Signers and speakers show distinct temporal kinematic signatures in their manual communicative movements

Instrumental movements are kinematically different from communicative movements. Kinematics differ when we use our hands to hammer a nail vs. using them to describe hammering a nail. Here, we ask whether communicative movements all share the same kinematic profiles and thus differ from instrumental movements in the same way. We compared spontaneous gestures produced by hearing speakers when they talk (co-speech gesture) to productive iconic signs produced by deaf signers when the signs superficially resemble co-speech gestures (classifier signs). We used motion tracking and kinematic analyses to disentangle the spatial and temporal kinematic patterns of communicative movements in 33 English-speakers and 10 American Sign Language (ASL) signers, using each group’s instrumental movements as a control. Participants copied a movement on an object performed by a model (instrumental movement) and then described what they did with the object (communicative movement). We found no instrumental-communicative differences between groups in spatial kinematics. However, for temporal kinematics, speakers’ co-speech movements were less rhythmic and jerkier than their instrumental movements; in contrast, signers’ communicative movements were more rhythmic and smoother than their instrumental movements. We thus found differences in the temporal (but not spatial) kinematic signatures of co-speech gestures vs. classifier signs, leading to 3 conclusions: (i) communicative movements do not always have the same kinematic signatures; (ii) since signers’ and speakers’ communicative movements have different kinematic features, even highly iconic signed movements cannot be considered entirely gestural; and (iii) we need fine-grained techniques to measure movement components when identifying the gestural aspects of sign.