Abstract
Humans detect changes in the air pressure and understand the surroundings through the auditory system. The sound humans perceive is composed of two distinct physical properties, frequency and intensity. However, our knowledge is limited how the brain perceives and combines these two properties simultaneously (i.e., intra-auditory integration), especially in relation to motor behaviors. Here, we investigated the effect of intra-auditory integration between the frequency and intensity components of auditory feedback on motor outputs in a constant finger-force production task. The hierarchical variability decomposition model previously developed was used to decompose motor performance into mathematically independent components each of which quantifies a distinct motor behavior such as consistency, repeatability, systematic error, within-trial synergy, or between-trial synergy. We hypothesized that feedback on two components of sound as a function of motor performance (frequency and intensity) would improve motor performance and multi-finger synergy compared to feedback on just one component (frequency or intensity). Subjects were instructed to match the reference force of 18 N with the sum of all finger forces (virtual finger or VF force) while listening to auditory feedback of their accuracy. Three experimental conditions were used: (i) condition F, where frequency changed; (ii) condition I, where intensity changed; (iii) condition FI, where both frequency and intensity changed. Motor performance was enhanced for the FI conditions as compared to either the F or I condition alone. The enhancement of motor performance was achieved mainly by the improved consistency and repeatability. However, the systematic error remained unchanged across conditions. Within- and between-trial synergies were also improved for the FI condition as compared to either the F or I condition alone. However, variability of individual finger forces for the FI condition was not significantly decreased as compared to I condition alone. This result indicates an improvement in motor performance is consistent with Bayesian estimation, and changes in multi-finger interaction mostly result in the enhanced motor performance. These findings provide evidence that the central nervous system can take advantage of the intra-auditory integration in a statistically optimal (Bayesian) fashion to enhance motor performance by improving multi-finger synergy.
Highlights
We found that intra-auditory integration enhanced motor performance quantified as overall mean-squared error (OMSE)
We found that the enhanced motor performance achieved by improving consistency and reproducibility of the virtual finer (VF) force with unchanged accuracy
We reported that motor performance could be enhanced through intra-auditory integration as evidenced by improved tracking of the target force by the VF force
Summary
For nearly every real-world case of motor behavior, the central nervous system (CNS) simultaneously receives information from a variety of sensory modalities, including visual, tactile, and auditory signals (Ernst and Banks, 2002; Stein and Stanford, 2008; Berniker and Kording, 2011), and is required to coordinate multiple effectors (e.g., motor units, muscles, joints, limbs) to effectively perform the movement (Babinski, 1899; Castiello, 1997; Alexandrov et al, 1998; Shim et al, 2003b). Concerning the second problem (motor redundancy), the control of multiple motor effectors has been studied at the level of muscles (Babinski, 1899; Smith, 1993; Ting and McKay, 2007), joints (Castiello, 1997; van der Kamp and Steenbergen, 1999; Latash and Jaric, 2002), and body segments (Alexandrov et al, 1998; Kim et al, 2012), including fingers (Latash, 2000; Latash et al, 2002, 2007; Shim et al, 2003b, 2005b, 2008). Our recent work showed that the removal of tactile feedback in multi-finger pressing tasks resulted in decreases in multi-finger synergies for online control (Koh et al, 2015) These studies suggest that sensory feedback plays critical roles in multi-finger synergies and sensory integration may systematically influence the synergistic interactions between motor effectors. Calibration was performed in a soundproof room by manipulating frequency from 20 to 10,000 Hz in 1 Hz increments and normalizing intensity at each increment
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