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Nobel prize winners with a new discovery: "It's possible to measure the inner life"
Grid cells were supposed to tell the brain only where it was. Now it turns out they also point towards where it wants to go.
Published
For 20 years, the grid cell was the closest thing neuroscience had to a fixed point.
May-Britt Moser and Edvard Moser found them in the rat brain in 2005, work that nine years later would lead to a Nobel Prize.
As an animal moves, these cells fire in a hexagonal pattern that tiles space the way graph paper lays a grid over the world.
They were the coordinate system, the load-bearing beam of the inner map, and their geometry was so regular it looked as though someone had drawn it into the brain with a ruler.
Grid cells always reported the same thing: You are here. So punctually that no one thought to ask what else they might have to say.
That is why it sparked curiosity when the research team reported in early 2025 that grid cells had been keeping a secret. Hidden inside their activity, in slivers of time too small for older equipment to catch, lay a restless motion.
"We wondered what they're used for"
Ten times a second, the cells sent a signal out from the animal's position and into the surrounding map. First to the right, then to the left, then right again. Like a decive that helps maintain a steady rhythm – a metronome.
The researchers called these steady probes sweeps, and on their screens they look like the cones of light from a pair of headlamps, shooting out from the head and scanning the terrain ahead of the animal.
It was a beautiful finding. It was also a finding that intrigued the researcher. A metronome is impressive, but it is also simple. It does the same thing over and over again.
"We saw these sweeps, which are there all the time, going right and left across the map, very rigid," Abraham Zelalem Vollan says about those early days.
"And then we wondered what they're used for. What their function is. Is it just something stereotyped that happens automatically with no regard for the surroundings, or are the sweeps actually being used to probe what's relevant to the rat here and now?"
The solution was a fishing rod
You cannot ask a rat what it is paying attention to. All a human being in a lab has access to is what the rat actually does.
If you want to know what the animal cares about, you first have to make it care about something, and do so in a way that is precise enough to measure.
The solution was almost comically low-tech. The researchers tied a piece of food to a string on a fishing rod and moved it fast and unpredictably across the arena while the rat gave chase.
Prey that darts and swerves forces a hunting animal to track it continuously and correct its course as it goes. At every moment, the researchers knew where the rat's attention was. It was over there, where the food was.
Now they could see whether the swipes were influenced by what was happening around the rat.
Here's how the researchers went about it
Vollan describes the method: Wire up the rat. Run the experiment. Let the computer analyse the recordings overnight. Then decode which location on the inner map the grid cells were representing and compare it to where the food actually was.
"It was almost a little too good to be true when we saw it. That it actually worked. What happened was exactly what we'd hoped for," he says.
The sweeps stopped behaving like a metronome.
When the rat spotted the moving bait, they narrowed from a wide angle to a focused beam, the rhythm quickened, and the beam tore free of the head direction and pointed at the food.
Over and over, they probed the narrow strip between hunter and prey. The headlamps had become a searchlight.
Attention runs ahead
Now and then, in the middle of a chase, the bait swings suddenly to one side and the rat loses track of it. For a few hundred milliseconds, the animal is at a loss. Then it finds the prey again and throws itself forwards.
It is inside these pauses that the researchers find what they are looking for. The sweeps have already turned. The beam points at the food's new position before the rat turns its head to follow.
"You can see that the sweeps are faster than the motor movement," says Edvard Moser.
We recognise this from our own experience, even if we rarely think about it. You know where you are about to look a moment before your gaze actually gets there.
What the recordings show is that this sensation can be measured. A signal deep within the brain's map swings towards something the animal has not yet seen, a beat before the muscles are told anything.
But this is where Vollan and his colleagues pause, because there is a more mundane explanation, and it is not easy to dismiss: Perhaps the sweeps have nothing to do with the food at all.
Points towards what really matters
Maybe it's only the brain preparing to turn the body, with the head lagging behind. In that case, the researchers have not found a beam that follows interest. They have found a twitch in a motor programme. The two explanations would produce almost identical data.
There is only one place to look for the difference. They need the rare moments when the bait swings in an arc behind the rat.
The animal then has to turn its head one way while the food continues in the other. And for the first time, prey and movement point in opposite directions.
The sweeps follow the food. Clockwise, around behind the animal, independent from the direction of the head.
