fMRI: a very critical review: Romansh note! (Introduction)

by David Turell , Thursday, February 18, 2016, 18:02 (3202 days ago) @ David Turell

Another reminder of what is measured, blood flow and oxygen consumption, nothing more:-http://www.the-scientist.com/?articles.view/articleNo/45366/title/Demystifying-BOLD-fMRI-Data/&utm_campaign=NEWSLETTER_TS_The-Scientist-Daily_2016&utm_source=hs_email&utm_medium=email&utm_content=26383650&_hsenc=p2ANqtz--h6TGTF6isCR92q5ugiju74xIdRiSTuZaZ3IOhs8Ie2etxK9VEykXHmtzwxQ2-s21hCtG87fjLVy4aayP0D5iV15W6lg&_hsmi=26383651/-“'What we do know, of course, is what MRI measures,” said Robert Turner, director emeritus of the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany. MRI measures the magnetization of hydrogen protons in water molecules excited by pulses of radio waves that lead their spins to temporarily align. “Over the next few tens of milliseconds,” Turner noted, “their orientations fan out again, and the magnetization we measure will quickly decrease.”-'But what can this tell us about brain activity?-"When hemoglobins—the iron-rich oxygen-carrying proteins in our blood—run out of oxygen, Turner explained, “they become paramagnetic,” disturbing the local magnetic field. This makes the protons spin out of phase more rapidly.” One might think this means BOLD fMRI highlights oxygen consumption by active neurons, but in reality, such activity is rarely measured.-"What BOLD does reveal is what usually happens next: fresh blood rushes into the area, flushing out paramagnetic deoxyhemoglobin and replacing it with new, oxygenated hemoglobin. Since this does not interfere with the proton spins, the result is a larger fMRI signal. So BOLD fMRI reflects a combination of changes in blood flow and oxygen consumption within the brain—not neuronal activity itself.-“This means that if BOLD shows you a large blob of activity, that doesn't necessarily mean that all the neurons in that region are spiking,” said David Attwell of University College London, one of the meeting's organizers. “So what we really need to know is how neurons are influencing bloodflow.”-***-
"But in some brains, BOLD may not work at all, Hillman cautioned. “In the developing brain of young animals, for example, we find that BOLD activity is very unusual,” she said. “Initially, the bloodflow response doesn't seem to be attuned to neural activity at all, so fMRI may be as good as blind.”-"Diseased brains can also skew results. “Pathology may affect the BOLD signal in the absence of any changes in neurons themselves,” said Bojana Stefanovic of Toronto's Sunnybrook Research Institute. In patients who suffered a stroke, for example, the amount of water may be reduced where cells have died, and increased by oedema in some of the surrounding tissues. The brain's bloodflow may also be altered by disruptions to the vasculature, for example, or the formation of scar tissue.-"The best way to deal with this depends on the research question, Stefanovic told The Scientist. “There's this idea that if we can link BOLD to neuronal activity—that would be nirvana,” she said. “Clinicians, however, are looking for measures with a clear link to symptoms. And, fortunately, there is no shortage of disease effects BOLD can sense.'”-***-“'Surprisingly, we found that while BOLD responds to expected reward and actual outcome separately, the dopamine response integrates them into one ‘better or worse' signal.'"-Comment: Still an indirect measurement of areas of the brain, nothing more.