Hardly a week passes without some sensational news about brain scans unleashing yet another secret of our cognitive faculties. Very recently I stumbled upon the news that according to recent research neuroscientists can tell, depending on your brain responses, whether you and your significant one will still be together in a few years: “You might hide it from friends and family. But you can’t hide it from neuroscientists”. The technique at the bottom of the study, just like the majority of studies making a big splash, is functional magnetic resonance imaging, fMRI.
Researchers have been struggling to unfold ‘what’s under the hood’ through the lens of Neuroscience and they have been finding all sorts of insights into human behavior. They have been looking at everything from how multitasking is harder for seniors to how people love talking about themselves. Neural basis of love and hatred, compassion and admiration have all been studied with fMRI, yielding colored blobs representing the corresponding love or hatred centers in our brains.
First a brief background: The fMRI technique measures brain activity indirectly via changes in blood oxygen levels in different parts of the brain as subjects participate in various activities. While lying down with head immobilized in a small confined chamber of the notoriously noisy MR scanner, subjects are shown experimental stimuli. They wear earplugs to reduce at least some part of the noise while performing these cognitive tasks.
Like other brain imaging techniques, fMRI technique capitalizes on the coupling of neural activity, cerebral blood flow and energy demand. BOLD (blood oxygen level-dependent) mechanism that forms the basis of the vast majority of the fMRI techniques was first described by Seiji Ogawa.
It is currently believed that when a cognitive task is performed, the area of neural activation becomes more perfused as a result of an increased need for oxygen. This, in turn, increases oxyhemoglobin concentration in the local tissue while the deoxyhemoglobin (hemoglobin without any bound oxygen) found in red blood cells decreases. Deoxy and oxyhemoglobin have different magnetic properties. Deoxyhemoglobin is paramagnetic and introduces an inhomogeneity in the local magnetic field of the hydrogen atoms and reduces the MR signal (MR signal comes from water molecules, hydrogen atoms in the water molecule to be precise). Oxyhemoglobin, on the other hand, is diamagnetic and has little effect. So a decrease in deoxyhemoglobin (i.e. an increase in oxyhemoglobin) would result in an increase in the received signal.
In a nutshell, certain parts of the brain “light up” when people are doing simple cognitive tasks in MR scanners as a result of neural activity and researchers seek correlative information about various brain regions associated with the task or stimulus in question.
Following Ogawa ‘s exciting work linking brain function to the received MR signal, BOLD mechanism was shown in humans by three different groups independently, which in turn started the flood of fMRI publications. The neuroscience and cognitive science communities all embraced brain imaging modalities, especially fMRI, with exuberance and as a result this technique expanded rapidly since its inception and has come to dominate research on the human brain.
Given the giant interest and investments in this technology and the flood of publications revealing countless correlations between the fMRI signals and brain functions, it is astonishing how little we know about the BOLD signal and its accurate interpretation. Roy and Sherrington noted, 120 years ago, that the blood flow was tightly coupled to neuronal metabolism, inspiring later generations of scientist to pursue this proposal to indirectly map the neural activity in the human brain.