## 2 Motivation and Hypotheses  
Previous studies focused primarily on the design and user evaluation of rhythmic synchronization techniques (e.g., [1,2,4,11]), while another focused on improving matching by modeling user behavior [3]. However, none of the above studies were conducted evaluating whether any external auditory stimulus rhythmically contrasting with the visual rhythm can somehow alter users' ability to synchronize. In fact, these interaction techniques are often used in scenarios where users may be required to synchronize with visual rhythmic patterns while listening to auditory stimuli (e.g., in a living room, while listening to music through the TV, radio, or media player) whose rhythm interferes with the visual rhythmic pattern. Therefore, we aim to investigate whether the presence of an interfering auditory stimulus may affect the ability to synchronize with the visual representation of a rhythm. The theory of selective attention [12] states that users are able to selectively focus on the stimulus of their interest (e.g., the visual representation of a rhythm) while ignoring interfering ones (e.g., disturbing background music). In addition, visual perception is generally dominant compared to auditory perception [13]. Consequently, the presence of a contrasting auditory stimulus should not significantly affect users' ability to synchronize; however, it is necessary to confirm this hypothesis through an appropriate evaluation with users. Based on the discussion above, the first research question is: to what extent does a contrasting auditory stimulus affect user performance when synchronizing with controls? In addition, none of the above studies evaluate interaction techniques with a large and varied number of users, and it is therefore unknown to what extent these techniques are suitable for a heterogeneous population in terms of age and gender. For example, three studies [1, 2, 3] involved 12 participants each, whose average ages were 27, 23, and 22, and the number of males always outnumbered females (nine males and three females in each study). Another study involved only eight participants [4], six males and two females,  
Figure 1: How rhythmic-synchronization-based interaction works: an animated control shows a rhythm, and user can select it by matching corresponding rhythm, e.g., using a binary sensor as a button to mark rhythm. In example, the rhythmic sequence is shown by a physical control using a series of LEDs arranged in a circular way. Once user synchronizes with given rhythmic sequence, lamp turns on. Figure 2: Control visible to user, with which it was required to synchronize. with an average age of 22 years. The last study mentioned above [11], finally, involved 11 participants of unspecified age and gender. Considering then that these studies involved few users, all young, and overwhelmingly male, it seems relevant to us to understand the extent to which interaction techniques based on rhythmic synchronization can be used by a heterogeneous population. In fact, an interaction technique should be usable by users of different ages and genders for it to be truly applicable in the real world. In the case of rhythm-based interaction techniques, however, there are several reasons why users might perform differently depending on age, gender. More specifically, these techniques show a cyclic rhythm pattern, which can have different speeds depending on system settings. Faster rhythms require greater synchronization ability--i.e., faster reflexes--than slower rhythms. Therefore, considering that these techniques require users to have certain reflex abilities to synchronize correctly with the displayed rhythm, we need to understand to what extent these reflex abilities vary depending on age and gender. Previous literature shows that response time to stimuli varies with age and gender [14; 15; 16], so one might expect older and female users to have more difficulty to synchronize with visual rhythm patterns than younger and male users.