The influence of artificially increased hip and trunk stiffness on balance control in man

Lightweight corsets were used to produce mid-body stiffening, rendering the hip and trunk joints practically inflexible. To examine the effect of this artificially increased stiffness on balance control, we perturbed the upright stance of young subjects (20–34 years of age) while they wore one of two types of corset or no corset at all. One type, the half-corset, only increased hip stiffness, and the other, the full-corset, increased stiffness of the hips and trunk. The perturbations consisted of combined roll and pitch rotations of the support surface (7.5 deg, 60 deg/s) in one of six different directions. Outcome measures were biomechanical responses of the legs, trunk, arms and head, and electromyographic (EMG) responses from leg, trunk, and upper arm muscles. With the full-corset, a decrease in forward stabilising trunk pitch rotation compared to the no-corset condition occurred for backward pitch tilts of the support surface. In contrast, the half-corset condition yielded increased forward trunk motion. Trunk backward pitch motion after forwards support-surface perturbations was the same for all corset conditions. Ankle torques and lower leg angle changes in the pitch direction were decreased for both corset conditions for forward pitch tilts of the support-surface but unaltered for backward tilts. Changes in trunk roll motion with increased stiffness were profound. After onset of a roll support-surface perturbation, the trunk rolled in the opposite direction to the support-surface tilt for the no-corset and half-corset conditions, but in the same direction as the tilt for the full-corset condition. Initial head roll angular accelerations (at 100 ms) were larger for the full-corset condition but in the same direction (opposite platform tilt) for all conditions. Arm roll movements were initially in the same direction as trunk movements, and were followed by large compensatory arm movements only for the full-corset condition. Leg muscle (soleus, peroneus longus, but not tibialis anterior) balance-correcting responses were reduced for roll and pitch tilts under both corset conditions. Responses in paraspinals were also reduced. These results indicate that young healthy normals cannot rapidly modify movement strategies sufficiently to account for changes in link flexibility following increases in hip and trunk stiffness. The changes in leg and trunk muscle responses failed to achieve a normal roll or pitch trunk end position at 700 ms (except for forward tilt rotations), even though head accelerations and trunk joint proprioception seemed to provide information on changed trunk movement profiles over the first 300 ms following the perturbation. The major adaptation to stiffness involved increased use of arm movements to regain stability. The major differences in trunk motion for the no-corset, half-corset and full-corset conditions support the concept of a multi-link pendulum with different control dynamics in the pitch and roll planes as a model of human stance. Stiffening of the hip and trunk increases the likelihood of a loss of balance laterally and/or backwards. Thus, these results may have implications for the elderly and others, with and without disease states, who stiffen for a variety of reasons.     

A motor learning strategy reflects neural circuitry for limb control

During motor skill acquisition, the brain learns a mapping between intended limb motion and requisite muscular forces. We propose that regions where sensory and motor representations overlap are crucial for motor learning. In primary motor cortex, for example, cells that modulate their activity for motor actions at a joint tend to receive input from that same portion of the periphery. We predict that this correspondence reflects a default strategy--a Bayesian prior--in which subjects tend to associate loads at a joint with motion at that joint (local sensorimotor association) when there is ambiguity regarding the nature of the load. As predicted, we found that in the presence of uncertainty, humans inappropriately generalized elbow loads as though they were based on elbow velocity. Generalization improved when we reduced uncertainty by decreasing coupling between elbow velocity and load during training. These results illustrate a key link between motor learning and the underlying neural circuitry.     

The effect of substrate stiffness on adult neural stem cell behavior

Adult stem cells reside in unique niches that provide vital cues for their survival, self-renewal and differentiation. In order to better understand the contribution of substrate stiffness to neural stem/progenitor cell (NSPC) differentiation and proliferation, a photopolymerizable methacrylamide chitosan (MAC) biomaterial was developed. Photopolymerizable MAC is particularly compelling for the study of the central nervous system stem cell niche because Young's elastic modulus (E_Y) can be tuned from less than 1 kPa to greater than 30 kPa. Additionally, the numerous free amine functional groups enable inclusion of biochemical signaling molecules that, together with the mechanical environment, influence cell behavior. Herein, NSPCs proliferated on MAC substrates with Young's elastic moduli below 10 kPa and exhibited maximal proliferation on 3.5 kPa surfaces. Neuronal differentiation was favored on the softest surfaces with E_Y < 1 kPa as confirmed by both immunohistochemistry and qRT-PCR. Oligodendrocyte differentiation was favored on stiffer scaffolds (>7 kPa); however, myelin oligodendrocyte glycoprotein (MOG) gene expression suggested that oligodendrocyte maturation and myelination was best on <1 kPa scaffolds where more mature neurons were present. Astrocyte differentiation was only observed on <1 and 3.5 kPa surfaces and represented less than 2% of the total cell population. This work demonstrates the importance of substrate stiffness to the proliferation and differentiation of adult NSPCs and highlights the importance of mechanical properties to the success of scaffolds designed to engineer central nervous system tissue.     

