Glossary of Terms

This will be an ongoing project with words added as they come to mind. Please note that this is not an ‘official dictionary’ but reflects my understanding of the terms – discussion and correction welcomed!

Active AVS
Applies to any session that is intended to support a particular mode of thought and is designed to be used in conjunction with deliberate application by the user. An example would be a beta study session which is used while studying or a theta meditation session when used in conjunction with conventional meditative techniques. Active AVS can be with or without biofeedback (closed-loop or open-loop).

Alpha (8-12Hz)
The alpha band was the first common feature of the brain recognised in EEG recordings. Alpha is the human brain’s natural state of awake rest. When the eyes are closed, or blinked, there is an almost immediate increase in alpha activity, between 9.5Hz and 10.5Hz. Alpha is the easiest range to entrain to, in fact, 10Hz is the only frequency where large scale entrainment is highly likely to occur. Alpha is associated with detached thought and is ideal for contemplation, mindfulness meditation, CBT-type therapy, anxiety and stress reduction and general relaxation.

Audiostrobe (AS)
Audiostrobe is a method of encoding light control signals in an audio file, developed and patented by Andrzej Slawinski, and currently the defacto standard for sound/light stimulation. It uses a 19.2kHz signal with the two stereo channels assigned to left/right or each of two colours and the amplitude corresponding to brightness. A similar method, Turbosonix, is used by some manufacturers. The compatibility between Audiostrobe and Turbosonix is not consistent.

AudioVisual Entrainment (AVE)
One of three terms, AVE/AVS/BWE, used interchangeably but with subtle distinctions. The common meaning is stimulus producing changes in brain wave pattern. AVE specifically applies to the use of sound and light to produce the phenomenon of entrainment. Therapeutic and developmental use of AVE relies on the principle that brainwave patterns observed in ‘normal’ subjects, or advanced practitioners of mind techniques, can serve as a model for our our brain activity, and that such patterns can be induced by exposure to corresponding stimulation.

AudioVisual Stimulation (AVS)
The second of three terms used interchangeably. AVS applies to the more general intent of using sound and light to alter perception or behaviour. Entrainment as such is not necessarily an objective, working on the basis that there are many techniques, such as ganzfeld, random/non-frequency specific stimulation or sensory overload, which can lead to useful outcomes. AVS is the term I prefer to work with, as it embraces techniques other than entrainment, although I’m beginning to see a need for a fourth term that embraces other stimulation modes, such as CES (cranial electrostimulation)  and TMS (transcranial magnetic stimulation) and others which are are entering the self-help/enthusiast’s domain.

Beta (>13Hz)
Beta dominates while we are engaged in anything that holds our attention – concentrating, studying, solving problems, performing memory tasks, etc. Low beta activity, often associated with excess theta, is implicated in depression and ADD/ADHD, with the imbalance often occurring asymmetrically between the brain hemispheres. Beta sessions are ideal for any form of active thought and for increasing overall energy and motivation. Beta waves tend to be of much lower amplitude than alpha, and large scale entrainment appears to be unusual.

Binaural Beat
A rhythmic sound heard when one tone is presented to the left ear and another similar tone is presented to the right ear. There is some debate over the mechanism by which these beats become audible, however it is generally understood that they are a ‘virtual’ sound created in the olivary nucleus, the organ responsible for phase discrimination and spacial location of sounds. The frequency of the beat perceived is the difference between the two tones, i.e. 200Hz left/210Hz right results in a 10Hz perceived beat. Headphones are almost essential for binaural beats, as the effect is dependent upon the tones arriving separately. Binaural beats are the most subtle and least effective means of inducing entrainment, however their gentle, relaxing sound may make them effective in other ways.

