As a rodent navigates an environment, its brain forms a mental map by firing a few select cells in the memory-forming region called the hippocampus. Janelia Farm group leader Albert Lee and colleagues found that these cells, which represent 25–50 percent of the total, are primed to take charge of the next piece of information destined for memory storage.
The cells, known as place cells, are considered analogous to cells in the human brain that store memories of people, places, facts, and events. Since most hippocampal neurons act as place cells in certain environments but remain silent in others, Lee was curious about what determines which cells take charge of new spatial memories. To find out, his group compared the electrical activity inside place cells with that in neighboring silent cells.
For a neuron to fire, inputs from neighboring cells must push its membrane potential (a difference in voltage between the interior and exterior of the cell) above a certain threshold. When Lee measured the electrical activity inside hippocampal cells as a rat explored a new maze, he found that place cells received more excitatory inputs than silent cells, but surprisingly they also had a significantly lower threshold for firing. Even before an animal enters a maze, its future place cells behave differently than others, responding to stimulation with a distinctly different firing pattern.
These results, published in the April 14, 2011, issue of Neuron, suggest there is some predetermination of place cell identity even before a new environment is encountered. “[The brain] has a certain pattern that it wants to have for the next memory that it gets,” Lee says. “It doesn’t care so much about the particular details of that thing, it just wants to assign it.” Since there is evidence that the human hippocampus also associates discrete sets of neurons with specific bits of information, Lee suspects this model of memory formation could help explain how those patterns are formed.