The tour guide in our brain

Researchers have uncovered a new class of brain cell that acts like the red pin on a Google map to tell you where you found things on past journeys.

These neurons, dubbed memory-trace cells, are the place markers that record whether you had that mouth-watering gelati opposite the Trevi Fountain or just up the road from the Pantheon.

On a more sombre note, they are clustered in a part of the brain that takes an early hit in the onset of Alzheimer’s disease and may well explain the appalling degradation of memory seen in that illness.

To unearth these very special neurons, the researchers, led by biomedical engineer Joshua Jacobs from Columbia University in the US, devised a clever video game, albeit one unlikely to rival Fortnite as a teen meme.

Players ride a trolley along a road bounded on each side by a brick wall which is divvied up into grey, blue and brown segments that act as reference points.

On the first run-through the player has to press a button when they reach an object, in one case an antique writing desk that could be a prop in a Stephen King spine tingler.

But it’s on the second pass that things get really interesting. This time the player travels the same road with the desk taken away – the task is to press the button when they reach the point where they think the desk was.

None of this would be out of the ordinary if the gamer was just your average punter. Jacobs’ players were, however, in a class of their own.

He enlisted 19 people with drug-resistant epilepsy who had electrodes inserted in their brains to map where seizures start and guide treatment. Tapping into those electrodes, the researchers could monitor activity right down to each solitary neuron.

They were curious about a very precise area called the entorhinal cortex. It’s a part of the brain that functions like a GPS, altering the firing rate of neurons as the physical location changes.

Jacobs and colleagues had a hunch the same area might also work as something of a tour guide, telling us not just what route we took, but what noteworthy ephemera we encountered along the way.

The brain responses in those epilepsy patients fell very much in line with that theory.

When patients pressed the button to indicate where they thought the desk was, those entorhinal brain cells began firing away to a very different tune.

The same thing happened when Jacobs re-ran the experiment with other objects on the road. Cell firing was unique to the point where each pattern could be used to decode the object being recalled.

“Our study demonstrates that neurons in the human brain track the experiences we are willfully recalling and can change their activity patterns to differentiate between memories,” says Salman Qasim, lead author on the study.

“They’re just like the pins on your Google map that mark the locations you remember for important events.” 

But the team notes those neurons are also implicated in disease.

“We found these memory-trace neurons primarily in the entorhinal cortex, which is one of the first regions of the brain affected by the onset of Alzheimer ‘s disease,” says Jacobs.

“Because the activity of these neurons is closely related to what a person is trying to remember, it is possible that their activity is disrupted by diseases like Alzheimer’s, leading to memory deficits,” he says.

“Our findings should open up new lines of investigation into how neural activity in the entorhinal cortex and medial temporal lobe helps us target past events for recall, and more generally how space and memory overlap in the brain.”

The study is published in the journal Nature Neuroscience.