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Musing, Speculations on Delta Frequencies in the EEG


A chapter from the Textbook of Neurofeedback, EEG Biofeedback and Brain Self Regulation
edited by Rob Kall, Joe Kamiya and Gary Schwartz
The E-book is Available on CD Rom

Musing, Speculations on Delta Frequencies in the EEG

Geoffrey Blundell


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.

Here we meet a stereotype: so far with men, it has always been the right hemisphere which switches off, the left going into overdrive, while with women it is the opposite.

In two cases involving women, they were in the midst of dealing with relationship problems. One knew enough about Mind Mirror patterns to realise that the assymetry portended fundamental changes in her life, but at that moment, with the left hemisphere not functioning, it was impossible to express anything meaningful in words. In the second case, I sat with a subject during guided imagery given by my colleague Max Cade which lasted for an hour. I quietly gave her very simple feedback in the form of a whispered "yes" and "no" as she hovered between symmetry and asymmetry. At the end she felt the feedback had shown her clearly the difference between the two modes. She said that in the asymmetric mode she could intend to say something but hear her voice saying something quite different. From this, she understood why it was so impossible to discuss the situation with her male friend and why he had found it impossible to make sense of what she was saying. He commented: "She seemed like the witch in the village". He knew enough about the technology to realise that the Mind Mirror was demonstrating clearly that at certain times it was meaningless to try to make verbal sense of what was happening between them. I believe that this simple demonstration helped them to come to a new understanding in their relationship; they are still together many years later.

With one healer who was working with a client, we noted a very interesting pattern. He showed an excellent state 5 pattern from his left hemisphere but the right showed only LFs, as shown in the diagram (pattern-2). He said that during this phase he was searching for ways to help his client. We were interested to note the pattern afterwards came into balance, a state he called "charging his batteries". We have seen this pattern once only but it shows the startling potential in the available plasticity of our mind.

Anna Wise has noted in her book that many of her clients show delta LFs while she is working with them. She herself had an experience at one of the Body, Mind and Spirit festivals in England. She was connected to the Mind Mirror on one of the stands devoted to healing. At the end of the session with one very effective healer, she was showing nothing but delta. She was able to move around and talk in this state. Remembering the feeling afterwards, she felt that she remained in it for hours.

When we first connect a subject to the Mind Mirror, there is almost always a LF response coupled with beta for the first five minutes. We call it the alpha-blocking response and on the Mind Mirror the visual pattern looks like two saucers back to back. (pattern-3) and confirms to us that the machine is functioning and the connections correctly made. As the subject's apprehension caused by the novelty of the situation of being connected to the Mind Mirror dies away over the next few minutes, so does the low-frequency response. It is not caused by the machine settling down because this only takes a few seconds and affects all the channels. At the peak period of a few years when Max Cade was giving courses in London, as many as forty subjects a week were being wired up on the Mind Mirror. The first response most subjects showed was the pattern just described which became our "standard event".

Len Ochs works with a strobe light which is offset slightly to the dominant EEG frequency. He also reports that high levels of LFs are found from his clients and that success in his therapy is accompanied by a reduction in these levels. He says: "The amplitude of theta and delta has been observed to rise as patients experience anger, sadness and hurt..." and "high-amplitude delta and theta seem to parallel the active influence of a past filled with anger, sadness, hurt, or head injury". His colleague Elaina M. Jannell says: "It is the lower frequency activity that is prominent in both physical and psychological trauma". Our only point of difference with Len Ochs is that we prefer not to use the term "delta" for these LF responses.

The first report which we found describing these LFs was in a 1975 paper on low frequencies during paranormal research by Dr Joel Witton published by the Toronto Society for Psychical Research (Ref 2). It is interesting to note that they are not linked to the success or failure of the research but rather to its novelty, a point that will be amplified in the discussion on the source of these LFs. There is no suggestion that LF waves are linked to paranormal effects, though it may be that Dr Whitton had hoped they might be some kind of carrier wave for psychic energy.

