The growth of stability: postural control from a development perspective

This study compared central nervous system organizational processes underlying balance in children of three age groups: 15-31 months, 4-6 years, and 7-10 years, using a movable platform capable of antero-posterior (A-P) displacements or dorsi-plantar flexing rotations of the ankle joint. A servo system capable of linking platform rotations to A-P sway angle allowed disruption of ankle joint inputs, to test the effects of incongruent sensory inputs on response patterns. Surface electromyography was used to quantify latency and response patterns. Surface electromyography was used to quantify latency and amplitude of the gastrocnemius, hamstrings, tibialis anterior, and quadriceps muscle responses. Cinematography provided biomechanical analysis of the sway motion. Results demonstrated that while directionally specific response synergies are present in children under the age of six, structured organization of the synergies is not yet fully developed since variability in timing and amplitude relationships between proximal and distal muscles is high. Transition from immature to mature response patterns was not linear but stage-like with greatest variability in the 4- to 6- year-old children. Results from balance tests under altered sensory conditions (eyes closed and/or ankle joint inputs altered) suggested that: (a) with development a shift in controlling inputs to posture from visual dependence to more adult-like dependence on a combination of ankle joint and visual inputs occurred in the 4- to 6-year-old, and reached adult form in the 7- to 10-year-old age group. It is proposed that the age 4-6 is a transition period in the development of posture control. At this time the nervous system (a) uses visual-vestibular inputs to fine tune ankle-joint proprioception in preparation for its increased importance in posture control and (b) fine tunes the structural organization of the postural synergies themselves.     

Perturbations of human posture: influence of impulse modality on EMG responses and cerebral evoked potentials

Leg muscle EMG responses and cerebral evoked potentials (CP), elicited by perturbations of stance while on a treadmill with split belts, were analyzed in order to study the relationship between compensatory leg muscle responses and afferent input to supraspinal centers. Various conditions of perturbation were used to establish the extent to which compensatory EMG responses and CPs show congruent behavior. Four different treadmill acceleration rates were applied in three different conditions (unilateral perturbation, directed forward or backward; bilateral perturbation, directed forward or backward; and opposing bilateral perturbation). EMG responses and CPs showed parallel increases in amplitude with increasing displacement velocity. The EMG responses showed distinct differences, predominantly in the response amplitude, between the different perturbation conditions, whereas the CPs were affected only to a minor degree. Tibialis anterior EMG responses were more closely related to the CP following forward perturbation than the corresponding gastrocnemius responses were to the CP following backward perturbation. We conclude that the EMG responses are more closely related than the CPs to displacement parameters and suggest that this is due to the further spinal processing of the afferent input needed to generate an appropriate EMG response. The closer relationship between the tibialis anterior response and CP may reflect a predominant central representation and control of tibialis anterior activation in the regulation of posture. The functional implications of these findings are discussed.     

Perturbations of Human Posture

Leg muscle EMG responses and cerebral evoked potentials (CP), elicited by perturbations of stance while on a treadmill with split belts, were analyzed in order to study the relationship between compensatory leg muscle responses and afferent input to supraspinal centers. Various conditions of perturbation were used to establish the extent to which compensatory EMG responses and CPs show congruent behavior. Four different treadmill acceleration rates were applied in three different conditions (unilateral perturbation, directed forward or backward; bilateral perturbation, directed forward or backward; and opposing bilateral perturbation). EMG responses and CPs showed parallel increases in amplitude with increasing displacement velocity. The EMG responses showed distinct differences, predominantly in the response amplitude, between the different perturbation conditions, whereas the CPs were affected only to a minor degree. Tibialis anterior EMG responses were more closely related to the CP following forward perturbation than the corresponding gastrocnemius responses were to the CP following backward perturbation.

We conclude that the EMG responses are more closely related than the CPs to displacement parameters and suggest that this is due to the further spinal processing of the afferent input needed to generate an appropriate EMG response. The closer relationship between the tibialis anterior response and CP may reflect a predominant central representation and control of tibialis anterior activation in the regulation of posture. The functional implications of these findings are discussed.     

Human locomotor adjustments during perturbed walking

The locomotor adjustment induced by step perturbation of human subjects walking on a treadmill was described by a quantitative analysis of the EMG activities of selected trunk and leg muscles and by rotations of leg joints. The role of the proprioceptive input in the EMG reaction was also evaluated. The perturbation was obtained by a rapid and unexpected increase of belt speed. The motor response showed the stereotyped characteristics of a motor automatism and was accomplished without affecting the basic motor pattern of the gait. The EMG adjustment showed short-latency reflex responses (40–60 msec) of muscles acting at the joints more directly affected by the perturbing stimulus. This result supports the hypothesis of a spinal neuronal mechanism involved in the rapid adjustment of gait. The activity of primary spindle afferents seems to play an important role in the production of the faster EMG responses.     

