Israeli scientists have proven the existence of a three-dimensional compass in mammalian brains, with the help of the humble fruit bat. And with the help of technology they invented to receive and measure signals from individual neurons in the batty brain.
That compass enables the bat, and our cats and us too for that matter, to orient ourselves in a three-dimensional environment - the flat surface and up and down, too. It is not orientation vis a vis magnetic north but in comparison with our environment. "In an office building, I would know where the various places are relative to myself. I wouldn't know where north is," grad student Arseny Finkelstein explains.
The technology the scientists invented consists of very slim flexible electrodes - "less than a hair in thickness" - that are implanted in the bat brain. The electrodes can measure signals from individual neurons and transmit them to a receiver. "We are the only lab that can record brain signals at the level of isolated neurons from a flying mammal," Finkelstein says.
Thusly Finkelstein and team demonstrated that the brain of the Egyptian fruit bat has special "3-D" neurons that can tell which way its head is pointing. These cells could be key to the bat navigating as it flies, says the team in a study published today in Nature.
Navigation relies on spatial memory: past experience of various locations, they explain. That spatial memory is formed in a primitive brain structure in the hippocampus, which we have, rats have and bats have.
"We think the navigation system that evolved in animals enables humans to remember location-based special memory," explains Finkelstein. It's not only remembering that you had been somewhere; but what you did there. "We think these compass and location cells constitute a sort of scaffolding on which we 'hang' our memories."
Vertigo such as that experienced by fighter pilots happens when this neuronic 3-D navigation system goes haywire, their study suggests. The pilots can't tell up from down, a condition that persists until they regain orientation (and then suddenly disappears). It's like what happens to you when you get lost in a vast shopping mall parking lot: you may not know left from right for a while, but when you suddenly regain orientation, you regain your navigational skill. You can blame your a blip in primitive 3-D cellular mechanism for that pleasure.
In the mammalian hippocampus, there are three types of brain cells key to navigation: “place” and “grid” cells, which like a biological GPS system, enable animals to keep track of their position - and “head-direction” cells. Those are the ones acting like a compass, responding when said beast turns its head.
Much work had been done on place and grid cells but practically none on the head-direction one. To change that, the researchers enlisted fruit bats with micro-electrodes implanted in their brains, to monitor their neuronal activity, and tracking devices that could tell when the bats moved their little heads.
Fruit bats. Their 'brain compasses' are like ours. Theirs help them fly. Our help us navigate shopping malls. Photo by Courtesy of Weizmann Institute
"Recordings made with the help of these microelectrodes revealed that in a specific sub-region of the hippocampal formation, neurons are tuned to a particular 3D angle of the head: Certain neurons became activated only when the animal’s head was pointed at that 3-D angle," the scientists explained.
The study also found that different nerve cells in the batty brain deal with vertical and horizontal direction: The scientists found that head-direction cells in one region of the hippocampal formation responded to the bat’s orientation relative to the horizontal surface, that is, facilitating the animal’s orientation in two dimensions. The cells that react to the vertical component of the bat’s movement – the 3-D orientation – were elsewhere in the hippocampus.
They suspect that we non-bat mammals use the 2-D head-direction cells when driving, for instance, and the 3-D cells when maneuvering in space – climbing a tree, falling out of one, or piloting a jet.
Though they only worked on fruit bats, the Weizmann team suggests their discovery is likely pertinent to all mammals.
Not relevantly but intriguingly, the hippocampus is named for an animal that has none – the sea horse. That is because it looks like a sea horse. Fish, like the sea horse, have a parallel structure called the pallium. Anyway, the hippocampus is a brain structure involved in memory formation and is bigger in smarter animals, like apes and rats, than in cognitively challenged ones like the echidna.
Our cousin the fruit bat
So why did they use bats? Several reasons, Finkelstein told Haaretz. "The most immediate one is that most of the research so far on how animals navigate and the nervous mechanism explaining navigation had been done in rats." But they were a lousy animal model for spatial navigation and orientation, he says.
Rats generally scurry two dimensions, on flat surfaces. "Most animals, including humans, actually navigate in a three-dimensional world, consisting of many surfaces at different heights. For instance now I'm on the second story of a building. If I had to calculate how to leave the building, via stairs or window, I have to consider a 3-D path," says Finkelstein. (Falling out of window is also considered a 3-D path.)
Possibly proto-people found their 3-D navigational skills more useful when swinging from trees but it's still useful, he sums up. Bats not only fly: they climb cave walls – in any case, they get about in three dimensions all the time, "so they give us access to the third dimension – height," Finkelstein explains.
Second reason: Bats are more like us, in the way they perceive space at least, than rats are.
"Rats and other rodents perceive space with help of senses – we call them proximal senses – that operate at short distances, smell for instance, or touch. Rats sample the environment with their whiskers. Bats on the other hand, and primates, perceive space mainly through distal senses – in the bat's case, sonar and vision."
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