Effect of force accuracy on hemodynamic response: an fNIRS study using fine visuomotor task

Abstract

Objective. Despite converging neuroimaging studies investigating how neural activity is modulated by various motor related factors, such as movement velocity and force magnitude, little has been devoted to identifying the effect of force accuracy. This study thus aimed to investigate the effect of task difficulty on cortical neural responses when participants performed a visuomotor task with varying demands on force accuracy. Approach. Fourteen healthy adults performed a set of force generation operations with six levels of force accuracy. The participants held a pen-shaped tool and moved the tool along a planar ring path, meanwhile producing a constant force against the plane under visual guidance. The required force accuracy was modulated by allowable tolerance of the force during the task execution. We employed functional near-infrared spectroscopy to record signals from bilateral prefrontal, sensorimotor and occipital areas, used the hemoglobin concentration as indicators of cortical activation, then calculated the effective connectivity across these regions by Granger causality. Main results. We observed overall stronger activation (oxy-hemoglobin concentration, p = 0.015) and connectivity (p < 0.05) associated with the initial increase in force accuracy, and the diminished trend in activation and connectivity when participants were exposed to excessive demands on accurate force generation. These findings suggested that the increasing task difficulty would be only beneficial for the mental investment up to a certain point, and above that point neural responses would show patterns of lower activation and connections, revealing mental overload at excessive task demands. Significance. Our results provide the first evidence for the inverted U-shaped effect of force accuracy on hemodynamic responses during fine visuomotor tasks. The insights obtained through this study also highlight the essential role of inter-region connectivity alterations for coping with task difficulty, enhance our understanding of the underlying motor neural processes, and provide the groundwork for developing adaptive neurorehabilitation strategies.

fNIRS-based adaptive visuomotor task improves sensorimotor cortical activation

“…During the visuomotor task, we observed typical hemodynamic activation on the contralateral and ipsilateral sensorimotor regions, which is consistent with studies reporting bilateral activation patterns induced by complex unimanual motor tasks, such as sequential finger movements [66] and the use of chopsticks [67]. Compared with simple movements or force control tasks, the visuomotor task required frequent transformations of the online visual feedback into force adjustment and mainly involved visual information processing, force error correction and their complex temporal interactions, leading to abundant recruitment of the bilateral sensorimotor areas [46]. Furthermore, our results demonstrated that the adaptive task difficulty can induce increased activation (i.e.…”

“…The visuomotor task required participants to move a pen-shaped tool along a planar ring path and to produce a constant force against the plane under visual guidance, the same as a previous work did [46]. During the task, the participants held the end effector of a haptic device (Touch, 3D Systems Inc., USA) with their right hands and drew a circle within the range of a circular ring.…”

“…During each trial, the actual exerted contact force was recorded at every sampling point (sampling rate 1000 Hz) and the force score representing the percentage of sampling points where the exerted force was within the specified range was computed to assess the force control performance. For more details on the force score computing see [46].…”

“…Based on the computed MNI coordinates, the 46 measurement channels covered six ROIs as follows: left prefrontal cortex (lPFC) (channels #1-4), right prefrontal cortex (rPFC) (channels #5-8), left sensorimotor cortex (lSMC) (channels #9-12, #17-23, and #31-34), right sensorimotor cortex (rSMC) (channels #13-16, #24-30, and # [35][36][37][38], left occipital cortex (lOC) (channels #39-42), and right occipital cortex (channels # [43][44][45][46]. More details on the probe design and the cortical projection coordinates of all channels in MNI space are presented in supplementary table 1.…”

“…We used hemodynamic signals from the bilateral sensorimotor cortex measured by functional near-infrared spectroscopy (fNIRS) to adjust the task difficulty in a trial-to-trial manner. The approach relied on evidence provided by a recent work indicating the inverted U-shaped effect of task difficulty on the hemodynamic responses measured by fNIRS and the assumption that improvement in sensorimotor cortical activation can benefit neurorehabilitation and fine motor skills [46]. Previous studies have demonstrated the feasibility of fNIRS as a platform for the development of NF systems to increase neural activity associated with motor imagery [47][48][49][50] or cognitive functions [51][52][53][54], as well as dynamic difficulty adjustment systems for mental arithmetic training [55] or musical learning [56].…”