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The human body is an electrochemical organism, although the electrical activities of the body are relatively weak. Nevertheless, a variety of different human electrical activities can be measured through electrodes taped to the skin or scalp. In scientific sleep and dream research, the most important measurements are EEGs (electroencephalograms), which are measurements of brain wave activity, and EOGs (electrooculograms), which measure eye movements distinguishing between rapid eye movement (REM) and non-REM sleep. A third measurement usually taken along with an EEG and EOG is an EMG (electromyogram), a measure of muscle tone in the neck that is typically made through electrodes attached to the chin. The EMG helps researchers identify periods of head movement that may interfere with EEG and EOG readings, and is also a secondary indicator of REM periods (because muscular activity is at a standstill during REM sleep).