Adaptation in motor strategies for postural control associated with sensory reweighting

The purpose of this study was to identify and differentiate the motor strategies associated with sensory reweighting adapted during specific sensory integration tasks by healthy young adults. Thirty-six subjects (age range: 21–33 years) performed standing computerized dynamic posturography balance tasks across progressively increasing amplitudes of visual (VIS), somatosensory (SOM) and both (VIS+SOM) systems perturbation conditions. Adaptation in the motor strategy was measured as changes in electromyographic (EMG) activities and joint angles. The contribution of the perturbed sensory input in maintaining postural stability was calculated to determine the sensory reweighting. A multivariate design was used to model a linear combination of motor adaptation variables that discriminates specific sensory integration tasks. Results showed a significant progressive decrease in postural sway per unit amplitude of sensory perturbation in each condition, indicating dynamic sensory reweighting. Linear discriminant function analysis indicated that the adaptation in motor strategy during the VIS condition was associated with increased activity of EMG and joint angles in the upper body compared to the lower body. Conversely, during the SOM and VIS+SOM conditions, the adaptation in motor strategy was associated with decreased activity of EMG and joint angles in the lower body compared to the upper body. Therefore, the adaptation in motor strategies associated with sensory reweighting were different for different sensory integration tasks.     

Aging Affects the Ability to Process the Optic Flow Stimulations: A Functional Near-Infrared Spectrometry Study

Abstract

Optic flow (OF) has been utilized to investigate the sensory integration of visual stimuli during postural control. It is little known how the OF speed affects the aging brain during the sensory integration process of postural control. This study was to examine the effect of OF speeds on the brain activation using functional near-infrared spectroscopy (fNIRS) and postural sway between younger and older adults. Eleven healthy younger adults (5M/6F, age 22?±?1-year-old) and ten healthy older adults (4M/6F, age 71?±?5-year-old) participated in this study. A virtual reality headset was used to provide the OF stimulus at different speeds. A forceplate was used to record the center-of-pressure to compute the amplitude of postural sway (peak-to-peak). Compared with younger adults, older adults showed significantly increased activation in the OF speed of 10?m/s and decreased activation in the OF speed of 20?m/s in the left dorsolateral prefrontal cortex. Older adults also showed decreased activation in the left temporoparietal region (VEST) in the OF speed of 20?m/s. A significant difference in peak-to-peak was found between groups. Our results indicated that age might be associated with the ability to process fast OF stimulation.     

Weighting and reweighting of visual input via head mounted display given unilateral peripheral vestibular dysfunction

We translated a well-established laboratory paradigm to study sensory integration into a Head-Mounted-Display (HMD). In the current study, a group of 23 individuals with unilateral vestibular dysfunction and 16 age-matched controls observed moving spheres projected from the Oculus Rift. We confirmed increased visual weighting with an unstable surface and decreased visual weighting (i.e., reweighting) with increased visual amplitude. We did not observe significant differences in gains and phases between individuals with vestibular dysfunction and age-matched controls. The vestibular group increased sway in mid and high frequencies significantly more than controls with the change in surface or visual amplitude. Mild visual perturbations within HMDs carry the potential to become a useful portable assessment of postural control in individuals with vestibular disorders.     

Task demand effects on postural control in older adults

The literature shows conflicting results regarding older adults' (OA) postural control performance. Differing task demands amongst scientific studies may contribute to such ambiguous results. Therefore, the purpose of this study was to examine the performance of postural control in older adults and the relationship between visual information and body sway as a function of task demands. Old and young adults (YA) maintained an upright stance on different bases of support (normal, tandem and reduced), both with and without vision, and both with and without room movement. In the more demanding tasks, the older adults displayed greater body sway than the younger adults and older adults were more influenced by the manipulation of the visual information due to the room movement. However, in the normal support condition, the influence of the moving room was similar for the two groups. These results suggest that task demand is an important aspect to consider when examining postural control in older adults.     

Differences in Rapid Initiation and Termination of Voluntary Postural Sway Associated with Ageing and Falls-Risk

The authors examined differences between young adults (n = 25) and healthy older adults (n = 48) in reaction time and the relations between center of pressure (COP) and center of mass (COM) motions during rapid initiation and termination of voluntary postural sway. Older adults were divided into low and high falls-risk groups based on Physiological Profile Assessment scores of sensorimotor function. Low falls-risk older adults had slower reaction times during anteroposterior sway initiation and decreased COP–COM separation during anteroposterior and medialateral sway initiation and anteroposterior continuous voluntary sway compared with young adults. High falls-risk older adults had slower initiation and termination reaction times in all response directions and decreased COP–COM separation during sway initiation and continuous voluntary sway in the anteroposterior and medialateral directions compared with young adults. Compared with low falls-risk older adults, high falls-risk older adults had slower initiation and termination reaction times in all response directions and decreased COP–COM separation during medialateral continuous voluntary sway. Reaction time and COP–COM measures significantly predicted group status in discriminant models with sensitivities and specificities of 72–100%. Overall, these findings highlight important associations of age-related declines in sensorimotor function related to an increased risk of falling with slower postural reaction time and reduced postural stability.     

