Final answer:
An EEG records the electrical impulses from the brain, providing a measure of a brain's electrical activity with an accuracy within milliseconds, which is especially helpful in the study of sleep disorders.
Step-by-step explanation:
An EEG, or electroencephalography, is a technique used to record and measure the electrical activity of the brain. By placing an array of electrodes around a person's head, this method is effective for understanding brain activity, particularly the frequency and amplitude of brainwaves, which are extremely useful for researchers to analyze, especially in sleep studies to help understand sleep patterns and diagnose sleep disorders. Unlike fMRI, which provides a three-dimensional map of active brain areas, EEG offers rapid responses that can track electrical impulses within milliseconds, thus providing a very accurate measure of the electrical activity occurring in the brain at any given moment.