Eccentric head positions bias random generation of leftward and rightward handle-bar rotations

We explored the effects of eccentric head positions on rapid rotations of a handle-bar in a modified randomization task. Subjects had to generate a random sequence of leftward and rightward handle-bar rotations; each pre-selected rotation was produced in response to a visual signal as rapidly as possible. Eccentric head positions induced a bias in that handle-bar rotations in the direction of the eccentric head position were chosen more frequently than handle-bar rotations in the opposite direction. This is consistent with previous evidence on a spatial coupling. In contrast to response selection, response initiation was not affected by eccentric head positions. The kinematic characteristics, however, differed: rotations in the direction of the eccentric head position were of larger amplitude and longer duration than rotations in the opposite direction. This difference may have been secondary to a difference in the start positions of the handle-bar, which were shifted in the direction opposite to the eccentric head position.     

Automatic motor activation by mere instruction

Previous behavioral studies have shown that instructions about stimulus–response (S-R) mappings can influence task performance even when these instructions are irrelevant for the current task. In the present study, we tested whether automatic effects of S–R instructions occur because the instructed stimuli automatically activate their corresponding responses. We registered the lateralized readiness potentials (LRPs) that were evoked by the instructed stimuli while participants were performing a task for which those mappings were irrelevant. Instructed S–R mappings clearly affected task performance in electrophysiological and behavioral measures. The LRP was found to deflect in the direction of the response tendency that corresponded with the instructed S–R mapping. Early activation of the instructed response was observed but occurred predominantly on slow trials. In contrast, response conflict evoked by instructed S–R mappings did not modulate the N2 amplitude. The results strongly suggest that, like experienced S–R mappings, instructed S–R mappings can lead to automatic response activation, but possibly via a different route.     

Effects of single trial of heart-rate biofeedback during ramp bicycling exercise

We examined whether a single trial of heart-rate biofeedback was effective to attenuate heart-rate responses during ramp exercise despite a lack of biofeedback conditioning. 35 healthy women exercised in two trials while they tried to attenuate their heart rate by watching biofeedback signals or they only exercised without biofeedback signals (Control trial). 17 subjects were able to attenuate the heart rate during Biofeedback trial (Can group) whereas the remaining subjects were not (Cannot group). In the Can group, the magnitude of heart-rate attenuation in all exercise time was equivalent to 11+/-3% of the preexercise heart rate. Since the heart-rate reduction was similar to that achieved after the heart-rate biofeedback conditioning in previous studies, it is likely that the single trial of heart-rate biofeedback was effective for almost half the subjects to attenuate the heart-rate responses during ramp exercise.     

The interplay of controlled and automatic processing in the expression of spontaneously inferred traits: A PDP analysis

Inferences made about actors influence subsequent processing about those actors. Three experiments conducted in the context of spontaneous trait inference (STI) making demonstrate that such influences occur can either occur via automatic processes or via controlled processes. Results from Experiment 1 demonstrated that processing goals manipulated prior to encoding actor behavior affected the extent to which STIs automatically influenced subsequent responses but did not alter the extent to which STIs influenced those responses via controlled processes. Results from Experiment 2 showed that the extent to which STIs affected subsequent responding via the action of controlled processes were more affected by a delay between exposure to an actor behavior and the response task than the extent to which STIs affected task performance via the action of automatic processes. Finally, results from Experiment 3 showed that participants' subjective experience of awareness of their trait inferences is related to estimates of the extent to which controlled processing is involved in the production of their future responses but not to estimates of the extent to which those responses are affected by automatic processing.     

Automatic and controlled cognitive responses to intergroup threat as assessed using the process dissociation procedure: a study of a low-status group from China

Xiang, L. & Zhao, Y. (2012). Automatic and controlled cognitive responses to intergroup threat as assessed using the process dissociation procedure: A study of a low-status group from China. Scandinavian Journal of Psychology 53, 280-285. Explicit and implicit methods are typically employed to investigate the respective controlled and automatic cognitive responses to intergroup threat. However, these may not be "process pure" measures of automatic or controlled responses. The present study used the process dissociation procedure to investigate the relative contributions of automatic and controlled cognitive responses to intergroup threat. Following exposure to a threat/no threat manipulation, fifty Chinese rural undergraduates who were regarded as low-status group members completed a recognition memory task, in which they were asked to identify all or select "old" (previously presented) trait words relating to an urban outgroup. The results showed that compared to the non-threat group, when rural members perceived threat from an urban group, their automatic response was a decrease in favoritism for this outgroup, but was not derogative, whereas the controlled response was neither positive nor negative. These findings are inconsistent with previous research using explicit and implicit methods. The reasons for this discrepancy are discussed.     

