## 4 User Evaluation Method
### Evaluation Application and Collected Data  
A web application was developed for the execution of the experiment. At the beginning, the application explained what the participants had to do (see procedure section below). Then, participants were allowed to declare gender and age so that the experiment could start.  
The experiment consisted of 60 trials, and for each of them, the application showed a rhythmic control (Figure 2) consisting of green and gray circles and a smaller dot moving within them, along with seven other hidden controls whose rhythms were shifted by one unit of time each. Therefore, there were eight controls active at the same time, seven of which were hidden to the user, while one randomly chosen was visible to the user for each trial (Figure 3).  
Participants were required to select the visible control by pressing the space bar when the dot was inside the green circles, making all green circles disappear in sequence. The green circles were restored to the visible state if the space bar was pressed when the dot was inside a gray circle or if it was not pressed when the dot was inside a green circle. The trial was considered completed when one of the following events occurred: (1) all the green circles disappeared (i.e., control successfully activated); (2) 10 s passed after the start of the trial (timeout interval after which a missed activation was recorded); or (3) an unwanted control was activated (i.e., activation of an incorrect control after which an error was recorded). Upon completion of a trial, a new trial was started. The rhythm speed of the controls was initially set to 2.8 s, as suggested by the previous study on SEQUENCE [1]. However, according to a pretest on 17 students (16M/1F) aged between 21 and 28 years (M: 24), we decided to accelerate the rhythm slightly since the error rate was very low. In fact, eight of them did not make mistakes, while the others made very few. Therefore, to make synchronization more difficult, we set the rhythm to 2.4 s. Note that in the previous study on SEQUENCE, the rhythm speed at 2.4 s caused an error rate of 20.66% [1].  
The application collected data on 60 trials split into two conditions consisting of 30 trials each, namely \"Sound\", in which the application played an auditory rhythm (drum loop) interfering with the visual rhythm, and \"Mute\", in which the application displayed  
Figure 3: Eight controls with different rhythm patterns, of which one was made visible to user while seven were hidden. Visible control changed randomly during experiment for any trial. If one of hidden control was unintentionally activated, a false positive would be recorded.  
the visual rhythm without playing any sound. The condition changed every three trials so that learning, i.e., carryover effect, was distributed equally among the conditions, without one condition being dominant over the other. In addition, the application required the user to complete nine additional trials, whose data were not recorded, at the beginning of the experiment. This allowed participants to familiarize themselves with the experiment before collecting any data. During the 60 trials actually recorded, the application collected data on (1) activation time, (2) false activation (accidental activation of one of the 7 hidden controls), and (3) missed activation (no control activated within 10 s--timeout interval).  
At the end of the experiment, the application asked to fill in a questionnaire about the ease of synchronization in both conditions (evaluated on a six-point ordinal scale) and a text field where they could write a comment on the experience. Finally, the collected data were sent to the server and saved as CSV for further analysis.