We believed at the time that the above report was the first that we had seen giving an independent corroboration of our findings. We ought to have read more thoroughly W. Grey Walter's 1953 book "The Living Brain" (Ref 3). In it he says: "As long ago as 1938, I suggested ... that delta waves represent a change which is usually ominous ... a defence mechanism ... Sometimes we may suspect them of paralysing the cortex by electrocution, as it were ..." He goes on to say that the need for these low frequencies, "these wardens of brain function", may lie in the necessity of "protecting the brain from the consequences of its own complexity". These comments seem dramatic because he was applying them to brain injury and organic disease, but in calling them "a warden of brain function", he seems to be suggesting that they might be caused the trauma of an injury as well as its physical impact.

The Source of the Low Frequency Potentials.

The brain contains upwards of 10,000 million neurons. Many operate in small groups dealing with aspects of specific tasks, these small groups are then found as components in higher order brain activities. Many are also connected in large groups which alter the whole mood or ambience of the brain. It is useful to use the computer programme as an analogy; when we are learning to recognise a word, we are in effect creating a programme for that functions. But the computer analogy breaks down in many ways; the brain is capable of carrying out many tasks simultaneously with a degree of complexity in both analogue and digital modes that cannot be envisioned in computers; and further we have no idea how the brain collates the results of separate processes and presents them as a unified experience to our consciousness.

When a neuron "fires", ie switches, it generates a voltage. When a large number fire together, the voltage is large enough to be measured at contacts placed on the surface of the head. These low frequency (LFs) voltages are called Event Related Potentials (ERPs) when they appear in response to a stimulus such as a flash of light. This technique has provided a very powerful non-invasive technique for studying brain function and developing cognitive maps of the brain. Two important qualities of a brain response have been identified: the time taken to complete a particular task - the latency; and the amplitude of the response which is linked to the element of surprise.

At this point we should note again that there are two categories of low-frequency response, firstly, the ERP just described which are responses to known events, and secondly those LFs generated by the brain's own reflections about events for which a cause may not be identifiable. Yet the brain processes seem to be the same in each case and as ERPs have been widely studied it is useful to see what can be learnt from them.

Consider first the time taken. For a single neuron the switching time is very short, a thousandth of a second or less. The time taken for a complete sequence, for example, word recognition, is very much longer and will involve thousands of neurons. Many of these sequences or subroutines appear to be used very often by the brain as elements in more complex tasks and can usefully be imagined as computer programmes which can be applied to many different functions. Various subroutines have been identified as the P100, P300, P550, N140, N400 etc. The N - negative and P - positive indicate the relative direction of the change of potential and the figure is the time taken in milliseconds. To remind you of the time scales involved 500ms equals half a second. The P300 response has been widely studied and appears as a part of many brain processes. It is visually recognisable on a graph as beginning with a negative going change of voltage followed by a larger positive one, the total response taking about 300 milliseconds to complete, hence its name. While there is on-going discussion about its precise function, it seems to be a recognition programme or routine which contacts various parts of the brain to find meanings and associations linked to the stimulus. If one says "the cat is green" then one finds a different response - the N400 after green. This second response must be linked to the contradiction of a green cat. While information is often stored near the area which needs it - action data will be found near the sensory motor cortex, other information such as colour may apply in many different contexts such as perception or hearing a description of an object. We know from PET scans which can indicate areas of brain function by change of blood flow, that seeing an object can activate many different centres in the brain, making available everything we know about it; the meaning the object conveys is very quickly linked to the original perception. The emotional "colour" is also added equally quickly.

The second quality of the event-related potential is its amplitude and this divides into two categories; those which are large enough to be recognised without special processing and those which are so small that they are overwhelmed or hidden by other brain activity such as the alpha and beta described in the previous chapter. When they relate to simple house-keeping events of the brain such as recognising a word, the amplitude may be a microvolt or less and impossible to recognise from a single stimulus. There are two techniques to separate these voltages so that the response can be seen; a filter will separate the response if the stimulus is repetitive; if not averaging techniques can separate the response. The repetitive stimulus may be flashing light or a clicking metronome. An analogue filter or digital one based on Fourier analysis at the same frequency as the light, will filter out other brain activity so displaying the desired response while eliminating other brain activity. The second method of separating these tiny LFs, averages the voltages from a stimulus repeated many, many times; the mean value of all the readings giving the true response. Other brain activity is averaged out to zero because it is not synchronised to the stimulus. Because both methods require that the stimulus is repeated many times; both suffer from habituation, an effect which means that the average of a number of responses may not be the same as one to the very first stimulus but we have no way of telling. They both require a known event as a starting point and therefore cannot be used to identify the Mind Mirror LFs.

