Some thoughts I had the last weekend. In this study:
Budzynski T, Jordy JE, Budzynski HK, Tang H, Claypoole K. Academic performance enhancement with photic stimulation and EDR feedback.
the researchers used photic stimulation of 14 and 22 Hz.
The result was not a peak at the entrained frequency but a rise of the dominant alpha frequency.
So the results of the study may be even more complicated to interpret.