How Can The Brain Be Imaged?

Technologies that allow us to view what is going on inside the brain are a fairly new and exciting development. This blog provides a very brief explanation of these techniques. There will be frequent references to this blog in future presentations of brain imaging studies.

One of the first techniques was Positron Emission Tomography (PET). PET imaging requires that a radioactive substance called a radiotracer been injected into the bloodstream. This radiotracer makes its way into the brain. The level of radioactivity is extremely low so that the individual undergoing the imaging is not put at risk. The individual lies down within the PET imaging machine and is asked to perform different tasks. A computer processes the data to produce 2- or 3 – dimensional images. The images show blood flow and oxygen and glucose metabolism in the tissues of the brain. These images reflect the amount of brain activity in the different regions of the brain.

Functional Magnetic Resonance Imaging (fMRI) is a more recent development that does not require the injection of radioisotopes into the blood stream. It is an enhancement of Magnetic Resonance Imaging where the individual lies on a table with her head inside a giant magnet. Protons inside the atoms in the brain align themselves with the magnetic field and are wacked temporarily out of alignment by a pulse of radio waves aimed at the brain. As the protons relax back into alignment again, they emit radio waves that a computer uses to create a brain snapshot. fMRI takes advantage of two more facts about the body: (1) blood contains iron and (2) blood rushes to a specific part of the brain when it is activated. As freshly oxygenated blood zooms into a region, the iron distorts the magnetic field enough for the scanner to pick it up.

Prior to the development of these imaging techniques, researchers were restricted to recording electrical activity in the brain from the scalps of humans. Still much valuable data was obtained and these techniques are still used today. Event-related potentials (ERPs) are electrical waveforms that are elicited by specific sights, sounds, or other stimuli. The P300 is a bump in the electrical waveform that occurs within one-third of a second after a person is exposed to a word or some other external stimulus. This heightened activity reflects the additional processing that the brain devotes to novel, distinctive events. Larger P300s tend to be associated with greater subsequent recall.[1]

[1] Reported in Schacter (1996).  Searching for memory:  the brain, the mind, and the past.    New York:  Basic Books.   p. 55.



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