Visuomotor error augmentation affects mediolateral head and trunk stabilization during walking

Prior work demonstrates that humans spontaneously synchronize their head and trunk kinematics to a broad range of driving frequencies of perceived mediolateral motion prescribed using optical flow. Using a closed-loop visuomotor error augmentation task in an immersive virtual environment, we sought to understand whether unifying visual with vestibular and somatosensory feedback is a control goal during human walking, at least in the context of head and trunk stabilization. We hypothesized that humans would minimize visual errors during walking – i.e., those between the visual perception of movement and actual movement of the trunk. We found that subjects did not minimize errors between the visual perception of movement and actual movement of the head and trunk. Rather, subjects increased mediolateral trunk range of motion in response to error-augmented optical flow with positive feedback gains. Our results are more consistent with our alternative hypothesis – that visual feedback can override other sensory modalities and independently compel adjustments in head and trunk position. Also, aftereffects following exposure to error-augmented optical flow included longer, narrower steps and reduced mediolateral postural sway, particularly in response to larger amplitude positive feedback gains. Our results allude to a recalibration of head and trunk stabilization toward more tightly regulated postural control following exposure to error-augmented visual feedback. Lasting reductions in mediolateral postural sway may have implications for using error-augmented optical flow to enhance the integrity of walking balance control through training, for example in older adults.     

Development of postural stability limits: Anteroposterior and mediolateral postural adjustment mechanisms do not follow the same maturation process

There is increasing evidence that indicates a critical transition period for the maturation of postural control from the ages of 6–7 years. Some studies suggest that this transitional period may be explained by a change from a ballistic toward a sensory strategy, but the cause remains unknown. The purpose of this study was to investigate the influence of the transition period on dynamic postural control in a natural self-initiated leaning task under different sensory conditions. We evaluated the center of pressure (COP) displacement during maximum leaning in four directions (forward, backward, rightward, leftward) under three sensory conditions (eyes open, eyes closed and eyes closed standing on a foam). Three groups were tested: young children (4 years old), older children (8–10 years old) and adults (21–42 years old). The maximum COP excursion along the anteroposterior and mediolateral axes and the COP amplitude were analyzed. Young children showed smaller maximum anteroposterior and mediolateral COP excursion than other groups. Older children also exhibited a significantly smaller maximum excursion along the mediolateral direction but performed similar to adults along the anteroposterior direction. In a similar manner, the analysis of the COP amplitude did not indicate any differences between the groups along the anteroposterior axis. In contrast, along the mediolateral axis, the results showed developmental differences. Furthermore, the effect of sensory conditions was similar across the children's groups. Our results suggest an important plasticity period for the maturation of postural control mechanisms. Notably, our findings support the idea that the postural mechanisms controlling the anteroposterior axis reach maturity before the mechanisms involved in controlling the mediolateral axis.     

Gait complexity is acutely restored in older adults when walking to a fractal-like visual stimulus

Typically, gait rehabilitation uses an invariant stimulus paradigm to improve gait related deficiencies. However, this approach may not be optimal as it does not incorporate gait complexity, or in more precise words, the variable fractal-like nature found in the gait fluctuations commonly observed in healthy populations. Aging which also affects gait complexity, resulting in a loss of adaptability to the surrounding environment, could benefit from gait rehabilitation that incorporates a variable fractal-like stimulus paradigm. Therefore, the present study aimed to investigate the effect of a variable fractal-like visual stimulus on the stride-to-stride fluctuations of older adults during overground walking. Additionally, our study aimed to investigate potential retention effects by instructing the participants to continue walking after turning off the stimulus. Older adults walked 8 min with i) no stimulus (self-paced), ii) a variable fractal-like visual stimulus and iii) an invariant visual stimulus. In the two visual stimuli conditions, the participants walked 8 additional minutes after the stimulus was turned off. Gait complexity was evaluated with the widely used fractal scaling exponent calculated through the detrended fluctuation analysis of the stride time intervals. We found a significant ~20% increase in the scaling exponent from the no stimulus to the variable fractal-like stimulus condition. However, no differences were found when the older adults walked to the invariant stimulus. The observed increase was towards the values found in the past to characterize healthy young adults. We have also observed that these positive effects were retained even when the stimulus was turned off for the fractal condition, practically, acutely restoring gait complexity of older adults. These very promising results should motivate researchers and clinicians to perform clinical trials in order to investigate the potential of visual variable fractal-like stimulus for gait rehabilitation.     

