Ears, heads, and eyes: When singers synchronise.

We examined the relationship between endogenous rhythms, auditory and visual cues, and body movement in the temporal coordination of duet singers. Sixteen pairs of experienced vocalists sang a familiar melody in Solo and two Duet conditions. Vocalists sang together in Unison (simultaneously producing identical pitches) and Round Duet conditions (one vocalist, the Follower, producing pitches at an eight-tone delay from their partner, the Leader) while facing Inward (full visual cues) and Outward (reduced visual cues). Larger tempo differences in partners' spontaneous (temporally unconstrained) Solo performances were associated with larger asynchrony in Duet performances, consistent with coupling predictions for oscillators with similar natural frequencies. Vocalists were slightly but consistently more synchronous in Duets when facing their partner (Inward) than when facing Outward; Unison and Round performances were equally synchronous. The greater difficulty of Rounds production was evidenced in vocalists' slower performance rates and more variable head movements; Followers directed their head gaze away from their partner and used bobbing head movements to mark the musical beat. The strength of Followers' head movements corresponded to the amount of tone onset asynchrony with their partners, indicating a strong association between timing and movement under increased attentional and working memory demands in music performance.