Effect of Preparation on Dual-Task Performance in Postural Control

The impact of divided attention on automatic postural responses: A systematic review and meta-analysis

Quick responses to a loss of balance or "automatic postural responses" (APRs) are critical for fall prevention. The addition of a distracting task- dual-tasking (DT), typically worsens performance on mobility tasks. However, the effect of DT on APRs is unclear. We conducted a systematic review and meta-analyses to examine the effects of DT on spatial, temporal, and neuromuscular components of APRs and the effect of DT on cognitive performance. A Meta-analysis of 19 cohorts (n = 329) showed significant worsening in spatial kinematic features of APRs under DT conditions (P = 0.01), and a meta-analysis of 9 cohorts (n = 123) demonstrated later muscle onset during DT (P = 0.003). No significant DT effect was observed for temporal kinematic outcomes in 18 cohorts (n = 328; P = 0.47). Finally, significant declines in cognitive performance were evident in 20 cohorts (n = 400; P = 0.002). These results indicate that, despite the somewhat reactive nature of APRs, the addition of a secondary task negatively impacts some aspects of the response. These findings underscore the importance of cortical structures in APR generation. Given the importance of APRs for falls, identifying aspects of APRs that are altered under DT may inform fall-prevention treatment approaches.

Functional Gait Can Be Affected by Noise: Effects of Age and Cognitive Function: A Pilot Study

Background: The ageing process may degrade an individual's balance control, hearing capacity, and cognitive function. Older adults perform worse on simultaneously executed balance and secondary tasks (i.e., dual-task performance) than younger adults and may be more vulnerable to auditory distraction.

Aim: The purpose of this study was to determine the effect of passive listening on functional gait in healthy older vs. younger adults, and to investigate the effect of age, functional gait, hearing ability and cognitive functioning on dual-task performance.

Methods: Twenty young and 20 older healthy adults were recruited. Functional gait (Functional Gait Assessment in silent and noisy condition), hearing function (audiogram; Speech in Babble test), and cognitive ability (Cambridge Neuropsychological Test Automated Battery) were measured.

Results: Overall, a significant difference between functional gait performance in silent vs. noisy conditions was found (p = 0.022), with no significant difference in dual-task cost between the two groups (p = 0.11). Correlations were found between increasing age, worse functional gait performance, poorer hearing capacity and lower performance on cognitive function tasks. Interestingly, worse performance on attention tasks appeared to be associated with a worse functional gait performance in the noisy condition.

Conclusion: Passive listening to multi-talker babble noise can affect functional gait in both young and older adults. This effect could result from the cognitive load of the babble noise, due to the engagement of attention networks by the unattended speech.

Timings of attentional switching to perturbation and postural preparation during transient forward or backward floor translation

Background

Relationships between the timings of attentional switching and postural preparation were investigated using a choice-reaction paradigm with transient floor translation (S2), with the direction indicated by a warning auditory signal (S1).

Methods

Thirteen healthy young adults participated in this study. S2 started 2 s after S1 onset while standing on the platform. The platform moved forward when S1 was a high tone, and backward when S1 was a low tone. In the S1–S2 period, attentional switching was evaluated by P3 component of event-related potential.

Results

A shift in the center of pressure in the anteroposterior direction (CoPap) or a continuous increase in postural muscle activation toward S2 was recognized as postural preparation. Changes in postural muscle activation were found just before the CoPap shift. P3 was observed about 250–650 ms after S1. Onset of postural preparation was significantly later (about 200 ms) than latency of P3 (p < 0.001) and correlated strongly with P3 latency (forward: r = 0.81, backward: r = 0.74, p < 0.01).

Conclusion

Postural preparation for S2 was demonstrated to start after attentional switching from S1 to S2.

Sensory and motoric influences on attention dynamics during standing balance recovery in young and older adults

This study investigated the impact of attention on the sensory and motor actions during postural recovery from underfoot perturbations in young and older adults. A dual-task paradigm was used involving disjunctive and choice reaction time (RT) tasks to auditory and visual stimuli at different delays from the onset of two types of platform perturbations (rotations and translations). The RTs were increased prior to the perturbation (preparation phase) and during the immediate recovery response (response initiation) in young and older adults, but this interference dissipated rapidly after the perturbation response was initiated (<220 ms). The sensory modality of the RT task impacted the results with interference being greater for the auditory task compared to the visual task. As motor complexity of the RT task increased (disjunctive versus choice) there was greater interference from the perturbation. Finally, increasing the complexity of the postural perturbation by mixing the rotational and translational perturbations together increased interference for the auditory RT tasks, but did not affect the visual RT responses. These results suggest that sensory and motoric components of postural control are under the influence of different dynamic attentional processes.

