Force matching by patients with unilateral focal cerebral lesions

Neural Oscillations and Functional Significances for Prioritizing Dual-Task Walking in Parkinson's Disease

Background

Task prioritization involves allocating brain resources in a dual-task scenario, but the mechanistic details of how prioritization strategies affect dual-task walking performance for Parkinson's disease (PD) are little understood.

Objective

We investigated the performance benefits and corresponding neural signatures for people with PD during dual-task walking, using gait-prioritization (GP) and manual-prioritization (MP) strategies.

Methods

Participants (N = 34) were asked to hold two inter-locking rings while walking and to prioritize either taking big steps (GP strategy) or separating the two rings (MP strategy). Gait parameters and ring-touch time were measured, and scalp electroencephalograph was performed.

Results

Compared with the MP strategy, the GP strategy yielded faster walking speed and longer step length, whereas ring-touch time did not significantly differ between the two strategies. The MP strategy led to higher alpha (8-12 Hz) power in the posterior cortex and beta (13-35 Hz) power in the left frontal-temporal area, but the GP strategy was associated with stronger network connectivity in the beta band. Changes in walking speed and step length because of prioritization negatively correlated with changes in alpha power. Prioritization-related changes in ring-touch time correlated negatively with changes in beta power but positively with changes in beta network connectivity.

Conclusions

A GP strategy in dual-task walking for PD can enhance walking speed and step length without compromising performance in a secondary manual task. This strategy augments attentional focus and facilitates compensatory reinforcement of inter-regional information exchange.

Asymmetry in grasp force matching and sense of effort

While asymmetries in upper limb force matching have been observed, the mechanisms underlying asymmetry in the sense of effort have not been conceptualized. The aim of this study was to investigate asymmetries in the perception and reproduction of grasp force. Forty-two young adults, 22 right-handed (RH) and 20 left-handed (LH), were, respectively, divided into three groups according to differences between their right and left-hand strength (left stronger than right, right stronger than left and right & left equivalent). A reference force, representing 20% of the maximal voluntary contraction (MVC) produced by the right or left hand, was matched with same hand (Ipsilateral Remembered--IR) or opposite (Contralateral Remembered--CR) hand. The matching relative error was 92% (for RH) and 46% (for LH) greater in the CR than IR condition. Asymmetries in matching were significant for RH participants only in the CR condition and were dependent on right/left differences in hand strength as shown by the constant error (CE). For this RH population, right-hand overshoot of the left-hand reference and left-hand undershoot of the right-hand reference were significant when the right hand was stronger than the left. Asymmetry remained significant when CE was normalized (%MVC). Asymmetry was reduced when the strength of each hand was equivalent or when the left hand was stronger than the right. These findings suggest that effort perception is asymmetric in RH but not in LH individuals. The hand x strength interaction indicates that asymmetry in force matching is a consequence of both a difference in the respective cortical representations and motor components, which confer a different "gain" (input-output relationship) to each system. The similarity with position sense asymmetry suggests that the gain concept may be generalized to describe some functional/performance differences between the two hand/hemisphere systems. The more symmetrical performance of the LH than RH group underlines that context specific influence of handedness, hemisphere dominance and hemispheric interactions modulate performance symmetries/asymmetries.