Simultaneity of certainty in Turkish Sign Language (TİD)

As collaborative discourse participants, interlocutors are expected to convey how certain they are about the proposition that they put forward in the discourse. In spoken language literature, speakers use various strategies to convey their certainty like attitude verbs, prosody, or gestures. Sign language literature reports that signers mostly modulate their manual (hands) signs and nonmanuals (face and body movements), yet there is no study investigating how both channels interact in the expression of certainty. The current study tests the degree to which different sentence types and nonmanuals affect the signer certainty in Turkish Sign Language (TİD) in a rating study. I show that signers interpret signer certainty by taking into account cues from both manual and nonmanual channels. Within the framework of threshold semantics, I discuss how sentences set the certainty threshold, and show how nonmanuals boost or deboost the threshold. Specifically, head nod increases the threshold while squint decreases it, thus providing further support for the dynamic nature of thresholds.