Muscle ceramide content in man is higher in type I than type II fibers and not influenced by glycogen content

The purpose was to determine the relation between visual feedback gain and variability in force and whether visual gain-induced changes in force variability were associated with frequency-specific force oscillations and changes in the neural activation of the agonist muscle. Fourteen young adults (19–29 years) were instructed to accurately match the target force at 2 and 10% of their maximal voluntary contraction with abduction of the index finger. Force was maintained at specific visual feedback gain levels that varied across trials. Each trial lasted 20 s and the amount of visual feedback was varied by changing the visual gain from 0.5 to 1,474 pixels/N (13 levels; equals ~0.001–4.57°). Force variability was quantified as the standard deviation of the detrended force data. The neural activation of the first dorsal interosseus (FDI) was measured with surface electromyography. The mean force did not vary significantly with the amount of visual feedback. In contrast, force variability decreased from low gains compared to moderate gains (0.5–4 pixels/N: 0.09 ± 0.04 vs. 64–1,424 pixels/N: 0.06 ± 0.02 N). The decrease in variability was predicted by a decrease in the power of force oscillations from 0–1 Hz (~50%) and 3–7 Hz (~20%). The activity of the FDI muscle did not vary across the visual feedback gains. These findings demonstrate that in young adults force variability can be decreased with increased visual feedback gain (>64 pixels/N vs. 0.5–4 pixels/N) due to a decrease in the power of oscillations in the force from 0–1 and 3–7 Hz.     

Magnified visual feedback exacerbates positional variability in older adults due to altered modulation of the primary agonist muscle

The purpose of this study was to determine whether magnified visual feedback during position-holding contractions exacerbates the age-associated differences in motor output variability due to changes in the neural activation of the agonist muscle in the upper and lower limb. Twelve young (18–35 years) and ten older adults (65–85 years) were instructed to accurately match a target position at 5° of index finger abduction and ankle dorsiflexion while lifting 10 % of their 1 repetition maximum (1RM) load. Position was maintained at three different visual angles (0.1°, 1°, and 4°) that varied across trials. Each trial lasted 25 s and visual feedback of position was removed from 15 to 25 s. Positional error was quantified as the root mean square error (RMSE) of the subject’s performance from the target. Positional variability was quantified as the standard deviation of the position data. The neural activation of the first dorsal interosseus and tibialis anterior was measured with surface electromyography (EMG). Older adults were less accurate compared with young adults and the RMSE decreased significantly with an increase in visual gain. As expected, and independent of limb, older adults exhibited significantly greater positional variability compared with young adults that was exacerbated with magnification of visual feedback (1° and 4°). This increase in variability at the highest magnification of visual feedback was predicted by a decrease in power from 12 to 30 Hz of the agonist EMG signal. These findings demonstrate that motor control in older adults is impaired by magnified visual feedback during positional tasks.     

Electroencephalographic coherence,aging, and memory: distinct responses to background context and stimulus repetition in younger,older, and older declined groups

The current study examines the EEG coherence of young, old, and old declined adults performing a visual paired-associates task. In order to examine the effects of encoding context and stimulus repetition, target pairs were presented on either detailed or white backgrounds and were repeatedly presented during both early and late phases of encoding. Younger adults were found to have lower levels of frontal–temporal and temporal–parietal coherence, but higher levels of frontal–parietal coherence, particularly for the gamma frequency band. A number of differential coherence responses to background context and early- versus late-encoding phases were also observed across the groups, particularly for lower alpha and upper alpha frequencies. Coherence–performance maps were generated to further explore topographical differences in the relationship between coherence and performance across groups. Results revealed a more diffuse pattern of negative coherence–performance relations in older declined adults. Results are discussed in light of the literature on age-related cognitive decline.     

