
The principle underlying these devices is a well establish principle in
psychology called 'entrainment,' whereby external sensory stimuli
influence gross electrical patterns of brain function.

They are 'experimental' in that people experiment with them and they
are _not_ widely (if at all) used in medicine for therapeutic purposes.  
Given the exception of TENS and similar units used for external electrical 
stimulation, usually for pain relief, not really a light and sound machine.

They are _not_ experimental in the sense of a specific medical 
category to that effect, as with experimental drugs, as the FDA does not 
specifically regulate medical devices in the way it does pharmaceuticals.   


There are few reliable studies of therapeutic or enhancement effects
for mind machines, other than those relaxation-related effects found with 
meditation or self-hypnosis as well.  Reported benefits are mostly anecdotal and
subjective so far, so it's hard to generalize about their potential value.

A pretty good general non-technical introduction to a wide variety
of these devices may be found in "Would the Buddha Wear a Walkman ?"
Some interesting background material, names of suppliers, and capsule reviews
of specific equipment.  


A more important question might be whether they have enough additional
value to be worth investing in.  'Biofeedback' was found to be a legitimate
and reliable effect experimentally under certain conditions, (in that
it demonstrated that we can influence physiological processes previously 
considered purely autonomic) but never panned out as a particularly valuable 
therapeutic tool because of the skill level required and the subtlety and
temporary nature of the effects in most cases.   Maybe someone else 
has more, there used to be a whole mailing list devoted to mind machines,
somewhere on the net.


						kind regards,