Modulation of afterhyperpolarization evoked by doublets and increasing number of stimuli in rat motoneurons.

The authors studied the influence of variable stimulation patterns on parameters of afterhyperpolarization (AHP) in rat spinal motoneurons using intracellular recording of antidromic action potentials. The action potentials analyzed were evoked by either (a) a single stimulus or pair of stimuli with gradually increasing interpulse intervals (IPI) of 5-20 ms or (b) an increasing number of stimulus pulses, from 1 to 4, at a constant IPI. It was demonstrated that modulations of AHP parameters after 2 pulses depended on the IPI, whereas the most significant changes in these parameters were found after the application of 2 or 3 stimuli in a series. The authors propose that changes in the AHP parameters have functional significance for motoneuronal firing pattern and therefore for motor unit force development during tetanic contractions.

Beta-range EEG-EMG coherence with isometric compensation for increasing modulated low-level forces.

Corticomuscular synchronization has been shown to occur in beta (15-30 Hz) and gamma range (30-45 Hz) during isometric compensation of static and dynamic (periodically modulated) low-level forces, respectively. However, it is still unknown to what extent these synchronization processes in beta and gamma range are modified with increasing modulated force. We addressed this question by investigating the corticomuscular coherence (CMC) between the electroencephalogram (EEG) and electromyogram (EMG) from the first dorsal interosseus muscle (FDI) as well as the cortical and muscular spectral power during a visuomotor task where different levels of a dynamic (modulated) force were used. Seven healthy right-handed female subjects compensated dynamic forces at 8, 16, and 24% of the maximal voluntary contraction (MVC) isometrically with their right index finger. Under the three conditions investigated, we found a broad-band CMC comprising both beta and gamma range and peaking at approximately 22 Hz within the beta band. This broad-band coherence increased linearly with higher force level. A separate analysis of the gamma range CMC did not show significant modulation of the CMC by the force levels. EEG and EMG spectral power did not show any significant difference among the three force conditions. Our results favor the view that the function of beta range CMC is not specific for low-level static forces only. The sensorimotor system may resort to stronger and also broader beta-range CMC to generate stable corticospinal interaction during increased force level, as well as when compensating for dynamic modulated forces. This finding re-enforces the importance of the beta-range EEG-EMG coherence in sensorimotor integration.

Role of proprioception and vision in handwriting.

The objective of this study is to better understand the role of proprioception in handwriting and test earlier conclusions stating that the automated shaping of letters was not impaired by the removal of visual control in deafferentation. To this aim we compared the performance of the deafferented patient GL, who suffers from a complete loss of cutaneous and proprioceptive sensation, with that of eight healthy age- and sex-matched subjects. The word "Parallele", written within a short sentence with and without visual control, was quantified using a digital writing tablet. Three of the 13 analyzed parameters were strikingly different in patient GL compared to healthy subjects, both with and without vision: increase of number of pen touches, increase in number of inversions in velocity, and decrease of mean stroke frequency. The changes in these three parameters indicate a strong impairment in automated behaviour in the absence of proprioception and touch. This impairment is also supported by the significantly longer movement duration, which is also significantly increased by the removal of visual control. The present study provides for the first time a quantification of handwriting in a completely deafferented patient and reveals the central role of proprioception for the storage, updating, and maintenance of skilled motor programs. The fact that the deficits are already present with visual feedback suggests that the role of vision in handwriting is only secondary.

Absence of gamma-range corticomuscular coherence during dynamic force in a deafferented patient.

