April 19, 2013
Studying How Brains Work When Bats Are On The Move
April Flowers for redOrbit.com - Your Universe Online
Two new studies, published this week in Science, examine the mechanisms of animal navigation using bat models.
The first study is the result of collaboration between University of Maryland (UMD), College Park and Boston University (BU). The findings, based on brain rhythms in bats and rats, challenges a widely-used model — based solely on rodents — of how animals navigate their environment.
The study authors say that neuroscientists, in order to get a clearer picture of the processes at work in the mammalian brain during spatial navigation, must closely study a broad range of animals instead of just one species.
The scientists reported significant difference between the brain rhythms of bats and rats in the part of the brain used for navigation. They found specialized cells used for processing spatial information in a region of the brain called the medial entorhinal cortex. This region is a hub of neural networks for memory and navigation.
Previous studies using rat models, shows that brain cells in this area fire continuously in a rhythmic electrical signal called a theta wave as the animals navigate through space. Some brain models treat theta waves as a key element of spatial navigation in all mammals. The problem, according to the research team, is that this idea is based solely on rodent research.
The team tested bat and rat brain tissue for rhythmic electrical responses at the cellular level, finding evidence for theta waves in the rat cells. However, the theta waves were not present in the bat cells.
"This raises questions as to whether theta rhythms are actually doing what the spatial navigation theory proposes," said UMD biology researcher Katrina MacLeod in a statement. "To understand brains, including ours, we really must study neural activity in a variety of animals."
Many common features of brain organization are shared between humans and other mammals. The differences in theta waves between bats and rats raises questions about how spatial information is represented in all brains.
BATS IN FLIGHT
The second study is the work of Dr. Nachum Ulanovsky and research student Michael Yartsev of the Weizmann Institute of Science's Neurobiology Department. The findings reveal how three-dimensional, volumetric, space is perceived in mammalian brains for the first time. A unique, miniaturized neural-telemetry system developed especially for this task was used to conduct the research. This enabled the measurement of single brain cells during flight.
There have been extensive studies attempting to answer the question of how animals orient themselves in space. Until now, however, experiments were only conducted in two-dimensional settings. These previous studies have found that orientation relies on "place cells" — neurons located in the hippocampus, which is a region of the brain involved in memory. Responsible for a specific spatial area, each place cell sends an electrical signal when the animal is located in that area. Collectively, the place cells produce full representations, or virtual memory maps, of whole spatial environments. The navigation of many animals, however, is carried out in three dimensions, unlike the laboratory experiments. Attempts to expand the scope of such experiments from two to three dimensions have been challenging.
The University of Arizona and NASA conducted one of the more famous efforts in this area. They launched rats into space aboard a space shuttle. Although the rats moved around in zero gravity, they still ran along a set of straight, one-dimensional lines. So far, other experiments attempting three dimensional projections onto two dimensional surfaces have not produced volumetric data, either.
The researchers concluded that in order to understand movement in three-dimensional, volumetric space, it is necessary to allow animals to move through all three dimensions — that is, to research animals in flight.
The Weizmann team studied the Egyptian fruit bat, which is very common in Israel. The bat is rather large, allowing the scientists to attach the wireless measuring system without restricting the bats' movements. It took Ulanovsky several years, in cooperation with a US commercial company, to develop the wireless, lightweight (four ounces, about seven percent of the weight of the bat) device containing electrodes that measure the activity of individual neurons in the bat's brain.
The team then had to find a way to adapt the behavior of their bats to the needs of the experiment. Bats tend to fly directly to their destination in a straight line, making their normal flight patterns one-dimensional. The experiment, however, required their flights to fill a 3D space.
A previous study by Ulanovsky's group provided the answer. This study tracked wild fruit bats using miniature GPS devices. They found that when bats arrive at a fruit tree, they fly around it using the full volume of space around the tree. In order to simulate this behavior in the lab -- an artificial cave equipped with an array of bat-monitoring devices — the team installed an artificial "tree" made of metal bars and cups filled with fruit.
Measuring the activity of place cells in the bats' hippocampus, the study revealed that the representation of three-dimensional space is similar to that in two dimensions. Even in 3D, each place cell is responsible for identifying a particular spatial area in the "cave" and sends an electrical signal when the bat is located in that area. All three dimensions of the cave are represented in the full population of place cells — left, right, up and down.
Examining the place cells closely allowed the team to answer a highly debated question: Does the brain perceive the three dimensions of space as "equal," that is, does it sense the height axis in the same way as that of length or width?
They found that the perception of all three dimensions is uniform, suggesting that each place cell responds to a spherical volume of space. The team does caution that for non-flying animals, the resolution of the different axes might not be perceived the same. The implications for humans are particularly interesting, as humans evolved from tree swinging primates who moved in 3D space, however, modern humans are ground-dwelling animals who generally navigate in a two-dimensional space.
The study provides novel insights into some basic functions of the brain, specifically navigation, spatial memory and spatial perception using innovative technology that allowed the first glimpse into the brain of a flying animal. Ulanovsky believes this trend towards more "natural" research is the future of neuroscience. Laboratories that simulate natural conditions — or even animals in their natural habitats — will form the neural basis for animal behavior investigations.