Specificity of postural sway to the demands of a precision task at sea

Mariners actively adjust their body orientation in response to ship motion. On a ship at sea, we evaluated relations between standing postural activity and the performance of a precision aiming task. Standing participants (experienced mariners) maintained the beam from a handheld laser on a target. Targets were large or small, thereby varying the difficulty of the aiming task. Targets were located in front of the participant or to the participant's right (requiring participants to look over the right shoulder), thereby varying the functional consequences (for the aiming task) of postural activity in different body axes. The torso was oriented toward the bow or toward the ship's side (athwartship), thereby varying the effects on postural activity of differential motion of the ship in its different axes. The weather was rough, producing high magnitudes of ship motion, which sometimes caused participants to step or stagger. Our manipulations influenced the magnitude and dynamics of head and torso movements, as well as the organization of movement in different axes. The results provide the first empirical confirmation that postural activity can be influenced by orientation of the torso relative to a ship. Despite powerful effects of ship motion, postural activity was influenced by variations in target location and in the difficulty of the aiming task, replicating subtle effects that have been observed on land. We discuss implications for hull design and the placement of workstations on ships.     

Effects of dual task difficulty in motor and cognitive performance: Differences between adults and adolescents

In the present study our aim was to compare dual-task performance in thirteen adolescents and fifteen young adults while concurrently performing a cognitive and a motor task. The postural control variables were obtained under three different conditions: i) bipedal stance, ii) tandem stance and iii) unipedal stance. The cognitive task consisted of a backward digit span test in which the participants were asked to memorize a sequence of numbers and then repeat the number in reverse order at three different difficulty levels (i.e. with 3, 4 and 5 digits). The difficulty of the cognitive task was seen to have different effects on adolescents and young adults. Adolescents seem to prioritize postural control during high difficulty postural conditions while a cross-domain competition model appeared in easy postural conditions.     

Effects of Visual Tasks and Conversational Partner on Personal and Interpersonal Postural Activity

The control of stance is influenced by activities that are engaged in during stance. We investigated simultaneous constraints imposed by individual-specific factors (such as the distance of visual targets) and of interpersonal factors arising from dyadic conversation. Each member of participant pairs looked at targets (drawings) that were similar to, but differed from, that of a conversational partner. Conversational partners conversed to identify the differing elements. In Experiments 1 and 2, members of each dyad conversed with each other or separately conversed with a confederate (an experimenter). We varied the distance of targets (Experiment 1) and their size (Experiment 2). In Experiment 3, target size could be the same for both members of a dyad (i.e., small-small, large-large) or could differ (i.e., small-large, large-small). Interpersonal postural coordination was stronger when members of dyads conversed with each other, replicating earlier studies, but this manipulation also influenced parameters of individual sway. In Experiment 3, interpersonal postural coordination also was influenced by variation in the size of the partner's target. Analysis of the sway of individuals revealed influences of target distance and size, replicating previous effects; however, these manipulations also influenced interpersonal postural coordination. Overall, the results indicate that postural activity was modulated simultaneously with respect to individual and dyadic parameters of the task situation. We argue that it may be useful, both theoretically and empirically, to interpret the effects of conversation on postural activity within the broader context of relations between postural control and the performance of suprapostural tasks.     

Interaction between the development of postural control and the executive function of attention

The authors examined the interaction between the development of postural control and the development of the executive function of attention in 13 children and 6 adults in dual-task conditions. Participants performed an attentionally demanding cognitive task and a postural task simultaneously. The authors equalized the attentional load of the cognitive task across age groups. Comparative changes in the center of pressure in dual- and single-task conditions indicated that dual tasks interfered with postural performance in the wide stance (WS) and the modified Romberg stance (RS). Children at 4-6 years of age (but not children at ages 7-12 years of age or adults) experienced postural control interference in both stance positions, but interference was greater in the RS (p = .018). For all participants, cognitive task performance in RS was unchanged from that in WS. The knowledge gained from the results of this study will contribute to the design and implementation of academic and preacademic programming for young children. Their performance of an intentionally demanding cognitive task would be enhanced by the provision of appropriately sized desks and chairs or their use of an alternate, less demanding position.     

Interaction Between the Development of Postural Control and the Executive Function of Attention

The authors examined the interaction between the development of postural control and the development of the executive function of attention in 13 children and 6 adults in dual-task conditions. Participants performed an attentionally demanding cognitive task and a postural task simultaneously. The authors equalized the attentional load of the cognitive task across age groups. Comparative changes in the center of pressure in dual- and single-task conditions indicated that dual tasks interfered with postural performance in the wide stance (WS) and the modified Romberg stance (RS). Children at 4-6 years of age (but not children at ages 7-12 years of age or adults) experienced postural control interference in both stance positions, but interference was greater in the RS (p = .018). For all participants, cognitive task performance in RS was unchanged from that in WS. The knowledge gained from the results of this study will contribute to the design and implementation of academic and preacademic programming for young children. Their performance of an intentionally demanding cognitive task would be enhanced by the provision of appropriately sized desks and chairs or their use of an alternate, less demanding position.     