An element of surprise will increase the ERP from a single stimulus to a level where it can be easily discerned on its own. Emanuel Donchin in his presidential address to the Society of Psychophysiological Research in 1980 (ref 4), showed how a video game could be used as a reproducible stimulus. In this game, the player bargains with the computer over the price of a car. During the game the computer changes tactics and the resulting ERP potentials are much greater due to the active involvement of the player with the bargaining. Instead of fractions of a microvolt, ERPs measuring tens of microvolts, were easliy shown on a graph without the distortion of filtering or averaging. Many examples of traces of graphs are given showing that the response may continue changing for a second or more. Donchin says:

"Rare events elicit a P300. The rarer the event , the larger the P300" and "The amplitude of the P300 can be used to measure the subjective probability or expectancy of events".

There is another response, very similar to the P300, which seems to play a part in the LFs which we see on the mind mirror: the Orienting Reflex (OR). The following description of the OR is taken from "Neuronal Models and the Orienting Reflex" by E.N. Solokov. "The OR is a special functional system which increases the discriminatory power of analysers....I believe that one of the mechanisms for this is a direct stimulation through the special descending pathways to the receptors from the reticular formation and from the cortex, which can change this discriminatory power." (Ref 5) Both seem to compare inner expectations or schemas for dealing with world with the effect of the stimulus; the mismatch manifesting as an ERP. A major difference between them is shown by habituation, ie how quickly the brain decides that the event is of no significance. In the main, the P300 only shows when the event is of an unexpected type, whereas for the OR the first stimulus is always unusual. One could say that the P300 is always habituated that is to say the event is immediately checked for inner significance and if none is found then there is no ERP, whereas the OR always checks the first presentation for significance and only then relaxes its guard if none is found. Even here the picture can be confused; it is well known that in a P300 study, a P300 response will be found from the very first presentation of the stimulus regardless of its significance. There is another difference; the OR may habituate ie no longer respond to a situation which is seen as benign, but it does not relax its guard. If the test involves attention to a series of tones then the absence of an expected tone may produce an OR. (ref?)

During a study, the stimulus may be a real external event or it may consist of imagining a real event. Studies have shown that perception and mental imagery involve some of the same internal representations, have functional similarities and will therefore show similar ERPs (Ref 6), therefore it is reasonable to suppose that the brain uses the same processes to find meaning in stimuli whether they come from a known external source or an internal. ERPs can be measured from the visual cortex both when an object is presented to a subject, or when the object is being imagined by the subject ie. being seen with the mind's eye.

We have shown that there have been many studies of brain processes which have made use of ERPs. Cognitive maps have been developed of how the brain evaluates and assembles information to present to our conscious awareness. Donchin says:

"ERP components are best viewed as manifestations of the activities of subroutines invoked during the informational transactions of the brain".

He makes it clear that he sees the P300 as part of a larger process, a "primary task" which makes use of subroutines. For us it is the "primary task", ie the whole routine, which is of greatest interest. The primary task is also accompanied by low- frequency (LF) changes. But we can no longer call it an event- related potential (ERP) if we do not know the timing of the primary stimulus. What does "primary stimulus" mean for us? These are the events described earlier: the airline pilot faced with a strobe light; the person being videoed without permission; the woman re-evaluating a relationship; the trainee healer exploring potential gifts; the subject taking part in psychic experiments: the traumatised clients seen by Len Ochs.

They are events which cannot be repeated for precise study. The event or its timing may not be predictable or when it can be predicted, it may not be ethical to use the opportunity.

For us, viewing LFs on the Mind Mirror through filters, the event must always be perceived as significant by the subject otherwise we would not see them. Perhaps in the future a way might be found to use alpha blocking as a marker and thus gain the possibility of using averaging techniques from many similar responses.


Method of Measurement of the slow potentials.

