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Mapping the Brain in Action : Functional magnetic resonance imaging, about Mapping the Brain in Action : Functional magnetic resonance imaging - Traditional lie-detector tests are not always dependable. Relying as they do on pulse, blood pressure, breathing rate, and skin conductance (sweating), they incriminate the nervously innocent while exonerating liars who keep their cool. But tellers of tall tales may find a new technique harder to fool. Functionalmagnetic resonance imaging (fMRI) reveals which parts of the brain are activated during different types of activity and it shows that the brains of liars are doing different things from those of truth-tellers.

How does fMRI work? fMRI builds on the earlier technology of magnetic resonance imaging (MRI). MRI makes use of the fact that every hydrogen atom in the body and there are two in every single water molecule inside us is a tiny magnet. (This is because, as we learned in Chapter 7, every accelerating charged particle, including the spinning proton in the nucleus of a hydrogen atom, produces a magnetic field.) Because of this, when living tissue is placed in the field of a very strong magnet typically one that is over 10,000 times more powerful than Earth’s magnetic field all the hydrogen atoms line up

in the field much the way a compass lines up with Earth’s magnetic field. A radio wave is then used to perturb the atoms, knocking them slightly out of line. As the atoms bounce back to their natural alignment within the field, they release a small amount of energy that can be detected and recorded. Because body tissues vary in water concentration, different tissues release different amounts of energy, allowing a very detailed image to be constructed. Like MRI, fMRI constructs images based on different concentrations of water molecules in different parts the body. With fMRI, however, the focus is on blood oxygen levels. Like all cells, neurons in the brain require more energy when they’re active, and so require more oxygen for cellular respiration. In order to accommodate this need, blood flow to active areas of the brain is increased. This increased flow is always in excess of what the active tissue requires, resulting in high local blood oxygen levels that can be detected and converted to an image of active brain areas.

So what happens in the brain when people lie? In one study, volunteers were given a playing card the five of clubs and a $20 bill. They were told they could keep the money if they managed to fool the computer into thinking they had a different card. fMRI maps were made of the brain while volunteers deceitfully denied having the five of clubs and while they truthfully denied having other cards. The maps were then compared. Lying caused increased activity in several areas of the brain (see figure), including those responsible for attention, inhibiting actions, and monitoring errors.

This suggests that lying requires the inhibition of a natural tendency toward truth-telling as well as increased effort and attention. Interestingly, truth-telling did not increase activity in any part of the brain. Lying appears, overall, to be much harder work than telling the truth. fMRI is much too costly to be used regularly for lie detection. However, it has proven invaluable in studies that look at the areas of the brain responsible for different sensations, emotions, and activities. fMRI has already contributed to our understanding of how we remember information, feel love, gamble, recognize faces, and respond to pain.






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