For the first time, scientists have detected grid cells in primates while performing a visual memory task.
Grid cells are a type of neurons that fire at multiple locations forming triangular patterns. Scientists had earlier identified grid cells in rats in 2005. But this is the first time that researchers have noticed grid cells in rhesus monkeys.
Researchers from the Yerkes National Primate Research Center, Emory University, monitored the monkeys by recording the electrical activities of the grid cells using electrodes. The electrodes were introduced into the monkeys' entorhinal cortex, a region of the brain that is located in the medial temporal lobe, while the monkeys were looking at images on a computer screen.
Experts also used infrared eye-tracking method to track the movements of the monkeys' eyes and identify which part of the image they were focusing on. Whenever the eyes focused on multiple distinctive locations, the neurons fired forming a triangular pattern.
In earlier research work involving rats, researchers noticed that the grid cells fired when the rats crossed the lines on an invisible triangular grid. But the new study shows that the grid cells fired when the monkeys just moved their eyes.
This suggests that the monkeys don't have to visit a place to construct a mental map of the place. Vision plays a major role in sensing things for primates, but for rodents their touch and smell are important.
Researchers hope the study will help in understanding how humans form and remember mental maps of the world. It could also shed light on how neurodegenerative diseases such as Alzheimer's remove those abilities.
"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 behavioral signs of Alzheimer's," Senior author Elizabeth Buffalo, from the Emory University School of Medicine and Yerkes National Primate Research Center, said in a statement.
"We think these neurons help provide a context or structure for visual experiences to be stored in memory," she said.
"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, a graduate student at Georgia Tech and Emory University.
"This is an exciting way of thinking about memory that may lead to novel treatments for neurodegenerative diseases."
Experts also noticed one more feature of the grid cells in primates. They noticed that the neurons' firing rate reduced when a monkey looked at the same image more than once. More neurons showed memory responses, moving from the posterior (rear) towards the anterior (front) of the entorhinal cortex. This suggests that the grid cells in monkeys not only map the visual field, but also aid in the memory function.
Researchers also noticed "theta-band" oscillations, where the grid cells fire in a way, from 3 to 12 times per second. They suggest that the theta oscillations play a significant role in generating grid cell networks in the brain and also to gather information from these cells.
While both rats and monkeys have theta oscillations, they were found occurring in short periods in monkeys. But it was observed they are not vital to the formation of the spatial representation, the way in which space is represented in the brain.
Grid cells were detected in both rodents and primates while performing various experiments. Although, there are some aspects of the grid cells in primates that are not seen previously in rodents, experts said it doesn't mean that the cells have a different nature in primates.
They are further planning to study how the monkeys move through virtual 3-D space and in real space, including changes in head or body positions to find how grid cells respond.
The findings of the study, "A map of visual space in the primate entorhinal cortex," are published in the journal Nature.