Recently, we studied corticomuscular coherence (CMC) in a visuomotor task and showed for the first time gamma-range (30-45 Hz) CMC during isometric compensation of a periodically modulated dynamic force. We speculated that for the control of such forces, the sensorimotor system resonates at gamma-range frequencies to rapidly integrate the visual and proprioceptive information and produce the appropriate motor command. In this study, we tested the role of the proprioceptive afferent feedback on gamma-range CMC by comparing the deafferented patient GL to six age- and sex-matched subjects during the performance of a visuomotor force task consisting of isometric compensation of static and dynamic forces applied on the finger. Patient GL presented no significant gamma-band CMC during dynamic force. Instead, she had only beta-range CMC as in the static force condition; concurrently, her performance was significantly worse than that of the controls in both conditions. This gives support to the conclusions of our previous paper and suggests that proprioceptive information is mandatory in the genesis of gamma-band CMC during the generation and control of dynamic forces.

Handwriting performance in the absence of visual control in writer's cramp patients: initial observations.

BACKGROUND: The present study was aimed at investigating the writing parameters of writer's cramp patients and control subjects during handwriting of a test sentence in the absence of visual control. METHODS: Eight right-handed patients with writer's cramp and eight healthy volunteers as age-matched control subjects participated in the study. The experimental task consisted in writing a test sentence repeatedly for fifty times on a pressure-sensitive digital board. The subject did not have visual control on his handwriting. The writing performance was stored on a PC and analyzed off-line. RESULTS: During handwriting all patients developed a typical dystonic limb posture and reported an increase in muscular tension along the experimental session. The patients were significantly slower than the controls, with lower mean vertical pressure of the pen tip on the paper and they could not reach the endmost letter of the sentence in the given time window. No other handwriting parameter differences were found between the two groups. CONCLUSION: Our findings indicate that during writing in the absence of visual feedback writer's cramp patients are slower and could not reach the endmost letter of the test sentence, but their level of automatization is not impaired and writer's cramp handwriting parameters are similar to those of the controls except for even lower vertical pressure of the pen tip on the paper, which is probably due to a changed strategy in such experimental conditions.

Oscillatory cortical activity during a motor task in a deafferented patient.

Little is known about the influence of the afferent peripheral feedback on the sensorimotor cortex activation. To answer this open question we investigated the alpha and beta band task-related spectral power decreases (TRPow) in the deafferented patient G.L. and compared the results to those of six healthy subjects. The patient has been deafferented up to the nose for 24 years but her motor fibers are unaffected and she can perform complex motor tasks under visual control. We recorded EEG (58 scalp positions) as well as the exerted force during a visuomotor task. The subjects had to maintain in precision grip an isometric force at 15% of the maximal voluntary contraction. In the patient we found a significantly higher alpha band spectral power during rest and larger alpha TRPow decreases during the motor task when compared to the healthy subjects' data. In contrast, we did not observe any significant differences between patient and controls for the beta band TRPow. The results indicate an altered functional alpha band network state in the patient, probably due to the chronic deafferentation leading to a deep 'idling' state of the contralateral sensorimotor area.

Is the movement-evoked potential mandatory for movement execution? A high-resolution EEG study in a deafferented patient.

During simple self-paced index finger flexion with and without visual feedback of the finger, we compared the movement-evoked potentials of the completely deafferented patient GL with those of 7 age-matched healthy subjects. EEG was recorded from 58 scalp positions, together with the electromyogram (EMG) from the first dorsal interosseous muscle and the movement trace. We analyzed the movement parameters and the contralateral movement-evoked potential and its source. The patient performed the voluntary movements almost as well as the controls in spite of her lack of sensory information from the periphery. In contrast, the movement-evoked potential was observed only in the controls and not in the patient. These findings clearly demonstrate that the movement-evoked potential reflects cutaneous and proprioceptive feedback from the moving part of the body. They also indicate that in absence of sensory peripheral input the motor control switches from an internal "sensory feedback-driven" to a "feedforward" mode. The role of the sensory feedback in updating the internal models and of the movement-evoked potential as a possible cortical correlate of motor awareness is discussed.

Electroencephalographic spectral power in writer's cramp patients: evidence for motor cortex malfunctioning during the cramp.

