Summary: Historically, scientific research has largely focused on the gray matter of the brain, leaving the equally important white matter understudied. However, a recent groundbreaking study has used fMRI to detect significant brain activity in white matter.
When subjects performed tasks, researchers observed increased BOLD signals throughout the white matter.
This discovery challenges conventional beliefs about the brain’s activity and emphasizes the potential importance of white matter in understanding various brain disorders.
- The Vanderbilt team, led by John Gore, Ph.D., utilized fMRI to identify BOLD signals, indicative of brain activity, in the white matter—previously a little-researched area.
- When subjects performed tasks during the study, there was a noticeable increase in BOLD signals in the white matter across the entire brain.
- Despite the current lack of full understanding about these white matter signals, they’re believed to hold valuable insights, especially since many brain disorders, including epilepsy and multiple sclerosis, disrupt the brain’s “connectivity.”
Source: Vanderbilt University
The human brain is made up of two kinds of matter: the nerve cell bodies (gray matter), which process sensation, control voluntary movement, and enable speech, learning and cognition, and the axons (white matter), which connect cells to each other and project to the rest of the body.
Historically, scientists have concentrated on the gray matter of the cortex, figuring that’s where the action is, while ignoring white matter, even though it makes up half the brain. Researchers at Vanderbilt University are out to change that.
For several years, John Gore, Ph.D., director of the Vanderbilt University Institute of Imaging Science, and his colleagues have used functional magnetic resonance imaging (fMRI) to detect blood oxygenation-level dependent (BOLD) signals, a key marker of brain activity, in white matter.
In their latest paper, published Oct. 12 in the Proceedings of the National Academy of Sciences, the researchers report that when people who are having their brains scanned by fMRI perform a task, like wiggling their fingers, BOLD signals increase in white matter throughout the brain.
“We don’t know what this means,” said the paper’s first author, Kurt Schilling, Ph.D., research assistant professor of Radiology and Radiological Sciences at VUMC. “We just know that something is happening. There truly is a powerful signal in the white matter.”
It is important to pursue this because disorders as diverse as epilepsy and multiple sclerosis disrupt the “connectivity” of the brain, Schilling said. This suggests that something is going on in white matter.
To find out, the researchers will continue to study changes in white matter signals they’ve previously detected in schizophrenia, Alzheimer’s disease and other brain disorders. Through animal studies and tissue analysis, they also hope to determine the biological basis for these changes.
In gray matter, BOLD signals reflect a rise in blood flow (and oxygen) in response to increased nerve cell activity.
Perhaps the axons, or the glial cells that maintain the protective myelin sheath around them, also use more oxygen when the brain is “working.” Or perhaps these signals are somehow related to what’s going on in the gray matter.
But even if nothing biological is going on in white matter, “there’s still something happening here,” Schilling said. “The signal is changing. It’s changing differently in different white matter pathways and it’s in all white matter pathways, which is a unique finding.”
One reason that white matter signals have been understudied is that they have lower energy than gray matter signals, and thus are more difficult to distinguish from the brain’s background “noise.”
The VUMC researchers boosted the signal-to-noise ratio by having the person whose brain was being scanned repeat a visual, verbal or motor task many times to establish a trend and by averaging the signal over many different white matter fiber pathways.
“For 25 or 30 years, we’ve neglected the other half of the brain,” Schilling said. Some researchers not only have ignored white matter signals but have removed them from their reports of brain function.
The Vanderbilt findings suggest that many fMRI studies thus “may not only underestimate the true extent of brain activation, but also … may miss crucial information from the MRI signal,” the researchers concluded.
About this neuroscience research news
Original Research: Closed access.
“Whole-brain, gray, and white matter time-locked functional signal changes with simple tasks and model-free analysis” by Kurt G. Schilling et al. PNAS
Whole-brain, gray, and white matter time-locked functional signal changes with simple tasks and model-free analysis
Recent studies have revealed the production of time-locked blood oxygenation level–dependent (BOLD) functional MRI (fMRI) signals throughout the entire brain in response to tasks, challenging the existence of sparse and localized brain functions and highlighting the pervasiveness of potential false negative fMRI findings.
“Whole-brain” actually refers to gray matter, the only tissue traditionally studied with fMRI. However, several reports have demonstrated reliable detection of BOLD signals in white matter, which have previously been largely ignored.
Using simple tasks and analyses, we demonstrate BOLD signal changes across the whole brain, in both white and gray matters, in similar manner to previous reports of whole brain studies.
We investigated whether white matter displays time-locked BOLD signals across multiple structural pathways in response to a stimulus in a similar manner to the cortex. We find that both white and gray matter show time-locked activations across the whole brain, with a majority of both tissue types showing statistically significant signal changes for all task stimuli investigated.
We observed a wide range of signal responses to tasks, with different regions showing different BOLD signal changes to the same task. Moreover, we find that each region may display different BOLD responses to different stimuli.
Overall, we present compelling evidence that, just like all gray matter, essentially all white matter in the brain shows time-locked BOLD signal changes in response to multiple stimuli, challenging the idea of sparse functional localization and the prevailing wisdom of treating white matter BOLD signals as artifacts to be removed.