ERPs are studied as graphs of a change of voltage against time with a known event as a time marker at the left hand side of the graph. To people used to working in this field, our method of studying these voltages by means of filters must appear very crude because so much information is lost. The response time is given by the frequency of the filters; an output from the 1.0 Hz filter translates into a response time of a second whereas the 0.5 Hz filter indicates a response lasting for two seconds. (Graph relating ERPs to filter outputs). The time information is the same but it is not in a form which we are used to seeing. The amplitude of the response will agree but from the filters we have lost the information about the polarity of the LF.

We gain the ability to relate the LFs to other responses of alpha, beta and theta on the Mind Mirror. This is useful because in a real-life situation we do not know when the internal event may arrive. Externally it might be a word or situation with unexpected associations or internally, from an old memory, and yet when it does we want to gain as much information as we can. The filters are always in circuit, filtering out extraneous information and waiting for the unknown event.


Artifacts rather than genuine LF response.

When we first reported seeing these LFs on the Mind Mirror in 1975, two experts were quite dismissive of our reports. One demonstrated that a similar response could easily be generated by asking the subject to try to turn his head against a restraining hand. A slow headroll could also produce false readings. The sources of this apparent "delta" reading were the powerful neck muscles which were activated by the subject trying to turn or rotate his head. Because they are near the surface of the skin these muscle-tension voltages are readily picked up by our contacts and can overwhelm the LFs we wish to read. Added to that, the neck muscles also tend to vibrate against a restraining hand at about the same frequency as the ones for which we are looking. The signals we wish to read are from neurons inside the skull, the bone of which, being soaked in cerebrospinal fluid, is a very good conductor of electricity and thus acts as a shield and severely attenuates the brain voltages we wish to measure. However, on the very first Mind Mirror we had anticipated this and had included a sensitive muscle tension warning detector which alerted us when muscles were in excessive tension. We also reduce the muscle tension pick-up by using bipolar input from contacts placed fairly near together, which helps to cancel out muscle-tension voltages.

A second source of false readings can be found in the input amplifier. In a well-designed amplifier, a high level of interfering signals can be present without affecting the wanted reading. In one that is poorly designed, the wanted and unwanted signals can easily get mixed up and produce a whole range of spurious readings; an effect known as intermodulation.

We have always been confident that our LF readings were accurately representing the subject's response. In rare cases when the muscle-tension indicator alerted us that there was a problem, it could always be solved by shoulder and neck massage to reduce the tension reading. We could then be sure that our observations were accurate. We were finally very confident when we observed some of the high-level asymmetric readings described earlier because it seemed unlikely that a subject would tense the muscles on one side of the neck without operating the warning detector. It was also easy to make a physical check of the neck muscles to see whether one side was more tense than the other.

What the Low Frequencies seem to be telling us.

Studies of large Event Related Potentials (ERPs) have shown that they can be measured at many points simultaneously from the brain as though it was making use of all its resources in trying to find meaning when the stimulus is unusual. Solikov, quoted earlier, has suggested that the Orienting Response (OR) magnifies the effects through its links to the reticular activating system (RAS) in the brain stem (Ref 5). This system controls many aspects of consciousness such as the response to pain, wakefulness and relaxation. It is reasonable to suppose that the effects of this reticular activity will manifest as ERPs even when the cause is unknown.

One can imagine the LFs as an orienting radar which checks for meaning both externally and internally. In the laboratory, they are large enough to be studied reliably when the player is being challenged by a computer game. When the event is internal, they seem to accompany attempts to find new solutions and in a personal crisis may indicate a renewal in our basic and fundamental way of being in the world. The events can range from resolving trauma after an accident, to interest in the paranormal, or choosing a new direction in our life. We believe that they can give a new window on how the brain functions and are a resource in our quest for understanding the nature of our mind.