Effects of anticipation certainty on preparatory brain activity and anticipatory postural adjustments associated with voluntary unilateral arm movement while standing

We examined the effects of anticipation certainty concerning which voluntary movement is required in response to a stimulus while standing on preparatory brain activity and anticipatory postural adjustments (APAs). Ten right-handed adults abducted their left or right arm rapidly in response to a visual imperative stimulus, based on the type of stimulus. A warning cue, which did or did not contain information about the side of arm abduction, was presented 2000ms before the imperative stimulus. Preparatory brain activity before arm abduction was quantified by the mean amplitude of the contingent negative variation 100ms before the imperative stimulus (late CNV amplitude). Compared with the low anticipation condition, in the high anticipation condition the following results were obtained only in the case of right arm abduction: (1) larger late CNV amplitude, (2) earlier postural muscle activities with respect to the focal muscle of arm abduction, and (3) smaller peak displacement of center of pressure during the abduction. These findings suggest that high anticipation of voluntary movement of dominant arm to a stimulus while standing influences preparatory brain activity before the movement, resulting in earlier APAs and thus smaller disturbance of postural equilibrium during the movement.     

Effects of age on listening and postural control during realistic multi-tasking conditions

Successful performance of balance-related activities requires the effective integration of sensory, cognitive, and motor processes that can be affected by age-related changes. Of these age-related sensory changes, the effects of declines in hearing on balance have not been well-studied despite the fact that hearing loss has now been acknowledged as a significant risk factor for falls. The goal of this study was to evaluate age-related differences in a “standing while listening” task within increasingly challenging conditions resembling those that are often encountered in realistic, everyday situations.This study used a dual-task paradigm in an immersive Virtual Reality street scene setting in which postural load (firm, compliant), listening load (number of talkers), and visual load (eyes open/closed) were manipulated. A multi-talker divided attention listening task was used. Postural performance was assessed using center of pressure (COP) path length, while listening performance was assessed using spoken word recognition accuracy.Results demonstrated that age-related differences were observed in postural performance when postural demands were the highest and in listening performance when listening demands were the highest. Proportional dual-task costs were more pronounced for postural task performance compared to listening task performance and were more pronounced for older compared to younger adults. Postural dual-task costs increased as a function of increasing listening loads. Removal of visual information improved listening task performance across both groups and reduced the dual-task costs to listening in older adults when listening demands were highest (resulting in dual-task benefits).Taken together, the findings support previously documented age-related declines in postural control and auditory processing, demonstrate that increasing listening demands may result in poorer balance, particularly in older adults, and provide additional insights into the interactive effects of age-related declines when sensory, motor, and cognitive challenges are incremented factorially.     

The effects of age on sensory thresholds and temporal gap detection in hearing, vision, and touch

Differences in sensory function between young (n 5 42, 18—31 years old) and older (n 5 137, 60—88 years old) adults were examined for auditory, visual, and tactile measures of threshold sensitivity and temporal acuity (gap-detection threshold). For all but one of the psychophysical measures (visual gap detection), multiple measures were obtained at different stimulus frequencies for each modality and task. This resulted in a total of 14 dependent measures, each based on four to six adaptive psychophysical estimates of 75% correct performance. In addition, all participants completed the Wechsler Adult Intelligence Scale (Wechsler, 1997). Mean data confirmed previously observed differences in performance between young and older adults for 13 of the 14 dependent measures (all but visual threshold at a flicker frequency of 4 Hz). Correlational and principalcomponents factor analyses performed on the data from the 137 older adults were generally consistent with task and modality independence of the psychophysical measures.     

Dynamic postural stability is not impaired by moderate-intensity physical activity in healthy or balance-impaired older people

Older people are increasingly being encouraged to be more physically active but this may lead to physiological fatigue, tiredness and other effects, which, at high levels, can adversely alter postural stability. However, older adults rarely perform physical activity at high intensities. This study aimed to determine whether a single bout of moderate-intensity physical activity, similar to that experienced during daily living, alters dynamic postural stability, particularly among those at risk of falling. Thirty-one healthy young, 33 healthy older and 21 balance-impaired older, adults performed a rapid, voluntary step-up task before and immediately after a 14 min, self-paced, moderate-intensity physical activity protocol. Timing of step components from vertical ground reaction forces, mediolateral displacement of center of pressure, and onset and amplitude of hip abductor muscle activity were recorded during the step task. All groups demonstrated the same changes after the activity, with slightly shorter weight-shift phase duration, smaller displacement of the center of pressure towards the stance leg during weight shifting, and earlier onset of stance leg gluteus medius activity. These changes indicate improved coordination of the step task after activity. Thus this study showed that dynamic postural stability is not adversely affected immediately following moderate-intensity physical activity, even among balance-impaired elderly.     

