August 4, 2009
The Brain’s fMRI Activity Is An Indication Of Expectations
That phrase not only gets your muscles pumping but also starts blood flowing to the appropriate places in the brain. However, researchers at Baylor College of Medicine found that while measurements of electrical activity showed peaks in the "ready" or anticipatory phase, the blood flow in the brain measured by functional magnetic resonance imaging (fMRI) didn't start until "go." The results could have application in some mental disorders such as schizophrenia.
"Your brain is always working to predict when things are likely to happen. For example, we expect a traffic light will turn green or a response from another person during a conversation will happen in a certain amount of time," said Dr. David Eagleman, assistant professor of neuroscience and psychiatry and behavioral sciences at BCM. "If the brain is not putting things together correctly in the time domain, what you get is cognitive fragmentation, which could be behind some mental disorders. Seeing how the brain constructs its perception of time helps us to understand what happens when that perception goes wrong."
The findings appear in the current edition of PLoS Biology.
"We found that the strength of the fMRI signal is not dependent on how long you waited for the go signal, but when you expected it to happen," said Eagleman.
Participants took part in a variety of experiments to measure how the brain encodes such expectations using fMRI. The interval between ready and go was between 4 to 12 seconds.
"When you're forced to wait a longer time for the go signal, the fMRI response is larger," said Eagleman. "But it also depends on the way we rig the probabilities. So in some experiments we make the go signal likely to arrive earlier, and in other blocks, later. And this is how we can show that this new fMRI response reflects expectation, not simply elapsed time."
Eagleman said it is almost as if the brain is "holding its breath," waiting for the precise moment to activate blood flow to a specific area.
Neural activity leads to changes in blood flow through a series of steps. Chemicals are produced that then tell the blood vessels that blood is needed in specific areas. Eagleman speculates that during certain waiting tasks, the steps are put on temporary hold before blood is called to the scene.
"The brain is an extremely efficient organ and waits for the right moment," said Eagleman.
Eagleman and his colleagues had thought their results might more closely mimic those found when experts measured electrical energy in neurons or nerve cells. Those electrophysiology experiments showed that electrical activity increased while the subject was waiting for the "go" command. "Electrophysiology is only looking at one aspect of how the brain works," Eagleman said. "We are approaching it from a different angle. It's very important when you find a different story using different technology."
The next step is to see how electrophysiology and fMRI activity relate to each other in these cases.
"These findings are solid, but puzzling. The (fMRI) imaging results disagree with current electrophysiological measurements during similar tasks," said Dr. P. Read Montague, professor of neuroscience and psychiatry and behavioral sciences at BCM. "We hope that this may open the door to a better understanding of exactly the kinds of information measured by modern neuroimaging methods."
Others who took part in the study include first authors Dr. Xu Cui, currently at the department of psychiatry and behavioral sciences at Stanford University, and Chess Stetson, now with the computation and neural systems program at the California Institute of Technology. Both are former graduate students at BCM. Montague is director of the Human Neuroimaging Lab and the Computational Psychiatry Unit at BCM where the analysis took place. In addition to directing the Laboratory for Perception and Action, Eagleman also directs Baylor College of Medicine's Initiative on Neuroscience and Law.
The research is supported by grants from the National Institute of Neurological Disorders and Stroke, the National Institute on Drug Abuse and the Kane Family Foundation.
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