6.3: Imaging brain function

Anatomical analyses are certainly a meaningful way to study aspects of the nervous system. But being able to measure and record brain function is important for answering different types of questions. After all, the brain of a recently deceased person might look identical anatomically to that of a living person, but they would exhibit significantly less function.

6.2.1 Electroencephalography (EEG)

Example questions answered:

“At what times of the night does the sleeping brain exhibit synchronized neuronal activity?”

“How does brain activity change when a person is having a seizure?”

The electroencephalogram (EEG) is one method by which we can observe electrical activity of the brain. When neurons send action potentials, several charged ions move across cell membranes, which causes a change in the electrical charge of the adjacent area. Some of these charges are very large, especially when several millions of neurons are sending action potentials at the same time. These currents produced by neurons of the outer surface of the brain (cortex) can be so large, that they are detectable outside the skull at the surface of the head. The EEG works by recording these changes at different areas of the brain. The technique was first used in the 1920s by psychiatrist Hans Berger, who invented the technique. The theory behind the EEG is the same used in an electrocardiogram, or ECG, to detect the electrical activity of heart cells.

Figure 6.13 The electroencephalogram (EEG) is a noninvasive measure of neuronal activity (top). The information gathered (bottom) reflects changes in electrical potential measured at the scalp.

To perform an EEG, a gel is applied onto small patches of the scalp. This gel is basically a sodium chloride gel that acts as a conductor, allowing the currents to be picked up by a series of adhesive electrodes. EEG systems used in hospitals for diagnosis or in laboratories for research studies may have anywhere between 20 to 128 electrodes. Each electrode is sensitive enough to detect voltage deflections as small as 10 microvolts. Each electrode is placed on a specific area of the scalp, and the information from each electrode runs into a computer. From there, the computer can then perform calculations to determine which cortical areas to the brain exhibit what patterns of activity.

EEG software is capable of examining the rapidly-fluctuating electrical potentials and dissecting out the several components buried within the complex signals. From one waveform, it can pick out high frequency components called beta waves (between 13 and 30 Hz), low frequency components called delta waves (between 0 and 4 Hz), and all frequencies in between.

The EEG is a noninvasive functional analysis method. Nothing permanent is done to the person when they get an EEG because it is only detecting information. Wearing a cap of electrodes may be annoying, but it is a harmless procedure. Because of the experimental set up for EEG, there are many cases where using an EEG is preferable. The EEG is a standard part of a polysomnogram, a series of tests that can be done to evaluate sleep disorders. In fact, our best and most reliable characterization of the phases of sleep rely on brain activity measures as recorded by an EEG (Chapter 13). Relatedly, EEGs are also used clinically when a person is under anesthesia as a tool to evaluate their level of unconsciousness.

Not only is the EEG noninvasive, but it is also relatively cheap - at least in comparison to many of the other methods described in this chapter. It does not require heavy equipment and is mobile. All the machinery needed to run an EEG can be contained within a backpack.

EEG is also useful for the diagnosis of a variety of brain disorders that are a result of aberrant cortical neural activity. The most well known of these is epilepsy. In epilepsy, when a person has a seizure, it is believed that large masses of neurons are sending action potentials across the cortex, producing very large signals that can be detected by scalp electrodes. Migraine, a debilitating condition, may be due to waves of unusual neural activity that sweep across the cortex. Other applications of EEG may include helping diagnose Alzheimer’s disease, depression, and possibly ADHD.

The EEG has great temporal resolution. Because it is detecting changes in currents, it is capable of sampling in the range of 10,000 Hz, meaning that for each electrode, 10,000 data points can be collected per second. Since action potentials happen on the time scale of milliseconds, being able to calculate changes in electrical potentials at such speeds gives us the ability to assess brain activity very precisely.

But, the EEG has poor spatial resolution, only detecting signals that originate in the outer most layer of the brain. More electrodes increase spatial resolution, but even at 128 electrodes, an EEG only has spatial resolution to the level of about 7 cubic centimeters.