A comparison of prehension force control in young and elderly individuals

Age-related changes were investigated in the control of precision grip force during the lifting and holding of objects with slippery (silk) and nonslippery (sandpaper) surface textures. Two groups of active elderly adults comprising individuals aged 69–79 years (n = 10), and 80–93 years (n = 10) together with a group of young adults aged 18–32 years (n = 10) participated in the study. Each subject lifted a free weight (3N) during which time gripping and lifting forces were monitored. The elderly subjects, especially the individuals in the 81–93 year group, had a larger number of fluctuations in the grip force rate curve and longer force application time than the younger subjects during lifting. The effect of prior experience with one surface on the following different surface was more pronounced in the younger subjects than the elderly subjects. These results suggest a decline in programmed force production capacity with increased age. The fingers of the elderly subjects were more slippery and they exhibited a greater safety margin of the grip force while holding the object than the younger adults. The overall results demonstrated that precision grip force control capacity declines with advancing age. It is suggested that this decline is due mainly to age-related changes in skin properties, and cutaneous sensibility functions, and in part to central nervous system function.     

Objective measurement of human visual acuity by visual evoked potentials

Electrophysiological studies were performed to measure the threshold (upper end of range) spatial frequency using visual evoked potentials and comparison with visual acuity neuron 26 healthy subjects. The aim of the present work was to create a method for objective measurement of visual acuity. This was addressed by initial measurements using a universally accepted method of visual stimulation and processing of electroencephalograms, which allows errors due to individual differences in visual system function to be minimized. These experiments yielded a strong correlation between the threshold spatial frequency of the test grid yielding an evoked potential on presentation and visual acuity, in degrees, expressed as the resolving ability of the visual system for this optotype. A logarithmic relationship was found between these values and an equation allowing automated calculation of visual acuity (resolving ability) from electrophysiological data was derived. The results were independent of the subject’s responses and therefore provides a maximally objective assessment of visual acuity. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 91, No. 8, pp. 956–969, August, 2005.     

Force and time gain interact to nonlinearly scale adaptive visual-motor isometric force control

This study examined the influence of force–time gain on the visual-motor control of isometric force. The spatial lengths on the computer screen representing the unit of elapsed time (time gain) and force (force gain) of the force output were compressed or extended in a crossed fashion while subjects produced index finger abduction force to a sinewave and constant force target that was 20 % of maximal voluntary contraction. The results revealed a U-shaped interactive influence of force–time gain on force performance, namely a particular combination of moderate force–time gains leads to optimal force performance. The nature of the interaction between the force and time gains also differed depending on the task demand. During constant force production, the best gain at one dimension (force or time) was invariant across the other dimension (time or force), whereas during sinewave force production, the best gain at one dimension varied with the gain at the other dimension. The results support the proposition that the control of force output is organized by the interactive influence of different categories of constraints where the influence of visual information gain depends on the dynamics of the force control and the task demand. The findings also provide implications for visual gain parameter settings for adaptive force control.     

Spatial organization of cortical electrical activity at different stages of a visual set in preschool and early school age

Parameters of the formation of a visual nonverbal set and the rate of its replacement with a new set were compared in children of three age groups: 5–6, 6–7, and 9–10 years. The vast majority of subjects (27 of 30 preschool children and 42 of 43 third-grade children) showed clear set effects. Age-related differences in set plasticity and the dynamics of reaction times to test stimuli were observed. The set was more rigid in children aged 5–6 years than in older children. Differences in the dynamics of the spatial organization of alpha and theta activity were seen in the anterior areas of the cortex at different stages of the set in children of different age groups. Analysis of cortical potentials coherence functions and behavioral parameters led to the hypothesis that the frontothalamic selective attention system and the corticohippocampal connection system responsible for the cortical processing of new visual information and episodic memory function are involved in organizing the visual set. A critical age (from six to seven years) was indentified in the formation of plastic types of visual nonverbal sets. Translated from Zhurnal Vysshei Nervnoi Deyatel’nosti imeni I. P. Pavlova, Vol. 58, No. 1, pp. 46–55, January–February, 2008.     

Information and force level interact in regulating force output during two and three digit grip configurations

The experiment examined the force fluctuations during two and three digit grip configurations to investigate the relationship between task performance and inter-digit individuation as a function of force level and visual information intermittency rate over ∼100-fold range (0.21–20 Hz). Subjects grasped an object with either the index finger (two digit grip) or the index and middle finger (three digit grip) opposing the thumb and produced isometric force to match a low and high total force level target. Force accuracy was lower at the large visual intermittency conditions and the higher force level. The force variability was lower in the three digit grip. Inter-digit individuation increased as a function of visual intermittency rate and was greater at the low force level. There was no improvement in performance or inter-digit individuation when visual feedback intermittency was greater than ∼6 Hz (∼150 ms). Linear regression between the measures of task performance and inter-digit individuation yielded a significant negative relationship that was only present in the two digit grip when visual feedback rate was 1.67 Hz or lower and in the three digit grip when the feedback rate was 10 Hz or lower. The greater biomechanical degrees of freedom in the three digit grip configuration enable the subject to use, more effectively, visual information feedback at faster timescales in maximizing task performance by increasing digit independence. The shift from visual to nonvisual dominated motor control processes is dependent on the interaction of informational and biomechanical degrees of freedom.     

