3.1.3. Procedure

Participants took part in one of two conditions, a dyadic condition and an individual condition (shown in Figure 8). Throughout the experiment, participants were told not to speak or use mouthing, and were told to remain seated for the duration of the task.

**Figure 8.** Stages in experiment 2, for the dyadic condition (**top**), identical to experiment 1, and the individual condition (**bottom**), where participants take part in 3 improvisation rounds.

All participants in both conditions were given 3 practice items before the main study began. Participants were shown a vignette and asked to describe it to the experimenter using gestures. Participants who did not produce an explicit gesture for the target item in the vignette (e.g., camera, baby) were instructed to do so by the experimenter. After successful completion of the practice items, the experimenter left the room. The participant controlled the progression of the experiment using the arrow keys to move from trial to trial. Participants were allowed to repeat each vignette as needed before responding. After the initial production stage, the experimenter re-entered the room only to set up the experiment for the following stage or to give instructions preceding the communication stage.

The dyadic condition largely replicated experiment 1 in structure. Participants first completed an initial improvisation stage before taking part in an interaction stage with a partner. However, in the interaction stage, participants were seated in the same room, facing each other, with the computer displaying the vignettes in sight. At each trial, both participants could see the target vignette playing on the computer. After they had watched the video, one participant, the designated gesturer, had to describe the vignette to their partner using only gestures. No feedback was required from their partner and no feedback was given from the experimenter. However, there were no other constraints on how participants could communicate with gestures during the session and paired participants were free to provide gestural feedback to each other, or enact repair strategies on their own productions. Note that this lack of constraints stands in contrast to experiment 1, where participants were physically separate and only interacted via webcam streams between computers. Once the gesturer had finished their gesture, the experiment proceeded to the next trial. Participants switched roles at each trial, producing a gesture on every other trial. Participants completed a total of 64 trials, each producing a gesture for all 32 items. Following the interaction round, participants completed a final improvisation round, identical to the first.

In the individual condition, participants completed three improvisation rounds, each using 3 different randomised sets of all 32 vignettes. Participants produced gestures to communicate each vignette to the camera without a partner, across all three rounds (i.e., no communication took place). There was a brief break period in between rounds while the experimenter set up the next stimulus set.
