Lisa Giocomo, an assistant professor of neurobiology in the School of Medicine, and Surya Ganguli, an assistant professor of applied physics, received a seed grant in 2014 to figure out exactly how the brain navigates. Some neurons behaved like they were supposed to, but the researchers found that most had noisy, error-prone navigation, much like a broken GPS.
The researchers were left with trying to figure out if the brain was able to correct its errors and the solution involved a type of neuron called a boundary cell. Boundary cells are found in the in the hippocampal formation of the brain.
Boundary cells fire when an animal such as a mouse or person nears walls and other landmarks. Ganguli, Giocomo and Kiah Hardcastle, a Stanford Neurosciences Institute graduate fellow, tracked the firing of neurons in mice that walked around a square box. The boundary cells helped reset wayward grid cells in the same way that seeing a familiar landmark in the world while lost helps people get reoriented.
Recently, the researchers wanted to figure out what else boundary cells could do and discovered that only a few of them could be categorized into predefined categories.
“There were all these cell types that didn’t have a name,” said Hardcastle in a press release. “They weren’t grid or border, head-direction or speed cells, which are the 4 main types. This started as an extension of previous work, but then it really took a left turn.”
Most neurons had a mix of navigational variables. Grid cells or head-direction cells also tracked speed. Speed cells could fire when a mouse moved quickly or slowly, but not at transitional speeds.
They also found that it was difficult to identify neuron types because each neuron responded differently.
“We didn’t see grid cells or speed cells or head-direction cells,” said Ganguli. “We saw this big continuum.”
The findings showed that the brain doesn’t use the same navigation tools as we thought it did and often could be compared to a compass transforming into a GPS while driving.
“The variables that the brain cares about may not be the same as the variables that the mind cares about. There may be a discrepancy between those. And if there is, then we somehow have to break free of the prejudices of our mind in order to understand the brain,” said Ganguli.
The study was funded by the New York Stem Cell Foundation, the James S. McDonnell Foundation, the Burroughs-Wellcome Trust, the Alfred P. Sloan Foundation, the McKnight Foundation, the Office of Naval Research and an NSF-IGERT grant from the Stanford Center for Mind, Brain and Computation and a grant from the Bio-X Interdisciplinary Initiatives Program. It was published online on April 6 in the Neuron journal.
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