When looking at all the promotional material for audio-visual stimulation products you could be forgiven for gaining the impression that brainwave entrainment is the best thing since sesame bagels. Unfortunately ‘entrainment’ has become a catchword that has come unstuck from its actual meaning.
When the brain receives a pulse of sound via the ears, there is an evoked potential visible by EEG in the region of the auditory cortex (temporal region). If this pulse is repeated at a consistent frequency the EEG will show a frequency following response (FFR) in the same region. Under certain circumstances, the conditions for which are unclear, other regions of the brain may fall into lockstep with the auditory cortex and a signal corresponding to the stimulus frequency may be seen in other areas of the cortex. When other areas start following the initial stimulus, then we have the phenomenon correctly referred to as entrainment.
Exactly the same sequence of occurs with visual stimulation in the form of flashes of light. First there’s the evoked response in the visual cortex and secondary visual areas, then there’s entrainment if areas of the brain not generally associated with vision start following.
Activation of the brain’s non-visual and non-auditory regions by a repetitive audio or visual stimulus may occur at the same frequency, which may be considered true entrainment, or it may occur at multiples of the stimulus rate, or entirely different frequencies.
Auditory entrainment is beset by a number of difficulties.
Firstly, the brain responds to any significant stimulus with an evoked potential. This means that anything in addition to the stimulus, such as background music or environmental sounds, has the potential to interfere with entrainment. As a general rule, the most entraining sound is a simple train of clearly audible pulses. Such a train of pulses is often referred to as an isochronic beat – bursts of tone at identical intervals. Isochronic beats consist of a carrier frequency (pitch) and a beat frequency (rate). In the simplest form, the carrier is turned on and off at the beat rate. Fancier forms use a shaped wave to modulate the carrier. Modulating the carrier with a sine wave has two major benefits over simple square wave switching – first, it sounds more pleasant, and second, it minimises harmonics which can result in the brain responding at multiples of the stimulus frequency.
Next, we have aesthetics. More often than not, people find the insistent isochronic beat unpleasant to listen to. This is dealt with by adding background sounds, such as noise or music, and reducing the relative volume of the beats. The result is reduced likelihood of entrainment. The other alternative is to use monaural or binaural beats.
Monaural beats are the result of simple mixing of two tones. A tone of 100Hz combined with one of 110Hz will result in a plainly audible 10Hz beat. Monaural beats are less effective than isochronics, but they do work, and they are gentler on the ears.
Binaural beats make use of an idiosyncrasy of the brain whereby two slightly different tones presented to each ear create a beat. In this case the 100Hz would be applied to the left ear only, and the 110Hz tone to the right ear only. Most people, though not all, will hear a 10Hz beat originating within their head. There are a few explanations for this, the most popular being that it arises from the olivary nucleus where the brain tries to establish positional information from the phase relationship of the sounds arriving at each ear. A less popular explanation, one more readily demonstrated by experimentation, is that the binaural beat is the same as a monaural beat, except that the mixing occurs in the medium of brainstuff.
Binaural beats are the mainstay of entrainment mass marketing. A large proportion of the products that are accompanied by the most hype are based on binaural beats. The sad, bad news is that binaural beats do not entrain. Several researchers (Siever: Entraining Tones and Binaural Beats) have demonstrated that binaurals do not entrain. My own tests with an EEG don’t even show evoked response over the temporal region. This does not mean that sessions based on binaural beats don’t work, they just don’t work the way the promotional material says they do.
The final problem with entrainment is that it mostly doesn’t happen and it’s not obvious whether it matters if it does or not. Photic stimulation with light glasses has the highest likelihood of causing measurable responses in non-visual parts of the brain. Adding audio does not significantly increase the response. Audio on its own, and isochronic beats in particular, can entrain, but its effects are much weaker than photic stimulus. When entrainment does happen, there’s no direct relationship between a stimulus frequency and a particular outcome.
