While measuring bodily and biochemical changes in the body is a source of important information, brain imaging technology has also given us significant insights into the causes of depression.
Brain imaging technologies are a group of non-invasive techniques that allow scientists to examine the whole brain or portions of it without having to perform surgery. Imaging procedures provide doctors with information about brain structure. This can include what different parts of the brain look like, as well as brain function and how the brain is behaving during different activities. Structural brain imaging techniques produce photographs or models of the brain. In contrast, functional imaging techniques produce "brain movies" that show how the various parts of the brain interact through time. Functional imaging technologies depend upon measurements of brain metabolism (e.g., oxygen and glucose use) and blood flow rates to make these movies possible. These imaging techniques include:
- Computed Axial Tomography (CT or CAT) - uses special x-ray equipment to measure the amount of radiation being absorbed throughout a person's body. This measurement is used to build a picture of the brain.
- Magnetic Resonance Imaging (MRI) - MRI uses radio frequency waves and a strong magnetic field to create 3D computer images of internal organs and tissues.
- Positron Emission Tomography (PET) - uses trace amounts of short-lived radioactive material (called a tracer) to map functional processes in the brain. This technique allows scientists to determine the metabolic rates of the brain by measuring oxygen and blood sugar (glucose) use. Areas of the brain that are active use more oxygen and glucose than areas that are not active. A computer records a 3D image of the brain, and the areas that are actively metabolizing sugar and oxygen "light up" with different colors.
- Functional MRI (fMRI) - also allows us to determine which parts of the brain are active. Rather than glucose levels, fMRI measures blood flow. Magnets in the fMRI scanner use the natural magnetic properties of blood and water in the body, and create a color-coded image on a computer screen. The fMRI image shows researchers which areas of the brain have the highest (more activity) and lowest (low activity) blood flows.
- Electroencephalography (EEG) - this is a measurement of the electrical activity of the brain and shows an electrical signal from a large number of neurons. By placing electrodes on the scalp, clinicians and researchers get a read-out in graph form called an electroencephalogram rather than an image.
- Magnetoencephalography (MEG) - an imaging technique used to measure the magnetic fields produced by electrical activity in the brain. It is a powerful technique because it measures ongoing brain activity on a millisecond-by-millisecond basis.
- Near infrared spectroscopy (NIRS) - an optical technique for measuring oxygen in the brain. It works by shining near-infrared light (700-900nm) through the skull and measuring how much light comes back out of the brain. The light that comes back out depends on how much oxygen is in the blood, so it is an indirect measure of brain activity.
Using these techniques, researchers have found that people with depression have less activity in certain parts of the brain. Scientists think that the prefrontal cortex enables us to regulate emotions. More specifically, it helps us inhibit inappropriate or crippling emotions. If our prefrontal cortex is less active, then negative emotions (such as depressed mood) may be displayed more frequently and more intensely. Functional brain imaging also suggests that certain parts of the brain work more slowly in people with depression. The activity in these areas is connected to our ability to focus on the outside world. This may explain in part, why people with depression are focused more on their own thoughts and internal feelings than their surroundings.
There is some evidence to suggest that parts of the brain work harder and possibly faster during periods of mania. Researchers have found abnormalities in areas of the brain with bipolar that are important for regulation of mood and logical connections between language and memory. Studies of this type are of the cutting-edge variety and will hopefully yield a precise understanding of which parts of the brain are most active during mania and depression, and how patients with bipolar disorder "think differently" (if, in fact, they do) compared to people without bipolar diagnoses.