Fast eye movements known as saccades, that allow us to scan a visual scene appear to act as a pacemaker for driving information about a scene into memory, say researchers at Emory University.
In Emory University’s Yerkes National Primate Research Center, scientists have noticed that in monkeys looking at images, the start of a saccade resets the rhythms of electrical activity called theta oscillations, in the hippocampus, a region of the brain key to formation of memory.
Tracking eye movements is already a promising basis for diagnosing brain disorders such as Alzheimer’s disease and schizophrenia. Further understanding of how eye movement rhythms organize memories could boost the accuracy and power of eye-tracking diagnoses.
Theta Oscillations and Saccades
“Both animals and humans seem to take in sensory information at this theta rhythm,” says senior author Elizabeth Buffalo. “But one striking difference between rodents and primates is the way they gather information about the external world. Rodents are much more reliant on the senses of smell and touch.”
Theta oscillations are electrical activity cycles in the brain, which occur between 3 to 12 times per second. Scientists have previously observed theta oscillations in the hippocampus of rodents, when the rodents were actively exploring, sniffing or feeling something with their whiskers.
Buffalo says the actions most similar to rodents’ sniffing and whiskering in primates are saccades. When our eyes scan text or explore a picture, the eyes’ focus tends to jump from point to point several times per second.
Buffalo and co-author Michael Jutras, looked at electrical signals in the hippocampi of two rhesus monkeys while the monkeys were examining different pictures, while researchers tracked their eye movements. The researchers saw that after a saccade, the electrical signals in the hippocampus display a more coherent rhythm (see above illustration).
The rhythm reset a saccade imposes may be a way to ensure the hippocampus is receptive to new sensory information, the researchers propose.
“The eye movements are acting like the conductor of the hippocampal orchestra,” Jutras says, “The phase reset might be a mechanism to ensure the ongoing theta rhythm is in sync with incoming visual information.”
Since all primates have an innate preference for novelty, monkeys tend to spend a longer time looking at new images and less time looking at repeated ones. The researchers inferred that the monkeys had a stronger memory of a given picture if, upon second viewing, they looked through it quickly. The theta rhythm reset was more consistent during the viewing of images that the monkeys remembered well.
“Based on this finding, we concluded that this resetting of the theta rhythm is an important part of the memory process,” Jutras says.
“This study has given us a better understanding of the function of the hippocampal theta rhythm, which has been well characterized in rodents but isn’t well understood in primates,” he says. “A future goal is to investigate the relationship between hippocampal theta and eye movements during memory formation and navigation in humans. This could be possible with epilepsy patients who undergo monitoring of hippocampal activity as part of their treatment.”
Oscillatory activity in the monkey hippocampus during visual exploration and memory formation
Michael J. Jutras, Pascal Fries, and Elizabeth A. Buffalo
10.1073/pnas.1302351110 PNAS July 22, 2013