Removal of visual feedback lowers structural variability of inter-digit force coordination during sustained precision pinch

This study examined the effects of visual feedback on inter-digit force coordination during a precision pinch. Sixteen healthy, right-handed subjects were instructed to pinch an instrumented apparatus for 1min with a stable force output. Visual feedback was provided for the first 30s and withdrawn for the second 30s. Detrended fluctuation analysis (DFA) and detrended cross-correlation analysis (DCCA) methods were used to quantify the time-dependent structures of each digit's force and of the force correlation between the digits. After removing visual feedback, the DFA scaling exponent, α_DFA, increased from 1.10±0.12 to 1.29±0.13 for the thumb and from 0.95±0.08 to 1.33±0.13 for the index finger (F_1,95=372.47, p<0.001); the DCCA scaling exponent, α_DCCA, increased from 1.00±0.08 to 1.33±0.13 (t₉₅=20.33, p<0.001). Structural changes were observed beginning with the first 5s epoch after the removal of visual feedback. The results provide evidence that removing visual feedback lowers the structural variability of inter-digit force coordination. This change is reflected in the high-level control strategy, resulting in the two digits being more tightly coupled under somatosensory feedback without visual inputs.