Sustained-attention capacity in young and older adults

The sustained-attention capacity of young (21-29 years) and older (65-78 years) adults was examined using a high-event rate digit-discrimination vigilance task presented at three levels of stimulus degradation. Increased stimulus degradation led to a reduction in the hit rate and to a greater decline in hit rate over blocks (increased vigilance decrement). Sensitivity (d') declined over blocks only at the highest level of stimulus degradation. Older adults had a lower hit rate and showed greater vigilance decrement than young adults, particularly when stimuli were highly degraded. However, both age groups showed the same pattern of stability in sensitivity when stimulus degradation was moderate, and sensitivity decrement over time when stimulus degradation was high. The results suggest that the process of sustained allocation of capacity--as reflected in temporal changes in sensitivity--operates similarly in young and older adults.     

Age-associated differences in global and segmental control during dual-task walking under sub-optimal sensory conditions

The ability to safely perform cognitive-motor dual-tasks is critical for independence of older adults. We compared age-associated differences in global and segmental control during dual-task walking in sub-optimal sensory conditions. Thirteen young (YA) and 13 healthy older (OA) adults walked a straight pathway with cognitive dual-task of walking-while-talking (WT) or no-WT under four sensory conditions. On randomly selected trials, visual and vestibular inputs were manipulated using blurring goggles (BV) and Galvanic Vestibular Stimulation (GVS), respectively. Gait speed decreased more in YA than OA during WT. Gait speed increased with GVS with normal vision but not BV. Step length considerably decreased with WT. Trunk roll significantly decreased only in OA with GVS in WT. Head roll significantly decreased with GVS regardless of age. Results indicate GVS-induced adaptations were dependent on available visual information. YA reduced their gait speed more than OA to achieve a similar pace to safely perform WT. GVS resulted in both age-groups to reduce head movement. However, with the addition of WT during GVS, OA also stiffened their trunk. Therefore, with increased attentional demands healthy OA employed different compensatory strategies than YA to maintain postural control.     

Influence of Destabilization on the Temporal Characteristics of “Volitional” Stepping

Previous work suggests that there may be fundamental differences between compensatory stepping responses evoked by postural perturbation and visually cued “volitional” stepping (e.g., gait initiation). In contrast to visual cueing, postural destabilization evokes an array of sensory inputs that are intrinsically linked to the mobilization of rapid compensatory responses. The hypothesis examined in this study was that this fundamental difference would lead to distinct changes in the temporal characteristics of the stepping response. Six healthy young adults were instructed to step quickly in response to either visual cueing or anterioposterior platform motion. Both forward and backward stepping responses were characterized, using measures of vertical ground reaction force. A stereotypical temporal patterning of the stepping response occurred in both stimulus conditions and both directions of stepping, and anticipatory postural adjustments were evident in all trials. However, postural destabilization led to a more rapid initiation and execution of the temporal pattern, in comparison with visually cued responses. The most pronounced effect was seen in the duration of the response, which was reduced by a factor of two, with approximately proportional foreshortening of both the preparatory and swing phases. The results suggest that sensory information conveying a state of instability has a distinct influence on the characteristics of a subsequent stepping reaction. The persistence of the anticipatory postural adjustments suggests that the failure to see this element of the response in previous studies may reflect fundamental differences between volitional and unplanned compensatory stepping.     

Influence of Destabilization on the Temporal Characteristics of lquot;Volitionalrquot; Stepping

Previous work suggests that there may be fundamental differences between compensatory stepping responses evoked by postural perturbation and visually cued "volitional" stepping (e.g., gait initiation). In contrast to visual cueing, postural destabilization evokes an array of sensory inputs that are intrinsically linked to the mobilization of rapid compensatory responses. The hypothesis examined in this study was that this fundamental difference would lead to distinct changes in the temporal characteristics of the stepping response. Six healthy young adults were instructed to step quickly in response to either visual cueing or anterioposterior platform motion. Both forward and backward stepping responses were characterized, using measures of vertical ground reaction force. A stereotypical temporal patterning of the stepping response occurred in both stimulus conditions and both directions of stepping, and anticipatory postural adjustments were evident in all trials. However, postural destabilization led to a more rapid initiation and execution of the temporal pattern, in comparison with visually cued responses. The most pronounced effect was seen in the duration of the response, which was reduced by a factor of two, with approximately proportional foreshortening of both the preparatory and swing phases. The results suggest that sensory information conveying a state of instability has a distinct influence on the characteristics of a subsequent stepping reaction. The persistence of the anticipatory postural adjustments suggests that the failure to see this element of the response in previous studies may reflect fundamental differences between volitional and unplanned compensatory stepping.     

