Humans exhibit an innate ability to synchronize their movements to music. The field of gait rehabilitation has sought to capitalize on this phenomenon by invoking patients to walk in time to rhythmic auditory cues with a view to improving pathological gait. However, the temporal structure of the auditory cue, and hence the temporal structure of the target behavior has not been sufficiently explored. This study reveals the plasticity of auditory-motor coupling in human walking in relation to 'complex' auditory cues. The authors demonstrate that auditory-motor coupling can be driven by different coloured auditory noise signals (e.g. white, brown), shifting the fractal temporal structure of gait dynamics towards the statistical properties of the signals used. This adaptive capability observed in whole-body movement, could potentially be harnessed for targeted neuromuscular rehabilitation in patient groups, depending on the specific treatment goal. P revious research investigating sensorimotor synchronization suggests that when rhythms are presented in different modalities, there appears to be an auditory advantage over other sensory pathways. Sejdic et al. showed that, compared to visual and haptic rhythms, auditory rhythms have the strongest influence on a person's gait. An evolutionary reason for this advantage has been presented, suggesting that bipedal gait and the associated sounds of locomotion influenced the evolution of human auditory-motor rhythmic abilities 2 . Traditional finger-tapping paradigms have refined our understanding of the neural circuitry that underpins auditory-motor coupling, however the mechanisms underlying this phenomenon in whole-body, functional movements are not well understood. This is due to the practical limitations associated with brain imaging and measurement techniques currently available for upright, whole body movements. However, despite these practical limitations that render investigations into neural circuitry somewhat elusive for whole body movement, there is still a pressing need to capitalize on the potential that auditory-motor coupling offers the field of neuromuscular rehabilitation. The authors describe a novel auditory-motor entrainment paradigm that could potentially be implemented for the rehabilitation of gait, in cases of brain injury and neurodegenerative disease. Synchronization of walking to rhythmic auditory cues has previously been investigated as a potential training tool to support functional independence in individuals with pathological gait [3] [4] [5] . Parkinson's disease and stroke, in particular, have been the main focus of auditory cueing investigations, with evidence to suggest that walking to a fixed tempo metronome may temporarily improve gait velocity, stride length, cadence and symmetry 6, 7 . However, these studies have not taken the temporal dynamics of gait parameters into account, i.e. the sequencing of gait parameters over time, which reflects the adaptive state of the neuromuscular system. Well-established motor learning principles suggest that variability in practice and feedback is required for better learning outcomes 8 , however this is largely absent from the field of auditory cueing for gait rehabilitation, which has focused predominantly on fixed tempo metronomes as the auditory cue. Gait variability can be defined as the natural stride-to-stride fluctuations that are inherent in normal, healthy gait 9 . These stride-to-stride fluctuations are thought to arise from interconnected, nonlinear processes that enable us to expand and maintain a large repertoire of movement strategies, allowing robust gait in dynamic, unpredictable environments. Fractal patterns observed in biological signals such as heart rate 10 , respiration 11 and walking strides 12 measured over time, indicate that the time intervals between events are not equal, nor are they independent. Rather, there is a relationship between these intervals that extends far forward and backward in time; in other words, exhibiting long-range correlations in the time series, or fractal fluctuations. The presence of these fractal processes in biological systems is theoretically referred to as ''complexity '' 13 , which describes an underlying order or pattern that is contained within a complex, variable system; a system that is capable of sudden
According to Hunt et al. REF , walking in time to a fixed rhythm like the one provided by the metronome changes the natural dynamics of the human gait.
The influence of auditory-motor coupling on fractal dynamics in human gait
Scientific Reports
Scientific Reports
Biology Medicine