Accessory stimulus modulates executive function during stepping task

Working memory load modulates anticipatory postural adjustments during step initiation

Working memory (WM) can influence selective attention. However, the effect of WM load on postural standing tasks has been poorly understood, even though these tasks require attentional resources. The purpose of this study was to examine whether WM load would impact anticipatory postural adjustments (APAs) during step initiation. Sixteen healthy young adults performed stepping tasks alone or concurrently with a WM task in a dual-task design. The stepping tasks involved volitional stepping movements in response to visual stimuli and comprised of simple and choice reaction time tasks and the Flanker task which consisted of congruent and incongruent (INC) conditions. In the dual-task condition, subjects were required to memorize either one or six digits before each stepping trial. Incorrect weight transfer prior to foot-lift, termed APA errors, reaction time (RT), and foot-lift time were measured from the vertical force data. The results showed that APA error rate was significantly higher when memorizing six-digit than one-digit numerals in the INC condition. In addition, RT and foot-lift time were significantly longer in the INC condition compared to the other stepping conditions, while there was no significant effect of WM load on RT or foot-lift time. These findings suggest that high WM load reduces the cognitive resources needed for selective attention and decision making during step initiation.

The Effect of Prior Knowledge of Color on Behavioral Responses and Event-Related Potentials During Go/No-go Task

In daily life, the meaning of color plays an important role in execution and inhibition of a motor response. For example, the symbolism of traffic light can help pedestrians and drivers to control their behavior, with the color green/blue meaning go and red meaning stop. However, we don't always stop with a red light and sometimes start a movement with it in such a situation as drivers start pressing the brake pedal when a traffic light turns red. In this regard, we investigated how the prior knowledge of traffic light signals impacts reaction times (RTs) and event-related potentials (ERPs) in a Go/No-go task. We set up Blue Go/Red No-go and Red Go/Blue No-go tasks with three different go signal (Go) probabilities (30, 50, and 70%), resulting in six different conditions. The participants were told which color to respond (Blue or Red) just before each condition session but didn't know the Go probability. Neural responses to Go and No-go signals were recorded at Fz, Cz, and Oz (international 10-20 system). We computed RTs for Go signal and N2 and P3 amplitudes from the ERP data. We found that RT was faster when responding to blue than red light signal and also was slower with lower Go probability. Overall, N2 amplitude was larger in Red Go than Blue Go trial and in Red No-go than Blue No-go trial. Furthermore, P3 amplitude was larger in Red No-go than Blue No-go trial. Our findings of RT and N2 amplitude for Go ERPs could indicate the presence of Stroop-like interference, that is a conflict between prior knowledge about traffic light signals and the meaning of presented signal. Meanwhile, the larger N2 and P3 amplitudes in Red No-go trial as compared to Blue No-go trial may be due to years of experience in stopping an action in response to a red signal and/or attention. This study provides the better understanding of the effect of prior knowledge of color on behavioral responses and its underlying neural mechanisms.

Effect of auditory stimulus on executive function and execution time during cognitively demanding stepping task in patients with Parkinson's disease

Start hesitation in patients with Parkinson's disease (PD) occurs predominantly during distractive and conflictual situations. The aim of this study was to investigate how differently an auditory stimulus (AS) influences execution function and execution time during a cognitively demanding stepping task in PD patients as compared to healthy controls. PD patients and healthy controls stepped forward in response to a visual imperative stimulus of an arrow. We applied a Simon task that comprised congruent and incongruent conditions. Direction and location of the arrow matched in the congruent condition, while they didn't in the incongruent condition. AS were randomly and simultaneously presented with the visual stimulus. An error in the direction of an anticipatory postural adjustment (APA), termed an APA error, and temporal parameters (reaction onset of APA and APA duration) were analyzed. As a result, the AS increased the APA error rate in the control group regardless of the condition, but they did not influence it in the PD group. The AS also speeded the reaction onset in both groups regardless of the condition. The APA duration was prolonged by the AS for the control group, while it was unaffected by the AS for the PD group in both conditions. These findings indicate that AS could facilitate a step initiation, conceivably by facilitating a stimulus identification process and increasing attentional control of stepping behavior, without influencing a decision-making process even in a cognitively demanding condition in patients with PD.

Modulation of EMG-EMG Coherence in a Choice Stepping Task

The voluntary step execution task is a popular measure for identifying fall risks among elderly individuals in the community setting because most falls have been reported to occur during movement. However, the neurophysiological functions during this movement are not entirely understood. Here, we used electromyography (EMG) to explore the relationship between EMG-EMG coherence, which reflects common oscillatory drive to motoneurons, and motor performance associated with stepping tasks: simple reaction time (SRT) and choice reaction time (CRT) tasks. Ten healthy elderly adults participated in the study. Participants took a single step forward in response to a visual imperative stimulus. EMG-EMG coherence was analyzed for 1000 ms before the presentation of the stimulus (stationary standing position) from proximal and distal tibialis anterior (TA) and soleus (SOL) muscles. The main result showed that all paired EMG-EMG coherences in the alpha and beta frequency bands were greater in the SRT than the CRT task. This finding suggests that the common oscillatory drive to the motoneurons during the SRT task occurred prior to taking a step, whereas the lower value of corticospinal activity during the CRT task prior to taking a step may indicate an involvement of inhibitory activity, which is consistent with observations from our previous study (Watanabe et al., 2016). Furthermore, the beta band coherence in intramuscular TA tended to positively correlate with the number of performance errors that are associated with fall risks in the CRT task, suggesting that a reduction in the inhibitory activity may result in a decrease of stepping performance. These findings could advance the understanding of the neurophysiological features of postural adjustments in elderly individuals.