Age-related differences in finger force control are characterized by reduced force production

It has been repeatedly shown that precise finger force control declines with age. The tasks and evaluation parameters used to reveal age-related differences vary between studies. In order to examine effects of task characteristics, young adults (18–25 years) and late middle-aged adults (55–65 years) performed precision grip tasks with varying speed and force requirements. Different outcome variables were used to evaluate age-related differences. Age-related differences were confirmed for performance accuracy (TWR) and variability (relative root mean square error, rRMSE). The task characteristics, however, influenced accuracy and variability in both age groups: Force modulation performance at higher speed was poorer than at lower speed and at fixed force levels than at force levels adjusted to the individual maximum forces. This effect tended to be stronger for older participants for the rRMSE. A curve fit confirmed the age-related differences for both spatial force tracking parameters (amplitude and intercept) and for one temporal parameter (phase shift), but not for the temporal parameter frequency. Additionally, matching the timing parameters of the sine wave seemed to be more important than matching the spatial parameters in both young adults and late middle-aged adults. However, the effect was stronger for the group of late middle-aged, even though maximum voluntary contraction was not significantly different between groups. Our data indicate that changes in the processing of fine motor control tasks with increasing age are caused by difficulties of late middle-aged adults to produce a predefined amount of force in a short time.     

Children's coordination of force output in a pinch grip task

This study examined the role of sensorimotor system noise in the organization of the force output of the thumb and index finger and the coordination between the two digits in an isometric pinch grip force task as a function of age (6, 8, 10, 18-22 years), feedback condition (with and without visual feedback information), and force level (5, 15, 25, and 35% of maximal voluntary force. With increases in age under the visual feedback conditions, the signal-to-noise ratio increased, the sequential structure of the force output signals became more irregular, the degree of coherence between the digits at higher force levels was enhanced, and the digits exhibited a greater degree of coherence across the higher frequencies of the power spectrum at all force levels. However, these age differences were either minimized or eliminated under conditions without feedback. These findings show that the age-related performance differences in grip force variability are primarily due to more effective use of visual information rather than age differences in intrinsic sensorimotor noise.     

The affordance of barrier crossing in young children exhibits dynamic,not geometric,similarity

Previous research has shown that adults perceive affordances like the passability of apertures, climbability or crossability of steps and graspability of objects. In this study, the affordance for stepping over or onto barriers was examined in young children. This was done by placing three distinct barriers (a foam obstacle, a gap, and a single step up), which were scaled to body size, in the walking paths of 4- and 6-year olds and adults, and observing how they crossed the barriers. Age-related differences in the scaling of these actions corresponded to levels of movement variability, indicating that children as young as 4 years old are sensitive to their own constraints and scale their actions accordingly. These results indicate that affordances are not directly related to leg geometry, but rather entail the dynamics of the developing perception–action system.     

Diminished task-related adjustments of common inputs to hand muscle motor neurons in older adults

