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Biomedical subjects

G N Gantchev

Publications and source records attributed to G N Gantchev.

At least 19 recordsLinked to original sources
PubMed1992

Modulation of somatosensory evoked potentials during isometric contraction in relation to hand use.

The aim of the study was to assess the effect of the hand used on the modulation of somatosensory evoked potentials (SEPs) during isometric contraction. SEPs to median nerve stimulation were assessed during dynamic (increase and decrease of force) and hold phases of isometric contractions. The experiments were carried out with the right or left hand. The changes in the SEP amplitude were found to vary with the hand used. When isometric contractions were performed by the right hand the amplitudes of the later components diminished during the dynamic phases, except for the component N140-P190 which increased. The amplitude of N55-P100 diminished during both the dynamic and hold phases. When isometric contractions were performed by the left hand the amplitudes of the early components diminished during the dynamic phases. These results show that the changes in SEP during isometric contraction depend on the hand used.

Adult↗
PubMed1992

Stance does interfere with movement related brain potentials.

Movements of the upper limb are well known to be preceded by electromyographic activity in leg and trunk muscles known as postural preadjustment. Brain potentials preceding the actual commencement of voluntary displacement were also described. The relationship between the occurrence of these two electrophysiological phenomena was investigated by performing two types of movement in three different body positions. Healthy subjects were free standing, sitting and standing with backsupport. When fast forward arm elevation was executed, the preceding Bereitschaftspotential was higher in amplitude at 500 msec before EMG onset and alongside midline scalp leads during sitting. The amplitude at 150 msec, immediately preceding movement, did not show significant differences related to body positions and there was no contralateral dominance. When index finger flexion was performed, it did not yield differences in the Bereitschaftspotential between sitting and standing positions. These findings suggest the existence of two separate stages of the preparatory process and an influence of the muscle mass involved in the stance perturbation during voluntary movement.

Adult↗
PubMed1991

Movement related brain potentials accompanying a non-productive voluntary action in humans.

This study assessed the effects of training and ability to execute a voluntary movement upon movement-related brain potentials (MRBPs). A self-paced thumb flexion initiated a sequence of autotriggered electrical stimuli over the median nerve that caused a twitch opposing the intended thumb extension. The MRBPs had earlier onsets during the first runs of skill acquisition than during later training sessions; they occurred earlier when they preceded a stimulus train than when they preceded a single stimulus; the onset was earlier over the vertex than over the premotor area. MRBP duration was longer during train stimulation when a voluntary effort had to be maintained against tetanic contraction. MRBP amplitude did not reflect task requirements under these experimental conditions.

Adult↗
PubMed1991

Somatosensory evoked potentials during standing posture on different support surface.

Somatosensory evoked potentials in response to stimulation of the posterior tibial nerve at the ankle were recorded during standing on stable ground or on unstable support surface (seesaw) or on support surface short in relation to foot length. During standing on the seesaw and on the short support surface a decrease in the amplitude of the early component (N32-P39) was observed. The amplitude of N49-P58 decreased during standing on the short support surface. The amplitude of the later components (N49-P58; P58-N76; N76-P117) decreased during standing on the seesaw in comparison to that during standing on the stable ground and on the short support surface. Thus, the attenuation of the cerebral potential during standing depend on the conditions for maintenance of posture.

Adult↗
PubMed1991

Spatio-temporal characteristics of SEP to tibial nerve stimulation.

Source derivation techniques have recently been applied to analyse the spatial distribution of brain potentials. Somatosensory evoked potentials (SEPs) to tibial nerve stimulation recorded at 8 sites were analysed by using 2 source derivation techniques: Laplacian and criterion, presented by Kossev et al. (1988). First SEP recorded as a function of time were averaged and thus their basic components P39, N49, P58 were enhanced. In most of the subjects these components had maximal amplitudes at Ez rather than Cz. These sites were taken as nodes and the source derivation techniques were applied for all SEP components. Spatial derivatives (potential gradients) between adjacent leads were also computed. The current density at the nodes for different time points of SEP was obtained using Laplacian technique. For the chosen electrodes places and subjects it was found that the maximal source density did not correspond always to the SEP peak amplitudes. According to the criterion used it might be supposed that the generators of P39, N49 and P58 were located close to the scalp surface. Because of the significant individual variety of the results obtained more research is needed for choosing the appropriate scalp sites and for the contribution of the base line when measuring the SEP amplitudes.

Adult↗
PubMed1990

Effect of body and foot positions on the somatosensory evoked potentials.

Somatosensory evoked potentials in response to stimulation of the posterior tibial nerve at the ankle were recorded during sitting and standing with variable foot positions. During standing a decrease in the amplitude of the early positive component was observed. The deviation of the foot from a horizontal position was associated with an increase in the amplitude of the early negative component. The combined influence of body and foot positions showed a decrease in the amplitude of both early and late components. The standing position induced changes in more components than the varied foot positions. This suggests that maintenance of the standing posture is a more complex task than the maintenance of the foot position itself.

Adult↗
PubMed1987

Postural adjustments associated with a fast voluntary arm movement at various foot positions.

