April 18, 2013
‘Place Cells’ In Brain Act As Internal GPS, Guiding Rats To Rewards
April Flowers for redOrbit.com - Your Universe Online
Scientists at Johns Hopkins University studying rats' ability to navigate familiar territory found that remembered spatial information is used by one particular brain structure to imagine routes that the rats then follow. Understanding why damage to that structure disrupts specific types of memory and learning could have implications for people with Alzheimer's disease and age-related cognitive decline. The model developed for this study may be useful in future research on higher-level tasks like decision-making because these mental trajectories guide the rats' behavior.
"For the first time, we believe we have evidence that before a rat returns to an important place, it actually plans out its path," says David Foster, assistant professor of neuroscience at the Johns Hopkins University School of Medicine. "The rat finds that location in its mind's eye and knows how to get there."
At least as far as navigation is concerned, the team found that the "mind's eye" is located in the hippocampus. The hippocampus is a structure composed of two banana-shaped segments on both sides of the brain under the cerebral cortex and is best known for creating memories. It is one of the first parts of the brain to sustain damage in patients suffering from Alzheimer's.
They focused the study on a group of neurons in the hippocampus called place cells. These cells are known to fire when animals are at a given location within a given environment. How and when the brain uses that information, however, was not previously understood.
Foster and his colleague Brad Pfeiffer, a postdoctoral researcher in Foster's lab, miniaturized an existing technology to implant 20 microwires into each side of the hippocampus in four rats. They were able to record electrical activity from as many as 250 individual place cells simultaneously.
The rats became familiar with the testing area over a two week training period. The 2-meters-square area was surrounded by a variety of objects that allow the rats to "place" themselves in relation to the objects outside. Inside, the testing area was filled with 36 tiny "dishes" placed at regular intervals in a grid. At any given time, one dish would be filled with the rats' reward: liquid chocolate.
The rats were subjected to as many as 40 sets of alternating "even" and "odd" trials in a day. Odd trials required foraging, for which the rats had to search through the arena to find a chocolate-filled dish in a random location. The even trials required the rats to return to the same "home," dish to find the chocolate. The researchers recorded the firing of their place cells as the rats fulfilled their tasks.
The study, published recently in an online issue of the journal Nature, found that as a rat performed an odd trial in which it traveled randomly through the box, different combinations of place cells fired at each location along the path. When the rat traveled to the same location in an even trial, however, the same set of cells fired every time. These firing combinations create a "mark" for each spot in the rat's brain like a waymark on a GPS, which can be reconstructed into a virtual map as needed.
"Home" place cells in the hippocampus fire in a sequence that creates a predictive path when a rat is about to go to a specific location. The rat then follows this path much similar to Hansel and Gretel following an imagined trail of bread crumbs.
According to Foster, "unlike a Hansel and Gretel bread crumb trail, which only allows you to leave by the same route by which you entered, the rats' memories of their surroundings are flexible and can be reconstructed in a way that allows them to 'picture' how to quickly get from point A to point B." The rats must already be familiar with the terrain between A and B, Foster says. Like a GPS, however, they don't have to have previously started at point A with the goal of reaching point B.
As elderly individuals experience cognitive decline, they commonly lose orientation and easily get lost. Previous research on aging mice found a failure in the ability of their place cells to distinguish between different environments. The findings of this new study suggest that defective place cells could affect a person's ability to "look ahead" and imagine a different scenario as well.
"The hippocampus seems to be directing the movement of the rats, making decisions for them in real time," says Foster. "Our model allows us to see this happening in a way that's not been possible before. Our next question is, what will these place cells do when we put obstacles in the rats' paths?"