A matter of style: Intra- and inter-individual motor variability as structural properties of human motor-perceptual organization

Despite its pervasive presence, intra- and inter-individual motor variability has traditionally been overlooked in the study of human motor behavior. Only in recent years, increasing attention has been devoted to the potential functional role that variability may play in motor-perceptual organization. Seminal findings have suggested that a certain degree of intra-individual motor variability, reflected in each person’s motor characteristics, can support cognitive operations, motor skill learning, and even motor rehabilitation. Nevertheless, existing evidence remains insufficient to provide a comprehensive understanding of the functional significance of motor variability. Motivated by these preliminary insights, the present thesis aims to consolidate the functional role of intra- and inter-individual motor variability in human motor behavior. In doing so, it contributes to moving the field beyond an exclusive focus on shared, average-based, movement regularities toward a joint consideration of both general motor architectures and individual-specific motor features - commonly referred to as motor style. The first study proposes a theoretical framework that systematically unpacks the factors contributing to variability in motor outcomes, decomposing it into meaningful and functionally interpretable (sub-)domains. The second and third studies operationalize motor variability through the validation of a multidimensional metric capable of capturing both intra- and inter-individual motor dissimilarities. These theoretical and methodological frameworks are subsequently leveraged in the fourth and fifth studies to examine the functional role of motor variability in both action execution and action observation. Specifically, the fourth study investigates the stability of intra- and inter-individual motor variability across movement domain and its contribution, together with the one of invariant movement laws, to the structure of motor individuality during action execution. The fifth study extends this investigation to action observation, examining how inter-individual motor variability (i.e., dissimilarity) influences predictive processes active during action understanding. Collectively, the findings of this dissertation demonstrate that intra- and inter-individual motor variability constitute structural properties of human motor-perceptual organization. Together with shared motor invariants, motor variability contributes to shaping human motor individuality as a multi-layered phenomenon which must be considered for future theoretical, translational, and rehabilitative advances within the motor cognition field.