Changes in Standing and Walking Performance Under Dual-Task Conditions Across the Lifespan

Simultaneous performance of a postural and a concurrent task is rather unproblematic as long as the postural task is executed in an automatic way. However, in situations where postural control requires more central processing, cognitive resources may be exceeded by the addition of an attentionally demanding task. This may lead to interference between the two tasks, manifested in a decreased performance in one or both tasks (dual-task costs). Owing to changes in attentional demands of postural tasks as well as processing capacities across the lifespan, it might be assumed that dual-task costs are particularly pronounced in children and older adults probably leading to a U-shaped pattern for dual-task costs as a function of age. However, these changes in the ability of dual-tasking posture from childhood to old age have not yet been systematically reviewed. Therefore, Web of Science and PubMed databases were searched for studies comparing dual-task performance with one task being standing or walking in healthy groups of young adults and either children or older adults. Seventy-nine studies met inclusion criteria. For older adults, the expected increase in dual-task costs could be confirmed. In contrast, in children there was only feeble evidence for a trend towards enlarged dual-task costs. More good-quality studies comparing dual-task ability in children, young, and, ideally, also older adults within the same paradigm are needed to draw unambiguous conclusions about lifespan development of dual-task performance in postural tasks. There is evidence that, in older adults, dual-task performance can be improved by training. For the other age groups, these effects have yet to be investigated.

First trial and StartReact effects induced by balance perturbations to upright stance

Postural responses (PR) to a balance perturbation differ between the first and subsequent perturbations. One explanation for this first trial effect is that perturbations act as startling stimuli that initiate a generalized startle response (GSR) as well as the PR. Startling stimuli, such as startling acoustic stimuli (SAS), are known to elicit GSRs, as well as a StartReact effect, in which prepared movements are initiated earlier by a startling stimulus. In this study, a StartReact effect paradigm was used to determine if balance perturbations can also act as startle stimuli. Subjects completed two blocks of simple reaction time trials involving wrist extension to a visual imperative stimulus (IS). Each block included 15 CONTROL trials that involved a warning cue and subsequent IS, followed by 10 repeated TEST trials, where either a SAS (TESTSAS) or a toes-up support-surface rotation (TESTPERT) was presented coincident with the IS. StartReact effects were observed during the first trial in both TESTSAS and TESTPERT conditions as evidenced by significantly earlier wrist movement and muscle onsets compared with CONTROL. Likewise, StartReact effects were observed in all repeated TESTSAS and TESTPERT trials. In contrast, GSRs in sternocleidomastoid and PRs were large in the first trial, but significantly attenuated over repeated presentation of the TESTPERT trials. Results suggest that balance perturbations can act as startling stimuli. Thus first trial effects are likely PRs which are superimposed with a GSR that is initially large, but habituates over time with repeated exposure to the startling influence of the balance perturbation.

Age-related differences in attentional cost associated with postural dual tasks: increased recruitment of generic cognitive resources in older adults

Dual-task designs have been used widely to study the degree of automatic and controlled processing involved in postural stability of young and older adults. However, several unexplained discrepancies in the results weaken this literature. To resolve this problem, a careful selection of dual-task studies that met certain methodological criteria are considered with respect to reported interactions of age (young vs. older adults)×task (single vs. dual task) in stable and unstable postural conditions. Our review shows that older adults are able to perform a postural dual task as well as younger adults in stable conditions. However, when the complexity of the postural task is increased by dynamic conditions (surface and surround), performance in postural, concurrent, or both tasks is more affected in older relative to young adults. In light of neuroimaging studies and new conceptual frameworks, these results demonstrate an age-related increase of controlled processing of standing associated with greater intermittent adjustments.

Startle induces early initiation of classically conditioned postural responses

Startling acoustic stimuli (SAS) induce the early release of prepared motor responses. The current study used SAS, in conjunction with a classical conditioning paradigm, to examine advanced motor preparation of conditioned postural responses (PRs). After generalized startle responses were induced, standing posture was perturbed in 2 blocks of 15 Conditioning trials, where in each trial the onset of a nonstartling auditory cue [i.e., a conditioned stimulus (CS)] preceded a leftward support-surface translation. Upon completion of each block, a single trial was conducted. After block 1, a CS-Only trial was used to induce conditioned PRs in the absence of balance perturbations. After block 2, a post-Conditioning Startle trial that involved a CS subsequently followed by a SAS was used to examine motor preparation of conditioned PRs. PRs were quantified in terms of center of pressure displacements, ankle and hip kinematics, as well as surface electromyography of proximal and distal bilateral muscle pairs. Results indicated that repeated experience with cued balance perturbations led to PR conditioning and, more importantly, motor preparation of PRs. Conditioning was evidenced in biomechanical and electromyographic responses observed in CS-Only trials, as well as the progressive changes to evoked response parameters during repeated Conditioning trials. SAS presented in post-Conditioning Startle trials evoked early onsets of biomechanical and electromyographic responses, while preserving relative response parameters that were each distinct from generalized startle responses. These results provide important insight into both the consequences of using cues in dynamic postural control studies and the neural mechanisms governing PRs.