Neuromuscular control of posture in the infant and child: is vision dominant?

This study explored the effects of vision and maturation on the characteristics of neuromuscular responses underlying balance control in both seated and standing children of five age groups (3 1/2-5 months, 8-14 months, 2-3 years, 4-6 years, and 7-10 years). A platform was used to unexpectedly disturb the child's balance in the anterior or posterior direction. Responses of the leg, trunk, and neck muscles were recorded using surface electromyograms. Vision was not required for the activation of these responses in any of the age groups tested. However, comparison of the muscle response latencies of standing children to posterior platform translation in the two visual conditions showed a significant reduction in latency for neck flexors in the 2- to 3-year-olds with vision removed and ann increase in the total number of monosynaptic reflexes. No reduction in latency was found in the older age groups. The hypothesis of a shift from an early long latency visual dominance to a shorter latency proprioceptive one during childhood is discussed. Postural control showed a cephalo-caudal developmental gradient with postural responses appearing first in the neck, then trunk, and finally, legs, as children developed from 3 to 14 months of age. A wide variety of response patterns was seen in the 3- to 5-month-olds, indicating that postural responses are not functional prior to experience with stabilizing the center of mass.     

Event-related potentials elicited by automatic targets: a dual-task analysis

The role of limited capacity processes in the detection of automatic targets was investigated in a dual-task paradigm using both behavioral and event-related brain potential (ERP) measures. An automatic detection task was paired with another concurrent discrimination while the relative importance of each task was systematically varied. The resulting performance operating characteristic (POC) showed that both the speed and accuracy of automatic detection responses were affected by the allocation of attention. Reductions in the accuracy of each task were accompanied by reductions in the amplitude of a late-positive component of the ERP (P300). In addition, the latency of the P300 component elicited by automatic targets was increased in dual-task conditions. A comparison of behavioral and ERP measures suggested the involvement of two separate limited-capacity processes in automatic detection: one concerned with the formation of an episodic representation of target occurrence and the other with the execution of rapid motor responses.     

Neuromuscular Control of Posture in the Infant and Child

This study explored the effects of vision and maturation on the characteristics of neuromuscular responses underlying balance control in both seated and standing children of five age groups (3½–5 months, 8–14 months, 2–3 years, 4–6 years, and 7–10 years). A platform was used to unexpectedly disturb the child’s balance in the anterior or posterior direction. Responses of the leg, trunk, and neck muscles were recorded using surface electromyograms.

Vision was not required for the activation of these responses in any of the age groups tested. However, comparison of muscle response latencies of standing children to posterior platform translation in the two visual conditions showed a significant reduction in latency for neck flexors in the 2- to 3-year-olds with vision removed and an increase in the total number of monosynaptic reflexes. No reduction in latency was found in the older age groups. The hypothesis of a shift from an early long latency visual dominance to a shorter latency proprioceptive one during childhood is discussed.

Postural control showed a cephalo-caudal developmental gradient with postural responses appearing first in the neck, then trunk, and finally, legs, as children developed from 3 to 14 months of age. A wide variety of response patterns was seen in the 3- to 5-month-olds, indicating that postural responses are not functional prior to experience with stabilizing the center of mass.     

Postural muscle responses following changing balance threats in young, stable older, and unstable older adults

The authors determined the postural muscle response to support surface perturbations, in relation to aging and level of stability of 16 young adults and 32 older adults who were classified into stable (SOA) and unstable (UOA) groups on the basis of their functional balance abilities. Forward and backward support surface translations of various amplitudes and velocities were used so that postural responses of the standing adults could be elicited. The thigh and leg postural muscle responses were recorded with surface electromyography (EMG). The older groups had significantly longer onset latency in the anterior postural muscles, smaller integrated EMG in the posterior muscles, and greater extent of integrated EMG attenuation over time. The UOA showed longer onset latency in the gastrocnemius following slow backward perturbation and used a greater percentage of the functional capacity of the gastrocnemius muscle than the SOA did. Those findings indicate that the SOA and UOA had limited ability to adapt to changing balance threats; the UOA were more limited than the SOA. When designing balance training programs, therefore, therapists should consider the adult's level of functional stability.     

