Introduction

We've learned more about the brain and how it functions in the past two decades than in all of recorded history. What is largely responsible for this explosion of information? The answer lies primarily in improved technology. Many years ago the only way brains could be studied was by the initially illegal method of autopsy. And while studying the brain after death provided an enormous amount of information—delineating the areas that allow us to produce and interpret speech, for example—it did little to increase our understanding of how information is processed and stored or why certain students have difficulty learning how to read. Today, imaging techniques allow us to look at the specific brain areas a person uses when recalling a noun versus a verb, or when listening to music versus composing a song. We literally can look inside a brain and see which areas are most active while the person is engaged in various mental activities.

The chronology of brain imaging includes many methodologies that, although older and more primitive, remain viable today. As mentioned, the first method was autopsy, which has been in use since the days of da Vinci and is still useful. Scientists have learned a great deal about what causes Alzheimer's Disease, for example, by studying the brain tissue of those who died of this disease. Scientists have also learned much about the link between structure and function by studying people who have had brain injuries, strokes, or other traumas.

Animal studies have long been used to increase our understanding of how the brain works. This is possible because all mammalian brains function in a similar manner. Even though many of the methods used with animals cannot be applied to human subjects, we will see later that these studies often are useful to increase our understanding of human brain functioning.

Brain Imaging Techniques

X-Rays

The journey to the present “electronic age” of imaging techniques began with the development of the x-ray, discovered in 1895. X-rays are high-frequency electromagnetic waves that easily penetrate nonmetallic objects. When they do, the atoms in the test object absorb some of the radiation, leaving the unabsorbed portion to strike and expose a photographic plate. The more dense objects show up lighter on the plate, while the less dense objects look darker. Although this process works well if we want to see whether a bone is broken (or what objects you are carrying in your luggage at the airport), it is of little use in depicting the brain and other parts of the body that are largely composed of soft tissue with little contrast in density between areas.

Computerized Axial Tomography (CAT) Scans

In the early 1970s, a technique was developed to increase the gradations in shades of grey from the approximately 25 of the normal x-ray to more than 200. This procedure is called computerized axial tomography (CAT) scanning. It uses x-ray technology but combines several two-dimensional images into a set of three-dimensional “slices.” The images resulting from a CAT scan look like a greyish x-ray but give a much clearer and more detailed picture of the brain. Neurologists and neurosurgeons routinely use these pictures to locate and determine the extent of tumors or lesions and the loss of tissue. As sophisticated and useful as they are, however, x-ray and CAT scans do not address the issue of function, which is the primary concern of those of us whose job is to understand the learning process.

Monitoring the Brain's Energy Consumption

To understand how some of the newer imaging techniques work, we need a little background information on the use of energy by the brain. Your brain is the “greediest” organ in the body. The resting brain uses oxygen and glucose at 10 times the rate of the rest of the body. Thus, even though the brain makes up less than 2.5 percent of total body weight, it is responsible for 20 percent of the body's energy consumption. The major sources of energy for the brain are oxygen and glucose, a simple carbohydrate. When certain areas of the brain are active, cells in those areas have a greater need for glucose and oxygen. Scientists realized that if they could trace the flow and consumption of either of these substances in the brain, they could tell which areas were working the hardest and therefore were responsible for a certain action. Around 1973, they began working to develop instruments that could construct an image of the brain by measuring the emissions given off as oxygen and glucose were consumed. Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) are two of the resulting brain-imaging methods. Figure 1.1 shows a comparison of three images of a slice of brain obtained by CAT, PET, and MRI.

Figure 1.1. Imaging Techniques

Note: This figure is not available for electronic dissemination.  Source: Gregory, R. (Ed.). (1987). The Oxford companion to the mind. New York: Oxford University Press, p. 348. Reproduced by permission, courtesy of Drs. Michael E. Phelps, Edward J. Hoffman, and John C. Mazziotta, UCLA School of Medicine.

Starting at 9 o'clock in the figure and moving clockwise are images obtained with standard photography, x-ray CAT, PET (most often created in color), and MRI.

