Print

6.6Measurement of Brain Functions

To understand the functions of the brain, it is necessary to observe its activity in real time. In recent years, research on active brains has been pursued even in new fields of study, such as education, psychology, etc.

 

Top of Page

6.6.1

Functional Nuclear Magnetic Resonance Imaging (fMRI)

fMRI has been developed to measure the interior of the brain in real time without X-rays. Our bodies are composed mostly of water and proteins, which contain much hydrogen. When placed in a strong magnetic field, hydrogen nuclei (protons) resonate with high-frequency radio pulses. When the irradiation ceases, the hydrogen nuclei return to their normal state. Nerve activation increases local blood flow by 20%–40%. Since oxygen is supplied by hemoglobin, the ratio of oxyhemoglobin increases in sites of activity. Deoxyhemoglobin is hemoglobin that has released its oxygen. Although deoxyhemoglobin causes unevenness in the magnetic fields of surrounding tissues and degrades the MRI signal, nerve activity increases the amount of oxyhemoglobin to a relatively high level, thus increasing the MRI signal strength. A disadvantage of fMRI is that what is observed is not real-time brain activity but a reaction that occurs several seconds after the neuron has been excited. Recent technological developments are gradually reducing this lag time.
Another disadvantage of fMRI is that because the signals are weak, the data that has been added many times gets displayed (including averaged-out data on many people), and there is a possibility of missing the reactions of particular brains of particular individuals.

Top of Page

6.6.2

Positron (Positive Electron) Emission Tomography (PET)

Another recent technology that is often used is PET. Unlike fMRI, the quantities of specific substances are measured using PET. Radiotracers are produced in an apparatus called cyclotron, injected into a blood vein, allowed to diffuse into the brain, and photographed. Since half-lives of radioactive isotopes in PET are very short, there is almost no effect on the human body. By this method, it is possible to trace changes in the quantities of specific substances, such as the distribution of dopamine and glucose in the brain. An example of PET application is Parkinson's disease, which occurs when neurons die in the substantia nigra. When administered into the blood veins, L-dopamine goes to the brain, becomes absorbed into cells of the substantia nigra, and gets converted to dopamine in the cells. When the brain is viewed by PET, only a signal from the substantia nigra is detected in Parkinson's disease patients. It is possible to determine from the size of the phosphorescent area whether the person is at a risk for developing Parkinson's disease. Figure 6-2 shows an image obtained by PET.

Top of Page

6.6.3

X-Ray Computed Tomography (CT)

X-Ray computed tomography, abbreviated X-ray CT, is used to examine the inner structures of the human body by X-rays. A beam of focused X-rays is passed through the head from one side, and the absorbance is measured on the opposite side. However, the inside structure can be accurately determined because the scan is actually performed at an angle. Thus, internal bleeding, infarctions, and the presence or absence of tumors can be detected. X-Ray CT is highly sensitive, but it cannot be used to observe the activity of substances in the human body in real time. Furthermore, X-rays pose the risk of radiation exposure.

Top of Page

6.6.4

Other Methods

Ultrasound is less invasive than other methods for examining the inside of the brain. However, ultrasound waves have low sensitivity and do not easily pass through the skull, and therefore, ultrasound is not a suitable device for examining the inside of the brain. Other methods are electroencephalography, which detects minute electrical currents that flow during neuron activity, and light topography, by which the brain is irradiated with light from outside the skull and blood flow is examined a few millimeters directly below the skull. Except for the detection of epilepsy by electroencephalography, these methods provide very little reliable data, and their sensitivity needs improvement.

column

Reviving Brain Function from a Persistent Vegetative State

Brain death is the cessation of brain stem function, spontaneous breathing, and heartbeat. In a persistent vegetative state, the cerebral cortex does not recover. But the brain stem remains normal, and spontaneous breathing and heartbeat remain normal even though the person is unconscious. A persistent vegetative state is not death. There are some reported cases that when someone spoke to a person in a vegetative state, a part of the person's brain showed changes in the bloodstream. There has also been a case in which a person in a vegetative state was able to answer "yes" or "no" by imagining playing tennis or walking around the house (these two processes activate different parts of the brain). Furthermore, there are cases where consciousness that has ceased for several years has been recovered by electrical stimulation deep inside the brain.
Moreover, if electrodes are implanted in the brain of a patient with severe amyotrophic lateral sclerosis (ALS) who exhibits volition but can hardly move a muscle, it is possible to create an interface through which such a person can still communicate his or her wishes just by thinking. In future, development of more sensitive noninvasive methods will probably make simple communication of wishes possible in persistent vegetative patients.
However, development of such a device poses dangers as well. For example, transcranial magnetic stimulation (TMS) stimulates neurons in the brain by generating a pulse magnetic field from a coil and inducing an electrical field in the brain. This method is used for treatment of Parkinson's disease and spinocerebellar degeneration. However, depending on the site of stimulation, this method can cause retrograde amnesia and epilepsy. Furthermore, this method can be used for mind control through induced amnesia; therefore, it must be strictly regulated.

Top of Page

next

prev