Zrinka Potocanac

Response inhibition during avoidance of virtual obstacles while walking

While walking, one often has to suppress and adjust a planned step in order to avoid a fall. Given that steps are preprogrammed this requires some form of motor inhibition. Motor inhibition is commonly tested in hand function and only recently attempts have been made to evaluate inhibition in the lower limbs, during step initiation. As adequate motor inhibition might play a role in avoiding falls a test to assess response inhibition during walking would be valuable. We developed a task in which subjects walked on a treadmill by stepping on projected patches of light, which could suddenly change color forcing the subjects to avoid it by shortening or lengthening their steps. The difficulty level was manipulated in 4 conditions by changing the distance available to respond. We hypothesized that larger demands on motor inhibition during walking would produce more failures and tested the performance of young adults (n=12) in order to establish the protocol for use in older adults. The failure rate on the walking test was analyzed. Reducing the available response distance by 150 mm from the easiest condition resulted in a significant increase in failure rates from 15.6% to 65.1%. Therefore, results indicate this novel test can be used to assess the level of motor inhibition during walking. Additionally, in comparison to previous literature on obstacle avoidance, our experiment shows that changing a precise aiming movement is considerably more challenging than changing the same movement executed automatically.

Gait asymmetry during early split-belt walking is related to perception of belt speed difference

Gait adaptation is essential for humans to walk according to the different demands of the environment. Although locomotor adaptation has been studied in different contexts and in various patient populations, the mechanisms behind locomotor adaptation are still not fully understood. The aim of the present study was to test two opposing hypotheses about the control of split-belt walking, one based on avoidance of limping and the other on avoiding limb excursion asymmetry. We assessed how well cerebellar patients with focal lesions and healthy control participants could sense differences between belt speeds during split-belt treadmill walking and correlated this to split-belt adaptation parameters. The ability to perceive differences between belt speeds was similar between the cerebellar patients and the healthy controls. After combining all participants, we observed a significant inverse correlation between stance time symmetry and limb excursion symmetry during the early phase of split-belt walking. Participants who were better able to perceive belt speed differences (e.g., they had a lower threshold and hence were able to detect a smaller speed difference) walked with the smallest asymmetry in stance time and the largest asymmetry in limb excursion. Our data support the hypothesis that humans aim to minimize (temporal) limping rather than (spatial) limb excursion asymmetry when using their perception of belt speed differences in the early phase of adaptation to split-belt walking.

Quick foot placement adjustments during gait: direction matters

To prevent falls, adjustment of foot placement is a frequently used strategy to regulate and restore gait stability. While foot trajectory adjustments have been studied during discrete stepping, online corrections during walking are more common in daily life. Here, we studied quick foot placement adjustments during gait, using an instrumented treadmill equipped with a projector, which allowed us to project virtual stepping stones. This allowed us to shift some of the approaching stepping stones in a chosen direction at a given moment, such that participants were forced to adapt their step in that specific direction and had varying time available to do so. Thirteen healthy participants performed six experimental trials all consisting of 580 stepping stones, and 96 of those stones were shifted anterior, posterior or lateral at one out of four distances from the participant. Overall, long-step gait adjustments were performed more successfully than short-step and side-step gait adjustments. We showed that the ability to execute movement adjustments depends on the direction of the trajectory adjustment. Our findings suggest that choosing different leg movement adjustments for obstacle avoidance comes with different risks and that strategy choice does not depend exclusively on environmental constraints. The used obstacle avoidance strategy choice might be a trade-off between the environmental factors (i.e., the cost of a specific adjustment) and individuals’ ability to execute a specific adjustment with success (i.e., the associated execution risk).

Online adjustments of leg movements in healthy young and old

Online movement adjustments are crucial for daily life. This is especially true for leg movements in relation to gait, where failed adjustments can lead to falls, especially in elderly. However, most research has focused on reach adjustments following changes in target location. This arm research reports two categories of online adjustments (see Gaveau et al., Neuropsychologia 55:25–40, 2014 for review). Small, frequently undetected, target location shifts invoke fast, automatic adjustments, usually without awareness. In contrast, large target location shifts can lead to slow, voluntary adjustments. These fast and slow adjustments presumably rely on different neural networks, with a possible role for subcortical pathways for the fast responses. Do leg movement adjustments also fall into these two categories? We review the literature on leg movement adjustments and show that it is indeed possible to discern fast and slow adjustments. More specifically, we provide an overview of studies showing adjustments during step preparation, initiation, unobstructed, and obstructed gait. Fast adjustments were found both during stepping and gait. In the extreme case, even step adjustments appear to be further modifiable online, e.g., when avoiding obstacles during tripping. In older adults, movement adjustments are generally slower and of smaller magnitude, consistent with a greater risk of falling. However, fast responses seem less affected by aging, consistent with the idea of independent parallel mechanisms controlling movement adjustments (Gomi, Curr Opin Neurobiol 18:558–567, 2008). Finally, putative neural pathways are discussed.

