Scientists rarely have the opportunity to study single cell behavior deep inside a living human brain. But a US team was offered the chance to make human brain recordings in epilepsy patients undergoing treatment that implanted electrodes deep inside their brains. The researchers discovered that humans, like other animals, appear to have a type of brain cell that behaves like a GPS.
Scientists had already discovered that the brains of rodents and nonhuman primates have “grid” cells that help the animals keep track of their relative location when navigating in an unfamiliar environment.
Grid cells send signals to another group of cells called place cells, and they both send signals to the hippocampus, an area of the brain that is important for forming memory. Together, the two groups of cells help make a mental picture of where the animal is in its environment.
But apart from a study published in 2010 that used non-invasive brain scans to suggest grid cells exist in human brains, this latest study is the first to show direct evidence of grid cell activity in human brains.
Joshua Jacobs, who runs a cognitive brain dynamics lab at Drexel University in Philadelphia, PA, and colleagues write about their findings in this week’s online issue of Nature Neuroscience.
Grid cells get their name from the triangular grid pattern they appear to use to represent spatial location. Imagine yourself standing on a patterned floor made of interlocking triangle shapes. As you walk around, you cross over from one triangle to another. Grid cells appear to map such a grid pattern because of the way their activity spikes as you traverse the grid.
This cell behavior, which is distinct from that of other brain cells, allows the brain to keep track of how far you have travelled from a starting point, or from your last turn. This type of navigation is called path integration.
Jacobs told the press:
“It is critical that this grid pattern is so consistent because it shows how people can keep track of their location even in new environments with inconsistent layouts.”
For their study, the team recorded brain activity in 14 epilepsy patients as they played a video game on a laptop at their hospital beds. The patients were undergoing a treatment that required them to have electrodes implanted deep inside their brains.
In the video game, the patients used a joystick to control a bicycle that they “rode” around a wide-open terrain. They had to navigate and retrieve objects from various points in an unknown environment, and then recall where the objects were located.
At first they had trial runs where they were able to see the objects from far away. Then the objects were invisible and they had to start from the center of the map, and find the objects. When the bicycle was right in front of an object, it became visible.
In the final stage of the game, the researchers asked the patients to cycle up to particular objects.
During these various stages of the game, the researchers observed the activity of individual cells in the patients’ brains. They saw how different grid cells fired up at different times, depending on where the patient was on their grid.
Jacobs says the triangular grid pattern appears to play a key role in navigation, explaining:
“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.”
Senior author Michael Kahana, a neuroscientist and professor of psychology at the University of Pennsylvania, adds:
“The present finding of grid cells in the human brain, together with the earlier discovery of human hippocampal ‘place cells,’ which fire at single locations, provide compelling evidence for a common mapping and navigational system preserved across humans and lower animals.”
The grid cells in the human patients were found in a part of the brain known as the entorhinal cortex, which is also where they have been found in animals.
The entorhinal cortex is one of the first brain regions to be affected in Alzheimer’s disease. Jacobs says understanding grid cells could help us better understand why people with Alzheimer’s become diosoriented and maybe even find a way to improve their brain function.
Written by Catharine Paddock PhD