Some people have thin wires in their brains. The wires are there for medical reasons: surgeons need to find the exact spot where activity goes wrong. While the wires sit there, researchers can listen to single neurons. Show a person a cartoon and one particular neuron stays silent. Show another cartoon, and another: silence. Show The Simpsons, and it fires.
Olga Ivashkina, a neurobiologist, counts this among the brightest finds from such patients. The same recordings turned up neurons for Jennifer Aniston and for the Sydney Opera House. A cell that answers to one cartoon and shrugs at all the others.
That changes what you are doing when you glance at a yellow face on a screen. Your eye does not hand your brain a heap of equal pixels and leave it there. Somewhere up the line, a cell is waiting for exactly this.
How does a neuron get that picky? Through a pyramid, as Ivashkina draws it. At the bottom, cells in the visual system split the scene into scraps. Some look only for dark and light, some for contrast, some for colour, some for movement and the direction it goes. Each layer feeds the next.
Faces get a department of their own, because people are social and a face tells you how someone feels. A crowd of neurons hunts for faces everywhere. Hence the sad smiley, and hence the pancake with holes that look like eyes and reads, for a moment, as a sceptical smiley.
At the top sit cells that stand for a whole thing: a Simpson, yellow, red flowers, red cars, cars in general. By then the signal has left the visual system for the associative areas, where neurons hold specific concepts.
This is the bright activity of these specialised or selective neurons, like a neuron that is specifically active when a person is shown the cartoon The Simpsons.
Это яркая активность этих специализированных или селективных нейронов, вроде нейрона, который специфически активен, когда человеку показывают мультфильм Симпсоны.
— Olga Ivashkina, Osnova, 38:01
Here the story could end, with one neuron per idea. Ivashkina does not let it.
A single neuron is never alone. The whole Simpsons image has a group of them, a hundred, a thousand, ten thousand. And there is a second way the brain can hold the same cartoon. The cartoon is broken into pieces, and a whole population of neurons carries it a bit at a time: one cell tied to the humour, another to yellow, "because it's all so yellow", another to something else. Only the whole crowd working together pins the cartoon down.
Animal experiments show both codes at once. The bright specialised cells are there, and so is the smeared population code. The brain seems to strike a trade-off between the two, though nobody has pinned down its terms.
Specialised cells are not a quirk of vision, either. Place cells work the same way. Walk down a long, dull underpass with nothing to catch the eye and you might expect the same few groups of neurons to hum on the whole way. Instead one place neuron works at one spot in the passage and another at its far end. Ivashkina is careful to say this was not guaranteed. The brain could have done without specific neurons. Earlier work on vision made researchers suspect it would not, and finding it was still a pleasure. It looks like a property of the whole brain.
Nor are these cells tied to pictures. In a study from the Netherlands Institute for Neuroscience in 2024, depth electrodes in the hippocampus of epilepsy patients picked up cells tuned to a particular person or character. They also fired when the patient read a pronoun that pointed back to that character. A pronoun referring back to the character also activated the cell.
So the picture Ivashkina is left with has two layers, and nobody can yet say how they fit. Neurons that stand for one thing and crowds that carry a thing in fragments sit in the same head, as the same tissue. Neurons and brain, Ivashkina says, are one and the same. Researchers just watch them in different ways. The open job is to work out how both kinds of coding for reality live side by side in one brain, in one environment.
