What is the grammatical role of "signifying" in the following sentences? And what does mean the "signifying fear"?
Intense contraction of facial muscles, such as those used in signifying fear heightens arousal.
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“Signifying” is the same as “signaling” or “showing” or “displaying” here. This sentence is saying that facial muscle contraction increases arousal and the muscle contraction might be done to signal fear.
“Signifying” is the present progressive form of the verb “to signify”. It’s being used as a noun here (indirect object of “used”) so you can also say it’s a gerund.