Any method whereby a biological parameter is measured and displayed such that a behaviour may be modified to bring about changes in that parameter. One of the most common forms of biofeedback is galvanic skin resistance (GSR), where the resistance of the skin, which varies with stress induced moisture, is measured as an indication of relaxation. Other common parameters are heart rate variability (heart rate fluctuates with stress) and temperature (learning to modulate peripheral temperature has been effective in alleviating migraine headaches). Feedback of brain functions is usually termed neurofeedback, and includes monitoring of electrical activity (EEG) and cerebral bloodflow (HEG – haemoencephalography by passive- or near-infrared). Biofeedback can be passive or active. Passive biofeedback simply reports the current value of the parameter and allows the  user to respond appropriately. Active biofeedback uses a stimulus guided by the measurement, adapting as the parameter changes. Biofeedback training involves some form of manual or cognitive task leading to desired changes in the measured parameter.

Brain Waves
When an EEG is connected to the scalp over areas of the brain, it is usual to see a signal comprising many frequencies between 0Hz and 40Hz. There is much activity above 40Hz, however the bulk of interest has thus far been below this frequency. The signal detected by the EEG is the sum of the activity of millions of neurons in the several square centimetres below the electrode, diffused and attenuated by the intervening tissue and bone. The frequency spectrum is arbitrarily divided into ranges roughly corresponding to the standard states of human conciousness; sleep/delta, dreamy/theta, awake/alpha, engaged/beta. When there is a conspicuous energy peak in the frequency spectrum, this is referred to as a dominant frequency. One objective of AVSis to induce activity at the stimulation frequency. When this synchronous activity occurs in regions of the brain not directly associated with the sense being stimulated, this is referred to as entrainment. Stimulation of matching frequencies in the visual cortex by photic stimulation is so consistent that it is used as a proof-of-sight method in comatose patients, and as a diagnostic in hysterical amblyopia (hysterical blindness). Therapeutic protocols aim to increase activity at particular frequencies, while suppressing activity in others. Neurofeedback is particularly effective in achieving this, however open-loop AVS has been shown to be effective in many cases.

BrainWave Entrainment (BWE)
The third interchangeable term along with AVE and AVS. BWE embraces all means by which a brain might be encouraged to follow an external stimulus to become entrained. BWE is the term that has captured the marketing mind and is used extremely liberally to describe anything that has beats in the common brainwave range. Much BWE promotional literature implies that entrainment is an automatic response to stimulus, making no distinction between the evoked and frequency following responses that will almost invariably be found over the sensory cortices, and the true entrainment response that is sometimes observed over non-sensory regions.

Closed-Loop AVS
A biofeedback device monitors response to the stimulus and alters the session parameters to optimize that particular response. Closed-loop sessions can be active or passive.

Delta (<3Hz)
Delta waves tend to be much larger than any of the others and arise spontaneously during deep sleep. There is some debate over whether delta can be entrained, however it occurs in deep meditation, and delta sessions can certainly be an aid in achieving delta meditation states. Although most dreams occur while theta dominates, during REM sleep, dreams can and do regularly occur during delta. It’s very difficult to identify the boundaries between waking delta and sleep, leading to questions of whether a deep meditator, exhibiting high delta, is actually ‘awake’ at all or whether they are in a state more akin to a lucid dream.

Electoencephalogram (EEG)
EEG refers to both the device used to detect and record brain waves and the recording itself. The EEG device consists of a number of electrodes that can be attached to the scalp, a corresponding number of precision, low noise amplifiers, often with filters to minimise common intereference, and some form of recording device. Modern EEGs are almost universally computer based, with the raw data from the amplifiers digitised and analysed using such techniques as Fourier analysis to extract individual frequencies. An EEG can be used in conjunction with a stimulus device to provide driven stimulation, or with feedback, including specialised games or puzzles, to provide neurofeedback training.

The phenomena whereby one oscillating system locks onto the similar frequency of a neighbouring oscillating system. In the case of brainwave entrainment, activity stimulated in the sensory regions may be picked up and followed by other regions. The activity in the sensory regions is not entrainment, it is evoked response or frequency following response (FFR)

Frequency Following Response (FFR)
When a suitable stimulus is delivered via the senses, a response will be seen in the regions of the brain primarily associated with that sense, referred to as an evoked potential or evoked response. When that stimulus is repetitive, the response will tend to be repeated at a matching frequency – this is called the frequency following response. Flashes of light will cause responses clearly detectable over the visual cortex at the back of the head. The response to sound pulses will often be evident over the temporal regions (side of head, above and behind the ears). Photic stimulation is dramatically more effective than audio stimulation, and pulses of sound are more effective than monaural or binaural beats. FFR is often confused with entrainment.