The purpose of this study was to quantify correlated motor unit activity during isometric, shortening and lengthening contractions of a hand muscle in older adults. Thirteen old subjects (69.6±5.9 years, six women) lifted and lowered a light load with abduction–adduction movements of the index finger over 10° using 6-s shortening and lengthening contractions of the first dorsal interosseus muscle. The task was repeated 10–20 times while activity in 23 pairs of motor units was recorded with intramuscular electrodes. The data were compared with 23 motor-unit pairs in 15 young (25.9±4.6 years, five women) subjects obtained using a similar protocol in a previous study. Correlated motor unit activity was quantified using time-domain (synchronization index; Common Input Strength) and frequency-domain (coherence) analyses for the same motor-unit pairs. For all contractions, there was no difference with age for the strength of motor-unit synchronization, although age-related differences were observed for synchronous peak widths (young, 17.6±7.4 ms; old, 13.7±4.9 ms) and motor-unit coherence at 6–9 Hz (z score for young, 3.0±1.8; old, 2.2±1.5). Despite increased synchrony during lengthening contractions and narrower peak widths for shortening contractions in young subjects, there was no difference in the strength of motor unit synchronization (CIS ~0.8 imp/s), or the width of the synchronous peak (~14 ms) during the three tasks in old subjects. Furthermore, no significant differences in motor-unit coherence were observed between tasks at any frequency for old adults. These data suggest that the strategy used by the central nervous system to control isometric, shortening, and lengthening contractions varies in young adults, but not old adults. The diminished task-related adjustments of common inputs to motor neurons are a likely consequence of the neural adaptations that occur with advancing age.     

Effects of visual deprivation on intra-limb coordination during walking in children and adults

Following the laws of planar covariation, intersegmental coordination is defined with respect to the vertical and heading direction. This vertical reference can be estimated using multisensory information, amongst others visual cues play a role. In the past it was already shown that visual deprivation and/or perturbation of visual information largely affect gait kinematics. The goal of this study is to investigate the impact of visual deprivation on intra-limb movement coordination. Children aged between 3 and 11 years and young adults are included in this study to investigate age-related differences in the visual control of locomotion. Intersegmental coordination was tested under two different conditions: full vision (FV) and no vision (NV). Heading direction and walking speed were taken into account. A significant interaction effect was observed between visual condition and age for walking speed. Between age groups, no differences are observed in the FV condition but in the NV condition children walk significantly slower than adults. This shows that the relative importance of visual information is age dependent. Between age groups significant differences were found in heading direction. Coordination was characterized using the planar covariation technique, by constructing thigh versus shank and shank versus foot angle–angle plots and by cross-correlation function analysis. Regardless of the presence or absence of visual information, the planarity index remains high, indicating that the laws of planar covariation hold in the absence of visual afferent information. On the other hand, the shape of the gait loop does show significant differences between FV and NV conditions. Changes in the shape of the gait loop are primarily determined by changes in the coupling between the thigh and shank elevation angles. The coupling between the shank and foot elevation angles is dependent upon walking speed and does not differ between FV and NV conditions. Between age groups significant differences are observed in covariation plane orientation.     

Greater amount of visual information exacerbates force control in older adults during constant isometric contractions

The purpose of this study was to compare control of force and modulation of agonist muscle activity of young and older adults when the amount of visual feedback was varied at two different force levels. Ten young adults (25 years ± 4 years, 5 men and 5 women) and ten older adults (71 years ± 5 years, 4 men and 6 women) were instructed to accurately match a constant target force at 2 and 30% of their maximal isometric force with abduction of the index finger. Each trial lasted 35 s, and the amount of visual feedback was varied by changing the visual angle at 0.05, 0.5, and 1.5°. Each subject performed three trials for each visual angle condition. Force variability was quantified as the standard deviation and coefficient of variation (CV) of force. Modulation of the agonist muscle activity was quantified as the normalized power spectrum density of the EMG signal recorded from two pairs of bipolar electrodes placed on the first dorsal interosseus muscle. The frequency bands of interest were between 5 and 100 Hz. There were significant age-associated differences in force control with changes in the amount of visual feedback. The CV of force did not change with visual angle for young adults, whereas it increased for older adults. Although older adults exhibited similar CV of force to young adults at 0.05° (5.95 ± 0.67 vs. 5.47 ± 0.5), older adults exhibited greater CV of force than young adults at 0.5° (8.49 ± 1.34 vs. 5.05 ± 0.5) and 1.5° (8.23 ± 1.12 vs. 5.49 ± 0.6). In addition, there were age-associated differences in the modulation of the agonist muscle activity. Young adults increased normalized power in the EMG signal from 13 to 60 Hz with an increase in visual angle, whereas older adults did not. These findings suggest that greater amount of visual information may be detrimental to the control of a constant isometric contraction in older adults, and this impairment may be due to their inability to effectively modulate the motor neuron pool of the agonist muscle.     

