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Sunday, September 26, 2021

Memory networks

Researchers from UC Davis and the University of Texas Health Science Center at Houston have revealed the various parts of the brain that work together to recollect memories.

According to Andrew Watrous, the lead author of the study and a UC Davis graduate student, other researchers and doctors have looked at the brain through fMRI scans, but with less direct methods.

“The problems [with past methods] are first, indirect measurements and second, slow resolution or time to develop,” Watrous said.

In contrast, the new method provides new insights by taking far more detailed brain scans and taking scans of a larger area.

“[We] recorded different areas of the brain simultaneously such as the frontal and parietal lobe and areas that were thought to be key in memory retrieval. [The] advantage is that we’re recording brain activity in various areas while we are spatially aware of them,” Watrous said.

Using these recordings, the researchers could record not only which parts of the brain were activated, but when they were activated as well.

“We were the first group to combine these recordings and graph theory,” said Arne Ekstrom, senior investigator and an assistant professor at the UC Davis Center for Neuroscience.

The approach of graphing the recordings of areas of the brain provided a fresh perspective for the study of memory recollection.

In order to record the activity, electrodes were placed inside the skull.

“We work with a neurosurgeon, such as Nitin Tandon. He dissects part of the skull and places the electrodes on the brain in multiple locations, puts the skull back on and the patients recover,” Ekstrom said.

The patients were individuals suffering from epilepsy. Due to their history, the researchers understood what parts of their brain were affected by epilepsy and how they might have been involved in recollecting memories.

In addition, according to Watrous, the neurosurgeon placed the electrodes on both the healthy and the epilepsy-affected parts of the brain to fully comprehend the process of memory recollection.

“You can place the electrodes on top of the brain and they can be read through the scalp. So what is unusual [about this method] is the number of electrodes placed, [providing] unprecedented access to the different signals into the brain,” Ekstrom said.

The more electrodes are used, the more accurate the readings will be, since each electrode is responsible for recording a smaller part.

Through the study, the researchers found that there were different frequencies regarding the type of memory, such as temporal versus spatial.

“The brain resonated at a lower frequency [when considering judgments about space in comparison to judgments about order or time],” Ekstrom said.

The researchers observed these frequencies by considering their oscillation as recorded by the machinery.

“Think of a wave on the ocean. A surfer at the top of the wave has a lot of potential energy that they use as they ride down. We can use these recordings to estimate the frequency components using a variety of methods, such as Fourier transform and wavelet transform,” said Christopher Conner, a researcher on the University of Texas team responsible for data collection.

“Low frequency waves have very high amplitude. These are the huge waves the surfer actually rides. They come once every 10 to 20 seconds. Very high frequency waves have small amplitude. These are the little ripples you see on the top of the wave — just a couple inches tall, hundreds of them per second,” Conner said. “If you look at the wave, there are more ripples at the top than the bottom. It’s the same way with our recordings. What we did was to see how the high amplitude, low frequency waves coordinated the smaller ripples between areas. To push the metaphor to the extreme: Tsunamis travel thousands of miles, ripples don’t. So if you want to send information a long way, use the low frequency range to do it.”

Those ripples or frequencies were recorded while patients learned about new environments or a specific order of activities in the day. When patients attempted to recollect those memories, the researchers would examine the frequencies and the areas involved in recalling the memory.

From this study, researchers glean another insight toward how the human brain works.

“In that sense, we must know how they communicate with one another. This work represents a crucial step in that direction — that areas can use different carrier frequencies to align information transfer,” Conner said.

For future expansion of the study, Ekstrom has considered studying two ideas: what happens if we disrupt the network, and the use of fMRI to map graphs of the human brain to facilitate understanding of the memory recollection process.

According to Conner, brain function can only be truly understood in the context of how the whole brain network operates.

VICTORIA TRANG can be reached at science@theaggie.org.

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