A new kind of navigational neuron has been discovered in the mammalian brain, and it fundamentally changes our understanding of how we relate to objects in our vicinity.
We already knew how mammals, including humans, locate themselves within an environment thanks to the Nobel Prize-winning discovery of place cells and grid cells. These cells are said to form the brain’s inner GPS system. The newly discovered “vector trace cells” add another layer of complexity to this system because their activity depends more on the objects in the environment rather than the environment itself.
The vector trace cells become active when we see an object. They help us judge how far that object is from us and also its relative distance to other objects we can see.
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But vector trace cells are active even when the object they have been tracking is no longer visible or has been removed by someone else, and they can remain in this active state for hours. In other words, these cells – assuming they are present in the human brain – may help us remember where we last saw an object.
Steven Poulter and his colleagues at the University of Durham, UK, working with researchers at University College London, found the new neurons accidentally. The team was working on an experiment that involved putting obstacles (wine bottles, in this case) in the path of rats, while monitoring activity in the rodents’ brains, particularly in a region of the hippocampus called the subiculum.
“One particular evening, I took away a couple of the bottles and I looked at the neuronal response – but it stayed the same,” says Poulter. He initially assumed there was an error in the software, but he soon realised that some types of cells were consistently firing as if the wine bottles were still there.
The team then confirmed, through four years of further experiments, that the cells that continued to fire were a never-before-seen type of neuron.
Colin Lever, a co-discoverer of the cells, says what the cells can’t do is identify objects. They act as distance-checkers, but the information they generate and store is the same regardless of whether the object in question is a mobile phone or an office desk.
The cells also don’t seem to differentiate between objects and obstacles. A brick that a rat can simply crawl over activates the neurons in the same way as a wine bottle that the animals must travel around. Even a line painted on the floor activates the neurons in a similar way.
Present in humans
Lever suspects the cells will be found in the human brain, noting that there is already indirect evidence for the presence of these cells in humans. He suggests they are vital for how we visualise our way around a room or space. He also speculates that, as the subiculum is one of the first areas of the brain to degenerate with the onset of Alzheimer’s, it could explain why forgetting where you left an object is often one of the first symptoms of the disease.
Alastair Smith at the University of Plymouth, UK, who wasn’t involved in the study, says the findings seem to “move us away from a system which tells you where you are in space and actually tells you where other things are in that space”.
Neil Burgess at University College London says the discovery of the cells is important, and that it verifies long-held theories about spatial memory. It is relatively rare for neurologists to find a class of cell that functions in a way that their theoretical models exactly predict, he says.
bioRxiv
