Brain waves occur at various frequencies, that is, some are quick, some quite slow. The classic names for these “EEG bands” are delta, theta, alpha and beta. The dominant wave pattern you see below is alpha; these waves happen between 8 and 13 times per second, or 8-13 Hertz (Hz). Alpha represents a sort of “idle” state, or “ready but not doing much” state and is normally fairly large over the back third of the brain when the eyes are closed and when you are awake. Alpha disappears when we either get mentally busy (e.g., open the eyes, start doing intense mental work even eyes closed) or when we become drowsy. Thus the presence of alpha can show the presence of an awake, resting state. If it is present at a fairly high voltage when the eyes are open, this would usually indicate an inattentive, daydreamy state. In fact we often see this sign in adolescents and adults with attentional difficulties.
When we get mentally busy and engaged, we should see alpha “block,” or reduce significantly in size. In its place we see mostly smaller, quicker “beta” waves. The beta family of waves happen at frequencies from 16-35 Hz, with higher frequencies known as “gamma”.
Delta and theta waves are relatively slow. Delta is usually defined as waves occurring from 1-4 times per second (1-4 Hz). Theta occurs at 4-7 Hz. During drowsiness, first alpha disappears, then the size of theta waves begins to increase. As sleep begins, theta waves get quite large, then become mixed with and eventually give way to slower delta waves.
The presence of delta and theta waves in the waking, eyes open EEG is normal, but only if the waves are fairly small. High amplitude slow waves can be signs of various neurological and psychological problems, ranging from epilepsy to ADHD.
For years all that was possible was recording these waves on paper with the traditional polygraph. Over the last 25 years, advances in signal processing made it possible to sample these waves many times per second (usually 128 or 256 samples per second; our current equipment samples at 4096 per second) and to analyze them in various ways. Using this technology we can now measure precisely the amplitude and frequency of waves of interest, be fairly exact about the scalp distribution of the waves, and even compare a client’s EEG to a normative life-span wide reference database that shows how the person’s brain activity compares, on the average across a particular task, to healthy people of similar age and same sex.