Influence of wearing an unstable shoe construction on compensatory control of posture

This study investigated the influence of wearing unstable shoe construction (WUS) on compensatory postural adjustments (CPA) associated with external perturbations. Thirty-two subjects stood on a force platform resisting an anterior-posterior horizontal force applied to a pelvic belt via a cable, which was suddenly released. They stood under two conditions: barefoot and WUS. The electromyographic (EMG) activity of gastrocnemius medialis, tibialis anterior, rectus femoris, biceps femoris, rectus abdominis, and erector spinae muscles and the center of pressure (CoP) displacement were acquired to study CPA. The EMG signal was used to assess individual muscle activity and latency, antagonist co-activation and reciprocal activation at joint and muscle group levels. Compared to barefoot, WUS led to: (1) increased gastrocnemius medialis activity, (2) increased total agonist activity, (3) decreased antagonist co-activation at the ankle joint and muscle group levels, (4) increased reciprocal activation at the ankle joint and muscle group levels, and (5) decrease in all muscle latencies. No differences were observed in CoP displacement between conditions. These findings demonstrate that WUS led to a reorganization of the postural control system associated to improved performance of some components of postural control responses.     

Walking challenges in moderate knee osteoarthritis: A biomechanical and neuromuscular response to medial walkway surface translations

Sensations of knee instability are self-reported in 60–80% of individuals with knee osteoarthritis. These sensations are most often reported during walking; however, it remains unclear how they affect knee joint biomechanics and muscle activation patterns as indicators of joint function. Perturbation paradigms may provide insight into how the knee joint responds to walking challenges. Thus, the purpose of this study was to determine how individuals with moderate medial compartment knee osteoarthritis respond to unexpected, 3 cm medial walkway surface translations during gait compared to an asymptomatic control group. It is hypothesized that individuals with knee osteoarthritis will demonstrate altered biomechanics, and elevated and prolonged muscle activation compared to the asymptomatic group. Twenty asymptomatic individuals and 20 individuals with knee osteoarthritis walked on a dual-belt instrumented treadmill. Participants experienced 24 unexpected medial/lateral, 1 cm/3 cm walkway translations during mid-stance on each leg. Joint motions, moments and maximal voluntary isometric contraction amplitude normalized muscle activations were analyzed for the 3 cm walkway translations. Discrete measures were extracted from each biomechanical waveform and Principal Component Analysis (PCA) was used to determine knee joint muscle activation patterns. PCA is a factorization method to reduce dimensionality of EMG envelopes into linearly uncorrelated principal patterns (PP1, PP2, PP3) that explain the largest possible variance in the dataset. PP1 is often interpreted as a feature that explains the overall amplitude, while PP2 and PP3 are features that explain the variance in temporal activation patterns (i.e. how activation patterns change over the gait cycle). Statistical significance was determined using Analysis of Covariance models (alpha = 0.05). In response to the medial 3 cm walkway translation, increased activation amplitudes in the hamstring and gastrocnemius, captured by PP1 were found in both groups, as well as alterations in temporal activation patterns (captured by combinations of PP2 and PP3 patterns) across all muscle sites (p < 0.05). No group differences were demonstrated in joint motion and moment discrete metrics (p > 0.05) in response to the 3 cm translation. These findings suggest that the medial 3 cm walkway translation posed a challenged to knee function, however the biomechanical and neuromuscular response was similar between individuals with moderate knee osteoarthritis and asymptomatic individuals.     

Tracking the displacement of objects: a series of tasks with great apes (Pan troglodytes, Pan paniscus, Gorilla gorilla, and Pongo pygmaeus) and young children (Homo sapiens)

The authors administered a series of object displacement tasks to 24 great apes and 24 30-month-old children (Homo sapiens). Objects were placed under 1 or 2 of 3 cups by visible or invisible displacements. The series included 6 tasks: delayed response, inhibition test, A not B, rotations, transpositions, and object permanence. Apes and children solved most tasks performing at comparable levels except in the transposition task, in which apes performed better than children. Ape species performed at comparable levels in all tasks except in single transpositions, in which chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) performed better than gorillas (Gorilla gorilla) and orangutans (Pongo pygmeaus). All species found nonadjacent trials and rotations especially difficult. The number of elements that changed locations, the type of displacement, and having to inhibit predominant reaching responses were factors that negatively affected the subjects' performance.     

