Yes, there are are studies which support the use of frequency "sweeps" vs. fixed frequencies; in fact this approach was included in a patent held by Dr. Harold Russell, Dan Vaughn and myself. One common misconception which has propagated in the LS field is that there are very precise frequencies which produce specific esoteric or medical effects. Though there is some basis for this (i.e., Tansey uses and touts 14 Hz neurofeedback for ADHD, though others more generally use the SMR range of which 14 Hz is a component), it is very unlikely that a fractional Hz will produce the same effect on everyone. That's why we made the Proteus programmable--the included sessions are intended as much as examples as anything else.
Back to frequency sweeps within a band, for example 9-12 Hz: you would be correct if you are suggesting that this is less effective than stimulation at a given user's "ideal" frequency within a band (for example, 10 Hz might be the "best" one for you). However, stimulating at a fixed frequency can lead to habituation: a lessening response to that frequency over time. So, varying the frequency is in part intended to sidestep stimulus induced habituation. Some studies have show that stimulating near an individual's ideal frequency can enhance amplitude at that frequency as well. I would post more details about these studies, except the FDA would consider this medical "labeling" and we do want to comply with their regulations. A good place to research LS studies is Pubmed (freely available online) and psychlit (fee based but usually available from State University libraries; I do my main research at the University of Washington).
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