[https://www.jstor.org/stable/1729929?sid=primo&seq=1 Science] article from 1970 showing that neural entrainment (or "frequency following response") is a neural phenomenon rather than artifact.
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[https://link.springer.com/article/10.1007/s00426-018-1066-8 Psychological Research] Meta-Analysis from 2018 showing a Hedge's g value of 1 for analgesia based on three studies between 1999 and 2010 (The Hedge's g statistic expresses the difference of the means in units of the pooled standard deviation)