300 cm⁻¹ — Isothiocyanates: Wasabi, mustard, horseradish.

2400–2550 cm⁻¹ — Sulfuraceous

4. Odor as a Chord, Not a Note

Most odorants have multiple dominant vibrational modes. The brain receives a spatial chord. Just as a C-major chord is more than the sum of its notes, a grapefruit odor is more than the sum of its molecules. It’s the relationship between the zones—their spacing, relative intensities, timing— that creates the perceptual object.

5. How Might Such a Map Form in the First Place?

Receptors tune to different vibrational ranges

If receptors use inelastic electron tunneling, each receptor isoform would be naturally tuned to a different vibrational energy. Thousands of isoforms → thousands of overlapping tuning curves.

Axons sort themselves by molecular preference

Neuronal development in the OB is guided by gradients of adhesion molecules and activity-dependent refinement. If receptors tuned to similar frequencies tend to fire together, their axons might converge into contiguous domains. Neural self-organization could turn vibrational similarity into spatial adjacency.

The cortex learns the chord structure

The piriform cortex is known for doing complex pattern synthesis.
Its job might be to integrate vibrational peaks into recognizable “odor objects,” treating each molecule as a small spectral fingerprint.

6. Where Ultrasound Fits In Again

If the OB is organized this way, ultrasound offers a direct way to:

• stimulate one vibrational domain (eliciting a pure odor “note”,

• stimulate multiple domains together (eliciting synthetic odor “chords”,

• or even disrupt the entire map (causing distortions or phantoms).

Ultrasound may simply “pluck” the glomerular clusters the way one might pluck a string.

This would explain why ultrasound can elicit odor percepts with surprising specificity—it’s activating a pre-existing, frequency-organized scaffold.

7. Why This Matters

A vibrational tonotopic olfactory system would be a fundamentally new sensory map, with implications far beyond chemoreception.

It would imply:

• Odors are spectral objects, not merely chemical categories.

• The olfactory system performs something like spectral mapping, analogous to the cochlea.

• Ultrasound and other physical stimuli could be used to probe or manipulate odor perception with unprecedented precision.

• It also reunifies chemistry and perception in a satisfying way: the IR vibrational spectrum—the chemist’s tool—would become the brain’s tool as well.

8. What’s next

The idea of a tonotopic vibrational map in the olfactory bulb is still complete speculation. But it may gain a new kind of plausibility if ultrasound can be shown to elicit odor percepts in a spatially specific that matches molecular vibrations

I plan to visit the discoverers in the next few days in the company of neuroscientists, perfumers and chemists to see whether the results match the vibrational theory.