of methodologies, the dynamic or functional imaging technologies,
measure neural activity within a living brain and provide information
about how this activity varies over time and different regions of the
brain. How is this accomplished?
When nerve cells send and receive signals, a great deal of electric
current flows. The current is produced by the movement of electric
charge associated with the flow of ions in axons and dendrites. These
moving electrical charges generate electromagnetic fields that pass
through the surrounding tissue and are detectable at the surface of
the head. If electrodes are attached to the scalp, electric field changes
having their origin in the neural activity of the brain can be recorded.
The method is called electroencephalography, or EEG. A graph of
brain electric field changes as a function of time is called an electroen-
cephalogram, sometimes referred to as a brain wave.
EEG gives a measure of neural activity averaged over large regions
of the cerebral cortex. Because of the electrical conductivity of the
brain and the skin, the electric fields generated by neural activity
get distorted and smooshed around as they pass from their source
locations within the brain to the surface of the scalp where they are
recorded. Thus, EEG may not be the best method for precise measure-
ment of the locations of neural activity in the brain.