In Left 4 Dead, test subjects had their sweat monitored, with values assigned to how much they were responding to the action.

ENGADGET

Researchers then conducted a behavioral test in which participants separately breathed the "exercise" sweat and the skydiving "stress" sweat while judging the emotions conveyed in pictures of faces on a computer screen.

CNN: Sweat may give off stress signals, study says

However, when the researchers performed a variation of this test, in which the earrings were exposed to artificial sweat for up to a week, nine of the 66 pieces of jewellery tested positive.

BBC: Earring