Mechanisms Controlling Head Stabilization in the Elderly During Random Rotations in the Vertical Plane

Frequency-related response characteristics of the mechanisms controlling stabilization of the head in 10 elderly subjects were compared with response characteristics in 8 young adults. Angular velocity of the head with respect to the trunk and EMG responses of 2 neck muscles were recorded in 10 seated subjects during pseudorandom rotations of the trunk in the sagittal plane at frequencies of 0.35 to 3.05 Hz. Subjects were required to actively stabilize their heads with (VS) and without (NV) visual feedback so that voluntary mechanisms and the influence of vision could be tested. Reflex mechanisms were examined when subjects were distracted by a mental calculation task during rotations in the dark (MA). Age emerged as an influential factor in the performance of head stabilization mechanisms, and decrements in performance were even more pronounced in the older as compared with younger elderly subjects. Age effects could be seen in the (a) diminished ability to voluntarily stabilize the head, particularly with the absence of vision, (b) impaired ability to stabilize the head when cognitively distracted, and (c) appearance of a resonant response of the head. Control of head stabilization shifted from reflex mechanisms to system mechanics, probably as a result of age-related changes in the integrity of the sensory systems. The elderly's system mechanics could not effectively compensate for the disturbances, however, and instability was the result.     

Mechanisms controlling head stabilization in the elderly during random rotations in the vertical plane

Frequency-related response characteristics of the mechanisms controlling stabilization of the head in 10 elderly subjects were compared with response characteristics in 8 young adults. Angular velocity of the head with respect to the trunk and EMG responses of 2 neck muscles were recorded in 10 seated subjects during pseudorandom rotations of the trunk in the sagittal plane at frequencies of 0.35 to 3.05 Hz. Subjects were required to actively stabilize their heads with (VS) and without (NY) visual feedback so that voluntary mechanisms and the influence of vision could be tested. Reflex mechanisms were examined when subjects were distracted by a mental calculation task during rotations in the dark (MA). Age emerged as an influential factor in the performance of head stabilization mechanisms, and decrements in performance were even more pronounced in the older as compared with younger elderly subjects. Age effects could be seen in the (a) diminished ability to voluntarily stabilize the head, particularly with the absence of vision, (b) impaired ability to stabilize the head when cognitively distracted, and (c) appearance of a resonant response of the head. Control of head stabilization shifted from reflex mechanisms to system mechanics, probably as a result of age-related changes in the integrity of the sensory systems. The elderly's system mechanics could not effectively compensate for the disturbances, however, and instability was the result.     

Choice as a function of local versus molar reinforcement contingencies.

Rats were trained on a discrete-trial probability learning task. In Experiment 1, the molar reinforcement probabilities for the two response alternatives were equal, and the local contingencies of reinforcement differentially reinforced a win-stay, lose-shift response pattern. The win-stay portion was learned substantially more easily and appeared from the outset of training, suggesting that its occurrence did not depend upon discrimination of the local contingencies but rather only upon simple strengthening effects of individual reinforcements. Control by both types of local contingencies decreased with increases in the intertrial interval, although some control remained with intertrial intervals as long as 30 s. In Experiment 2, the local contingencies always favored win-shift and lose-shift response patterns but were asymmetrical for the two responses, causing the molar reinforcement rates for the two responses to differ. Some learning of the alternation pattern occurred with short intertrial intervals, although win-stay behavior occurred for some subjects. The local reinforcement contingencies were discriminated poorly with longer intertrial intervals. In the absence of control by the local contingencies, choice proportion was determined by the molar contingencies, as indicated by high exponent values for the generalized matching law with long intertrial intervals, and lower values with short intertrial intervals. The results show that when molar contingencies of reinforcement and local contingencies are in opposition, both may have independent roles. Control by molar contingencies cannot generally be explained by local contingencies.     

Speed and accuracy of compensatory responses to limb disturbances

This study examines the speed and accuracy of compensatory responses to flexion-extension perturbations of the wrist in the horizontal plane. In Experiments 1 and 2 the subjects were required to establish an initial flexion or extension force of approximately 15% maximum at a prescribed initial muscle length. The perturbations changed the load force by +/-5% in both simple and choice reaction protocols. The results showed that the latencies to compensate for the perturbation were longer when the direction of disturbance was unknown (i.e., choice effect) and when the perturbation unloaded the muscle (i.e., directional effect). Accuracy constraints on the compensatory response increased movement time and reduced the variability of latency without affecting mean latency. In Experiment 3, a visual stimulus generated a comparable choice effect on latency to that produced by the perturbations, but no directional effect in relation to the preload was apparent. Our behavioral analysis of compensatory responses triggered by wrist perturbations confirms that these responses are susceptible to variables that influence the initiation of voluntary movements. Our analysis also demonstrates a directional preload effect that is stimulus specific.