The influence of visual illusions on grasp position

 Visual size illusions have been shown to affect perceived object size but not the aperture of the hand when reaching to those same objects. Thus, vision for perception is said to be dissociated from vision for action. The present study examines the effect of visual-position and visual-shape illusions on both the visually perceived center of an object and the position of a grasp on that object when a balanced lift is required. The results for both experiments show that although the illusions influence both the perceived and the grasped estimates of the center position, the grasp position is more veridical. This partial dissociation is discussed in terms of its implications for streams of visual processing. Received: 17 November 1997 / Accepted: 11 September 1998     

The influence of conceptual knowledge on visual discrimination

Does conceptual knowledge about objects influence their perceptual processing? There is some evidence for interactions between semantic and visual knowledge in tasks requiring both long-term memory and lexical access. Here we assessed whether similar perceptual/semantic interactions arise during sequential visual matching, a task that does not require access to semantic information. Matching of two-dimensional or three-dimensional novel objects was facilitated when the objects were associated with arbitrarily assigned distinctive artificial semantic concepts as compared to similar semantic concepts. In contrast to prior demonstrations, this effect was obtained in a task that did not require naming objects, and was not affected by participants rehearsing consonant strings, suggesting a direct influence from semantic associations on visual object recognition.     

在线调整对到达运动中人体上肢刚性特性的影响

目的研究在线调整与运动中人体上肢刚性值之间的关系,提高人体刚性值测量的精度以及评判在线调整能力。方法设计5类不同类型的上肢平面到达运动,通过施加快速扰动的方法,对运动过程的前期、前中期、中期、中后期和后期共5个位置的刚性值进行测量,探讨刚性值在受到在线调整影响时的变化规律,以及刚性幅值与运动精度之间的关系。结果运动过程中刚性一直变化,而且刚性值的变化影响了最终的运动误差。运动过程中在线调整的发生会引起刚性幅值的改变,尤其是运动后期的刚性幅值,但是目前没有发现这一变化与在线调整发生的时间或者内容存在密切的联系。结论在线调整在到达运动中扮演了重要的角色。考虑到在线调整的发生会引起刚性幅值的变化,在临床上,通过对患者在特定实验中刚性幅值的测量,结合其他医学诊断方法,可以更精确地表明患者当前在线调整功能的状况。     

The influence of visual information on habituation of the electrodermal and the visual orienting reaction.

In this study the influence of the information value of visual stimuli on habituation of the visual orienting reaction (VOR) and the skin conductance reaction (SCR) was investigated. 28 subjects received two blocks of 14 trials. Half the subjects received the higher information condition first and then the lower information, the other half received the conditions in the reversed order. Subjects fixated the stimuli with the higher information value longer than the stimuli with the lower information value during the 14 trials and habituated slower. This effect of information was absent in the second block and VOR habituation was also faster in this block. There was no difference in SCR amplitudes nor in SCR habituation scores between the two information conditions, neither in block 1 nor in block 2. The results of this study are discussed in relation with a two-stage model of the OR.     

The influence of verbal encoding on the memory of visual information

The previous research which investigated the effect of verbal encoding on the memory of visual information has yielded contradictory results: facilitation and inhibition of performance by verbal encoding. However, it has not been elucidated why and how these conflicting results were brought about. The main purpose of this study is to show that verbal encoding not only facilitates but also impairs performance by taking task demand and characteristics of visual representations into account. Another purpose is to investigate how verbal encoding would influence the memory of color information, which is irrelevant for the memory task. In Experiment 1, the results showed that verbal encoding could have two opposite effects in the same task. In Experiment 2, the results showed that the effect of verbal encoding on the memory of color would differ from that on the memory of the shapes of pictures.     

The influence of postural threat on the cortical response to unpredictable and predictable postural perturbations

This study investigated the effects of postural threat on the cortical response associated with postural reactions to predictable and unpredictable perturbations to upright stance. Postural threat was manipulated by having individuals stand on an elevated surface to alter the context in which the postural task was performed. Ten healthy young adults experienced a series of predictable and unpredictable trunk perturbations when standing at ground level and at the edge of a platform located 3.2 m above the ground. Participants felt less confident, more fearful of falling, more anxious and less stable when standing at the high surface height. Unpredictable perturbations generated a large negative potential (N1) which was increased by 84% when standing at the high compared to low surface height. The magnitude of change in this potential was related to the magnitude of change in balance perceptions, such as confidence and fear. Predictable perturbations did not generate a N1 potential but instead produced an anticipation-related potential prior to the perturbation. This cortical activity observed in response to predictable perturbations was not influenced by postural threat. A large N1 potential was observed for a 'surprise' perturbation that followed a series of predictable perturbations. There was a trend for the amplitude of this potential to be increased when standing at the high compared to low surface height. The results of this study provide evidence for the modulating influence of psychological factors related to postural threat on the cortical activity associated with postural reactions to unpredictable perturbations.     

