Generalization and transfer of contextual cues in motor learning.

We continuously adapt our movements in daily life, forming new internal models whenever necessary and updating existing ones. Recent work has suggested that this flexibility is enabled via sensorimotor cues, serving to access the correct internal model whenever necessary and keeping new models apart from previous ones. While research to date has mainly focused on identifying the nature of such cue representations, here we investigated whether and how these cue representations generalize, interfere, and transfer within and across effector systems. Subjects were trained to make two-stage reaching movements: a premovement that served as a cue, followed by a targeted movement that was perturbed by one of two opposite curl force fields. The direction of the premovement was uniquely coupled to the direction of the ensuing force field, enabling simultaneous learning of the two respective internal models. After training, generalization of the two premovement cues' representations was tested at untrained premovement directions, within both the trained and untrained hand. We show that the individual premovement representations generalize in a Gaussian-like pattern around the trained premovement direction. When the force fields are of unequal strengths, the cue-dependent generalization skews toward the strongest field. Furthermore, generalization patterns transfer to the nontrained hand, in an extrinsic reference frame. We conclude that contextual cues do not serve as discrete switches between multiple internal models. Instead, their generalization suggests a weighted contribution of the associated internal models based on the angular separation from the trained cues to the net motor output.

Unchanged muscle fiber conduction velocity relates to mild acidosis during exhaustive bicycling.

Muscle fiber conduction velocity (MFCV) has often been shown to decrease during standardized fatiguing isometric contractions. However, several studies have indicated that the MFCV may remain constant during fatiguing dynamic exercise. It was investigated if these observations can be related to the absence of a large decrease in pH and if MFCV can be considered as a good indicator of acidosis, also during dynamic bicycle exercise. High-density surface electromyography (HDsEMG) was combined with read-outs of muscle energetics recorded by in vivo (31)P magnetic resonance spectroscopy (MRS). Measurements were performed during serial exhausting bouts of bicycle exercise at three different workloads. The HDsEMG recordings revealed a small and incoherent variation of MFCV during all high-intensity exercise bouts. (31)P MRS spectra revealed a moderate decrease in pH at the end of exercise (~0.3 units down to 6.8) and a rapid ancillary drop to pH 6.5 during recovery 30 s post-exercise. This additional degree of acidification caused a significant decrease in MFCV during cycling immediately after the rest period. From the data a significant correlation between MFCV and [H(+)] ([H(+)] = 10(-pH)) was calculated (p < 0.001, Pearson's R = -0.87). Our results confirmed the previous observations of MFCV remaining constant during fatiguing dynamic exercise. A constant MFCV is in line with a low degree of acidification, considering the presence of a correlation between pH and MFCV after further increasing acidification.

Age-related changes in motor unit number estimates in adult patients with Charcot-Marie-Tooth type 1A.

BACKGROUND: Charcot-Marie-Tooth disease type 1A (CMT1A) is known as a demyelinating hereditary neuropathy. Secondary axonal dysfunction is the most important determinant of disease severity. In adult patients, clinical progression may be because of further axonal deterioration as was shown with compound muscle action potential (CMAP) amplitude reductions over time. The motor unit number estimation (MUNE) technique may be more accurate to determine the number of axons as it is not disturbed by the effect of reinnervation. The purpose of this study was to investigate the number and size of motor units in relation to age in patients and controls. METHODS: In a cross-sectional design, we assessed arm and hand strength and performed electrophysiological examinations, including CMAP amplitudes and MUNE of the thenar muscles using high-density surface EMG in 69 adult patients with CMT1A and 55 age-matched healthy controls. RESULTS: In patients, lower CMAP amplitudes and MUNE values were related to hand weakness. The CMAP amplitude and MUNE value of the thenar muscles were significantly lower in patients than in controls. CMAP amplitudes declined with age in controls, but not in patients. MUNE values declined with age in both patients and controls. CONCLUSIONS: The age-dependent decrease in the number of motor units was not significantly different between patients with CMT1A and controls, indicating that loss of motor units in adult patients is limited.

The 2nd Berlin BedRest Study: protocol and implementation.