Positron Emission Tomography (PET) Scans

PET is one of the most exciting advances in brain imaging. This technique allows scientists to picture the anatomical areas that become active while a person performs various mental tasks. The subject is injected with a small amount of radioactive glucose, which the blood carries to the brain. The subject is placed in a PET scanner and asked to engage in a series of mental activities such as listening to words, saying words, or generating words. Subjects are given a noun and must generate a verb that they associate with that noun. The brain areas responsible for these various activities will use much more of the radioactive glucose than other areas. When this happens, the radioactive material emits antimatter particles called positrons, which collide with the brain's electrons and produce gamma rays. These gamma rays travel through the skull and can be detected by sensors outside the head. From this information, a computer constructs colored images (tomographs). The areas of the highest glucose use and, therefore, of greatest activity show up in white, red, and yellow, while areas of lesser use glow as green, blue, and purple (Posner & Raichle, 1997, p. 18).

PET does have several drawbacks. For one thing, because it requires the injection of a radioactive tracer, a person is generally allowed only one scanning session (usually 12 scans) a year. For another, neurons fire in milliseconds, but it requires about 40 seconds to obtain the data necessary to build an image of activity with PET. Therefore, how long an area remains active and the sequence of the activation of neural networks is not captured with this methodology. Third, while a PET scan gives an excellent picture of overall activity in the brain, it does not show the specific area in which the activity is occurring.

Functional Magnetic Resonance Imaging (fMRI) Technology

fMRI is one of the newest brain-imaging techniques to address some of PET's shortcomings. To understand how it works, we must first look at the basic MRI technology. A large part of the human body is water, which is made of magnetically polarized molecules. An MRI takes advantage of the fact that the hydrogen atoms in the body's water can be made to behave like tiny magnets if they are placed in a strong magnetic field. A beam of radio waves fired from the MRI scanner will make the molecules of water in the body resonate and give off radio signals of their own. These waves are detected by sensors, and the information is then assembled into an image by a computer (Greenfield, 1997). The imaging of specific organs by this technique far surpasses the detail produced by CAT because the spatial resolution is much finer. In brain research, MRI is used widely to locate tumors and lesions, or to identify other areas of abnormalities.

The primary goal of fMRI is to show not only structures of the brain but also neural activity. First used in England in 1986, fMRI scanning in the United States has expanded over the past few years, partly because MRI scanners now are widely available and partly because they are much less expensive than PET scanners.

fMRI works much like a standard MRI. The subject is asked to engage in an activity such as tapping a finger or listening to a sound. The parts of the brain that are responsible for these activities will cause certain neurons to fire. These neural impulses require energy, so more blood flows to these regions. The oxygen in the blood changes the magnetic field so that the radio signal emitted becomes more intense. The fMRI scanner detects and measures these changes in intensity and produces a computer image. By subtracting this image from an image of the brain at rest, the computer produces a detailed picture of the brain activity responsible for moving a finger or listening to a tone. The scanner produces a rapid series of images, which result in a sort of “movie” of brain activity. The latest scanners can produce four images every second. The human brain reacts to a stimulus in about half a second, so the rapid scanning of fMRI can clearly show the ebb and flow of activity in various parts of the brain as it reacts to different stimuli or undertakes different tasks. A powerful fMRI can thus assemble a functional image of an entire brain in two to six seconds, compared with one minute for a PET scan. In addition, the fMRI can be repeated within seconds, while PET takes nine minutes for the radiation to dissipate (Carter, 1998). fMRI is also less invasive than PET because it does not require the introduction of a radioactive substance into the body.

Electroencephalography (EEG)

Even though the speed of fMRI scanning is impressive, this technique cannot capture the much faster fluctuations in electrical activity that occur as neurons communicate with one another. In order to follow the moment-to-moment changes in neuronal activity, the scientists must turn to other methods such as EEG.