The effects of background visual roll stimulation on postural and manual control and self-motion perception

The effects of background visual roll stimulation on postural control, manual controlf andselfmotion perception were investigated in this study. In the main experiment, 8 subjects were exposed to wide field-of-view background scenes that were tilted and static, continuously rotating, or sinusoidally rotating at frequencies between 0.03 and 0.50 Hz, as well as a baseline condition. The subjects performed either a postural control task (maintain an upright stance) or a manual control task (keep an unstable central display horizontally level). Root-mean square (RMS) error in both the postural and manual control tasks was low in the static tilt condition and extremely high in response to continuous rotation. Although the phases of the postural and manual responses were highly similar, the power and RMS error generated by the sinusoidal visual background stimulation peaked at a lower frequency in the postural task. Vection ratings recorded at the end of the postural and manual trials somewhat paralleled tbafrequency tuning differences between tasks, which a subsequent experiment showed to be the result of the differential motion of the central display rather than the differential positioning of the subject. In general, these results show that the dynamic characteristics of visual orientation systems vary according to the specific motor and/or perceptual system investigated.     

Adaptive perception of changes in affordances for walking on a ship at sea

Ocean waves cause oscillatory motion of ships. Oscillatory ship motion typically is greater in roll (i.e., the ship rolling from side to side) than in pitch (i.e., tipping from front to back). Affordances for walking on a ship at sea should be differentially influenced by ship motion in roll and pitch. When roll exceeds pitch, the maximum walkable distance within a defined path should be greater when walking along the ship’s short, or athwart axis than when walking along its long, or fore-aft axis. When pitch exceeds roll, this relation should be reversed. We asked whether such changes in ship motion would be reflected in judgments of direction-specific affordances for walking. Participants (experienced maritime crewmembers) judged how far they could walk along a narrow path on the ship deck. On different days, sailing conditions were such that the relative magnitude of pitch and roll was reversed. Judgments of direction-specific affordances for walking mirrored these changes in ship motion. The accuracy of judgments was consistent across directions, and across changes in ship motion. We conclude that experienced maritime crewmembers were sensitive to dynamic variations in affordances for walking that were, themselves, a function of dynamic properties of the animal-environment system.     

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.     

Attentional Demands Associated With Postural Control Depend on Task Difficulty and Visual Condition

The authors aimed to assess the effect of vision on variations in attentional resources allocated to postural control during tasks of various levels of difficulty. Test-retest reliability of postural and cognitive parameters was also evaluated. Twenty adults stood quietly on a force platform during 30-s trials (primary task). Twelve conditions involving combinations of three bases of support, two vision conditions, and the presence or absence of a simple reaction time task (secondary task) were tested. Baseline trials of the reaction time task were also performed with the participants seated. Reaction time and postural parameters demonstrated good to excellent test-retest reliability in most conditions. Postural control was altered by the reduction of the base of support and by the absence of vision. Maintaining an upright stance increased reaction time compared with a seated position, indicating that quiet standing tasks required some attention even in young adults. Changes in postural steadiness were correlated with changes in reaction time, showing a significant relationship between the difficulty of the postural task and the attentional resources allocated to postural control. However, reaction time increased with the reduction of the base of support only without vision. This dual task paradigm showed that vision can compensate for the increase in attentional demands during the most difficult postural tasks.     

Multisensory postural control in adults: Variation in visual,haptic, and proprioceptive inputs

Maintaining balance is fundamentally a multisensory process, with visual, haptic, and proprioceptive information all playing an important role in postural control. The current project examined the interaction between such sensory inputs, manipulating visual (presence versus absence), haptic (presence versus absence of contact with a stable or unstable finger support surface), and proprioceptive (varying stance widths, including shoulder width stance, Chaplin [heels together, feet splayed at approximately 60°] stance, feet together stance, and tandem stance) information. Analyses of mean velocity of the Centre of Pressure (CoP) revealed significant interactions between these factors, with stability gains observed as a function of increasing sensory information (e.g., visual, haptic, visual + haptic), although the nature of these gains was modulated by the proprioceptive information and the reliability of the haptic support surface (i.e., unstable versus stable finger supports). Subsequent analyses on individual difference parameters (e.g., height, leg length, weight, and areas of base of support) revealed that these variables were significantly related to postural measures across experimental conditions. These findings are discussed relative to their implications for multisensory postural control, and with respect to inverted pendulum models of balance. (185 words).     

