Fractal dynamics in dexterous tool use: the case of hammering behavior of bead craftsmen

Dexterous behavior exhibits exquisite context sensitivity, implying the efficacy of exploration to detect the task-relevant information. Inspired by the recent finding that fractal scaling of exploratory movements predicts how well the movements sample available perceptual information, we investigate the possibility that dexterity of craftsmen would be characterized by fractal (long-range) temporal correlation properties of fluctuations in their movement wielding a tool. A reanalysis of hammering behavior involved in stone beads production in India (Nonaka & Bril, 2012) revealed the presence of long-range, power-law correlations, as part of multiplicative cascades operating over a wide range of time scales. In the unfamiliar condition using unusual material, the wielding behavior of highly skilled experts displayed a significant increase of long-range temporal correlations, whereas that of less experts exhibited a significant loss of long-range correlations and reduced heterogeneity of scaling properties over time, which robustly discriminated the groups with different skill levels. Alterations in long-range correlation properties of movement fluctuations are apparently associated with changes in the situation differently depending on the level of expertise.