Abstract
BackgroundDifferent types of sound cues have been used to adapt the human gait rhythm. We investigated whether young healthy volunteers followed subliminal metronome rhythm changes during gait.MethodsTwenty-two healthy adults walked at constant speed on a treadmill following a metronome sound cue (period 566 msec). The metronome rhythm was then either increased or decreased, without informing the subjects, at 1 msec increments or decrements to reach, respectively, a low (596 msec) or a high frequency (536 msec) plateaus. After 30 steps at one of these isochronous conditions, the rhythm returned to the original period with decrements or increments of 1 msec. Motion data were recorded with an optical measurement system to determine footfall. The relative phase between sound cue (stimulus) and foot contact (response) were compared.ResultsGait was entrained to the rhythmic auditory stimulus and subjects subconsciously adapted the step time and length to maintain treadmill speed, while following the rhythm changes. In most cases there was a lead error: the foot contact occurred before the sound cue. The mean error or the absolute mean relative phase increased during the isochronous high (536 msec) or low frequencies (596 msec).ConclusionThese results showed that the gait period is strongly “entrained” with the first metronome rhythm while subjects still followed metronome changes with larger error. This suggests two processes: one slow-adapting, supraspinal oscillator with persistence that predicts the foot contact to occur ahead of the stimulus, and a second fast process linked to sensory inputs that adapts to the mismatch between peripheral sensory input (foot contact) and supraspinal sensory input (auditory rhythm).
Highlights
The effect of an imposed external rhythm on human motion has been extensively studied in the last decades [1, 2]
A custom-made metronome based on an Arduino Uno (Arduino SpA, Italy) with a custom-made software program written in Visual Basic (Microsoft Visual Studio, USA) generated a pulse that triggered an infrared Light Emitting Diode (LED) within the performance volume and a beep sound
Subjects adapted the step length to the subliminal rhythm changes with the constraint of maintaining the treadmill speed, changing the step length (Fig. 3)
Summary
The effect of an imposed external rhythm on human motion has been extensively studied in the last decades [1, 2]. Most of the work in the analysis of rhythmic motion focused on a simple finger tapping task. It has been shown that subjects modulated their responses to subliminal phase shifts of 5° [8] These studies suggest one might expect changes in the auditory cortex interstimulus for subliminal fluctuations of 10 msec in sound interval. This is much faster than any motor evoked response and imply that the auditory system might provide a way to interrogate the motor system below conscious perception timeframes. We investigated whether young healthy volunteers followed subliminal metronome rhythm changes during gait
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