Entrainment occurs quite readily at 10Hz or thereabouts, which is the dominant resting frequency for most people. An EEG will almost invariably show a sharp increase in alpha activity at around 10Hz when an individual closes their eyes and relaxes. There is also a distinct burst of alpha associated with each blink of the eyes (blink response). Entrainment to any other frequency is both less in amplitude and likelihood. Below 2Hz entrainment becomes extremely unlikely – I have not seen it on an EEG and others have made the same observation.
So why, if entrainment is so unpredictable, are brainwave entrainment devices and software selling like hotcakes and accompanied by endless testimonials claiming effectiveness across a vast range of applications? Well, the answer is simple – audio-visual stimulation works. How it works is not obvious, but it does work.
There are a number of approaches.
One is the emulation of a known good brainwave pattern, another is using a frequency that is associated with a particular mental state or known physiology.
In the first case EEG recordings are taken from individuals adept at the practise of a particular activity. Monks have been recorded whilst in deep mediation and have shown elevated theta and delta, leading to the idea that theta and delta are good for mediation. The same monks when meditating on compassion have shown elevated gamma, resulting in gamma being associated with ‘higher’ consciousness.
In the second case there are two branches.
One is the observation of abnormal brainstates. Depression, for example, shows reduced beta activity and stimulation with beta frequencies has been shown in numerous trials to be helpful. ADD/ADHD shows a similar lack of beta accompanied by excess theta, and again, beta stimulus has shown its worth.
The other is the observation that certain neurotransmitters are released or particular structures within the brain are activated when particular frequencies dominate. Alpha frequencies are associated with serotonin release. The hypothalamus seems particularly responsive to sub-delta frequencies, leading to reductions in chronic pain.
Much of the information relating to this case has been extrapolated from successful research with neurofeedback. In neurofeedback the brainwave activity is monitored in real time and the subject performs exercises to cause changes to the dominant frequency. The validity of such extrapolation to passive AVS use is uncertain.
The third approach is the most useful for those who wish to design their own sessions. Each of the four major brainwave bands is naturally associated with distinct states – beta is alert and engaged, alpha is detached and relaxed, theta is dreamy, delta is asleep or deeply meditative. Knowing this, a frequency in the band associated with the desired activity can be chosen. There are no ‘magic’ frequencies and each individual will respond differently. Pick a frequency and try it out, using your mind to do the types of things associated with that frequency band, fine tuning as necessary.
Lot’s of people pick up an AVS device or CD with the expectation of a quick fix. Mostly it won’t happen. Some protocols (notably anxiety reduction) do seem to work first time every time, but most are progressive. Popular wishes, such as increased IQ or overcoming procrastination, are rarely fulfilled by passive use of AVS – the best way to increase your IQ is to use your brain, and the best way to overcome procrastination is to take action. The same applies to many applications – the AVS itself may or may not be directly beneficial, but it is always an excellent focus for creative visualisation and personal introspection.
AVS is a versatile tool for addressing a wide range of behavioural, psychological and physiological issues. Marketers have jumped the gun in offering specific explanations for its effectiveness. There are a limited number of applications where trials have shown a distinct correlation between AVS and positive outcomes, however the majority of supporting evidence is anecdotal or extrapolated from successes with neurofeedback.
At this point it would be easy to conclude that AVS is something of a scam. While there are some completely outlandish and unreasonable claims made, there are authentic positive results. Unlike many alternative therapies, such as homeopathy, where there is no active principle and no evidence whatsoever of physiological response, AVS does have clearly detectable effects on cortical activity. The causal relationship between this activity and the outcomes is unclear, but at least there is a set of mechanisms that may end up providing adequate explanation.
In my opinion, the greatest value to be derived from an AVS device is in taking a break from everything else and exercising the brain with pure thought. Having pleasing sounds and beautiful patterns of light blocking out the sights and sounds of the mundane world is a little piece of bliss accessible any time you have a spare half hour. The benefits of simple relaxation for the body, mind and soul should not be underestimated, and if this is all there is to explain the effectiveness of AVS, then it is enough.