Long-term bed-rest is used to simulate the effect of spaceflight on the human body and test different kinds of countermeasures. The 2nd Berlin BedRest Study (BBR2-2) tested the efficacy of whole-body vibration in addition to high-load resisitance exercise in preventing bone loss during bed-rest. Here we present the protocol of the study and discuss its implementation. Twenty-four male subjects underwent 60-days of six-degree head down tilt bed-rest and were randomised to an inactive control group (CTR), a high-load resistive exercise group (RE) or a high-load resistive exercise with whole-body vibration group (RVE). Subsequent to events in the course of the study (e.g. subject withdrawal), 9 subjects participated in the CTR-group, 7 in the RVE-group and 8 (7 beyond bed-rest day-30) in the RE-group. Fluid intake, urine output and axiallary temperature increased during bed-rest (p < .0001), though similarly in all groups (p > or = .17). Body weight changes differed between groups (p < .0001) with decreases in the CTR-group, marginal decreases in the RE-group and the RVE-group displaying significant decreases in body-weight beyond bed-rest day-51 only. In light of events and experiences of the current study, recommendations on various aspects of bed-rest methodology are also discussed.

Modeling Trans-Spinal Direct Current Stimulation in the Presence of Spinal Implants.

Trans-spinal direct current stimulation (tsDCS) is a technique considered for the treatment of corticospinal damage or dysfunction. TsDCS aims to induce functional modulation in the corticospinal circuitry via a direct current (DC) generated an electric field (EF). To ensure subject safety, subjects with metallic implants are generally excluded from receiving neural dc stimulation. However, spinal injuries often require spinal implants for stabilization. Our goal was to investigate implant imposed changes to EF and current density (CD) magnitude during tsDCS. We simulated the EF and CD, generated by tsDCS in the presence of spinal rods for two electrode configurations and four implant locations along the spinal cord. For each scenario, a no-implant condition was computed for comparison. We assessed changes in EF and CD at the implant location and the EF inside the spinal cord. Our results show that implant presence was able to influence peak CD, compared to the no-implant condition. Nonetheless, the highest calculated CD levels were a factor six lower than those thought to lead to hazardous tissue-damaging effects. Additionally, implant presence did not considerably affect the average EF inside the spinal cord. Our findings do therefore not indicate potentially unsafe CD levels, or significant alterations to stimulation intensity inside the spinal cord, caused by a spinal implant during tsDCS. Our results are relevant to the safety of transcutaneous spinal stimulation applied in the presence of metallic spinal implants.

The addition of the MEP amplitude of finger extension muscles to clinical predictors of hand function after stroke: A prospective cohort study.

BACKGROUND: Within the first 72 hours after stroke, active finger extension is a strong predictor of long-term dexterity. Transcranial magnetic stimulation may add prognostic value to clinical assessment, which is especially relevant for patients unable to follow instructions. OBJECTIVE: The current prospective cohort study aims at determining whether amplitude of motor evoked potentials of the extensor digitorum communis (EDC) can improve clinical prediction after stroke when added to clinical tests. METHODS: the amplitude of motor evoked potentials of the affected EDC muscle at rest was measured in 18 participants within 4 weeks after stroke, as were the ability to perform finger extension and the Fugl-Meyer Motor Assessment of the upper extremity (FMA_UE). These three determinants were related to the FMA_UE at 26 weeks after stroke (FMA_UE26), both directly, and via the proportional recovery prediction model. The relation between amplitude of the motor evoked potentials and FMA_UE26 was evaluated for EDC. For comparison, also the MEP amplitudes of biceps brachii and adductor digiti minimi muscles were recorded. RESULTS: Patients' ability to voluntarily extend the fingers was strongly related to FMA_UE26, in our cohort there were no false negative results for this predictor. Our data revealed that the relation between amplitude of motor evoked potential of EDC and FMA_UE26 was significant, but moderate (rs = 0.58) without added clinical value. The other tested muscles did not correlate significantly to FMA_UE26. CONCLUSIONS: Our study demonstrates no additional value of motor evoked potential amplitude of the affected EDC muscle to the clinical test of finger extension, the latter being more strongly related to FMA_UE26.

