The discussion of the Feroni article also errs in a manner making it seem more rigorous than it was.

Feroni did not measure the response of "muscles". He measured the response of the left zygomatic major muscle, which is associated with smiling. He did not measure the response of any muscles associated with frowning.

The dependent variable in the Feroni was the level of contraction of the left zygomatic major muscle averaged over several post-stimulus presentation times when responding to sentences in each of the 8 expermental conditions. So long as no artifacts were present and a steady baseline for comparison was (which excluded 6.7% of the trials in research with a small number of trials per experimental condition).