EEG is an imaging tool that has been in use for more than half a century. It measures electrical patterns created by the oscillations of neurons. On an ongoing basis, even during sleep, these electrical signals are constantly flashing throughout the brain. The tissues of the body conduct electricity well, so that sensors placed on the scalp can detect the impulses passing from the brain through the skull and the scalp. The electroencephalograph amplifies the signals and records them on a monitor or paper chart. You are probably familiar with the term brain waves, the name given to these various patterns of electrical activity.

Brain-wave frequency is measured by recording the number of cycles or oscillations per second. The more oscillations per second, the higher the frequency of the wave. During wakefulness, the waves are small and fast and are called alpha waves. Oscillations at the highest frequency, beta waves, occur during attention (beta I waves) and intense mental activity (beta II waves). When we become drowsy and enter into light sleep, the waves slow down and are called theta waves. Entering into deep sleep produces large, slow waves known as delta waves.

Electroencephalography has provided a valuable tool for both researchers and clinicians, especially in the fields of epilepsy and sleep physiology, but it is also used in education-related issues such as language processing. Paula Tallal, language expert at Rutgers University, has used EEG along with MRI to determine that children with normal language skills have “lopsided” brains; that is, the left hemisphere is larger and more active than the right. This makes sense, since we know that in most people the left hemisphere specializes in language processing and the production of speech. Tallal has discovered, however, that the brains of children with language disorders often have balanced brains with both left and right hemispheres nearly equal in size and activity. She determined that the underpowered left hemisphere was not fast enough to adequately process language at normal speeds. A program, Fast Forward, developed by Tallal and Michael Merzenich at the University of California, San Francisco, has been successful in speeding up the processing levels in many children with this brain-based language delay (Tallal, 2000).

On the Horizon

Several newer imaging techniques promise to give us even more detailed pictures of the brain and how it functions. Among these are single-photon emission computerized tomography (SPECT), near-infrared-spectroscopy (NIRS), and magnetoencephalography (MEG). Multimodal imaging, which combines two or more techniques, is becoming increasingly popular.

Research is in progress to see whether neural differences exist between dyslexics and non-impaired readers (Shaywitz, 1999). Another area of great concern, to parents as well as teachers, is attention deficit hyperactivity disorder (ADHD). Early studies support the idea that an underlying neurological dysfunction is linked to the behaviors of children and adults with this disorder. Attempts to understand autism, eating disorders, obsessive-compulsive disorder (OCD), and other problems that affect students' school performance are the focus of numerous current neuroscientific studies.

Interpreting Brain Imaging for Educational Purposes

Will the day come when educators will have ready access to brain-imaging machines to assist them in diagnosing reading or attention problems? It may not be too outrageous to think so. But until that happens, our best bet is to educate ourselves about how these various methodologies work and to understand what they can and cannot do for us. Rarely does neuroscience prove that a particular classroom strategy works, but the information coming from the neurosciences certainly can provide a more informed basis for the decisions we make in our schools and classrooms.

For example, PET scans of a reader show that much more frontal lobe activity occurs when the subject reads silently than when he or she is reading aloud to others. Activity in the frontal lobes often indicates higher-level thinking. On the other hand, the scan of the student reading aloud glows brightly in the motor area of the brain that governs speech, while showing little activity elsewhere. One way to interpret these scans is that there is more comprehension of what is being read when one reads silently. Do these scans prove that students should never read aloud? Of course they don't. Armed with this information, however, teachers are able to make more informed decisions about how to balance silent and oral reading to obtain both diagnostic information on decoding problems and how to enhance comprehension of what is being read.

Synapse Strengtheners

Skim back over this chapter, then close the book and see if you can explain the major differences between a PET scan and an fMRI scan. If you are reading this book as part of a study group, ask each person in the group to identify one common student learning problem and speculate which brain-imaging technique might provide the most information toward understanding the problem and why. Using the diagram of the scans in Figure 1.1, explain to someone who has not read the book how these images were obtained and what they show. Explain how the new brain-imaging techniques affect our thinking about educational practice but do not necessarily prove that certain strategies work.