| In each 3-pound human brain there are something like 86 billion neurons and trillions of connections. The ever-shifting networks they form process our inner and outer worlds, but how exactly they create what we call experience is fuzzy at best. For a high-resolution view of what neurons are doing, neuroscientists would need to poke and prod inside living people's skulls with invasive techniques — an unethical prospect in almost all circumstances. Instead, researchers mostly study the brain from the outside, at low resolution.
One key tool is functional magnetic resonance imaging (fMRI), which measures blood flow in the brain as a proxy for brain activity. (When neurons fire, they require more oxygen, causing a surge of blood to active areas.) An older tool is the electroencephalograph (EEG), which uses electrodes placed on a person's head to record the electrical activity of neuronal populations. While these techniques are crucial to modern research, they create only coarse pictures of what's happening in the brain.
Rarely, neuroscientists get the opportunity to probe deeper thanks to the generous participation of patients, especially those undergoing surgery for epilepsy. To monitor their seizures, some patients have electrodes implanted into their brain tissue. With informed consent, neuroscientists can use these electrodes to record the activity of subpopulations of neurons, or even individual cells. They can then conduct experiments with the patients to see how neurons respond to scents, math problems, memory tasks and much more.
What's New and Noteworthy
Earlier this year, I reported a piece about concept neurons, single cells that fire for specific concepts, such as the actor Jennifer Aniston. When a person sees a photo of Aniston, reads her name or hears it spoken aloud, the same neurons respond. Each concept cell can code for dozens of different but often related concepts. And each person has a different set of concepts and cells that encode them, based on what they have been exposed to. Without single-cell electrode studies with epilepsy patients, we would not know that concept cells exist at all.
Concept cells respond to smells as well. In a 2024 study, the neurobiologist Florian Mormann at the University of Bonn examined how neurons behaved when epilepsy patients smelled various odors, such as licorice or coffee. The work confirmed how the human brain processes scents, which previously had been studied primarily in rodents. Mormann's team found that neurons in the piriform cortex, a brain region associated with odor processing, responded not only to scent molecules reaching receptors in the nose, but also to pictures or words related to the scent. This suggests that smell is processed by concept neurons that help recognize sensory information in abstract concepts.
Using similar techniques, Mormann and his colleague Andreas Nieder, a neuroscientist at the University of Tübingen in Germany, discovered how the brain encodes small numbers. Specific neurons fire for their "favorite" numbers, and they fire more precisely for 1 through 4 than for larger numbers. In a follow-up study, neuroscientists found that the brain places zero on a mental number line with the other numbers, but represents it more distinctively. "[Zero] is simply an eccentric outlier and represented as such in the brain," Nieder said. | 
