Scientists at Yerkes National Primate Research Center, Emory University, have identified grid cells, neurons that fire in repeating triangular patterns as the eyes explore visual scenes, in the brains of rhesus monkeys.
The finding has implications for understanding how humans form and remember mental maps of the world, as well as how neurodegenerative diseases such as Alzheimer's erode those abilities.
This is the first time grid cells have been detected directly in primates. Grid cells were identified in rats in 2005, and their existence in humans has been indirectly inferred through magnetic resonance imaging.
Grid cells' electrical activities were recorded by introducing electrodes into monkeys' entorhinal cortex, a region of the brain in the medial temporal lobe.
At the same time, the monkeys viewed a variety of images on a computer screen and explored those images with their eyes.
Infrared eye-tracking allowed the scientists to follow which part of the image the monkey's eyes were focusing on. A single grid cell fires when the eyes focus on multiple discrete locations forming a grid pattern.
"The entorhinal cortex is one of the first brain regions to degenerate in Alzheimer's disease, so our results may help to explain why disorientation is one of the first behavioural signs of Alzheimer's," said senior author Elizabeth Buffalo.
"We think these neurons help provide a context or structure for visual experiences to be stored in memory," Buffalo said in a statement.
"Our discovery of grid cells in primates is a big step toward understanding how our brains form memories of visual information," said first author Nathan Killian.
In the experiments in which rats' grid cells were identified, the cells fired whenever the rats crossed lines on an invisible triangular grid.
"The surprising thing was that we could identify cells that behaved in the same way when the monkeys were simply moving their eyes," Buffalo said.
"It suggests that primates don't have to actually visit a place to construct the same kind of mental map," she added.
Another aspect of grid cells not previously seen with rodents is that the cells' responses change when monkeys are seeing an image for the second time.
Specifically, the grid cells reduce their firing rate when a repeat image is seen. Moving from the posterior (rear) toward the anterior (front) of the entorhinal cortex, more neurons show memory responses.
"These results demonstrate that grid cells are involved in memory, not just mapping the visual field," Killian said.
The study was published in the journal Nature.