Repeatability and reliability of muscle relaxation properties induced by motor cortical stimulation.

Impaired muscle relaxation is a feature of many neuromuscular disorders. However, few tests are available to quantify muscle relaxation. Transcranial magnetic stimulation (TMS) of the motor cortex can induce muscle relaxation by abruptly inhibiting corticospinal drive. The aim of our study was to investigate whether repeatability and reliability of TMS-induced relaxation are greater than voluntary relaxation. Furthermore, effects of sex, cooling, and fatigue on muscle relaxation properties were studied. Muscle relaxation of deep finger flexors was assessed in 25 healthy subjects (14 men and 11 women, age 39.1 ± 12.7 and 45.3 ± 8.7 yr, respectively) with handgrip dynamometry. All outcome measures showed greater repeatability and reliability in TMS-induced relaxation compared with voluntary relaxation. The within-subject coefficient of variability of normalized peak relaxation rate was lower in TMS-induced relaxation than in voluntary relaxation (3.0% vs. 19.7% in men and 6.1% vs. 14.3% in women). The repeatability coefficient was lower (1.3 vs. 6.1 s-1 in men and 2.3 vs. 3.1 s-1 in women) and the intraclass correlation coefficient was higher (0.95 vs. 0.53 in men and 0.78 vs. 0.69 in women) for TMS-induced relaxation compared with voluntary relaxation. TMS enabled demonstration of slowing effects of sex, muscle cooling, and muscle fatigue on relaxation properties that voluntary relaxation could not. In conclusion, repeatability and reliability of TMS-induced muscle relaxation were greater compared with voluntary muscle relaxation. TMS-induced muscle relaxation has the potential to be used in clinical practice for diagnostic purposes and therapy effect monitoring in patients with impaired muscle relaxation. NEW & NOTEWORTHY Transcranial magnetic stimulation (TMS)-induced muscle relaxation demonstrates greater repeatability and reliability compared with voluntary relaxation, represented by the ability to demonstrate typical effects of sex, cooling, and fatigue on muscle relaxation properties that were not seen in voluntary relaxation. In clinical practice, TMS-induced muscle relaxation could be used for diagnostic purposes and therapy effect monitoring. Furthermore, fewer subjects will be needed for future studies when using TMS to demonstrate differences in muscle relaxation properties.

Modeling trans-spinal direct current stimulation for the modulation of the lumbar spinal motor pathways.

OBJECTIVE: Trans-spinal direct current stimulation (tsDCS) is a potential new technique for the treatment of spinal cord injury (SCI). TsDCS aims to facilitate plastic changes in the neural pathways of the spinal cord with a positive effect on SCI recovery. To establish tsDCS as a possible treatment option for SCI, it is essential to gain a better understanding of its cause and effects. We seek to understand the acute effect of tsDCS, including the generated electric field (EF) and its polarization effect on the spinal circuits, to determine a cellular target. We further ask how these findings can be interpreted to explain published experimental results. APPROACH: We use a realistic full body finite element volume conductor model to calculate the EF of a 2.5 mA direct current for three different electrode configurations. We apply the calculated electric field to realistic motoneuron models to investigate static changes in membrane resting potential. The results are combined with existing knowledge about the theoretical effect on a neuronal level and implemented into an existing lumbar spinal network model to simulate the resulting changes on a network level. MAIN RESULTS: Across electrode configurations, the maximum EF inside the spinal cord ranged from 0.47 V m-1 to 0.82 V m-1. Axon terminal polarization was identified to be the dominant cellular target. Also, differences in electrode placement have a large influence on axon terminal polarization. Comparison between the simulated acute effects and the electrophysiological long-term changes observed in human tsDCS studies suggest an inverse relationship between the two. SIGNIFICANCE: We provide methods and knowledge for better understanding the effects of tsDCS and serve as a basis for a more targeted and optimized application of tsDCS.

Relation between muscle fiber conduction velocity and fiber size in neuromuscular disorders.