Frequency Independent Sessions
AVS sessions that use a number of discrete frequencies or a range of frequencies to activate multiple functions of the brain simultaneously or consecutively. There is very little research published in this area, however a number of developers, including myself, have developed novel techniques and enjoyed spectacular results. My own mini-app, BrainForm, is an example. Chuck Davis uses an undisclosed variant in his pRoshi product. Frequency layering is a commonly promoted technique – there is very little to support layering, and many audio mixing issues that make it extremely problematic.

Gamma (>20Hz)
Also referred to as high beta, gamma is typically seen as low amplitude signals over small regions. Gamma activity is associated with high orders of consciousness; memory binding, inspired thoughts, extreme concentration.

All sounds, all waveforms, include harmonics. Harmonics are multiples of the base frequency. Harmonics are the part of a sound that gives it its unique character compared to a pure sine wave, which has the least harmonics. Square waves, or any wave shape with a steep leading edge (fast rise time) will generate many harmonics. For this reason, square waves often sound harsh and raspy, and there is some concern that square photic pulses may increase the likelihood of photically induced seizure in predisposed individuals.

Isochronic Beat
Any string of pulses separated by equal time periods. In AVS this term is applied to the beats heard when a carrier tone at a frequency usually referred to as the pitch, is amplitude modulated by another signal, the frequency of which is commonly referred to as the rate, or entrainment frequency. The shape of the modulating wave can be chosen to create a variety of effects, ranging from the smooth, gentle sine, through triangle, square, etc. to custom shapes that might sound like drumbeats, clicks, etc. Some studies indicate that the evoked response is stimulated on the leading edge of the pulse and steep rise times tend to be more effective in eliciting a response. Beats heard during a ramp between frequencies are, by definition, not isochronic, because the period between them is changing. Such beats, or any random or variable rate, are just plain pulses.

Many AVS/entrainment products claim to use “sophisticated layering techniques”. While frequency independent sessions and multifrequency session can be effective, layering is probably the technique most fraught with technical complications and least likely to deliver meaningful results.

The use of one signal to alter another signal (carrier) in such a way that the original signal can be extracted from (demodulated) the composite signal. Common types of modulation are:
Amplitude Modulation – the amplitude (volume) of the carrier follows the modulating waveform. Used to create isochronic beats.
Frequency Modulation – the frequency of the carrier follows the modulating waveform. Best know for its use in broadcasting.
Pitch Modulation – a special case of frequency modulation relevant to AVS. The key distinction between pitch modulation and frequency modulation is the frequency range of the carrier – for “pitch” to be meaningful the carrier has to fall within the audible spectrum.

Monaural Beat
A monaural beat sounds similar to a binaural beat, however the mixing of the two tones occurs externally. Monaural beats can be used without headphones. Monaural beats are more effective than binaurals, but less effective than pulses.

The particular type of Biofeedback using measurement of brain activity. Some of the best information about brain wave disorders and the use of feedback/stimulus to correct them has been derived from neurofeedback research and studies. The extent to which lessons learned from neurofeedback can be applied to open-loop AVS stimulation is debatable, however work with ADD/ADHD, depression and anxiety are all extremely encouraging.

Noise is a random distribution of signal amplitude versus over time. The highest and lowest frequencies present in noise represent the limits of its bandwidth. High pass, low pass and bandpass filters are used to seelct specific noise frequency bands. Special filters, affecting the distribution of frequencies and/or the rate of change between samples, creat special types of noise, including white, pink and brown noise. White noise has a flat power spectral density, sounding the most harsh and hissy. With pink noise, which sounds much softer than white noise, the power spectral density is inversely proportional to the frequency. Brown noise has a spectral density proportional to the inverse of the square of the frequency – more energy at lower frequencies – and sounds softer and deeper.

Open-Loop AVS
This term applies to any stimulus that is applied without biofeedback to verify and optimize the response.

