The Nervous System and Measurement of Its Activity
This chapter focuses primarily on the source and nature of the electrical activity of the nervous system and how it is measured. Included is a brief presentation of some neuroanatomy and neurophysiology necessary to understand the activity being measured. The material presented here forms the background for understanding some of the physiological bases of the EEG and the event-related brain potentials discussed in subsequent chapters. As outlined in chapter 1, for the sake of convenience, the nervous system may be divided into central (brain and spinal cord) and peripheral systems (nerves outside central nervous systems). These two systems are discussed briefly in separate sections of this chapter.
Electrical activity of the brain is produced by billions of brain cells called neurons. Activity is never absent in the living brain. Neurons are always active--when we are asleep or awake, active or passive, during meditation or hypnosis. The entire nervous system is dependent on neurons for its activity. These cells are the functional units of the brain, spinal cord, and all of the peripheral nervous system.
Although there is general agreement among scientists that neurons are the source of brain electrical activity, the exact nature of their contribution is an area of contention. For example, Noback and Demarest ( 1975) proposed that the EEG is produced by electrical activity at synapses (where brain cells transmit information) and by electrical activity within brain cells. They suggested further that recordings made from the scalp reflect the algebraic summation of excitatory and inhibitory activities that occur in underlying brain tissue. That is, some brain cells produce excitation and some reduce the level of activity; the resultant is the record called the EEG.
Elul ( 1972) made a strong argument for the position that the EEG is a resultant of activity within nerve cells in the cerebral cortex. He indicated that analyses of correlations between gross EEGs (recorded from many cells) and the activity of individual nerve cells suggest that the activity observed, at a given instant, is due to the synchronized firing of a relatively small number of cerebral neurons. Thus, according to Elul, the EEG is produced through the intermittent synchronization of cortical neurons, with different neurons becoming synchronized in successive instants. This implies that the EEG represents a series of bursts of aggregate neuronal activity, with each burst being the synchronized activity of different groups of cortical neurons. According to Elul, these bursts of activity are what we see in the EEG recording. He also pointed out that subcortical areas, such as the thalamus, have an influence over the EEG, because it is known that the thalamus plays a role in the production of at least one type of