Brain's internal mileage clock identified in rats and humans

Scientists have for the first time pinpointed the brain's internal mileage clock by recording activity in rats that were allowed to run in a small arena, with a parallel test in humans suggesting the same mechanism exists in people. The findings, published in Current Biology, tie the regular firing of grid cells in the brain's navigation system to the ability to gauge the distance traveled.

In the rat study, researchers trained the animals to run a set distance in a rectangular arena and rewarded them with a piece of chocolate cereal when they reached the target and returned to the start. When the rats moved the distance, the grid cells fired in a regular pattern—approximately every 30 centimeters traveled. The more regular that firing pattern was, the better the animals were at estimating the distance they had to go to get that treat, explained Prof James Ainge of the University of St Andrews.

Crucially, when the researchers altered the arena's shape, the regular firing became erratic and the rats' distance estimates worsened. The scientists likened this disruption to losing visual landmarks in fog: navigation becomes harder when the signal is irregular. "It's fascinating," said Prof Ainge. "They seem to show this sort of chronic underestimation. There's something about the fact that the signal isn't regular that means they stop too soon."

To test the idea in humans, researchers scaled up the experiment by building a 12-meter by 6-meter arena in the university's student union and asking volunteers to walk a set distance, then return to the start. In a symmetrical, rectangular box, participants estimated distance with high accuracy. When the walls were moved to change the arena's shape, they began making mistakes in distance judgments in the same pattern seen in rats. "Rats and humans learn the distance estimation task really well, then, when you change the environment in the way that we know distorts the signal in the rats, you see exactly the same behavioural pattern in humans," said Prof Ainge.

Those results strengthen the view that humans share the same internal mileage clock as rats. The cells involved are located in the entorhinal cortex, one of the first brain areas affected in Alzheimer's disease. "The specific brain cells we're recording from are in one of the very first areas that's affected in Alzheimer's," Ainge said. The researchers noted that diagnostic games already exist to test navigation and said they would be interested in adapting similar tests to probe distance estimation.

The work, published in Current Biology, could help explain why people misjudge distances in darkness or fog and why navigation breaks down in neurodegenerative conditions. By directly linking grid-cell timing to distance estimation, the study provides a concrete mechanism for how the brain builds and updates its internal map during movement. The scientists say the findings open avenues for early diagnostic approaches, potentially incorporating distance-estimation tasks into cognitive assessments for at-risk individuals. Researchers emphasise that while the results are promising, further work is needed to translate these findings into clinical tools, including large-scale trials and the development of practical, noninvasive tests.

The study thus advances a long-debated area of neuroscience: how the brain converts physical movement into a spatial representation that helps us navigate real-world environments. By showing that a ticking mileage clock is not only present in rats but also aligns with human behavior, the researchers provide a clearer picture of the neural choreography that underpins everyday navigation and its vulnerabilities in disease.