radioactive tracers can measure substances other than blood flow. Specifically, they can be used to measure the activity of different neurotransmitters, brain chemicals that are centrally involved with psychological processes.
Brain imaging technology allows us to view the workings of a living brain, an extraordinary accomplishment possible only in the last few decades.
How does an fMRI work?
Instead of relying on radioactive decay to image the brain, fMRI measures properties related to magnetic activity. MRI scanners do this by creating a large magnetic field. A typical research MRI scanner has a field strength of 3 Tesla, which is 60,000 times the strength of the Earth’s magnetic field! Most fMRIs measure changes in the magnetic field related to the level of oxygen in the blood, commonly referred to as the blood oxygenation level dependent or BOLD signal. Therefore, like PET scans, fMRIs use a measure of blood flow as an indicator of general activity in specific regions of the brain. The scanners take multiple scans simultaneously, covering the entire brain, with a new scan about every 2 seconds. As mentioned above, fMRI is cheaper than PET, is far more fine-grained across time and space and does not expose subjects to radioactivity. But it also has limitations. The strength of the magnetic field can be very dangerous if any metal is in the body. Therefore fMRI cannot be used in people with metal pins, pacemakers, or other metal inserts in their body. Moreover fMRI is very sensitive to motion and so is difficult to use with children or other restless people.
What is the difference between a structural MRI and functional MRI (fMRI)?
The technology of structural MRI (generally just referred to as MRI) preceded the technology of functional MRI (fMRI). Like fMRI, MRI technology measures magnetic properties in the brain. Instead of measuring brain function—that is, how the brain behaves—it measures brain anatomy. In very fine detail, it shows us what the physical brain looks like. Because it can see right through bones, it is able to give us a beautiful picture of brain tissue under the skull. This is extremely important when evaluating patients for brain damage, for example, after a stroke or a car accident. It can also tell us whether different groups of people differ in the anatomy of their brain, for example, whether chess players have larger parietal lobes than non-chess players (parietal lobes process spatial information). However, because people do not tend to differ that much in their brain structure in the absence of dramatic injury or disease, fMRI scans are used more frequently than MRI scans in psychological research.
What does an EEG measure?
Electroencephalography (EEG) is one of the oldest methods of studying the brain in living people. It is used to measure electrical activity in the brain, which is a reflection of brain cell (or neuronal) activity. EEG is non-invasive, in that there is no need to insert anything into the body. Electrodes are placed throughout the scalp in order to measure electrical activity across the brain. Because the electrodes sit on the scalp and do not directly touch brain tissue, they are most sensitive to electrical activity in the parts of the brain that are closest to the scalp. Unfortunately, it is impossible to identify exactly where the electrical signals are originating. In this way, EEG is far inferior to fMRI and PET with regard to mapping brain activity in space. Nonetheless, EEG is very sensitive to time and can provide a precise measure of the change in brain activity over time. EEG is also cheaper than fMRI or PET and is very easy to administer. EEG is most frequently used to assess for seizure activity as well as for changes in brain firing patterns during sleep.
What other kind of brain imaging technologies are there?
As technology continues to advance, more and more sophisticated techniques to study the brain have become available. Similar to EEG, magnetoencephalography (MEG) measures brain activity at the scalp, only it measures magnetic rather than electrical activity. Another non-invasive method, near-infrared spectroscopy (NIRS) uses a light source to measure changes in the amount of oxygen in the blood. More invasive techniques are also used. In deep brain stimulation (DBS), electrodes are implanted directly into the brain. These depth electrodes can monitor and stimulate electrical activity in very small areas of the brain. Because deep brain stimulation involves brain surgery, it is only performed as part of the surgical treatment of patients with brain diseases such as Parkinson’s disease or epilepsy. In non-human animals, however, electrodes are frequently implanted in the brain to measure the behavior of specific brain regions or even single neurons during various experimental conditions.
How many neurons are in a voxel?
Avoxel is the smallest unit of measurement of an fMRI. It is a three-dimensional cube. In MRI scans, the smallest unit of measurement is called a pixel. A voxel is approximately 2.5 millimeters on each side. Each voxel covers approximately one million neurons (or brain cells).
What advances have been made in gene studies?
In recent years, there has been an explosion in our ability to study the impact of specific genes. Since the entire human genome was decoded in the early 2000s, our capacity to unlock the secrets of our genes has greatly expanded. Most genetic studies are called linkage studies. This means that scientists attempt to link a specific behavioral trait, condition or abnormality to an altered form of a specific gene, called a candidate gene. The different varieties of a specific gene are called polymorphisms. Considerable research has investigated genetic polymorphisms that affect key neurotransmitters related to many psychological processes and psychiatric disorders. More specifically, multiple polymorphisms have been linked to the brain’s processing of dopamine, serotonin, norepinephrine, and acetylcholine. These neurotransmitters are known to play important roles in psychiatric disorders, such as schizophrenia, depression, and attention deficit disorder.
What are the limitations of gene studies?
Although the progress in modern genetics is fairly breathtaking, we should not overestimate what gene studies can tell us. What genetic research has shown us very clearly is that the genetic contributions to human psychology are extremely complicated. Multiple genes contribute to any one psychological trait (polygenicity) and most genes serve multiple functions (pleiotropy). Likewise, interactions among various different polymorphisms and different genes impact the expression of genes in complicated and as of yet unknown ways. Finally, genetic studies cannot account for the enormously powerful effect of environment on all aspects of psychology. Therefore while it is exciting when research shows a correlation between polymorphism A in gene B and a particular psychological trait, this likely explains only a small part of the whole picture.
What are the problems with correlational studies?
Most neurobiological studies are correlational; they show that certain brain patterns can occur together with certain behaviors. For example, patients with high levels of depression show reduced activity in their frontal lobe. This tells us that there may be a relationship between some specific brain activity and a psychological condition. It does not, however, tell us much about the nature of the relationship between the two. We cannot know if the brain activity caused the psychological condition or if a third factor caused both the brain activity and the psychological condition. Nor do we know how closely related the brain activity is to the psychological condition. For example, many travelers carry books to read on their vacations, but the vacation can take place without the books and the books can be read outside of travel.
What are association, necessity, and sufficiency in neurobiological research?
In a 2011 article about the use of neuroscience techniques in psychological research, Joseph Kable proposed that research into brain-behavior relationships should assess association, necessity, and sufficiency. Association refers to correlation. This means that two phenomena have been shown to occur together. People show increased activity in their insula on brain scans when asked to spend money. People with a certain genetic polymorphism are more likely to enjoy