To determine the relation between muscle fiber conduction velocity (MFCV) and muscle fiber diameter (MFD) in pathological conditions, we correlated invasively measured MFCV values with MFD data obtained from muscle needle biopsies in 96 patients with various neuromuscular disorders. MFCV was significantly correlated with MFD and independent of the underlying disorder. Pathological diameter changes were fiber-type dependent, with corresponding MFCVs. A linear equation expresses the relation well: MFCV (m/s)=0.043.MFD (microm)+0.83. We conclude that fiber diameter determines MFCV largely independent of the underlying neuromuscular disorders studied.

Improved evaluation of back muscle SEMG characteristics by modelling.

Surface EMG (SEMG) as non-invasive method is a valuable tool in functional studies of movement co-ordination. The interpolation of the SEMG power (EMG mapping) gives information about intra- and inter-muscular co-ordination. It has been shown that SEMG maps of low back pain patients and healthy subjects differ. The only major drawback to SEMG is that volume conduction of muscle tissue, fat, and skin decreases the spatial and temporal resolution of signals. To improve the interpretation of SEMG signals, we have applied high pass filtering of cross covariance functions, which has proved to be useful in increasing the spatial resolution, to SEMG data of the back region. Experimental data demonstrate that SEMG signals from the back extensors show only rarely signs of action potential propagation. This behaviour, also described in the literature, can be explained by a model assuming short, deep muscle fibres, having bipolar end effects, with overlapping positions parallel to the fibre direction. This condition is fulfilled by the mm. multifidii et rotatores which are part of the m. erector spinae. Although the model is simplistic, the agreement between simulations and experiments is good.

Discrimination of speech sound contrasts determined with behavioral tests and event-related potentials in cochlear implant recipients.

Cortical potentials evoked with speech stimuli were investigated in ten experienced cochlear implant (CI, type Nucleus 24M) users using three different speech-coding strategies and two different speech contrasts, one vowel (/i/-/a/) and one consonant (/ba/-/da/) contrast. On average, results showed that, compared to subjects with normal hearing, P300 amplitudes were smaller; however, most latencies were within the normal range. Next, individual P300 measures in response to the two speech contrasts were compared to behavioral discrimination scores. Significant within-subject differences in P300 amplitudes and latencies were found for the three speech coding strategies. These differences were in agreement with the behavioral, strategy-dependent discrimination of the speech contrasts.

Phonological effects on the auditory N400 event-related brain potential.

We report 3 experiments exploring the responsiveness of the auditory N400 event-related potential to the phonological relations between word or non-word targets and preceding prime words. When subjects had to decide whether primes and targets rhymed, non-rhyming words produced greater negativity in the N400 time range than rhyming words. The same effect was obtained when these targets were spoken by another voice than the prime words, suggesting that the effect is determined by phonological factors, and not merely by a physical-acoustic mismatch (Experiment 1). In the rhyming task, the differential N400 for non-rhyming vs. rhyming words was equally pronounced for non-rhyming vs. rhyming non-words (Experiment 2). In a lexical decision task on the same stimuli, a difference between non-rhyming and rhyming targets was obtained for words, but not for non-words (Experiment 3). The results show that the auditory N400 is sensitive to phonological variables. It is further proposed that phonological effects on the auditory N400 are not manifestations unique to phonological processes that demand conscious attention, but may also reflect operations that are performed automatically during auditory word recognition.

Possible mechanisms of muscle cramp from temporal and spatial surface EMG characteristics.

In this study, the initiation and development of muscle cramp are investigated. For this, we used a 64-channel surface electromyogram (EMG) to study the triceps surae muscle during both cramp and maximal voluntary contraction (MVC) in four cramp-prone subjects and during cramp only in another four cramp-prone subjects. The results show that cramp presents itself as a contraction of a slowly moving fraction of muscle fibers, indicating that either the spatial arrangement of the motoneurons and muscle fibers is highly related or that cramp spreads at a level close to the muscle. Spectral analyses of the EMG and peak-triggered average potentials show the presence of extremely short potentials during cramp compared with during MVC. These results can also be interpreted in two ways. Either the motoneurons fire with enlarged synchronization during MVC compared with cramp, or smaller units than motor units are active, indicating that cramp is initiated close to or even at the muscle fiber level. Further research is needed to draw final conclusions.

Influence of motoneuron firing synchronization on SEMG characteristics in dependence of electrode position.