If we try to describe LFs, we find that we quickly reach levels of brain process where we cannot trust the words used by the Left Hemisphere (LH). This may be because the process is a feeling or an intuition rather than a fact or it may that the LH does not know but will invent an explanation rather than admit that it might be losing control. This has been demonstrated in split brain research which came about through trying to control very severe cases of epilepsy by severing the corpus callosum, the major link between the two hemispheres. It was hoped that fit could be confined to one hemisphere and allow the subject to function with the other. A subject who has had this operation can be shown a rude picture in such a way that it is only visible to the right hemisphere. This may well produce an embarrassed laugh. If one then asks the subject why they laughed, one is dealing now with the left hemisphere (LH) and one quickly finds that it does not have the foggiest notion of why the laugh happened. Rather than admit this, LH will invent and improvise with the weakest of excuses such as "it was the strange expression on your face" or "it was the tone of your voice".

There is another effect which demonstrates the difficulty of finding out what is wrong with someone by asking them. This is known as the false memory syndrome. Traumatic memories from the past may be blanked out so thoroughly that verbal or visual memory traces seem to have vanished but they can nevertheless be held as an emotional code in the deeper levels of the brain. When the intellect tries to decode these emotions back into facts, it may well have to invent situations, based on partial memories, to explain the feelings. These explanations are likely to be wrong. However, if the subject was connected to the Mind Mirror while this explanation was being given, and one saw high levels of low frequencies, one would know right away that there was a discrepancy and that a more accurate explanation might be available.

A German doctor Georg *(don't add the e)* W. Groddeck, in his "The Book of the It" (ref 8-7) expresses very well the challenge of trying to describe what may be happening in deeper levels of the brain. He influenced the psychoanalyst Freud, whose book "The Ego and The Id" pays tribute to Groddeck. Freud however saw the psyche as made up of the conscious and unconscious parts whose behavioral components could be analysed into psychological categories, whereas for Groddeck, both the conscious and unconscious seemed to be a function of something else which he chose to call the "It"; an unknowable principle which animates our lives and actions. For Freud and also our civilisation the ego is supreme. The components of the ego can be neatly described in the terminology of psychoanalysis but Groddeck did not believe that this could help us to understand the nature of the forces within the human organism which influence health and disease. To Groddeck "the ego appeared as a contemptible mask fathered on us by the intellect" which persuades us that we are motivated by forces within the control of our conscious mind. In practice we have no idea how we digest food, why we chose a particular career, or how we become ill or stay well but to him these were expressions in the language of the It. One can of course say that a germ caused the illness, but this only part of the story - we have all had the experience of staying well in the middle of a flu epidemic, so the germ cannot be the only cause.

For Groddeck "the assertion 'I live' only expresses a small and superficial part of the total experience of 'I am lived by the It". However he does not see "It" as a thing but rather that it may be identified, for example, in the process of setting aside the ego for a moment to work a little with the unknown. In all humility he says: "I am not inviting you to follow me, but to follow yourself." Perhaps the low frequency EEG can illuminate some of the dark recesses of our mind until finally in Groddecks words - we are able to grasp the full majesty and terror of the "It".

Future Research.

If alpha blocking or beta increase could be used as a time marker for the low frequencies then we should see it followed by the same kinds of change which have been found in the study of ERPs. This would mean that whole armoury of responses - the P300, N400 etc - could be applied directly to the way the brain processes self-initiated events.

It is likely that when learning a new skill, learning to drive a car for example, fairly high levels of LFs will be seen but once one has learnt to drive without thinking about it, these will vanish.

Where filters have been used to display ERPs as spectral responses, for example in research on visually evoked potentials, then spectral frequencies have of course be found in the low frequency bands but they are also found in higher frequency bands as well. (Ref 8) Does this mean some of the responses we see in, for example, the alpha and theta bands are contributed by potentials evoked by internal events? Would this offer some explanation of the difficulty in finding an agreed meaning for alpha? Could these LFs be the source of so called "bad theta" often found in ADD clients?

When I began this chapter, I was of course very confident that we had not been mistaken in considering that the LFs were mirroring the way people explore their unconscious to find new meaning as though they represent a kind of internal scanning of the internal environment. But, until I explored the literature more thoroughly, I had not realised that our observations related so closely to conventional ERP studies. So, it has been very illuminating to write this section which has also acted for the basis of a presentation "Low Frequencies on the Mind Mirror" at an EEG conference in Key West Florida in February 1996. It has also been encouraging to note that in similar contexts others have noted that LFs were giving useful information.