Co-speech gesture projection: Evidence from truth-value judgment and picture selection tasks

Two main analyses have been proposed to explain how co-speech gestures interact with logical operators. According to the Supplemental analysis (Ebert & Ebert 2014), co-speech gestures have the same semantic status as appositive relative clauses. According to the Cosuppositional analysis (Schlenker To appear a; b), co-speech gestures trigger a particular kind of presupposition. The sentence John will not use the stairs, produced with an UP gesture (finger pointed upwards) is argued to give rise to the conditional presupposition that if John were to use the stairs, he would use the stairs in an upwards trajectory. Both the Supplemental and Cosuppositional analyses predict that inferences triggered by co-speech gestures should project out of the scope of operators, but not quite in the same way. We present an experimental investigation of the projection properties of the inferences arising from the co-speech gestures UP and DOWN in six different linguistic environments (plain affirmative and negative sentences, modal sentences containing might, and quantified sentences containing each, none, and exactly one). Applying a reading detection analysis (Cremers & Chemla 2017) to the responses of a Truth Value Judgment Task and a Picture Selection Task, we find evidence for existential projection of the gestural inferences in the scope of each, none, and exactly one, and, to some degree, local accommodation of the inferences. These results can be derived by the Cosuppositional analysis, in combination with an analysis of presupposition projection such as Beaver (2001), which predicts existential projection out of quantified structures; on the other hand, both findings are difficult to reconcile with the Supplemental analysis. Our projection results bring gestural inferences and verbal presuppositions closer together, but a remaining puzzle is why in quantified structures we obtain existential rather than universal inferences (Chemla 2009), the latter being the more standard finding in the presuppositional literature (though see Tieu et al. 2016 for evidence of universal projection of the same gestural inferences).