Anchoring : moving from theory to therapy

Limb movements are often rhythmic in nature, exhibiting sinusoidal-like oscillations. Nevertheless, the 'back and forth' trajectories are seldom perfectly harmonic and symmetric but often show discrepancies between the two half cycles, for example in terms of differences in peak velocity, fluency, and local reductions of spatial variability. A central tenet of the notion of anchoring, as advanced in coordination dynamics, is that such kinematic differences are informative about the underlying control and the prevailing informational, musculoskeletal, and intentional constraints that influence the way in which movements are coordinated. In particular, points or regions of reduced variability have been interpreted to serve as control or anchor points for the complete movement cycle. The work reported in this thesis examines the combined effect of informational and musculoskeletal constraints on anchoring and seeks to translate these insights to clinical applications. A series of studies on rhythmic unimanual visuomotor tracking (Chapters 2-4) and auditory-motor coordination (Chapter 5) are reported aimed at uncovering the informational and musculoskeletal underpinnings of anchoring. These fundamental insights motivated two further studies, by way of spadework for therapeutic applications, of the effects of acoustic cues on gait coordination (Chapters 6 and 7), which in turn motivated the development of an instrumented treadmill for movement-dependent visual and acoustic cueing in a rehabilitation setting (Chapters 8 and 9). The first experiment, presented in Chapter 2, examined the effects of correct and transformed visual feedback on tracking performance (accuracy and stability) and visual search behavior in unimanual rhythmic visuomotor tracking. The results indicated that tracking performance was aided by visual feedback manipulations resulting in coherently grouped (i.e., in-phase) visual motion structures. Visual search patterns revealed that smooth pursuit eye movements only occurred at lower oscillation frequencies while at higher frequencies gaze was typically fixated at a specific location. Gaze fixations at the endpoints were accompanied by reduced motor variability at