The frequency content of the surface electromyography (SEMG) signal, expressed as median frequency (MF), is often assumed to reflect the decline of muscle fiber conduction velocity in fatigue. MF also decreases when motor unit firings synchronize, and we hypothesized that this effect can explain the electrode-dependent pattern in our previous recordings from the trapezius muscle. An existing motoneuron (MN) model describes the afterhyperpolarization following a spike as an exponential function on which membrane noise is superimposed. Splitting the noise into a common and an individual component extended the model to a MN pool with a tunable level of firing synchrony. An analytical volume conduction model was used to generate motor unit action potentials to simulate SEMG. A realistic level of synchrony decreased the MF of the simulated bipolar SEMG by approximately 30% midway between endplate position and tendon but not above the endplate. This is in accordance with experimental data from the biceps brachii muscle. It was concluded that the pattern of decrease of MF during sustained contractions indeed reflects MN synchronization.

pH heterogeneity in tibial anterior muscle during isometric activity studied by (31)P-NMR spectroscopy.

The occurrence of pH heterogeneity in human tibial anterior muscle during sustained isometric exercise is demonstrated by applying (31)P-nuclear magnetic resonance (NMR) spectroscopy in a study of seven healthy subjects. Exercise was performed at 30 and 60% of maximal voluntary contraction (MVC) until fatigue. The NMR spectra, as localized by a surface coil and improved by proton irradiation, were obtained at a high time resolution (16 s). They revealed the simultaneous presence of two pH pools during most experiments. Maximum difference in the two pH levels during exercise was 0.40 +/- 0.07 (30% MVC, n = 7) and 0.41 +/- 0.03 (60% MVC, n = 3). Complementary two-dimensional (31)P spectroscopic imaging experiments in one subject supported the supposition that the distinct pH pools reflect the metabolic status of the main muscle fiber types. The relative size of the P(i) peak in the spectrum attributed to the type II fiber pool increases with decreasing pH levels. This phenomenon is discussed in the context of the size principle stating that the smaller (type I) motor units are recruited first.

Changes in muscle fiber conduction velocity indicate recruitment of distinct motor unit populations.

To obtain more insight into the changes in mean muscle fiber conduction velocity (MFCV) during sustained isometric exercise at relatively low contraction levels, we performed an in-depth study of the human tibialis anterior muscle by using multichannel surface electromyogram. The results show an increase in MFCV after an initial decrease of MFCV at 30 or 40% maximum voluntary contraction in all of the five subjects studied. With a peak velocity analysis, we calculated the distribution of conduction velocities of action potentials in the bipolar electromyogram signal. It shows two populations of peak velocities occurring simultaneously halfway through the exercise. The MFCV pattern implies the recruitment of two different populations of motor units. Because of the lowering of MFCV of the first activated population of motor units, the newly recruited second population of motor units becomes visible. It is most likely that the MFCV pattern can be ascribed to the fatiguing of already recruited predominantly type I motor units, followed by the recruitment of fresh, predominantly type II, motor units.

A thin, flexible multielectrode grid for high-density surface EMG.

Although the value of high-density surface electromyography (sEMG) has already been proven in fundamental research and for specific diagnostic questions, there is as yet no broad clinical application. This is partly due to limitations of construction principles and application techniques of conventional electrode array systems. We developed a thin, highly flexible, two-dimensional multielectrode sEMG grid, which is manufactured by using flexprint techniques. The material used as electrode carrier (Polyimid, 50 microm thick) allows grids to be cut out in any required shape or size. One universal grid version can therefore be used for many applications, thereby reducing costs. The reusable electrode grid is attached to the skin by using specially prepared double-sided adhesive tape, which allows the selective application of conductive cream only directly below the detection surfaces. To explore the practical possibilities, this technique was applied in single motor unit analysis of the facial musculature. The high mechanical flexibility allowed the electrode grid to follow the skin surface even in areas with very uneven contours, resulting in good electrical connections in the whole recording area. The silverchloride surfaces of the electrodes and their low electrode-to-skin impedances guaranteed high baseline stability and a low signal noise level. The electrode-to-skin attachment proved to withstand saliva and great tensile forces due to mimic contractions. The inexpensive, universally adaptable and minimally obstructive sensor allows the principal advantages of high-density sEMG to be extended to all skeletal muscles accessible from the skin surface and may lay the foundation for more broad clinical application of this noninvasive, two-dimensional sEMG technique.

