Siri has nothing on us: How do brain cells tell us where we’re going?
New findings provide a more complex profile of the brain’s “internal GPS”
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How do humans and other animals find their way from A to B? This apparently simple question has no easy answer. But after decades of extensive research, a picture of how the brain encodes space and enables us to navigate through it is beginning to emerge. Earlier, neuroscientists had found that the mammalian brain contains at least three different cell types, which cooperate to encode neural representations of an animal’s location and movements.
But that picture has just grown far more complex. New research now points to the existence of two more types of brain cells involved in spatial navigation — and suggests previously unrecognized neural mechanisms underlying the way mammals make their way about the world.
Earlier work, performed in freely moving rodents, revealed that neurons called place cells fire when an animal is in a specific location. Another type — grid cells — activate periodically as an animal moves around. Finally, head direction cells fire when a mouse or rat moves in a particular direction. Together, these cells, which are located in and around a deep brain structure called the hippocampus, appear to encode an animal’s current location within its environment by tracking the distance and direction of its movements.
This process is fine for simply moving around, but it does not explain exactly how a traveler gets to a specific destination. The question of how the brain encodes the endpoint of a journey has remained unanswered. To investigate this, Ayelet Sarel of the Weismann Institute of Science in Israel and her colleagues trained three Egyptian fruit bats to fly in complicated paths and then land at a specific location where they could eat and rest. The researchers recorded the activity of a total of 309 hippocampal neurons with a wireless electrode array. About a third of these neurons exhibited the characteristics of place cells, each of them firing only when the bat was in a specific area of the large flight room. But the researchers also identified 58 cells that fired only when the bats were flying directly toward the landing site.
“We have discovered a totally new kind of neuron, which we are calling ‘goal-direction cells’,” said Nachum Ulanovsky, senior author of the study, published this week in the journal Science. The findings, he added, “fill a critical gap in our understanding of the neural basis of navigation” by explaining how the brain encodes navigational goals.
The new cells, it turns out, continued to fire when the landing site was hidden from the bats’ view by a curtain. “The bats knew where the goal was, but could not echolocate or see the goal behind the curtain, but the goal-direction neurons still represented the hidden goal. This means that the representation of goals in the bat hippocampus was not merely sensory-based, but was memory-based.”
Neuroscientist Hugo Spiers, who studies the cellular basis of spatial navigation at University College London, said the findings are “hugely important” — but he does not think goal-direction neurons are a new cell type. “To me the results show that place cells are capable of more diverse information processing than previously thought,” he said.