Transient and sustained incentive effects on electrophysiological indices of cognitive control in younger and older adults

Preparing for upcoming events, separating task-relevant from task-irrelevant information and efficiently responding to stimuli all require cognitive control. The adaptive recruitment of cognitive control depends on activity in the dopaminergic reward system as well as the frontoparietal control network. In healthy aging, dopaminergic neuromodulation is reduced, resulting in altered incentive-based recruitment of control mechanisms. In the present study, younger adults (18–28 years) and healthy older adults (66–89 years) completed an incentivized flanker task that included gain, loss, and neutral trials. Event-related potentials (ERPs) were recorded at the time of incentive cue and target presentation. We examined the contingent negative variation (CNV), implicated in stimulus anticipation and response preparation, as well as the P3, which is involved in the evaluation of visual stimuli. Both younger and older adults showed transient incentive-based modulation of CNV. Critically, cue-locked and target-locked P3s were influenced by transient and sustained effects of incentives in younger adults, while such modulation was limited to a sustained effect of gain incentives on cue-P3 in older adults. Overall, these findings are in line with an age-related reduction in the flexible recruitment of preparatory and target-related cognitive control processes in the presence of motivational incentives.     

Age-related changes in deterministic learning from positive versus negative performance feedback

Feedback-based learning declines with age. Because older adults are generally biased toward positive information (“positivity effect”), learning from positive feedback may be less impaired than learning from negative outcomes. The literature documents mixed results, due possibly to variability between studies in task design. In the current series of studies, we investigated the influence of feedback valence on reinforcement learning in young and older adults. We used nonprobabilistic learning tasks, to more systematically study the effects of feedback magnitude, learning of stimulus–response (S–R) versus stimulus–outcome (S–O) associations, and working-memory capacity. In most experiments, older adults benefitted more from positive than negative feedback, but only with large feedback magnitudes. Positivity effects were pronounced for S–O learning, whereas S–R learning correlated with working-memory capacity in both age groups. These results underline the context dependence of positivity effects in learning and suggest that older adults focus on high gains when these are informative for behavior.     

The response-signal method reveals age-related changes in object working memory

Sixteen healthy young adults (ages 18-32) and 16 healthy older adults (ages 67-81) completed a delayed response task in which they saw the following visual sequence: memory stimuli (2 abstract shapes; 3,000 ms), a blank delay (5,000 ms), a probe stimulus of variable duration (one abstract shape; 125, 250, 500, 1,000, or 2,000 ms), and a mask (500 ms). Subjects decided whether the probe stimulus matched either of the memory stimuli; they were instructed to respond during the mask, placing greater emphasis on speed than accuracy. The authors used D. L. Hintzman & T. Curran's (1994) 3-parameter compound bounded exponential model of speed-accuracy tradeoff to describe changes in discriminability associated with total processing time. Group-level analysis revealed a higher rate parameter and a higher asymptote parameter for the young adult group, but no difference across groups in x-intercept. Proxy measures of cognitive reserve (Y. Stern et al., 2005) predicted the rate parameter value, particularly in older adults. Results suggest that in working memory, aging impairs both the maximum capacity for discriminability and the rate of information accumulation, but not the temporal threshold for discriminability.     

Age Inequalities in Recognition Memory: Effects of Stimulus Presentation Time and List Repetitions

It has been suggested that age-related slowing in the execution of mental operations in time-limited visual information processing tasks can affect subsequent memory performance. To investigate this proposal, recognition memory for Chinese symbols displayed in rapid serial visual presentation (RSVP) was assessed in 30 young adults and 33 older adults. The effects of presentation times (500, 1000, 2000, 2500, 3000, and 6000 ms) on target recognition were examined across six study-test trials for each participant. Recognition accuracy was higher for young adults than for older adults, especially at the shorter stimulus durations. Analysis of the time-accuracy functions describing the stimulus durations and amount of repetitions required to equate the performance of younger and older adults indicated that a limited-time mechanism provided an incomplete account for age-related differences in memory for rapidly presented information.