'Grid cell' linked to orientation Aug 4th 2013, 18:54
Sense of direction has been traced to a nerve cell in the brain that acts like an in-built GPS.
The “grid cell” keeps track of multiple co-ordinates on an internal map and helps the brain to navigate without landmarks.
Although previously identified in rats, bats and monkeys, scientists now have compelling evidence that grid cells also exist in humans. They may even play a bigger role in the human brain than in rodents.
The discovery arose from a study of people with electrodes placed deep in their brains as part of a radical epilepsy treatment. Recordings of electrical signals from the implants revealed patterns of grid cell activity as the patients played a computer game that involved riding bicycles in an open virtual landscape.
Grid cells provide a system of navigation called “path integration” which tracks distances and turns. It helps us sense how far we have travelled from a starting point, or last change in direction.
“Each grid cell responds at multiple spatial locations that are arranged in the shape of a grid,” said study leader Dr Joshua Jacobs, from Drexel University in Philadelphia, US. “This triangular grid pattern thus appears to be a brain pattern that plays a fundamental role in navigation. Without grid cells, it is likely that humans would frequently get lost or have to navigate based only on landmarks. Grid cells are thus critical for maintaining a sense of location in an environment.”
Fourteen epilepsy patients in hospital took part in the study, reported in the journal Nature Neuroscience. They used joysticks attached to bedside laptops to steer their virtual bicycles across a large square arena overlooked by a range of mountains.
The patients were first asked to navigate to four clearly visible objects. Then the exercise was repeated, but this time the objects remained invisible until they were located. The volunteers had to find their way back to each object in turn, in random order. Finally, they were asked to ride directly from one object to another. Again, the objects were made invisible until their locations were reached.
At the same time, electrical nerve signal “spikes” were recorded from the epilepsy implants. Evidence of grid cells was mostly seen in two brain regions, the entorhinal cortex – where they had previously been found in rats – and cingulate cortex. Significant numbers of grid-like neurons were also identified in the hippocampus, which plays a central role in memory.
The entorhinal cortex is one of the first brain areas to be affected by Alzheimer’s disease. Understanding grid cells may shed light on why people with Alzheimer’s often become disorientated, and possibly point the way towards new treatments, say the researchers.
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