The influence of aging and frontal function on the neural correlates of regulative and evaluative aspects of cognitive control

The influence of aging and frontal function on the neural correlates of regulative and evaluative control was examined by means of event-related brain potentials (ERPs). The behavioral data indicated that interference was greater for older than for younger adults and that this difference was mediated by frontal function. The ERP data revealed effects of aging on the neural correlates of both regulative and evaluative control. Prestimulus neural activity was correlated with response time and frontal function, and these relationships were moderated by the response-to-stimulus interval (RSI); the poststimulus data also revealed age-related differences in the neural correlates of evaluative control that interacted with RSI. These data support predictions derived from the context processing deficit theory of aging.     

The influence of neural regulation of the liver on experimental atherosclerosis

Summary The author studied the effect of liver denervation on the lipid metabolism and the intensity of anatomical changes in rabbit aortas during cholesterol administration. Animals in which laparatomy was performed under the same conditions as denervation served as controls. Some animals received cholesterol in sunflower-seed oil, others — mixed with vegetables. Some tendency to intensification of the usual changes occurring in the lipid metabolism following alimentary cholesterol load was detected in animals after liver denervation when cholesterol was administered in sunflower-seed oil. Liver denervation considerably inhibited the development of anatomical changes in the aorta irrespective of the method of cholesterol administration. The mechanism of this inhibitory effect is still obscure. Additional histological investigations of A. E. Seranova have demonstrated destruction of only individual nervous fibers. Leucocytic infiltration is seen in the connective tissue of the porta hepatis, vascular adventitia and around the nerve stems; hypertophy of the fibroblastic elements is noted in the epineurium of the nerve stems with proliferation and hypertrophy of the Schwann cells.It may be suggested that inhibition of experimental atherosclerosis in the aforementioned experiments depended mainly on the stimulation of the nervous conductors of the liver.(Presented by Academician V. N. Chernigovskii) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 51, No. 4, pp. 48–51, April, 1961     

The control of rapid limb movement in the cat

In cats trained to track a moving display by making rapid, isometric force adjustments, responses are characterized by extremely short reaction times (60-70 ms) and a stereotyped temporal configuration. The animal uses early derivatives of display movement to scale force responses to target stimuli of different sizes according to a learned relationship between initial display motion and required force (Ghez and Vicario 1978a, 1978b). In the present study we altered that relationship by using double stimulation and delayed feedback to assess the animals' ability to update their responses. In experiments where a second target stimulus followed the first after a controlled interval (15-120 ms) on random trials, the animal modified its response in the appropriate direction with little or no increase in reaction time. When the second stimulus called for a return to baseline, the animal aborted the ongoing response. When the second stimulus called for a doubling of force, the animal increased its phasic force output; however, this increase was not sufficient to reach the new target level and late responses were emitted. The control response which followed each experimental double stimulation trial showed consistent differences from other controls in the amplitude of both peak force and peak dF/dt. Control responses following trials calling for a return were reduced in size; those following stimuli requiring response doubling were increased. We concluded that the experimental trials not only elicited modification of ongoing responses but also caused the animal to alter its internalized gain function relating initial display derivatives to required force. In experiments where feedback was delayed after giving a first target stimulus such that the compensatory display failed to reflect the animal's initial response, the animal emitted a new updated response 70-80 ms after the first. The display trajectory which caused the cat to update its response on delayed feedback trial was identical to that of control trials with long reaction times. In this case, however, the information eliciting response updating had to be derived as a difference between the actual display trajectory and that expected by the animal, based on its experience with the tracking task. This suggests that the animal develops an internal model of display properties which is used to determine when a new response is required.     

