Animals navigate by calculating their current position based on how long and how far they have traveled and a new study on treadmill-running rats reveals how: neurons called grid cells integrate information about time and distance to support memory and spatial navigation, even in the absence of visual landmarks. The findings, published in an open-access article in the November 4, 2015 issue of Neuron, challenge currently held views of the role of grid cells in the brain. The article is titled “During Running in Place, Grid Cells Integrate Elapsed Time and Distance Run.” "Space and time are ever-present dimensions by which events can be organized in memory," says senior study author Howard Eichenbaum, Ph.D., a psychologist and neuroscientist at Boston University. "These findings support the view that memory evolved as a common function in mammals using circuits that organize events in space, time, and potentially many other dimensions of experience." Past research has shown that grid cells receive information from other cells about the direction traveled. But until now, there was no direct evidence showing that grid cells signal distance or time, leaving the role of these variables in path integration merely speculative. In the new study, Dr. Eichenbaum and first author Benjamin Kraus, Ph.D., also of Boston University, and colleagues, addressed this question by placing rats on treadmills while recording the activity of grid cells. The researchers kept either the run duration or run distance fixed, while varying the speed, in order to disentangle the influence of these factors on cell firing. During treadmill running, 92% of grid cells fired at specific moments or distances while the rats ran in place.
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