In some cases, the rat sits completely still while the inner beam points at the prey's actual position and not at where the body is about to go. The sweep is not preparing an action. It's pointing at what matters, in this case food.
This may explain the brain's sense of direction
If the beam can break free from the direction of the head, something must be doing the uncoupling. This is where the discovery, which could have remained a curiosity about grid cells, becomes something more structural. A claim about how the brain is wired.
The classical account of the brain's sense of direction runs through the head direction cells.
These are neurons connected in a ring that behave somewhat like a compass needle, with one key difference: instead of pointing north, they point in whichever direction the head is facing. They are found across large parts of the brain in a wide range of species, including humans.
When the sweeps tracked the bait as it swung around behind the rat, the head direction cells in the thalamus and presubiculum went on pointing where the head was facing, indifferent to the food.
The sweeps are steered by a different group of cells, in a cortical region called the parasubiculum. These cells are also arranged in a ring, and they are also concerned with direction, but with a twist that Edvard Moser is careful to phrase precisely.
Two parallel circuits
"It isn't that this ring overrides the compass. There are two parallel circuits," he says.
One is the head direction compass, bolted to the head axis.
"The other, in the parasubiculum, can uncouple from the body's orientation. That ring can be turned anywhere. The default is that it's aligned with head direction. When nothing else is going on, it is. But then it can uncouple from head direction and instead couple onto a goal, or a target, or whatever it might be. Those are the ones that steer the sweeps," the researcher explains.
So the architecture is two rings. One is always fastened to the body and keeps track of the direction of the head. The other is usually fastened to the body as well, but it can tear loose and anchor itself to whatever the animal is preoccupied with.
"One is always attached to the body. The other is always attached to the mind. Sometimes they overlap," Edvard Moser summarises.
The rats were wheeled around in little carts
A worry hangs over the chase experiments. A piece of food hurtling past is a blazing signal to every sense.
Naturally, the brain lights up in response. Perhaps the sweep is simply being driven around by raw sensory information? Impressive enough, but at its core a reflex.
What if the food were not there at all? Vollan spent a long time looking for a moment when the beam swings towards something without being pulled by external activity, when the only thing steering it is the animal's own plan.
He describes it as a long series of attempts that did not work.
"I was messing about trying to find a way to get them to walk backwards, or to move them this way and that way," he says.
He lifted rats, wheeled them around in little carts, kept trying new things. Analyse overnight. Plot the signal. Watch it fail. Back to the lab, try something else.
"You feel your way forward. You do something, then you look at what worked, and then you change it," he says.
A dead end worked
What finally worked was a corridor with a dead end with a reward at the end, too narrow for the rat to turn around in. To get out again, the animal had to reverse back down the tunnel while its head still faced forwards.
The behaviour was already there and needed no training. Rats build burrows underground, and moving backwards through tight spaces is natural for them.
Here head direction and direction of travel part company, with nothing outside to lure the beam. If the sweep is a slave to the senses, it should go on pointing forwards, where the head is aimed. If it can be steered from within, by the wish to get out, it should turn.
It turned.
The moment before the rat began to reverse, the inner direction signal rotated 180 degrees and locked onto the direction of travel, towards the mouth of the tunnel. The head faced one way. The searchlight faced the other.
The body followed the beam. A brain signal driven not by any outside stimulus but by an intention formed in the mind. Perhaps a message from the frontal lobe that the plan has changed, I want out of here, handed to the mental map to be carried out.
You can see how this works below:
Is this a form of attention?
The two experiments point at the same mechanism from opposite directions. The chase shows that it can be driven by sensory input.
The corridor shows that it can be driven from within, by plans and goals. A double anchoring of exactly this kind is what you would expect of a system that has something to do with attention.
"I actually think it is a form of attention," says Edvard Moser.
The study puts it more carefully. There it is called attention-like, and the sweeps are described as a focusing mechanism for gathering information from the surroundings.
The distance between those two formulations says something about how high the bar sits in this field.
Attention is a demanding concept in the scientific literature. It is not enough to show that the brain gathers richer information from one place. You also have to show the other side of it, that the animal is actively overlooking the rest.
That choosing one thing costs you something elsewhere. A faster reaction where you are paying attention. Missed events where you were not.
