Abstract
Error amplification (EA) feedback is a promising approach to advance visuomotor skill. As error detection and visuomotor processing at short time scales decline with age, this study examined whether older adults could benefit from EA feedback that included higher-frequency information to guide a force-tracking task. Fourteen young and 14 older adults performed low-level static isometric force-tracking with visual guidance of typical visual feedback and EA feedback containing augmented high-frequency errors. Stabilogram diffusion analysis was used to characterize force fluctuation dynamics. Also, the discharge behaviors of motor units and pooled motor unit coherence were assessed following the decomposition of multi-channel surface electromyography (EMG). EA produced different behavioral and neurophysiological impacts on young and older adults. Older adults exhibited inferior task accuracy with EA feedback than with typical visual feedback, but not young adults. Although stabilogram diffusion analysis revealed that EA led to a significant decrease in critical time points for both groups, EA potentiated the critical point of force fluctuations , short-term effective diffusion coefficients (Ds), and short-term exponent scaling only for the older adults. Moreover, in older adults, EA added to the size of discharge variability of motor units and discharge regularity of cumulative discharge rate, but suppressed the pooled motor unit coherence in the 13–35 Hz band. Virtual EA alters the strategic balance between open-loop and closed-loop controls for force-tracking. Contrary to expectations, the prevailing use of closed-loop control with EA that contained high-frequency error information enhanced the motor unit discharge variability and undermined the force steadiness in the older group, concerning declines in physiological complexity in the neurobehavioral system and the common drive to the motoneuronal pool against force destabilization.
Highlights
Impairment of force steadiness control is linked to unsafe visuomotor tasks in older adults (Lodha et al, 2016)
Post hoc analysis indicated that only the older group exhibited a greater total error in the error amplification (EA) condition than in the control condition (p = 0.002)
Post hoc analysis indicated that only the older group exhibited a greater size of force fluctuations in the EA condition than in the control condition (p < 0.001)
Summary
Impairment of force steadiness control is linked to unsafe visuomotor tasks in older adults (Lodha et al, 2016). An age-related increase in force fluctuations indicate impairment of precision force control for force production (Temprado et al, 2017), partly because prolonged delay in a feedback loop interferes with timely responses to tracking deviation (Kennedy and Christou, 2011; Tracy et al, 2015). As force fluctuations reflect an additive accuracy control mechanism to remedy trajectory deviations (Slifkin et al, 2000), dimensional changes in force fluctuations with advanced age links to a decline in error detection with feedback and/or feedforward processes (James and Kooy, 2011) depending on the environmental contexts (Morrison and Newell, 2015). Age-related increase in force fluctuations is primarily attributable to increase in motor unit discharge variability (Laidlaw et al, 1999; Tracy and Enoka, 2002; Tracy et al, 2005), underlying altered properties of spinal motor neurons and greater variability of descending drive to motor neurons (Hunter et al, 2016)
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