6.2.2 Positron emission tomography (PET scan)

Example questions answered:

“Which areas of the brain decrease in activity when a person experiences mild cognitive impairment?”

“Do people with substance use disorder have a high density of opioid receptors?”

The positron emission tomography (PET) scan is an application of nuclear medicine best known for its use in the medical setting for the diagnosis of cancer. The scanner itself is a large circular device that looks similar to the CT scanner. Before the scan, a radioactive compound called a tracer is injected into the bloodstream. This tracer is a compound where an atom is substituted with a radioactive isotope, such as tritiated hydrogen (³H) or fluorine-18. The tracer is chemically unstable, and it spontaneously emits positrons. When those positrons interact with the electrons of nearby molecules, two gamma particles are emitted in perpendicular directions. These gamma particles are then detected by the PET scanner as the person moves through the machine.

Figure 6.14 When a positron (e⁺) emitted by the tracer compound (FDG) interacts with nearby electrons, gamma rays ( ) are emitted at perpendicular angles which are detected by the PET scanner (gray boxes).

Figure 6.15 The positron emission tomography (PET) scanner (top) is quiet compared to the CT scan or MRI. It can generate data sets (bottom) that visualize the approximate location of highlyenergetically demanding tissue (yellow and orange), such as a brain tumor.

A common tracer that is used for imaging brain function is fluorodeoxyglucose-F18, or FDG. FDG is a radioactive analog of glucose, one of the main sources of cellular energy. Highly metabolically active areas of the body take FDG into the cells. Energetically demanding areas of the brain require more energy to do their functions (the same reasons that tumors, rapidlyreproducing patches of tissue, appear very bright in PET scan analyses). In order to produce the energy needed for increased neuronal activity, the brain changes perfusion levels by dilating blood vessels in order to bring more glucose to those areas. Therefore, when an area of the brain increases in energetic demand, that change can be detected by identifying the increase in glucose movement.

PET scans can be effective at diagnosing and identifying the location of tumors in the nervous system. It also provides an overall picture of brain activity, which may be useful in diagnosing disorders of cognitive deficits, like dementia associated with Alzheimer’s disease or frontotemporal dementia. PET scans have also been used to image the activity of specific brain areas as a person performs behavioral tasks, but this use of PET scan has largely been replaced by functional magnetic resonance imaging (fMRI; see section 6.2.3).

A second application of PET scanning is to visualize levels of receptors in vivo. This experimental approach relies on the nature of the interaction between specific radioactivelylabeled compounds and receptors (such as [11C] raclopride and dopamine receptors, or [18F] ASEM and acetylcholine receptors). Once the radiolabeled compound binds to the receptor, the PET scan is able to detect the location of the radioactive signal and the strength of that signal.

The downside of the PET scan as a diagnostic tool is similar to a limitation of the CT scan. A person is exposed to radioactive compounds and gamma wave radiation, which are potentially mutagenic.

In images produced by a PET scan, it is often difficult to identify boundaries between tissue, even between dramatically different internal organs. To make up for this deficit, PET scans are frequently performed simultaneously with an anatomical analysis like a CT scan.

PET scans generally have very poor spatial and temporal resolution. With PET scanning, you can really only see differences between areas if the volume is in the range of 5-10 cm3 . Anything smaller than a cubic centimeter will be impossible to detect with current PET scanners. As to the speed of the PET signal, it often takes tens of seconds or minutes before a change in signal can be observed and detected.

6.2.3 Functional magnetic resonance imaging (fMRI)

Example questions answered:

“Do neurons in the right hemisphere cingulate gyrus increase in activity when a person sees their loved ones?”

“Which areas of the brain change in activity when a person is planning a motor action?”

