The findings discovered in flying bats change what we knew about the neural activity that allows us to navigate in space
A joint announcement by the Hebrew University and the Weizmann Institute.
More than a decade ago, a "brain GPS" system was discovered in the brains of rodents. The discovery excited the world of science and earned its researchers a Nobel Prize in 2014. However, until now this system has been studied in animals that move on the ground. In a new study published today in the scientific journal Nature, scientists from the Weizmann Institute of Science together with researchers from the Hebrew University studied this system in flying bats and discovered that the three-dimensional space is represented in their brains in a fundamentally different way than they thought until now. A theoretical model to explain the findings, which was developed in cooperation with the researchers of the Hebrew University and the University of California in San Diego for the benefit of the study, suggests that the classic theories about the brain's GPS system will have to recalculate a route.
Deep in the brain of mammals, including humans, hides the area responsible for our ability to orientate in space. In this area, called the entorhinal cortex, are the grid cells, which make up the "brain GPS" system. As we move through a room, a lattice cell is activated in our brain each time we pass through one of several areas. These activity areas are arranged in a hexagonal lattice pattern that lines the floor, so that each activity area is surrounded by six activity areas around it. The symmetrical and repeating pattern that these areas of activity create on the floor of the room provides the brain with a sort of "millimeter paper" that is used to calculate position and distance. The area of the brain where the lattice cells are located is the first to be affected in Alzheimer's disease - and it is possible that spatial disorientation, one of the initial signs of the disease, is a result of damage to the lattice cells and the loss of the arrangement of the millimeter paper.
The secret of the hexagons
Many scientists have tried to decipher the secret of the hexagons of the cells of the lattice - such a symmetrical and repetitive arrangement is astonishing when it is discovered in biology, and many theories have been written about it. Mathematically, the best way to "pack" circles in two dimensions is in a hexagonal arrangement - similar to the arrangement of the honeycomb - and the activity areas of the lattice cells are indeed circular. Therefore, the researchers in the laboratory of Prof. Nahum Ulanovsky in the Department of Neurobiology at the Weizmann Institute expected to see a similar pattern of activity in XNUMXD as well. "We hypothesized, like many other researchers, that when we study the brain representation of the three-dimensional environment, we will envision spheres, instead of circles, arranged in a symmetrical pyramid, like an orange grove on display at the jade," explains Prof. Ulanovsky.
To test their hypothesis, the scientists, led by research student Gili Ginosar along with faculty scientist Dr. Liora Less, recorded the neural activity of lattice cells in bats while they were flying freely in a room the size of a spacious living room, with a miniaturized wireless device on top of them. To make sure that the flight path would indeed be XNUMXD, the researchers placed feeding stations with banana paste, which is especially loved by fruit bats, at different heights and at different points around the perimeter of the room. As the data began to flow, they noticed that the lattice cells were not behaving as expected at all. "The perfectly regular hexagonal lattice that characterizes the representation of the surface in two dimensions has disappeared as if it had never been," Ginoser says.
Instead, the spherical activity areas in their XNUMXD version were arranged like marbles in a box, an arrangement that is less organized than the "pyramid of oranges". Although the spheres were not arranged in a perfect global arrangement, the researchers found a local arrangement - a fixed distance between each sphere and its nearest neighbors.
To provide a mechanistic explanation for these findings, a member of the research team for theorists - Dr. Yonatan Elhadef, a former post-doctoral researcher in Prof. Ulanovsky's laboratory and currently an independent researcher at the University of California, San Diego, and Professors Haim Sompolinsky and Yoram Burke from the Hebrew University of Jerusalem. Together they put together a theoretical model based on principles from the world of statistical physics used to describe the interactions between particles. The theoretical model revealed that the spherical activity zones that represent the activity of the lattice cells in XNUMXD, behave similarly to particles - the activity zones are "attracted" to each other as they get closer to each other, but "repelled" when they get too close; This balance of forces can explain the existence of a local order that keeps the spherical activity areas at a fixed local distance, without an orderly global organization.
The surprising experimental findings, along with the theoretical model, offer us a new way to examine the neural basis for navigation in XNUMXD and the role that lattice cells have in this cognitive process. While previous models assumed that the representation of lattice cells in XNUMXD mimics the XNUMXD representation, the work of Prof. Ulanovsky and his partners and the "marbles in a box" model they developed, indicate a much more complex reality. Given that XNUMXD space is not represented by an ordered lattice, it seems that the classic theories about the cerebral GPS system in mammals are likely to be reexamined.
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