After more than a century of trying to infer brain function by looking at behavior or the effects of brain damage, we now have an array of tools that allows researchers to pinpoint the location of various functions as they happen. The ﬁrst modern technique to be developed is computer-assisted tomography (CT or CAT scan), which takes a series of X-ray pictures of the brain, treating it as a series of layers. The pictures are fed into a computer that enhances the pictures with color to make various brain structures easier to see.
CT scans have actually been rendered nearly obsolete for purposes of studying brain function by the development of positron-emission tomography (PET scan). This technique provides a live view of brain activity as it happens, rather than static photographs. The technique is fairly simple in theory. Radioactive glucose is injected into the bloodstream. When it reaches the brain, more of the radioactive glucose enters cells that are highly active (therefore metabolizing more glucose) than will enter relatively inactive ones. When the brain is viewed in the scanning device, with computers enhancing the image by using different colors for different areas of activity, the areas that are more active will be easily visible due to the presence of the radioactive substance. Such imaging has allowed researchers to identify exactly which regions of the brain, enhanced with different colors, are stimulated by different activities.
Although colorful PET pictures can give a clear idea of the level of activation of various brain areas, they do not show details of the brain’s physical structure. Magnetic resonance imaging (MRI, sometimes also called nuclear magnetic resonance or NMR) uses a radically different technique to do just that. The MRI exposes the brain to a powerful magnetic ﬁeld and measures the resulting radio-frequency wave pattern to provide astonishingly clear pictures of the anatomical details of the brain.
The newest player, functional magnetic resonance imaging (fMRI), combines the best features of MRI and PET scans, by monitoring changes in blood ﬂow that reﬂect changes in the activity level of the neurons in different parts of the brain. This allows a real-time picture of the activity in the brain, like the PET scan, but with the added photorealism of the MRI.
- Engel, S. A. “Using Neuroimaging to Measure Mental Representations: Finding Color-Opponent Neurons in Visual Cortex.” Current Directions in Psychological Science, 8 (1999): 23–27.