Neuroscience and Brain Imaging — The Modern Approach
The development of computerized tomography (CT), or “cat scans,” heralded a major change in the way neuroscientists study the brain. CT is basically an x-ray procedure that produces pictures of “slices” of the brain taken from rotated angles. The pictures have limited resolution, but CT can be highly effective in spotting brain damage and tumors. Magnetic resonance imaging (MRI) is better: here, higher-resolution pictures are produced by sending pulses of radio waves through the brain to a magnetic coil.
However, neither approach can provide information about ongoing brain activity and functions. The truly remarkable breakthroughs came with the advent of more sophisticated techniques and computers capable of imaging brain activity as it occurs. Brain imaging has become such a common part of popular culture — often discussed on television crime programs and in magazine articles — that it is easy to overlook how important and revolutionary this technology really is.
Positron Emission Tomography
In addition to oxygen, the brain runs on a steady supply of glucose (blood sugar), and positron emission tomography (PET) takes advantage of this by the injection of glucose treated with harmless amounts of radioactivity. As neural cells metabolize this glucose, special detectors placed on the head record the activity, and computer enhancement yields color images of the level of the activity in different areas of the brain. As a simple example, different areas might be more active when participants are looking at an object, thinking about it, or talking about it. However, the time required for metabolism produces delays of a minute or more between when the brain activity occurs and when it is displayed, which limits the accuracy of PET. Given the speed at which neural impulses travel — about 100 miles per hour — the researcher is always looking at the past instead of the present.
Functional Magnetic Resonance Imaging
Functional magnetic resonance imaging (fMRI) is much better and is the current method of choice. Here, precision detectors transmit information about areas of naturally occurring oxygen metabolism in the brain for computer enhancement, and the delay is seconds instead of a minute or more. Researchers are able to get a much clearer look at the brain and evaluate much smaller structures than they can using a PET scan.
It appears that brain-imaging technology will provide the basis for the lie detectors of the not-so-distant future. If specific areas of the brain are consistently active or certain types of ERPs (event-related potentials) appear when people lie, brain-imaging techniques could be much more reliable than today's polygraphs.
Because no “invasive” radioactive substances are involved, fMRI can also be used with any kind of participants — including infants. And the resolution is much finer, although the precision is still on the order of at least a few hundred thousand neurons that might or might not be doing the same thing. It's a safe bet, however, that researchers will continue to refine this method.
Electroencephalography has been around for a long time. Originally, it consisted of placing two or three electrodes on the skull to measure overall brain electrical activity, such as during wakefulness or varying levels of sleep. In its present form, called quantitative electroencephalography (QEEG), the number of electrodes has greatly increased, a skullcap standardizes their location, and the much more localized measurements are subjected to computer analysis. This yields event-related potentials (ERPs), that is, minute electrical changes that are studied to localize brain activity — with only milliseconds of delay. This speed also allows QEEG to be used to mark timing as a cross-reference for fMRI, thus increasing its accuracy.