Posts tagged theta oscillations
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(Image caption: In the two brain regions IPF (lateral prefrontal cortex) and V4, a region of the visual system, the brain activity oscillates in a specific frequency range. Credit: © Stefanie Liebe, MPI for biological Cybernetics)
Synchronous oscillations in the short-term memory
School children and university students are often big fans of the short-term memory – not least when they have to cram large volumes of information on the eve of an exam. Although its duration is brief, short term memory is a complex network of neurons in the brain that includes different brain regions. To store the information, these regions must work together. Researchers from the Max Planck Institute for Biological Cybernetics in Tübingen have now discovered that the participating regions must be active at the same time to enable us to form short-term memories of things that happen.
When we see something, signals from the eyes are processed in areas of the cerebral cortex located at the back of the head. For short-term memory, in contrast, regions in the front part of the cerebral cortex must be active. In order for us to remember something we have seen briefly, these far-apart regions of the brain must collate their information.
How this works can only be examined in apes. Scientists from Nikos Logothetis’s Department at the Max Planck Institute for Biological Cybernetics in Tübingen measured the electrical activity in an optic region and in the front area of the brain while the animals had to remember different images.
In the process, the scientists observed electrical vibrations, known as theta-band oscillations, in the two regions of brain. Surprisingly, these oscillations did not arise independently, but were synchronous. The more synchronously active the regions, the better the animals were able to remember an image.
Accordingly, the functioning of short-term memory can be envisaged as two revolving doors: While the memory is at work, the two doors move in time with each other and, in this way, facilitate the more effective exchange of information.
The study shows how important synchronised brain oscillations are for the communication between the different regions of the brain. Almost all higher intellectual capacities result from the complex interplay of specialised neuronal networks in different parts of the brain.
Eye movement rhythm important to eye-tracking diagnoses
Quick eye movements, called saccades, that enable us to scan a visual scene appear to act as a metronome for pushing information about that scene into memory.
Scientists at Yerkes National Primate Research Center, Emory University, have observed that in monkeys exploring images with their eyes, the onset of a saccade resets the rhythms of electrical activity (theta oscillations) in the hippocampus, a region of the brain important for memory formation.
Tracking eye movements is already a promising basis for diagnosing brain disorders such as Alzheimer’s disease and schizophrenia. A deeper understanding of how the rhythm of eye movements orchestrate memories could bolster the accuracy and power of eye-tracking diagnoses.
The findings were published this week in Proceedings of the National Academy of Sciences, Early Edition.
Senior author Elizabeth Buffalo was a researcher at the Yerkes National Primate Research Center and an associate professor of neurology at Emory University School of Medicine and is currently associate professor of physiology and biophysics at Universpity of Washington in Seattle. The first author of the paper is postdoctoral fellow Michael Jutras„ who is now an instructor at the University of Washington.
Theta oscillations are cycles of electrical activity in the brain occurring between 3 to 12 times per second. Scientists have previously seen theta oscillations in the hippocampus in rodents, when the rodents were actively exploring, sniffing or feeling something with their whiskers.
"Both animals and humans seem to take in sensory information at this theta rhythm," Buffalo says. "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."
She says the actions that are most comparable 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 Jutras examined electrical signals in the hippocampi of two rhesus monkeys while the monkeys were looking at a variety of pictures and the researchers tracked their eye movements. The researchers observed that after a saccade, the electrical signals in the hippocampus display a more coherent rhythm.
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.”
Scientists have previously hypothesized that theta oscillations in the hippocampus set the stage for memory formation. The researchers tested this idea by presenting the monkeys each image twice during a viewing session. Because 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."
(Source: news.emory.edu)