Coherent brain frequency measurement

[Gary Goldberg MD <goldberg@VM.TEMPLE.EDU>]

I have been very interested in a technique described in the following paper
for determining a nonlinear 'resonance' in the human brain's response to
auditory stimulation (usually around 40 Hz for awake, unanesthetized human
subjects):

Sapsford DJ. Pickworth AJ. Jones JG. A method for producing the coherent
frequency: a steady-state auditory
evoked response in the electroencephalogram. [Journal Article] Anesthesia &
Analgesia. 83(6):1273-8, 1996 Dec.

The technique involves sweeping auditory stimulation frequencies and
identifying the frequency of stimulation at which the power in the elicited
steady-state auditory evoked potential is maximum.  The authors claim that
this technique defines a 'coherent frequency' in the brain's response--i.e.
a frequency of stimulation to which the brain is already 'tuned' and
therefore is likely to respond maximally.  What is interesting about this
'coherent frequency' is that it can be changed in a systematic manner
through the administration of an anesthetic and therefore can be used to
assess how deeply anesthetized an individual brain has become.
I think the idea is very powerful and potentially of some fundamental
physiologic significance--the idea that the human brain has a definable
resonance at least in response to auditory stimulation that is manipulable
neurochemically could have some interesting and important clinical
implications.
I am just not sure about the methodology that is used by Sapsford et al to
define this frequency and wonder whether there may be more standard methods
for identifying resonances in a presumably highly nonlinear system like the
human brain.  There is a phenomenon in nonlinear systems called a 'soliton'
and I am not sure whether this type of phenomenon could be examined with
methods that have been developed for evaluating the emergence of solitons
in nonlinear systems rather than the method used by Sapsford et al which
approaches the phenomenon as a resonance in a linear system.
If anyone on the list has an interest in this area of research or has ideas
about how to evaluate this phenomenon with neurophysiologic methods, please
answer me privately.   Thanks.
--GG