Passive AVS
Passive AVS applies to any session that is used without feedback or deliberate intent to use the session time . Many therapeutic session work regardless of intent as they use entrainment to stimulate rhythms that are deficient in the user. Recreational AVS is usually passive as the session itself has no purpose other than enjoyment of the light and sound. Therapeutic sessions are often passive. Passive AVS is usually open-loop.

Sensorimotor Rhythm (SMR)
A band of frequencies around 12-16Hz associated with sensorimotor function. Sessions based on these frequencies can be useful for achieving a calm, alert state, improving dexterity, and establishing healthy sleep patterns. SMR, falling in the high alpha/low beta range, can be an excellent compromise when dealing with anxious depression, lower alpha commonly worsening depression and higher beta often inducing anxiety symptoms.

Schumann Resonance
In 1952 William Otto Schumann predicted a range of background frequencies arising from the waveguide effect of the earth’s ionosphere on the electrical discharges associated with storms, etc. This background noise occupies the range of 3-69Hz, with distinct peaks at 7.83, 14.3, 20.8, 27.3 and 33.8 Hz. 7.83Hz is popular in AVS sessions for relaxation, wellbeing and some trance-type states. Although correspondence between these frequencies and any particular human response/behaviour has not been conclusively determined, anecdotal evidence suggests that this correspondence does exist.

Sub-Delta (<1Hz)
The low end of the delta band is extremely difficult to entrain, however exposure to rhythms in this range are considered to calm the hypothalumus, providing pain relief and deep body relaxation. While upper delta frequencies tend to lead to drowsiness and sleep, this effect diminishes in the sub-delta range, and sub-delta sessions can be used at any time of day with little difficulty returning to routine function.

Theta (4-7Hz)
When theta dominates, we experience the dreamy state between sleep and wakefulness. At this time we are particularly open to suggestions, thus its application in hypnosis and NLP. Unusual states of consciouness abound here, with hypnogogia, lucid dreaming, OOBE, astral projection and suchlike being reported. Theta is excellent for meditation, and can be valuable in accessing deep memories. Excessive theta is associated with depression and ADD/ADHD whilst deficiency is implicated in addictive behaviours.


  • Saskia  On September 5, 2009 at 8:46 pm

    Excellent post, very useful! What I’m missing: AVS and BWE, both could obviously refer to topics already covered but will confuse newbies (like they did me) and need explanation.

    • craigtavs  On September 6, 2009 at 8:24 am

      Thanks, Saskia, I shall attend to that immediately.

  • avspro  On September 15, 2009 at 12:46 pm

    Hi Craig,

    Why Gamma waves are not present in the sessions (Proteus, Procyon ect …) According to research these waves have interesting effects.

    • craigtavs  On September 15, 2009 at 12:52 pm

      Hi Jocelyn,
      It’s only relatively recently that ‘useful’ information about the effects of gamma frequencies has started to emerge. While there’s well established protocols based on the other bands, anything gamma is going to be experimental.
      Most machines are quite capable of working at these frequencies (Procyon up to 75Hz), so altering existing sessions is an easy option.

  • M.C. Shaw  On August 12, 2012 at 4:51 am

    Thanks for a very helpful glossary. It would be helpful to have definitions for two terms used in the text: passive AVS and open-loop AVS.

    • CraigT  On August 12, 2012 at 8:50 am

      Thanks for your request. You will now find entries for Actve, Passive, Closed-Loop and Open-Loop AVS.

  • CraigT  On September 7, 2010 at 3:38 pm

    Thankyou – I have corrected the spelling of “Andrzej” – thanks for drawing the paragraph to my attention.


  • By What is AudioStrobe? | Dreaming Life on September 7, 2010 at 1:41 pm

    […] “Audiostrobe is a method of encoding light control signals in an audio file, developed and patented by Adrezj Slawinski, and currently the defacto standard for sound/light stimulation.” (source) […]

  • By What is AudioStrobe? | on April 30, 2011 at 7:13 pm

    […] “Audiostrobe is a method of encoding light control signals in an audio file, developed and patented by Adrezj Slawinski, and currently the defacto standard for sound/light stimulation.” (source) […]

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