An electroencephalogram (EEG) is a test used to detect abnormalities related to electrical activity of the brain. This procedure tracks and records brain wave patterns. Small metal discs with thin wires (electrodes) are placed on the scalp, and then send signals to a computer to record the results.
An electroencephalogram (EEG) is a very safe test. The electrical activity of your brain is recorded, but at no time is any electrical current put into your body.
The brain is a very complex system. The frontal cortex, the region where most of your conscious thoughts and decisions are made, conducts much less than a tenth of the total activity in the brain.
Planning, modeling of your surroundings, interpretation of sensory inputs up to and including your perception of reality, memory processing and storage and the basic drivers of your moods and emotions occur in many functional regions distributed around the brain, including the visual cortex at the rear, temporal cortex at the sides, parietal cortex behind the crown of your head and the limbic system deep inside the brain. The limbic system controls your basic moods and emotions, your fight/flight response and deeper long-term memory encoding as well as control of basic bodily functions such as breathing and heartbeat.
Most of these deeper functions interact intimately with different parts of the cortex (the outer layer which is accessible to EEG measurements) however the interaction is quite complex and distributed. In order to map the true activity of the brain it is very important to measure signals from many different cortical structures located all around the brain surface. It is not possible to map these signals purely from the frontal and temporal regions. Determination of the user's complete mental state is very poorly approximated unless signals from the rear of the brain are also considered.
With proper coverage and electrode configuration, it is possible to reconstruct a source model of all important brain regions and to see their interplay. Alternative systems missing these critical signals will tell less than half of the story. Generally they are restricted to determining level of consciousness, the amount and intensity of processing and (in some cases) the left/right hemispheric imbalance in frontal signals. While these are useful in some contexts, they provide a very limited and inaccurate view of the user's state of mind.