Effects of Gait Variations on Grip Force Coordination During Object Transport

In object transport during unimpeded locomotion, grip force is precisely timed and scaled to the regularly paced sinusoidal inertial force fluctuations. However, it is unknown whether this coupling is due to moment-to-moment predictions of upcoming inertial forces or a longer, generalized time estimate of regularly paced inertial forces generated during the normal gait cycle. Eight subjects transported a grip instrument during five walking conditions, four of which altered the gait cycle. The variations included changes in step length (taking a longer or shorter step) or stepping on and over a stable (predictable) or unstable (unpredictable support surface) obstacle within a series of baseline steps, which resulted in altered frequencies and magnitudes of the inertial forces exerted on the transported object. Except when stepping on the unstable obstacle, a tight temporal coupling between the grip and inertial forces was maintained across gait variations. Precision of this timing varied slightly within the time window for anticipatory grip force control possibly due to increased attention demands related to some of the step alterations. Furthermore, subjects anticipated variations in inertial force when the gait cycle was altered with increases or decreases in grip force, relative to the level of the inertial force peaks. Overall the maintenance of force coupling and scaling across predictable walking conditions suggests that the CNS is able to anticipate changes in inertial forces generated by gait variations and to efficiently predict the grip force needed to maintain object stability on a moment-to-moment basis.

Postural prioritization defines the interaction between a reaction time task and postural perturbations

Concurrent demands for postural and cognitive control processes are now known to induce interference, e.g., information processing speed may decrease during postural adjustment. It is less clear whether postural control may, at least in many situations, take precedence over cognitive control ("postural prioritization"). The purpose of this study was to determine if postural dual-task effects are the result of a postural prioritization effect. Twelve young subjects (6 female; 24.1 +/- 4.1) performed a discrete choice reaction time (RT) task in combination with a platform perturbation. To assess the effect of postural prioritization on RT and center of pressure (COP) parameters, destabilizing perturbations were randomly interspersed with non-destabilizing perturbations. Furthermore, stimulus order and the time interval of the RT stimulus relative to the platform perturbation were manipulated. COP and RT data obtained in these manipulations were compared to single-task baseline data. The results suggested that, irrespective of the degree of threat to postural stability, postural task processes are prioritized. Furthermore, anticipation of a postural stimulus negatively affects RT. However, once a perturbation commences subsequent RTs are speeded. Postural reactions were unaffected by a concurrent RT task, however. The RT stimulus acted as a cue to initiate biomechanical adaptations for an upcoming perturbation.

Cognitive demands and cortical control of human balance-recovery reactions

A traditional view has been that balance control occurs at a very automatic level, primarily involving the spinal cord and brainstem; however, there is growing evidence that the cerebral cortex and cognitive processing are involved in controlling specific aspects of balance. The purpose of this review is to summarize recent literature pertaining to the cognitive demands and cortical control of balance-recovery reactions, focussing on five emerging sources of evidence: 1) dual-task studies demonstrating that concurrent performance of cognitive and balance-recovery tasks leads to interference effects; 2) dual-task studies that have examined the temporal dynamics associated with the reallocation of cognitive resources to the balance-recovery task; 3) visual attention studies that have inferred contributions of visual attention based on gaze measurements and/or manipulations to occlude vision; 4) measurements of brain potentials evoked by postural perturbation; and 5) use of transcranial magnetic stimulation to alter contributions from specific cortical areas.

Unilateral reaction time task is delayed during contralateral movements

Performing unlearned unimanual tasks when simultaneously carrying out another task with the contralateral hand is known to be difficult. The dual task interference theory predicts that reaction time will be delayed if the investigated task is performed in the course of ongoing contralateral movements. Ballistic movements can be performed at maximal speed in simple reaction time (SRT) experiments when subjects have adequately prepared the motor system needed for movement execution. When fully prepared, activation of subcortical motor pathways by a startling auditory stimulus (SAS) triggers the whole reaction. In this study, we have examined dual task interference with reaction time in eight healthy volunteers. They were presented with a visual imperative signal to perform unilateral SRT either in a baseline condition (control trials) or while carrying out contralateral rhythmic oscillatory movements (test trials). A SAS was introduced in 25% of the trials in both conditions. SRT was significantly delayed in the interference test trial when compared to control trials either with or without SAS (P<0.001). Control and test trials with SAS were significantly faster than those without SAS in both conditions (P<0.001). However, there were no significant differences in the percentage SRT shortening induced by SAS or in the percentage SRT delay observed in the test trials. Our results suggest that performing rhythmic oscillatory movements with one limb slows SRT in the contralateral limb and that this effect is likely related to motor preparation changes. The effect described here can be of interest for physiological studies of interlimb coordination and the mechanisms underlying the dual task interference phenomenon.