“You are here.” It’s the phrase that you’ll find on almost any map, punctuated with the ubiquitous oversized arrow. It is the salient mark in a sea of confusing lines, shapes and labels that provides orientation and a sense of direction. Since the release of Google Maps and smart phones, many of us have become accustomed to having a boundless map in the palms of our hands, one that constantly updates according to our position in the world, complete with a large arrow. But in the absence of a map, directory, or an oversized arrow, how do you find your way? Where is the internal map in your brain and how does it store information about the world in a sea of connected neurons? Neuroscientists have been asking these questions for nearly thirty years now, and we only have a vague idea of how the brain forms internal representations of the outside world.
In the late 1970’s researchers were recording electrical activity from the hippocampus of rats as they explored an open maze. The hippocampus was known to be an important region of the brain for the formation and storage of memories, and, consequently they wanted to see how neurons behaved as a rat explored a new environment. Interestingly, the researchers found that some neurons would become activated when the rat was only in a specific location of the maze. Because these neurons where only activated in one place of the maze, the researchers called them place cells. When enough place cells were recorded from a single rat, researchers found that the whole maze was mapped out and each point within the maze had a corresponding place cell. Put another way, there was a neuron waving a big “you are here” arrow wherever the rat went in the world! The image below shows a cartoon of hundreds of hippocampal neurons, with one neuron storing the information about where the mouse is located in the maze.
Using the electrical read out of place cells in the hippocampus, researchers have learned a lot about how the brain maps out the world that we live in. We know that place cells form a map of the world inside the hippocampus and when these cells are damaged, rats and humans show impairments in spatial learning and memory. Until recently, these place cells have only been observed by recording the electrical activity that they produce when they become activated. In the March 2013 issue of Nature Neuroscience, however, researchers in the laboratory of Mark Schnitzer at Stanford University demonstrated a tiny (2 gram) microscope that could be mounted directly on a mouse’s head so that the mouse could move freely around a maze. The Schnitzer lab used a genetically modified mouse in which neurons fluoresce whenever they become activated, which allowed them to optically record the activity of place cells while the mouse explored a maze. This paper confirmed that a small group of neurons in the hippocampus form a neural map of the external world and that each point in space is represented by only a few neurons.
This paper provides a great step forward in the study of how the brain maps out the world we live in and how even individual neurons play a role in letting you know where you are.
Written by Ryan Jones.
Ziv Y., Burns L.D., Cocker E.D., Hamel E.O., Ghosh K.K., Kitch L.J., Gamal A.E. & Schnitzer M.J. (2013). Long-term dynamics of CA1 hippocampal place codes, Nature Neuroscience, 16 (3) 264-266. DOI: 10.1038/nn.3329
Image made by Ryan T. Jones.