Figure 1. Experimental paradigm.
A. Throughout the session, treadmill speed remained fixed at a rate deemed comfortable by the participant. During each approximately minute-long trial, participants first walked for ~10 sec without auditory cues, then walked for 10–18 sec while attempting to synchronize their foot falls to brief cue tones delivered at their then-prevailing step rate and phase. Thereafter, beginning at a right heel strike (S0), a sudden tempo shift occurred in the pacing cue sequence, the first tone indicating the new tempo by being early or late relative to the participant’s heel strike (S0). In response, participants were instructed to adapt their step phase (percent latency difference from cue tone onsets), and step rate or step-onset asynchrony (SOA) as quickly as possible, so as to again synchronize their steps to the ensuing cue tones delivered at the new tempo. To do this, the constant pace of the treadmill movement required that they adjust their step length appropriately. After 30–70 steps synchronized to the auditory cues at the new preferred tempo, the next trial began immediately, beginning again with 10 sec of uncued walking during which participants were asked to return to their most comfortable (preferred) step rate. B. Snapshot of events during tempo shifts in step advance (blue background) and step delay (red background) conditions, showing schematic steps S−2 to S3 and cues C−2 to C3 around tempo shifts signaled by the timing of C0. The tempo shift always occurred relative to a right step (S0), the first deviant tone (C0) indicating the new tempo by being early (e.g., on average 47 ms before S0 in step-advance trials) or late (e.g., on average 165 ms after S0 in step-delay trials). Distance between heel strikes represent SOAs -- i.e. the distance between S1 and S2 represents SOA1,2.