Auditory stimulus has a larger effect on anticipatory postural adjustments in older than young adults during choice step reaction

PURPOSE

The study aim was to compare the influence of an auditory stimulus (AS) on anticipatory postural adjustments (APAs) between young and older adults during a choice step reaction.

METHODS

Sixteen young and 19 older adults stepped forward in response to a visual imperative stimulus of an arrow. We used a choice reaction time (CRT) task and a Simon task which consisted of congruent and incongruent conditions. The direction of the presented arrow and its spatial location matched in the congruent condition while they did not in the incongruent condition. The AS was presented randomly and simultaneously with the visual stimulus. Incorrect weight shifts before lifting off the foot, termed APA errors, stepping errors, temporal parameters, and APA amplitudes were analyzed.

RESULTS

The APA error rate was higher in trials with than without AS in all task conditions for the older group, while this increase occurred only in the incongruent condition for the young group. The stepping error rate was also increased in the presence of AS in the incongruent condition for the older group. Reaction times were faster with AS in both groups. The APA amplitude of erroneous APA trials became larger with AS in the incongruent condition for both groups, and this effect appeared greater for the older group.

CONCLUSIONS

The effect of AS on APAs is larger in the elderly during a choice step reaction. In the presence of incongruent visual information, this effect becomes even greater, potentially inducing not only APA errors but also stepping errors.

Moving Forward by Stimulating the Brain: Transcranial Direct Current Stimulation in Post-Stroke Hemiparesis

Stroke remains a leading cause of disability worldwide, with a majority of survivors experiencing long term decrements in motor function that severely undermine quality of life. While many treatment approaches and adjunctive strategies exist to remediate motor impairment, many are only efficacious or feasible for survivors with active hand and wrist function, a population who constitute only a minority of stroke survivors. Transcranial direct current stimulation (tDCS), a type of non-invasive brain stimulation, has been increasingly utilized to increase motor function following stroke as it is able to be used with stroke survivors of varying impairment levels, is portable, is relatively inexpensive and has few side effects and contraindications. Accordingly, in recent years the number of studies investigating its efficacy when utilized as an adjunct to motor rehabilitation regimens has drastically increased. While many of these trials have reported positive and promising efficacy, methodologies vary greatly between studies, including differences in stimulation parameters, outcome measures and the nature of physical practice. As such, an urgent need remains, centering on the need to investigate these methodological differences and synthesize the most current evidence surrounding the application of tDCS for post-stroke motor rehabilitation. Accordingly, the purpose of this paper is to provide a detailed overview of the most recent tDCS literature (published 2014-2015), while highlighting these variations in methodological approach, as well to elucidate the mechanisms associated with tDCS and post-stroke motor re-learning and neuroplasticity.

Performance monitoring and response conflict resolution associated with choice stepping reaction tasks

Choice reaction requires response conflict resolution, and the resolution processes that occur during a choice stepping reaction task undertaken in a standing position, which requires maintenance of balance, may be different to those processes occurring during a choice reaction task performed in a seated position. The study purpose was to investigate the resolution processes during a choice stepping reaction task at the cortical level using electroencephalography and compare the results with a control task involving ankle dorsiflexion responses. Twelve young adults either stepped forward or dorsiflexed the ankle in response to a visual imperative stimulus presented on a computer screen. We used the Simon task and examined the error-related negativity (ERN) that follows an incorrect response and the correct-response negativity (CRN) that follows a correct response. Error was defined as an incorrect initial weight transfer for the stepping task and as an incorrect initial tibialis anterior activation for the control task. Results revealed that ERN and CRN amplitudes were similar in size for the stepping task, whereas the amplitude of ERN was larger than that of CRN for the control task. The ERN amplitude was also larger in the stepping task than the control task. These observations suggest that a choice stepping reaction task involves a strategy emphasizing post-response conflict and general performance monitoring of actual and required responses and also requires greater cognitive load than a choice dorsiflexion reaction. The response conflict resolution processes appear to be different for stepping tasks and reaction tasks performed in a seated position.

Preparatory state and postural adjustment strategies for choice reaction step initiation

A loud auditory stimulus (LAS) presented simultaneously with a visual imperative stimulus can reduce reaction time (RT) by automatically triggering a movement prepared in the brain and has been used to investigate a movement preparation. It is still under debate whether or not a response is prepared in advance in RT tasks involving choice responses. The purpose of the present study was to investigate the preparatory state of anticipatory postural adjustments (APAs) during a choice reaction step initiation. Thirteen young adults were asked to step forward in response to a visual imperative stimulus in two choice stepping conditions: (i) the responding side is not known and must be selected and (ii) the responding side is known but whether to initiate or inhibit a step response must be selected. LAS was presented randomly and simultaneously with the visual imperative stimulus. LAS significantly increased the occurrence rates of inappropriately initiated APAs while reducing the RTs of correct and incorrect trials in both task conditions, demonstrating that LAS triggered the prepared APA automatically. This observation suggests that APAs are prepared in advance and withheld from release until the appropriate timing during a choice reaction step initiation. The preparatory activity of APAs might be modulated by the inhibitory activity required by the choice tasks. The preparation strategy may be chosen for fast responses and is judged most suitable to comply with the tasks because inappropriately initiated APAs can be corrected without making complete stepping errors.