The control of rapid limb movement in the cat

Summary Electromyographic (EMG) activity associated with rapid voluntary limb movements exhibits a characteristic three burst pattern. The first burst is in the agonist muscle (AG1), the second is in the antagonist (ANT) and the third is again in the agonist (AG2). The present study was undertaken to determine whether ANT and AG2 reflect preprogrammed commands or responses to stretch consequent upon limb displacement. To answer this question EMG activity of agonist and antagonist muscles was examined in cats performing a tracking task. To dissociate centrally programmed muscular events from their intended mechanical consequences, isometric and anisometric conditions were presented in either a predictable or unpredictable sequence. A torque motor was used to control limb trajectory and to impose passive angular displacements.Whereas AG1 was present under both isometric and anisometric conditions, ANT and AG2 required limb displacement and were time locked to movement parameters. ANT occurred within 15 ms following the onset of acceleration. Its magnitude varied linearly with this parameter and inversely with AG1. Passive displacements stretching the antagonist elicited responses with similar latencies and greatest magnitude for a given acceleration. AG2 was only present in underdamped movements with terminal oscillations and typically occurred when the position reached its peak and the velocity recrossed zero. Its magnitude was a function of both limb deceleration and of intended force.The data indicate that both ANT and AG2 represent responses to muscles stretch whose amplitudes are modulated by descending commands. Reciprocal mechanisms operating at a spinal level could account for the reduction of the antagonist response as a function of intended force. The increased sensitivity of late stretch responses in the agonist with higher intended forces is compatible with motoneuron facilitation by tonic descending commands. It is proposed that the stretch evoked responses function to dampen terminal oscillations which ensue from rapid displacement of the mass of the limb against elastic forces of muscle and soft tissue.Supported by NIH Grant NS 15750     

Adaptation of movement endpoints to perturbations of visual feedback

We investigated the extent to which humans can quickly adapt their goal-directed arm movements to perturbed feedback. We predicted that the magnitude of adaptation to a changed relationship between vision and kinesthesia would depend on the type of perturbation, being largest when the perturbation can be generalized within egocentric frames of reference. To test this prediction we asked subjects to align a real 5-cm cube so that they could feel, but not see, with a simulation that they saw via a mirror. Subjects made successive movements between target locations in a sequence of adaptation and test phases. During adaptation phases, subjects received continuous visual feedback about the position of the real cube. The feedback was either veridical or perturbed. The perturbations were consistent with either a uniform translation, a scaling or a rotation. The latter two were relative to a central position between all the targets. During test phases, subjects received no visual feedback. We compared test movement endpoints after perturbed feedback with ones after veridical feedback. We found about 40% adaptation to translation, 20% to scaling and 10% to rotation. This difference in magnitude is consistent with the ease with which the transformation can be generalized within egocentric frames of reference. Changing the task so that it required different arm postures did not change the magnitude of adaptation, so postural configuration of the arm does not appear to be critical. Nevertheless, transfer to the unexposed arm was incomplete for translations and rotations, though it was complete for scaling, suggesting that at least part of the adaptation is posture based. We conclude that the adaptation to different kinds of perturbations not only differs in extent but also involves different (egocentric) mechanisms. Electronic Publication     

The influence of substrate stiffness on the behavior and functions of Schwann cells in culture

Solid tissues in the body possess a range of stiffness and provide cells with an instructive microenvironment. Scaffolds in tissue engineering should be rationally designed to become an adhesion substrate friendly to cells. Schwann cells are the principal glial cell in the peripheral nervous system and used as support cells for generating tissue-engineered nerve grafts. Although an important mechanical cue, substrate stiffness, has been documented to make significant effects on many types of cells cultured on the substrate, such a study for Schwann cells is still lacking. In this study, we investigated cell adhesion, survival, proliferation, migration, cytoskeleton, and neurotrophic actions of Schwann cells cultured on polyacrylamide gel substrates with different stiffness, and determined an optimal elastic modulus value for these substrates. Our data not only highlight the importance of substrate stiffness in the crosstalk between Schwann cells and surrounding microenvironment, but also introduce a new parameter, in addition to biocompatibility, biodegradability, and neuroaffinity, for designing scaffolds in nerve tissue engineering.     

The influence of sudden perturbations on trunk muscle activity and intra-abdominal pressure while standing

Unexpected ventral and dorsal perturbations and expected, self-induced ventral perturbations were delivered to the trunk by suddenly loading a vest strapped to the torso. Six male subjects were measured for intra-abdominal pressure (IAP) and intra-muscular electromyography of the transversus abdominis (TrA), obliquus internus abdominis (OI), obliquus externus abominis (OE) and rectus abdominis (RA) muscles. Erector spinae (ES) activity was recorded using surface electromyography. Displacements of the trunk and head were registered using a video-based system. Unexpected ventral loading produced activity in TrA, OI, OE and RA, and an IAP increase well in advance of activity from ES. Expected ventral loading produced pre-activation of all muscles and an increased IAP prior to the perturbation. The TrA was always the first muscle active in both the unexpected and self-loading conditions. Of the two ventral loading conditions, forward displacement of the trunk was significantly reduced during the self-loading. Unexpected dorsal loading produced coincident activation of TrA, OI, OE, RA and ES. These results indicate a response of the trunk muscles to sudden expected and unexpected ventral loadings other than the anticipated immediate extensor torque production through ES activation. It is suggested that the increase in IAP is a mechanism designed to improve the stability of the trunk through a stiffening of the whole segment.