Rachel -- Basically, Higbee cites specific controlled scientific studies that show sleep learning doesn't work. The studies show that the amount of retention differs depending on how you define "sleep." In light sleep/just nodding off stages, there is some recall of audio-playback material, but as a person descends deeper and deeper into full sleep, there is virtually no recall or retention of material.
As for subliminals, Higbee starts by examining what is always referred to as the original, successful application of subliminals (even PRWMS). He states that the original movie theater "eat popcorn, drink coke" experiment has never ever been repeated successfully. By this he means that no similar test under controlled circumstances has been able to show a quantifiable increase in the elicited behavior. (BTW, other posters have referred to websites that call into question not only the validity of the actual movie-theater experiment but whether they even happened at all). Higbee then briefly reviews other experiments on the efficacy of subliminals, all of which show no correlation between the contents of the tape and outward behaviors or attitudes.
On the similar topic, I recall a post that said the "silent subliminal" technology developed by Lowery Associates of Norcross GA (and used in the upper levels of the Centerpointe Holosync program) is the "only subliminal proven to work." I've seen this same claim on the Centerpointe website, but I've never seen the "proof." (BTW, I am a dedicated Holosync user. I believe Holosync is effective, but even Bill Harris downplays the importance of the subliminals in the program).
I know subliminals are really popular on this board and folks have strong views on this, so other thoughts in the spirit of an open discussion are welcome.
regards,
Publius