EPs ~ experiencer insights

We managed to induce four smells, replicated in two people:

🌬️ The sensation of fresh air, with a lot of oxygen
🗑️ The smell of garbage, like few-day-old fruit peels
🎇 An ozone-like “electric” sensation, like you’re next to an air ionizer
🔥 A campfire smell of burning wood

The system responsible for the perception of smells is interesting beyond aromas. It’s an unfiltered and less processed channel than vision or hearing. It’s also closely connected to brain regions such as the hippocampus, which is why smells bring up such strong memories.

It can also modulate the overall brain state. For example, ammonia-based smelling salts can increase alertness by activating similar nerves (the trigeminal nerve in that case), literally reviving fainted people!

We distinguish between a smell and a sensation here because, subjectively, they feel different. The smells are strong and localized to the noise, almost like you could sniff around and find the source. The sensations are more diffuse: a weak, slow-onset impression of a smell, often paired with other (likely placebo) feelings, such as a light tingling on the face.

Both smells and sensations are strongest on a light in-breath, so we tested by sitting there, with a probe to the forehead, mildly sniffing. Sometimes there is a slight waft of a smell that comes on over a few breaths, and sometimes it just hits you. The first time Albert smelled the garbage, he jerked his eyes open thinking a garbage truck just drove in! This was indoors.

Many of these scents correspond not to specific receptor types but rather combinations of receptors. We think this is because the focal spot is pretty large—300kHz ultrasound in tissue has a wavelength of 5mm, while the adult human olfactory bulb is roughly 6-14mm in length The olfactory bulb can vary in size by up to 3x, depending on “age and olfactory experience”, so perhaps (we’re making this up) with more usage your olfactory bulb might actually get bigger, leading to better resolution stimulation!.

We found different scents by steering the beam over ~14 mm (20 degrees at 4 cm radius). The distance between freshness and burning was ~3.5 mm. We ensured that the effect was not placebo with an auditory mask (blasting music through airpods) so you don’t hear the probe, though you cannot distinguish the different focal spots through sound anyways. We then tested discrimination in a trial where Thomas selected the focal spots, and Lev was naming the scents. You can check out the full video here.

It is remarkable that we could induce different scents with such little steering (40% of the diffraction-limited focal spot size And potentially even higher, because there was some dead space in between the focal spots, where you don’t feel anything.). This suggests that the resolution we have access to is much higher than the spatial resolution of the ultrasound (a kind of super-resolution for neurostim!) In particular, we do not need single-neuron resolution to find an independent basis of scents, upon which we can construct our latent space. To improve this system, the next steps are a more stable setup, increased frequency, more play with focal location, spot size, and stimulus waveform.