Quick foot placement adjustments during gait are less accurate in individuals with focal cerebellar lesions

Online gait corrections are frequently used to restore gait stability and prevent falling. They require shorter response times than voluntary movements which suggests that subcortical pathways contribute to the execution of online gait corrections. To evaluate the potential role of the cerebellum in these pathways we tested the hypotheses that online gait corrections would be less accurate in individuals with focal cerebellar damage than in neurologically intact controls and that this difference would be more pronounced for shorter available response times and for short step gait corrections. We projected virtual stepping stones on an instrumented treadmill while some of the approaching stepping stones were shifted forward or backward, requiring participants to adjust their foot placement. Varying the timing of those shifts allowed us to address the effect of available response time on foot placement error. In agreement with our hypothesis, individuals with focal cerebellar lesions were less accurate in adjusting their foot placement in reaction to suddenly shifted stepping stones than neurologically intact controls. However, the cerebellar lesion groups foot placement error did not increase more with decreasing available response distance or for short step versus long step adjustments compared to the control group. Furthermore, foot placement error for the non-shifting stepping stones was also larger in the cerebellar lesion group as compared to the control group. Consequently, the reduced ability to accurately adjust foot placement during walking in individuals with focal cerebellar lesions appears to be a general movement control deficit, which could contribute to increased fall risk.

Reliable estimation of inhibitory efficiency: To anticipate, choose, or simply react?

Response inhibition is an important executive process studied by clinical and experimental psychologists, neurophysiologists and cognitive neuroscientists alike. Stop‐signal paradigms are popular because they are grounded in a theory that provides methods to estimate the latency of an unobservable process: the stop‐signal reaction time (SSRT). Critically, SSRT estimates can be biased by skew of the response time distribution and gradual slowing over the course of the experiment. Here, we present a series of experiments that directly compare three common stop‐signal paradigms that differ in the distribution of response times. The results show that the widely used choice response (CR) and simple response (SR) time versions of the stop‐signal paradigm are particularly susceptible to skew of the response time distribution and response slowing, and that using the anticipated response (AR) paradigm based on the Slater‐Hammel task offers a viable alternative to obtain more reliable SSRT estimates.

Comparison of Two Muscle Activity Detection Techniques from Surface EMG Signals Applied to Countermovement Jump

The goal of this research was to compare two methods for automatic muscle activity detection from surface EMG signals: an algorithm based on wavelet transform and a simple threshold based algorithm (low- and high-pass filtering and comparison with a threshold). A total of 120 recorded surface EMG signals, obtained by measurements on 12 different leg muscles of professional handball players performing countermovement jump, were analyzed using both algorithms. An experienced researcher visually determined onset/offset times and these were used as a reference. For each signal bias between evaluated algorithms and visual evaluation of muscle activity was calculated. The results indicate that the algorithm based on wavelets is more appropriate for the analysis of fast dynamic movements. Low level of background noise presented a problem whit establishment of an appropriate activity threshold. For the wavelet based algorithm this was dealt with by adding low level of white noise to the raw EMG signals. This procedure improved the validity of the activity threshold defined for EMG signals with low levels of background noise therefore improving the results of the algorithm. However, it does not solve the issue of inappropriate threshold when using the simple threshold based algorithm.

Small, movement dependent perturbations substantially alter postural control strategy in healthy young adults

Postural control is commonly investigated by observing responses to perturbations. We developed a perturbation paradigm mimicking self-generated errors in weight shifting, which are a common cause of falling among older adults. Our aim was to determine the effects of this small, but complex, perturbation on postural sway of healthy young adults and evaluate the role of vision and cognition during movement dependent perturbations. Fifteen participants stood hip-width apart with their eyes open, closed and while performing two different cognitive tasks. Participants were continuously perturbed by medial-lateral (ML) support surface translations corresponding to, and hence doubling, their own center of mass sway. We analyzed the standard deviation (SD), root mean square (RMS), range, and mean power frequency (MPF) of center of pressure displacements. ML postural sway increased due to the perturbation (SD p ≤ .001, range p < .001, RMS p ≤ .001, MPF p < .001). Cognitive load increased the ML sway range (p = .048). Lack of vision increased ML MPF (p = .001) and anterior-posterior (AP) range (p < .001), SD (p < .001), and RMS (p = .001). Significant interaction of vision with the perturbation was found for the ML range (p = .045) and AP SD (p = .018). The perturbation specifically affected ML postural sway. Increased MPF is indicative of a postural control strategy change, which was insufficient for fully controlling the increased sway. Despite being small, this type of perturbation appears to be challenging for young adults.

Holding a Handle for Balance during Continuous Postural Perturbations—Immediate and Transitionary Effects on Whole Body Posture

When balance is exposed to perturbations, hand contacts are often used to assist postural control. We investigated the immediate and the transitionary effects of supportive hand contacts during continuous anteroposterior perturbations of stance by automated waist-pulls. Ten young adults were perturbed for 5 min and required to maintain balance by holding to a stationary, shoulder-high handle and following its removal. Center of pressure (COP) displacement, hip, knee and ankle angles, leg and trunk muscle activity and handle contact forces were acquired. The analysis of results show that COP excursions are significantly smaller when the subjects utilize supportive hand contact and that the displacement of COP is strongly correlated to the perturbation force and significantly larger in the anterior than posterior direction. Regression analysis of hand forces revealed that subjects utilized the hand support significantly more during the posterior than anterior perturbations. Moreover, kinematical analysis showed that utilization of supportive hand contacts alter posture of the whole body and that postural readjustments after the release of the handle, occur at different time scales in the hip, knee and ankle joints. Overall, our findings show that supportive hand contacts are efficiently used for balance control during continuous postural perturbations and that utilization of a handle has significant immediate and transitionary effects on whole body posture.