Edvard Moser mentions the famous experiments in which people counting basketball passes fail to notice a person in a gorilla suit walking straight through the picture.
"In practice you can go a long way towards saying that this is attention. Gathering information from one place means you are not gathering it from another. To choose is to leave out," he says.
Bats do something similar
No animal takes in the world evenly and photographically. They aim their senses.
The rat sweeps its whiskers, the snake flicks its tongue, the dolphin steers its sonar. The closest parallel, and the one the researchers keep returning to, is the bat.
It maps the dark by throwing out clicks in an alternating left-right pattern. As it closes in on they prey, it adjusts the rate, the direction, and the width of its calls.
It narrows its acoustic attention onto the target. Compare that to a rat's sweep narrowing in on a piece of food, and the likeness is almost uncanny.
The only difference is where the probing happens.
"The bat's echolocation happens in external space, while the sweeps happen in the inner map," says Vollan.
The brain has taken the trick the bat uses to probe the outside world and turned it inward, onto its own model of space.
It is tempting to read this as a technical story about navigation. But when an inner map can search, select, and follow what matters to the animal, we come close to a much older question: How much of our experience comes from outside, and how much is created within?
"It's possible to measure the inner life"
The idea that the mind does not passively receive the world but actively shapes experience goes back to Immanuel Kant. Space and time, he argued, are not things we find in the world but structures the mind itself lays down in order to be able to experience anything at all.
Grid cells have long been interpreted as a biological echo of that idea, a coordinate system we carry with us into every new room.
"We have an arsenal inside us of inner things. That we can now measure. That's what's so exciting about this work, that it's possible to measure the inner life," says May-Britt Moser.
"Attention contains a great deal of what we now call active sensing," says Edvard Moser.
In other words, that you sense from where you need it.
"The finding shows that the sense of place is also subject to active sensing," he adds.
What happens when they sleep?
The researchers recorded rats during REM, the phase of sleep in which humans have their most vivid dreams. Nothing comes in through the eyes. Nothing moves. And the map runs anyway.
Grid cells trace long paths across the inner map, with sweeps that narrow as the speed increases, exactly as they do during a chase. While the drama plays out in the rat's brain, the body lies perfectly still.
Whatever is steering the beam here, it is not the outside world. There is no outside world to act in.
Vollan is careful when talking about what this shows and does not show. The paths do not look like a simple replay of places the rat has been. They may run through places that never existed. 'We don't know,' is a phrase he uses often and on purpose.
May-Britt Moser goes a little closer to the edge of the wonder.
"You can almost imagine that they're dreaming they're out running, or that something is coming up behind them. We're scratching at the door of something. We begin to feel the texture of what's in there. Something we don't know," she says.
It's something you do
The brain regions this concerns are among the first to fail in Alzheimer's.
"As cells die, holes can open up in the inner map. The delicate coordination of thousands of cells that the sweeps require may be one of the first things to go," says May-Britt Moser.
But she is emphatic that this is speculation. What she would rather talk about is something much closer to everyday life.
"A sense of place isn't something you have. It's something you do," she says.
People have come to her convinced they were born without any sense of direction, as though it were a missing organ. But she insists that a normal brain has the equipment. What is missing lies in its use:
The habit of noticing stable landmarks and anchoring the mental map to them. She prefers that word, anchoring. An unanchored map floats and slips. A map fastened to something stable is a map you can navigate by.
The map is something you actively use
The person who 'has no sense of direction' has usually just never anchored it, often because their attention was somewhere else entirely, on people and animals and everything that moves, rather than on where the roads, the houses, and the mountains lie.
"It isn't a verdict. It's a recipe. If you think you were born with a poor sense of direction, you have to live with it. If you understand that the map is something you build and fasten yourself, you can do something about it," she says.
And there the two threads meet. The sweeps are how the mental map reaches out towards the world. They are how you relate places to one another, and to yourself. Now we see that they also turn towards whatever preoccupies the animal.
The philosophical question is whether we have an inner model of the world or whether we are continually making it through action. The rat answers by refusing to take sides. The map is there in advance. But it counts for little until it is aimed at the world and put to use.
Reference:
Vollan et al. Adaptive modulation of theta sweeps in the brain's navigation circuit, Science, 2026. DOI: 10.1126/science.aef4184
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