Age-related differences in inter-digit coupling during finger pinching

The present study was designed to examine the finger-pinch force control, digit force sharing and digit coupling relations of 13 young and 14 older adults. Subjects performed four isometric tri-digit finger-pinch force production conditions reflecting all combinations of mean force level (20 and 40% MVC) and target shape (constant and sinusoidal). Older adults had significantly reduced force control, as indicated by their greater levels of absolute and relative force variability and targeting error than young adults. The age-related loss of relative force control was more pronounced at low (20% MVC) than high (40% MVC) forces, and to a lesser extent, in sinusoidal than constant force conditions. Older adults had significantly greater peak and proportional power below 1.5 Hz than young adults, with this especially pronounced in constant force conditions. Digit force sharing results indicated that the index finger’s contribution to total force was increased and the middle finger’s contribution reduced in older than young adults. The results of the cross-correlation analyses revealed that older subjects had a significantly reduced level of coupling between the middle finger and the target force, thumb force and EMG signals, with longer time lags in comparison to young adults. These differences in force sharing and middle finger force coupling were more pronounced in sinusoidal than constant force conditions. Overall, these results suggest that the older adults’ reduced force control reflected age-related differences in the sharing and coupling of the finger forces. The results also highlighted that tasks of this nature display a degree of task-dependency, with these overall differences in digit force output and coupling not consistently observed across all force conditions.     

Manipulation of a fragile object by elderly individuals

We investigated strategies of healthy elderly participants (74–84 years old) during prehension and transport of an object with varying degrees of fragility. Fragility was specified as the maximal normal force that the object could withstand without collapsing. Specifically, kinetic and kinematic variables as well as and force covariation indices were quantified and compared to those shown by young healthy persons (19–28 years old). We tested three hypotheses related to age-related changes in two safety margins (slip safety margin and crush safety margin) and indices of force covariation. Compared to young controls, elderly individuals exhibited a decrease in object acceleration and an increase in movement time, an increase in grip force production, a decrease in the correlation between grip and load forces, an overall decrease in indices of multi-digit synergies, and lower safety margin indices computed with respect to both dropping and crushing the object. Elderly participants preferred to be at a relatively lower risk of crushing the object even if this led to a higher risk of dropping it. Both groups showed an increase in the index of synergy stabilizing total normal force produced by the four fingers with increased fragility of the object. Age-related changes are viewed as a direct result of physiological changes due to aging, not adaptation to object fragility. Such changes in overall characteristics of prehension likely reflect diminished synergic control by the central nervous system of finger forces with aging. The findings corroborate an earlier hypothesis on an age-related shift from synergic to element-based control.     

Effects of visual gain on force control at the elbow and ankle

Visual feedback is essential when minimizing force fluctuations. Varying degrees of somatotopic organization have been shown in different regions of the brain for the upper and lower extremities, and visual feedback may be processed differently based on the body effector where feedback-based corrections are used. This study compared the effect of changes in visual gain on the control of steady-state force at the elbow and ankle. Ten subjects produced steady-state isometric force to targets at 5 and 40% of their maximum voluntary contraction at seven visual gain levels. Visual gain was used effectively at both joints to reduce variability of the force signal and to improve accuracy, with a greater effect of visual gain at the elbow than the ankle. Visual gain significantly decreased the regularity of force output, and this effect was more pronounced at the elbow than the ankle. There were accompanying changes in the proportion of power in the 0–4, 4–8, and 8–12 Hz frequency bins of the force signal across visual gain at the elbow. Changes in visual gain were accompanied by changes in both agonist and antagonist electromyographic (EMG) activation at the elbow. At the ankle joint, there were only changes in agonist EMG. The results suggest better use of visual information in the control of elbow force than ankle force and this improved control may be related to the changes in the pattern of agonist and antagonist activation.     

