Complex impact of stimulus envelope on motor synchronization to sound.

The human brain tracks temporal regularities in acoustic signals faithfully. Recent neuroimaging studies have shown complex modulations of synchronized neural activities to the shape of stimulus envelopes. How to connect neural responses to different envelope shapes with listeners' perceptual ability to synchronize to acoustic rhythms requires further characterization. Here, we examine motor and sensory synchronization to noise stimuli with periodic amplitude modulations (AM) in human participants (14 females, 10 males). We used three envelope shapes that varied in the sharpness of amplitude onset. In a synchronous motor finger-tapping task, we show that participants more consistently align their taps to the same phase of stimulus envelope when listening to stimuli with sharp onsets than to those with gradual onsets. This effect is replicated in a sensory synchronization task, suggesting a sensory basis for the facilitated phase alignment to sharp-onset stimuli. Surprisingly, despite less consistent tap alignments to the envelope of gradual-onset stimuli, participants are equally effective in extracting the rate of amplitude modulation from both sharp and gradual-onset stimuli, and they tapped consistently at that rate alongside the acoustic input. This result demonstrates that robust tracking of the rate of acoustic periodicity is achievable without the presence of sharp acoustic edges or consistent phase alignment to stimulus envelope. Our findings are consistent with assuming distinct processes for phase and rate tracking during sensorimotor synchronization. These processes are most likely underpinned by different neural mechanisms whose relative strengths are modulated by specific temporal dynamics of stimulus envelope characteristics.Significance Statement Ample evidence demonstrates synchronized neurophysiological activity to the temporal regularities of sounds. This phenomenon has been proposed to reflect neural responses to onset edges in acoustic signals. Here, we examine listeners' ability to behaviorally synchronize to stimuli with sharp or gradual onsets. In two experiments, we show that while the sharp amplitude onsets facilitate temporal phase alignment between participants' behavioral output and the stimulus envelope, sharp onsets are not essential for tracking the rate of auditory rhythms in the acoustic input. The dissociation between phase and rate tracking suggests distinct underlying neural mechanisms that are separately modulated.