A surface EMG electrode for the simultaneous observation of multiple facial muscles.

With previous surface electromyography (sEMG) electrodes it has been difficult to combine small outer dimensions and secure skin attachment. We resolved this problem by developing a new skin attachment technique that yields firm electrode fixation without requiring an acrylic housing. Consequently, we could reduce the outer electrode dimensions to 4-mm diameter and only 1.5-mm thickness. In a bipolar montage, this electrode allows an inter-electrode distance of 8 mm. This improves measurement selectivity and, because of the small dimensions, makes possible the non-invasive observation of multiple facial muscles with a minimum of obstruction. Our new technique was tested on a group of 11 professional trumpeters. They were instructed to perform a series of muscle-specific facial poses and to play exercises on their instruments while EMG signals were recorded simultaneously from seven different perioral muscles. Although the skin attachment was subjected to high stress during trumpet playing, more than 98% of electrode placements yielded a secure mechanical and electrical connection. Muscle selectivity of the signals recorded during the facial poses was similar to that obtained in a previous investigation using intra-muscular fine-wire electrodes. Crosstalk in the perioral area was estimated to be lower than 25%. The availability of an unobstructive sEMG electrode for simultaneously observing multiple facial muscles opens up a wide range of applications (e.g. in speech research, psychophysiology and orthodontics).

Automatic decomposition electromyography in idiopathic inflammatory myopathies.

Automatic decomposition electromyography (ADEMG) is a commercially available software package with installed reference values that enables the objective measurement of motor unit action potentials (MUAPs). To assess the diagnostic yield of this package in idiopathic inflammatory myopathies (IIM) we performed bicepts brachii ADEMG in 17 patients with polymyositis, dermatomyositis and inclusion body myositis. Results were compared with those in 12 controls, and with the results of conventional EMG of the biceps and other muscles. Decreased mean values for MUAP duration occurred significantly more frequently in IIM patients than in controls; other MUAP characteristics did not differ. In IIM patients, decreased mean amplitude and increased mean number of turns occurred significantly less frequently on ADEMG than did corresponding abnormalities on conventional biceps EMG. Decreased mean values for duration and amplitude, and increased mean values for number of turns were seen significantly less often on ADEMG than corresponding abnormalities on conventional EMG of four different, individually chosen muscles. Overall evaluation of ADEMG resulted in a diagnosis of "possible myopathy" in 1 and "probable myopathy" in 8 patients, whereas overall evaluation of conventional EMG led to a diagnosis "suggestive of IIM" in 13 patients. We conclude that, although measurement of mean MUAP duration might be valuable in IIM diagnosis, our results do not favour the use of biceps brachii ADEMG and the installed reference values for the diagnosis of IIM. We suggest modifications to improve ADEMG's applicability.

Overlap of attention and movement-related activity in lateralized event-related brain potentials.

OBJECTIVE: In tasks that involve lateralized visuospatial attention and a lateralized motor response, the associated brain electrical potentials, i.e. the attention-related N2pc and the lateralized readiness potential, typically overlap at central scalp sites. The manifestation of the N2pc at central electrode sites is commonly attributed to electric volume conduction effects, assuming the N2pc to be generated in occipito-temporal brain areas. We evaluated this explanation in a simulation study. METHODS: Using a forward modeling approach with a realistically shaped volume conduction model, we calculated the range of amplitude ratios between occipital and central electrode sites when a single source is assumed in area V4 or in area TO, at the temporo-occipital convexity. RESULTS: A comparison of the simulated amplitude ratios with reported data indicates that volume conduction effects from the investigated source origins in the occipito-temporal region are insufficient to explain the experimental data. CONCLUSIONS: We conclude that the anterior spread of the N2pc from its occipito-temporal maximum to central electrode sites is probably due to simultaneous attention-related activity in posterior and central brain areas.