A chapter from the Textbook of Neurofeedback, EEG Biofeedback and Brain Self Regulation edited by Rob Kall and Joe Kamiya
Musing, Speculations on Delta Frequencies in the EEG
This article was written around 1996 by Geoff.
If you have worked with EEG for any length of time you have almost certainly had moments when the machine was apparently giving very bizarre readings. After a while the readings become normal and you perhaps think the problem was caused by a poor head contact or some other similar defect. This explanation is quite likely of course because the EEG voltages being measured are very small. Typically they may be as little as a few microvolts (millionths of a volt). Possible interfering voltages such as the electric light supply are more than a 100 volts. If a tiny fraction of that voltage was able to enter the the input of the sensitive EEG amplifier due to a poor head contact, then the reading would be be distorted. In practice, with careful contact placement and correct grounding, this will hardly ever be a problem.
Yet, with 20 years experience, I have met situations where I could not find any reasonable explanation for the malfunction in terms of faulty contacts or apparatus. It gradually became clear to me that the source of the unusual voltages might be the client who was in some kind of crisis. I could find nothing about this in any text books except advice that the source might be excessive tension in neck and shoulders muscles. This possibilty was easy to reject, by using an EMG detector or better still by massage of the any tense muscles.
The EEG equipment I am using uses a "Mind Mirror (TM)" analysis and display of the various frequencies in the EEG waveform. This makes diagnosis of unusual patterns more easy to recognise and explains why it was easier for me to reject the usual explanations of apparent malfunction. However, its amplifiers are quite conventional and similar to others on the market, therefore my experiences are valid for all other types of EEG equipment since the difference is only in how the output is used and displayed.
A short description of the Mind Mirror (TM) frequency analysis and display follows. Signals from the occipital lobes of the two hemispheres are analysed into various bands of frequencies which are collectively known as aplha, beta, theta and delta. The display show amplitude of the various frequencies as points moving away from the centre of the screen, zero amplitude is shown by two straight lines at the center of the screen. This enables patterns of frequencies to be more easily recognised in relation to each other (pattern 1). When anomalies appear, for example, low frequency signals appearing in the delta bands, the integrity of the rest of pattern may used to decide whether or not they are genuine responses which have a meaning.
We first noted LFs in 1975 when we began our studies with healers; the Mind Mirror was often measuring responses extending below 1Hz. Low Frequencies (LFs) found alone in deep sleep are called delta but our subjects were wide awake and simultaneously showing other frequencies such as alpha and beta. Clearly these LFs cannot be the delta waves of sleep. At first we ignored them because we thought their source might be muscle tension. We had no idea at that time that they might be close relatives of responses which are well known and have been very widely studied as Evoked Potentials (EPs) or Event Related Potentials (ERPs). ERPs appear in a response to a known external stimulus; we believe the LFs on the Mind Mirror are responses to unknown internal stimuli ie the ordinary thoughts and images which arise in our mind continuously.
Usually ERPs are very small and difficult to separate from other responses such as muscle tension unless the event has more than average significance. It follows that LF events large enough to be seen on the Mind Mirror will only be easily identifiable in unusual circumstances. But over many years of study, occasions have arisen often enough for us to be very sure that the LFs are valid representations of processes happening in the subject. Here are some situations where we have seen them.
We first noted these LFs during our studies with healers and thought that they must be an inevitable problem when measuring such tiny voltages from the brain. But we began to note that they were not random and did appear to relate to the situations. Trainee healers seemed to show them always, while with practising healers it seemed to depend on the situation. In psychic experiments, they always appeared. Finally, we had such obvious examples of them appearing in traumatic experiences that we could no longer ignore them. But demonstrating and measuring large LF responses to personal problems cannot be repeatable for ethical reasons - we had to be lucky enough to have a subject connected to the Mind Mirror while a situation was actually developing.
The largest amplitudes of low frequencies were nearly always from one hemisphere only, while the other would appear to be cut off. The first time I saw such a response was from an airline pilot who had not passed a routine health check; a test which he had failed involved measurement of his EEG while he was looking into a high-power stroboscopic light. He had come to Max Cade for relaxation therapy hoping that this would enable him to pass this test. He was wired up on the Mind Mirror and Max had already taken him through several relaxation exercises. I then set up the strobe light, but before switching it on, I saw that the Mind Mirror had "gone wrong" - the left hemisphere readings were off the Mind Mirror scale and there was no reading at all on the right. Because it was the first time I had seen such a response, I thought there was a technical fault in the equipment. I desperately tried to save the situation, checking all the connections and batteries, and was still doing this when the response quite slowly came back to normal symmetry. Only then did I realise that there was, in fact, no fault in the Mind Mirror; that it had indeed correctly shown the pilot's brain rhythm response when he realised we were going to use a strobe light.
On another occasion we had measured the EEG response of a Tibetan lama during a teaching retreat. He had given a reasonable account of the meaning of the different rhythms, describing his subjective impressions while watching the Mind Mirror; for the low frequencies he gave "beyond concept", meaning beyond verbal description. One of his followers, despite being suspicious of machines, wanted to be checked now that the lama had seemingly given the machine his blessing. He came back to the caravan where we were staying during the retreat and we duly connected him up. He was very wary and distrustful and when the local church bell suddenly struck shattering the quiet of the countryside, the left LF reading instantly increased until it was off the scale of the Mind Mirror at more than 160 microvolts compared with the usual 5 or 10. Meanwhile his right hemisphere switched off and was showing a level of only a few microvolts. The pattern quickly settled down to normal symmetry and then another burst happened. It seems that this time he may have responded to the fact that someone was watching his Mind Mirror pattern through the caravan window. As it was dark and he had his back to the window, he could not have seen the person but maybe he unconsciously noted hearing a movement outside. Afterwards, the only rational explanation he could find for his strange subjective experience was: "The Mind Mirror must have been feeding electrical signals into my head as well as measuring them".
On another occasion we were running a group in Holland. The organiser of the group had a video camera and wanted to capture typical Mind Mirror patterns for future teaching sessions. One subject heard the switch of the camera being operated and instantly the pattern showed the same unusual asymmetric pattern. The subject probably felt the camera was an invasion of his privacy since he had not given his assent to be filmed.