Alcoholism risk and the P300 event-related brain potential: modality, task, and gender effects

The P300 event related potential (ERP) was elicited from young adults at low- (LR) and high-risk (HR) for alcoholism (n = 24/risk group), with equal numbers of male and female subjects employed. The effects of stimulus modality, task difficulty, and gender were assessed with analogous auditory and visual paradigms (tone location/head rotation and tone matching/shape matching). Although task performance was generally comparable across risk groups, LR subjects produced smaller P300 amplitudes than HR subjects for the visual matching tasks. No consistent risk group P300 latency effects were obtained. Gender, task difficulty, and scalp topography differentially modulated the risk group outcomes across paradigms. These findings suggest that P300 amplitude reflects risk-for-alcoholism in male and female young adults, but the specific task conditions and electrode location affect the strength of risk group differences.     

Exploring how arm movement moderates the effect of task difficulty on balance performance in young and older adults

Emerging evidence highlights that arm movements exert a substantial and functionally relevant contribution on quiet standing balance control in young adults. Ageing is associated with “non-functional” compensatory postural control strategies (i.e., lower limb co-contraction), which in turn, may increase the reliance on an upper body strategy to control upright stance. Thus, the primary purpose of this study was to compare the effects of free versus restricted arm movements on balance performance in young and older adults, during tasks of different difficulty. Fifteen young (mean ± SD age; 21.3 ± 4.2 years) and fifteen older (mean ± SD age; 73.3 ± 5.0 years) adults performed bipedal, semi-tandem and tandem balance tasks under two arm position conditions: restricted arm movements and free arm movements. Centre of pressure (COP) amplitude and frequency were calculated as indices of postural performance and strategy, respectively. Especially in older adults, restriction of arm movement resulted in increased sway amplitude and frequency, which was primarily observed for the mediolateral direction. Further, increasing balance task difficulty raised the arm restriction cost (ARC; a new measure to quantify free vs. restricted arm movement differences in postural control) that was more prominent in older adults. These findings indicate the ARC provides a measure of reliance on the upper body for balance control and that arm movement is important for postural control in older adults, especially during tasks of greater difficulty.     

How providing more or less time to solve a cognitive task interferes with upright stance control; a posturographic analysis on healthy young adults

Contrasted postural effects have been reported in dual-task protocols associating balance control and cognitive task that could be explained by the nature and the relative difficulty of the cognitive task and the biomechanical significance of the force platform data. To better assess their respective role, eleven healthy young adults were required to stand upright quietly on a force platform while concomitantly solving mental-calculation or mental-navigation cognitive tasks. Various levels of difficulty were applied by adjusting the velocity rate at which the instructions were provided to the subject according to his/her maximal capacities measured beforehand. A condition without any concomitant cognitive task was added to constitute a baseline behavior. Two basic components, the horizontal center-of-gravity movements and the horizontal difference between center-of-gravity and center-of-pressures were computed from the complex center-of-pressure recorded movements. It was hypothesized that increasing the delay should infer less interaction between postural control and task solution. The results indicate that both mental-calculation and mental-navigation tasks induce reduced amplitudes for the center-of-pressure minus center-of-gravity movements, only along the mediolateral axis, whereas center-of-gravity movements were not affected, suggesting that different circuits are involved in the central nervous system to control these two movements. Moreover, increasing the delays task does not infer any effect for both movements. Since center-of-pressure minus center-of-gravity expresses the horizontal acceleration communicated to the center-of-gravity, one may assume that the control of the latter should be facilitated in dual-tasks conditions, inferring reduced center-of-gravity movements, which is not seen in our results. This lack of effect should be thus interpreted as a modification in the control of these center-of-gravity movements. Taken together, these results emphasized how undisturbed upright stance control can be impacted by mental tasks requiring attention, whatever their nature (calculation or navigation) and their relative difficulty. Depending on the provided instructions, i.e. focusing our attention on body movements or on the opposite diverting this attention toward other objectives, the evaluation of upright stance control capacities might be drastically altered.     

Stance Width Influences Postural Stability and Motion Sickness

We examined the influence of stance width (the distance between the feet) on postural sway and visually induced motion sickness. Stance width influences the magnitude of body sway, and changes in sway precede the subjective symptoms of motion sickness. Thus, manipulation of stance width may influence motion sickness incidence. Participants (healthy young adults) were exposed to complex, low-frequency oscillation of a moving room. Participants stood with their feet 5 cm, 17 cm, or 30 cm apart. During exposure to visual motion, the widest stance (30 cm) was associated with reduced incidence of motion sickness. For all stance widths, motion sickness was preceded by significant changes in motion of the head and torso. The results support the postural instability theory of motion sickness and suggest practical implications for the prevention of motion sickness. Adoption of wider stance may decrease the risk of motion sickness in operational situations.     

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.     