Figure 6.16 (1) Protons spin around an axis, aligning randomly at rest. (2) In the presence of a magnetic field B₀, their spin aligns with or directly opposite of the magnetic field. (3) Exposure to radiowaves knocks the protons out of their alignment into a high energy state. (4) As the protons return back to their alignment with the magnetic field, they release energy.

The functional magnetic resonance imaging (fMRI) technique is probably the most well-known method of studying brain activity. Because fMRI can be performed while a person is engaged in a task, many research studies use fMRI as a means to correlate behavior with activity patterns in specific parts of the brain.

An fMRI machine basically consists of two main components. As with the CT scan or PET scan described above, the fMRI machine is a circular tunnel through which a person on a table moves. As the person moves through the scanner, an extremely powerful magnet revolves around their head. The power of a typical magnet used in a hospital fMRI may be as powerful as 10,000 gauss (1 Tesla), strong enough to lift a car. The more powerful fMRI machines can be as powerful as 100,000 gauss (10 Tesla). The stronger the magnets, the better the spatial resolution that the machine can produce (our current best spatial resolution is on the order of millimeters). The second component is a device that emits radio waves. Just like the magnet, the radio emission device revolves around the head.

Both the magnetic field and the radio waves are crucial for the fMRI to work. They both interact with protons, the charged subatomic particles that have a small polarized direction because they spin around an axis. Initially, when these protons enter into a strong magnetic field, each proton charge aligns either with or directly against the direction of the field. Then, when hit by a radio wave, the protons lose their alignment, going into a high energy state. The protons then fall back to a low energy state as they return to alignment with the magnetic field. This process is what is being detected by the fMRI.

As it turns out, the protons in oxygenated hemoglobin are sensitive to the magnetic field (diamagnetic) while deoxygenated hemoglobin s not (paramagnetic). Because of this difference, we are able to use sensitivity to a magnetic field as a measure of changes in oxygenation levels. Like the PET scan, the fMRI hinges on the idea that more active areas of the brain have different metabolic demands than less active areas of the brain. When there is more activity in one area of the brain, the neurons in that area need more oxygen, and the adjacent blood vessels react by dilating. This change in blood flow is detected by the fMRI, and is called the blood oxygenation level-dependent signal, or BOLD signal. Temporal resolution of the fMRI is limited by the speed of blood vessel dilation, which occurs on the order of seconds to tens of seconds.

The main reason fMRI is useful in so many research applications is that you are able to visualize brain activity real-time during the performance of complex behavioral tasks. You can present specific visual stimuli to a person in an fMRI scan and evaluate which parts of the brain changes in activity. For example, seeing pictures of faces causes increased blood flow into the fusiform face area. You can ask a person to perform a gambling task and evaluate the areas of the prefrontal lobe that are responsive to risk taking.

Figure 6.17 The fMRI scan detects the location of blood flow changes as a person pays attention to different visual stimuli (top vs. bottom).

Although the technique has a great capacity for analyzing brain function, the nature of the machine itself presents limitations. The scanning tunnel is very small and claustrophobic, making it difficult to safely study anxiety or panic disorder without endangering the patient. The machine can also be very loud, which is not trivial if you are interested in studying younger patients. The use of a tremendously powerful magnetic field presents a different set of limitations. At the risk of severe injury or death, the patient entering the scanner cannot have any magnetosensitive implants, such as metallic aneurism clips or shrapnel. Even older generation tattoos have trace amounts of metal that cause burns when exposed to the magnets of the fMRI machine.

The data collected by fMRI can be very difficult to analyze and are frequently subject to false positives. The change in BOLD signal measured in an fMRI is very small: one study estimates that perfusion increases by only 0.4% even under the greatest activation. Using standard fMRI analysis methods, an infamous study demonstrated that even a dead salmon in an fMRI exhibits brain activity that is similar to those reported in other fMRI studies.

FMRI also assumes that increased blood flow is directly correlated with the amount of neural activity, which may not always be the case.