Web Bit 42-1: How do neuroscientists study the living brain?
By Allan Tobin and Jennie Dusheck
The human brain contains more than 100 billion neurons, each of which has some complicated pattern of ion currents. As ions pass in and out of the membranes of these neurons, other ions also move in the extracellular space. Each of these currents is so small and there are so many neurons with independent inputs and outputs that it would seem unlikely that they would form any pattern. But they do. And we can study the voltage changes associated with these currents by placing electrodes on the surface of the scalp. Remarkably, the measured voltaged changes reflect the state of the brain as a whole, in particular with regard to the state of arousal.
A record of the changes in voltage with time is called an electroencephalogram, or EEG. The EEG reveals the state of attention. The EEG pattern of a sleeping person is distinct from that of a waking person, and someone involved in mental activity has a different pattern than someone who is quietly vegetating. The pattern also changes during sleep. The alterations are associated with periods of dreaming and rapid eye movements (or REM).
About 1 percent of the population suffers from epilepsy, a condition in which seizures disrupt normal activity. The seizures may cause massive muscle contractions (in grand mal epilepsy) or may bring about a brief but noticeable lapse in attention (in petit mal epilepsy). Such seizures result from electrical storms in the brain, in which a large number of neurons appear to fire action potentials in step with one another. The EEG during a seizure reflects such abnormal electrical activity, showing large periodic waves that are unusual in a waking person. Even between seizures, there may be occasional abnormalities in the EEG pattern of someone who suffers from epilepsy.
The passage of electrical currents through a neuron requires the restoration of the resting potential, a process that requires energy. Parts of the brain that are more active at a particular time use more energy and take up more glucose from the blood. Another way of visualizing the activity of the brain in a living person measures differences in glucose uptake.
The visualization of glucose uptake depends on the use of a special glucose derivative, called 18 F-fluoro-deoxyglucose (FDG). This compound enters cells along with glucose, but the cells cannot use it to produce energy. Instead, it accumulates in the cells. More active cells absorb more FDG than inactive cells, so the pattern of FDG uptake reflects the activity of the cells. A complicated and expensive procedure, called positron emission tomography (PET), can detect the presence of radioactive 18 F. A PET scan can produce a picture of the distribution of energy use in the brain.
For example, the visual cortex of a person kept in the dark takes up little glucose. White light increases activity in the primary visual cortex, but a complex picture stimulates still more activity, not only in the primary visual area but in the association areas as well. Similarly, a subject who is listening to a Sherlock Holmes story shows greater activity in the parts of the brain known to be involved in hearing and in memory.