Q. Why is navigation an interesting problem to think about and study?
A. The word “navigation” conjures up ships on the ocean, but the really difficult problems with navigation are when straight paths don’t work. Then figuring out the specific sequences of turns and moves that will get you from A to B can present a difficult problem, requiring memory of the environment (a map) but also a way to figure out a reasonable path through the environment (a way to read the map).
Q. How does the brain represent space?
A. Neurons (brain cells) in the hippocampus represent space in a very particular way. In any given environment (eg a room), many of the millions of neurons in the hippocampus will be active – sending electrical signals to other neurons – but in a very specific pattern. Each neuron has a favorite place in the room, about the size of the animal, in which the cell will be active. Across the millions of cells, there is something like a patch-work quilt of cells all representing different places, and collectively telling the rest of the brain where the animal is in the room. These cells are often referred to as “place cells”. Note, this isn’t anything physically unique about the cells, but rather refers to what the cells do. Other cells of exactly the same physical type but in other parts of the brain do different things and are not place cells.
Q. What’s missing from previous studies?
A. Just responding in the current location is like a map in which you can only look at the bit of the map corresponding to where you are right now: it doesn’t help you figure out how to get anywhere.
Q. What’s new in this study?
A. We were interested in population events spanning a fraction of second (1/8 to ¼) in which half or more of all the principal neurons in the hippocampus are active at the same time, something which happens every second or so whenever an animal stops running. The specific patterns of activity in these events have been hard to decode, but we used extra high density electrical recordings which allowed us to record from more neurons simultaneously in a freely behaving animal than has been done before. This allowed us to see new patterns of activity across the population that were not known about before.
David Foster has a PhD from Edinburgh University in the U.K., and did postdoctoral training at the Massachusetts Institute of Technology in Cambridge, Mass. Since 2008, he has been Assistant Professor in the Solomon H. Snyder Department of Neuroscience at The Johns Hopkins University School Of Medicine in Baltimore, and a member of the Brain Science Institute. Dr. Foster is an Alfred P. Sloan Research Fellow and a NARSAD Young Investigator, and his work has additionally been supported by grants from the Whitehall Foundation and the National Institutes of Health. Learn more about Dr. Foster