Subjective Reports and Postural Performance Among Older Adult Passengers on a Sea Voyage

We sought to evaluate changes in subjective experience and postural performance among older adult passengers during the first 2 days of a sea voyage. On a vacation cruise, volunteer passengers gave verbal ratings of subjective bodily stability and awareness of ship motion followed by performance on the tandem Romberg test while facing fore-aft and athwartship. Data were collected when the ship was at the dock and on each of the first 2 full days at sea. Ship motion reduced subjective bodily stability and performance on the Romberg test and increased awareness of ship motion. On the first day at sea, Romberg performance was more strongly impacted by motion of the ship in roll (i.e., when facing fore-aft) than in pitch (i.e., when facing athwartship). Also on the first day at sea, subjective bodily stability was correlated with Romberg performance when facing fore-aft but not when facing athwart. In summary, at the beginning of the voyage older adult passengers on a sea voyage exhibited consistent changes in subjective awareness and postural performance. Subjective reports were correlated with postural performance in ways that appeared to be functional. We suggest that this finding may help to illuminate the role of conscious awareness within ecological analyses of perception and action.     

Postural costs of suprapostural task load

In an immersive visualization experiment, participants performed a conjunction search task while standing either in open (heels 10 cm apart, feet at a comfortable angle) or closed stance (feet pressed together). In the world-frame condition, the search display maintained its position in space as the participant swayed, generating optic flow informative about sway. In the head-frame condition, the display maintained constant distance and orientation with respect to the participant's head, providing no visual information about sway. In both conditions, participants (surprisingly) searched faster while in the more difficult closed stance. Interpretation of this result is unclear. Participants also swayed more as search-load increased, and made more errors in the high search-load condition. It is suggested that this performance tradeoff is a result of the sharing of a limited-capacity, modality-non-specific spatial-attentional resource between postural and suprapostural tasks.     

Measurement of auditory cues in drivers' distraction

A driving simulator was used to examine the effects on driving performance of auditory cues in an in-vehicle information search task. Drivers' distraction by the search tasks was measured on a peripheral detection task. The difficulty of the search task was systematically varied to test the distraction caused by a quantified visual load. 58 participants completed the task. Performance on both search tasks and peripheral detection tasks was measured by mean response time and percent error. Analyses indicated that in-vehicle information search performance can be severely degraded when a target is located within a group of diverse distractors. Inclusion of an auditory cue in the visual search increased the mean response time as a result of a change in modality from auditory to visual. Inclusion of such an auditory cue seemed to influence distraction as measured by performance on the peripheral detection task; accuracy was lower when auditory cues were provided, and responses were slower when no auditory cues were provided. Distraction by the auditory cue varied according to the difficulty of the search task.     

Coupling of postural and manual tasks in expert performers

The purpose of this study was to investigate the integration of bimanual rhythmic movements and posture in expert marching percussionists. Participants (N = 11) performed three rhythmic manual tasks [1:1, 2:3, and 2:3-F (2:3 rhythm played faster at a self-selected tempo)] in one of three postures: sitting, standing on one foot, and standing on two feet. Discrete relative phase, postural time-to-contact, and coherence analysis were used to analyze the performance of the manual task, postural control, and the integration between postural and manual performance. Across all three rhythms, discrete relative phase mean and variability results showed no effects of posture on rhythmic performance. The complexity of the manual task (1:1 vs. 2:3) had no effect on postural time-to-contact. However, increasing the tempo of the manual task (2:3 vs. 2:3-F) did result in a decreased postural time-to-contact in the two-footed posture. Coherence analysis revealed that the coupling between the postural and manual task significantly decreased as a function of postural difficulty (going from a two-footed to a one-footed posture) and rhythmic complexity (1:1 vs. 2:3). Taken together, these results demonstrate that expert marching percussionists systematically decouple postural and manual fluctuations in order to preserve the performance of the rhythmic movement task.     

Interaction of Perception and Action in Discrete and Continuous Rapid Aiming Tasks

Previously, we have shown that discrete and continuous rapid aiming tasks are governed by distinct visuomotor control mechanisms by assessing the combined visual illusion effects on the perceived and effective index of difficulty (ID). All participants were perceptually biased by the combined visual illusion before they performed the rapid aiming tasks. In the current study, the authors manipulated the order of performing perceptual and motor tasks to examine whether perceptual or motor experience with the illusory visual target would influence the subsequent perceived and effective ID in discrete and continuous tapping tasks. The results supported our hypothesis showing that perceptual experience with the illusory visual target in the discrete condition reduced the effective ID in the subsequent discrete tapping task, and motor experience with the illusory visual target in the continuous condition reduced the illusion effects on the perceived ID in the subsequent perceptual judgment task. The study demonstrates the coinfluence of perception and action, and suggests that perception and action influence one another with different magnitude depending on the spatial frame of reference used to perform the perceptuomotor task.