We investigated cortical activation as reflected in task-related spectral power (TRPow) changes in 8 writer's cramp patients during writing on a digital board and during isometric contraction and compared them to those of 8 age-matched healthy subjects. Scalp EEG was recorded over the contralateral primary sensorimotor area (SM1(c)), and from the ipsilateral sensorimotor area (SM1(i)). The electromyogram (EMG) was recorded from the Extensor Digitorum Communis (Extensor), Flexor Digitorum Superficialis (Flexor), and First Dorsal Interosseous (FDI) muscles. We analyzed (1) handwriting performance, (2) changes in the TRPow confined to alpha and beta band, and (3) the EMG spectral power during both tasks, writing and isometric contraction. During writing, all patients developed writer's cramp. The handwriting in writer's cramp patients was associated with significantly less reduction of the beta-range TRPow and lower frequency of the TRPow reduction compared to controls. No significant differences between patients and controls for the alpha band TRPow reduction during handwriting were observed. During writing, the patients showed higher EMG spectral power than the controls but this difference was at the border of significance. The present results indicate disorder in the motor execution system, in writer's cramp patients, associated with impaired functional beta-network state of the contra- and ipsilateral sensorimotor cortices, most probably due to inadequate modulation of the intracortical inhibition associated with writing.

Cortico-muscular coupling in a patient with postural myoclonus.

We investigated the cortico-muscular coherence in a patient with posturally induced cortically originating negative myoclonus. We recorded simultaneously 50 channels EEG and EMG from quadriceps and biceps femoris muscles of the left upper leg. Three experimental conditions were investigated with the patient in a seated position: (i) recording during rest (Rest), (ii) recording while the patient had to hold his left leg horizontally stretched out (Postural), and (iii) recording while the patient had to hold his left leg horizontally stretched out against a vertical force (Postural against force). Coherence, phase difference and cumulant density were computed as indicators for cortico-muscular coupling. The cortical component preceding the silent period was shown by averaging and was reconstructed. During postural and postural against force conditions, the EEG over the vertex was significantly coherent with EMG, in alpha (7-15 Hz) and beta range (15-30 Hz). The strongest coherence peak was at 21 Hz. No high-frequency coherence was observed. The phase difference and the cumulant density estimate corresponded to a 32 ms time lag between motor cortex and muscles, with EEG leading. The broadening of the coherence spectrum at which the motor cortex drives the muscles together with the excessive coherence levels and the giant SEP could reflect the hyperexcitability of the sensorimotor cortex. The frequency content of the coherence may be characteristic for this type of myoclonus. The results lend support to the view that the frequency analysis may have some diagnostic potential in cortical myoclonus.

Activation of cortical areas in music execution and imagining: a high-resolution EEG study.

Neuroimaging studies have shown that execution of a musical sequence on an instrument activates bilateral frontal opercular regions, in addition to bilateral sensorimotor and supplementary motor areas. During imagining activation of the same areas without primary sensorimotor areas was shown. We recorded EEG from 58 scalp positions to investigate the temporal sequence and the time course of activation of these areas while violin players prepared to execute, executed, prepared to imagine, or imagined a musical sequence on a violin. During the preparation for the sequence in three of seven musicians investigated the bilateral frontal opercular regions became active earlier than the motor areas and in one of them simultaneously with the motor areas. In two of the musicians a rather variable pattern of activation was observed. The frontal opercular regions were also strongly involved throughout the period of music execution or imagining. The supplementary motor area was involved in both preparation for the sequence and during execution and imagining of the sequence. The left primary sensorimotor area was involved in the preparation and termination of the musical sequence for both execution and imagining. The right sensorimotor area was strongly involved in the preparation for and during the execution of the sequence. We conclude that the bilateral frontal opercular regions are crucial in both preparation for and during music execution and imagining. They may have "mirror neurone" properties that underlie observation or imagining of one's own performance. The motor areas are differentially activated during the preparation and execution or imagining the sequence.