Removal of visual feedback alters muscle activity and reduces force variability during constant isometric contractions

The purpose of this study was to compare force accuracy, force variability and muscle activity during constant isometric contractions at different force levels with and without visual feedback and at different feedback gains. In experiment 1, subjects were instructed to accurately match the target force at 2, 15, 30, 50, and 70% of their maximal isometric force with abduction of the index finger and maintain their force even in the absence of visual feedback. Each trial lasted 22 s and visual feedback was removed from 8–12 to 16–20 s. Each subject performed 6 trials at each target force, half with visual gain of 51.2 pixels/N and the rest with a visual gain of 12.8 pixels/N. Force error was calculated as the root mean square error of the force trace from the target line. Force variability was quantified as the standard deviation and coefficient of variation (CVF) of the force trace. The EMG activity of the agonist (first dorsal interosseus; FDI) was measured with bipolar surface electrodes placed distal to the innervation zone. Independent of visual gain and force level, subjects exhibited lower force error with the visual feedback condition (2.53 ± 2.95 vs. 2.71 ± 2.97 N; P < 0.01); whereas, force variability was lower when visual feedback was removed (CVF: 4.06 ± 3.11 vs. 4.47 ± 3.14, P < 0.01). The EMG activity of the FDI muscle was higher during the visual feedback condition and this difference increased especially at higher force levels (70%: 370 ± 149 vs. 350 ± 143 μV, P < 0.01). Experiment 2 examined whether the findings of experiment 1 were driven by the higher force levels and proximity in the gain of visual feedback. Subjects performed constant isometric contractions with the abduction of the index finger at an absolute force of 2 N, with two distinct feedback gains of 15 and 3,000 pixels/N. In agreement with the findings of experiment 1, subjects exhibited lower force error in the presence of visual feedback especially when the feedback gain was high (0.057 ± 0.03 vs. 0.095 ± 0.05 N). However, force variability was not affected by the vastly distinct feedback gains at this force, which supported and extended the findings from experiment 1. Our findings demonstrate that although removal of visual feedback amplifies force error, it can reduce force variability during constant isometric contractions due to an altered activation of the primary agonist muscle most likely at moderate force levels in young adults.     

Age-related deterioration of the representation of space in human auditory cortex

One of the principal auditory disabilities associated with older age is difficulty in locating and tracking sources of sound. This study investigated whether these difficulties are associated with deterioration in the representation of space in the auditory cortex. In psychophysical tests, half of a group of older (>60 years) adults displayed spatial acuity similar to that of young adults throughout the frontal horizontal plane. The remaining half had considerably poorer spatial acuity at the more peripheral regions of frontal space. Computational modeling of electroencephalographic responses to abrupt location shifts demonstrated marked differences in the spatial tuning of populations of cortical neurons between the older adults with poor spatial acuity on the one hand, and those with good spatial acuity, as well as young adults, on the other hand. In those with poor spatial acuity, cortical responses contained little information with which to distinguish peripheral locations. We demonstrate a clear link between neural responses and spatial acuity measured behaviorally, and provide evidence for age-related changes in the coding of horizontal space.     

Developmental Changes in Measures of Hierarchical Stimulus Recognition in Conditions of Directed Attention in Children Aged 5–10 Years

Reaction times and the accuracy of recognition of hierarchical letter stimuli at the local and global levels were studied in 95 children of four age groups (5–6, 6–7, 7–8, and 9–10 years) and 10 adults. Overall, children of all age groups and adults recognized hierarchical stimuli at the global level more accurately and rapidly than at the local level (the global advantage effect) and showed slowing of reactions on recognition of incongruent stimuli at the local level (the global interference effect). At younger ages (5–6 years), there were significant individual differences in measures of recognition of hierarchical letters – in some children of this age (seven of 37), the global advantage effect on recognition of incongruent stimuli was absent. Significant progressive changes in the accuracy of recognition of hierarchical stimuli at both the local and global levels were seen on development from 6–7 to 7–8 years, as well as in adults a compared with children aged 9–10 years. Different developmental dynamics in the speed of recognition of hierarchical stimuli were seen at the local and global levels: the speed of recognition of small letters increased significantly in each sequential age group, starting from 6–7 years, while recognition of large letters showed no significant decrease in reaction times in children aged 9–10 years as compared with those aged 7–8 years. In the two youngest age groups (5–6 and 6–7 years), the type of hierarchical stimulus was found to influence recognition speed: reaction times increased significantly, independently of the level of recognition, using both target (incongruent stimuli) and neutral elements at the irrelevant level. It is suggested that the nonlinear developmental dynamics of measures of the recognition of hierarchical stimuli at preschool and early school age are determined by the maturation of the mechanisms processing sense-specific information and by the formation of executive functions, especially those associated with selective extraction of significant signals.