The aim of the study was to further investigate postural adjustments associated with a fast voluntary arm elevation at various foot positions. The subjects stood upright on a force platform and participated in 3 sessions according to foot position: horizontal position, dorsal and plantar flexion of 6 degrees. EMG activity of anterior deltoid muscle (DA) and of ipsilateral and contralateral soleus (SD, SS), biceps femoris (BFd, BFs) and ipsilateral anterior tibial (TAd) muscles, initial acceleration (Acc) and stabilogram change (Sg) were recorded. The results showed that postural preadjustment was provided by a co-contraction of SOLd and TAd, preceding 31-37 ms the time onset of DA, observed in all foot positions. Time onset of EMG activity in BFd and BFs followed 105-134 ms after EMG onset of DA, showing some dependence on foot position. The co-contraction of SD and TAd presumably has postural function with the role to maintain stability of the body against the increased postural requirements associated with a very fast arm movement. The preceding activity of BFd and BFs and their participation in postural preadjustments is probably not necessary, since the co-contraction of SD and TAd has ensured fixation of the ankle joint.

Adult↗
PubMed1986

Muscular synergies during different conditions of postural activity.

With the view to studying the muscular synergies in maintaining of upright posture EMG activity of soleus (SOL), tibialis anterior (TA), peroneus longus (PL), biceps femoris (BF), vastus lateralis (VL) and gluteus medius (GM) muscles during easy standing, voluntary leaning and induced body oscillations in frontal and sagital plane were recorded. The data showed that during easy standing and during small external disturbances the vertical posture was maintained mainly by the activity of SOL (triceps surae muscle) which was modulated by the modification of the external force field. TA remained silent. PL muscle was slightly active as a synergist of SOL. The pattern of muscular synergies during fast voluntary leaning and induced body oscillations differed as compared to the muscular synergies at easy standing. The activity of all muscles studied increased and became continuous with exception of that of the TA. The latter was silent but was involved earlier when gravity center passed behind its projection. VL being a synergist of TA during slow sway became continuously active as a synergist of SOL during the fast sway. Typical changes in muscular synergies appeared when induced body oscillations were imposed during horizontal and rotational movements of the platform. The various patterns of muscular synergies during pronation and supination of the foot depended on the knee joint angle. The amplitude of body leaning forward and backward and its variability as well as the plane of the lateral sway depended on the biomechanical variables and related motor programs for postural control.

Electromyography↗
PubMed1986

Somatosensory evoked potentials upon electrical stimulation triggered by voluntary movement.

The autotriggered signal was presented with a varying delay. SEP obtained without movement of the thumb (control series), SEP of autotriggered stimuli and movement-related brain potentials (MRBP) without stimuli were registered. The EEG was recorded from C3 + 2 and C3 = 2. The following results were obtained: the amplitude of all components diminished when the stimulation was autotriggered compared to the control series; the suppression was most pronounced with a stimulus delay about 150-200 ms; the effect of the suppression did not disappear up to 500 ms after the beginning of the movement.

Brain↗
PubMed1986

Motor unit activity during different functional states of the neuromuscular system.

The study showed that the mean interspike interval (ISI) and the variability of ISIs increased with fatigue for both low and high threshold motor units (MUs). The recruitment threshold tension decreased for both the low and high threshold MUs while the recruitment threshold in % of maximal voluntary contraction changed selectively because of decrease of muscle force. During ischaemia after an initial decrease of mean ISI and ISIs' variability there was an essential increase of variability of ISIs at almost the same mean ISI followed by a considerable final increase of both parameters. MUs' recruitment thresholds lowered with ischaemia more rapidly in the beginning and especially in the final stage of ischaemia.

Action Potentials↗
PubMed1984

Changes in the Bereitschaftspotential due to subsequent voluntary autotriggered stimulus.

The Bereitschaftspotential (BP) has been studied in two different tasks: simple voluntary isometric flexion/opposition of the thumb and same movement, followed by a mild electrical shock over the median nerve, autotriggered by the movement itself. When the two tasks were organized in separate series, the subject being told about the outcome of the movement beforehand, it was found that: The BP started later; Its amplitude was smaller; The maximum negatively (N2) was smaller, when expecting electrical shock compared to simple voluntary contraction. The decrease was more pronounced over the postcentral areas. When the sequence of the autotriggered electrical shocks was equiprobably intermixed with the simple voluntary presses in single series it was found that virtually no difference existed between different potential measures. This differential influence of equal physical requirements with different subjective sequels upon the potentials related to voluntary movements implied the preferential role of the subject's attitude and state of preparedness upon the brain potentials. Both the elevated stress level and the selective suppression in specific cortical areas might take place in tasks with predictable unpleasant electrical shock.

Action Potentials↗
PubMed1983

Changes in the H-reflex amplitude with different characteristics of the preceding conditioned stimulus.

The post-stimulus inhibition of the H-reflex was studied in human subjects with the following experimental sets: Presentation of a pair of stimuli, with intensity of the conditioned stimulus, S1, of 30% Hmax, and with varying intensities of the test stimulus, S2, from threshold to maximal values. Varying intensity of S1 from threshold values, 30% Hmax, 50% Hmax, and varying intensity of S2 from threshold to maximal values, for the H2-reflex-curve to be obtained. Varying intensity of S1 from 0.9 to 1.15-1.20 threshold and constant intensity of S2 of 30-40% Hmax. In the first two experiments the curve "intensity of S1/H2-reflex amplitude" at different intensities of S1 and different delays of S2 has been investigated. In the third experiment the H2-reflex recovery with diminishing the S1 to subthreshold values has been investigated. The results have shown: H2-reflex was strongly suppressed even with threshold values of S1. H2-reflex-curve after a conditioned stimulus was significantly lower to the H-reflex-curve after a single stimulus. The recovery of the H-reflex amplitude after S1 was observed with subthreshold values of S1. The possible role of the presynaptic inhibition for the suppression of the H1-reflex with the presentation of a pair of stimuli has been considered.

Adult↗
PubMed1983

Synchronization of movement-related cerebral potentials for averaging.

A software-oriented technique is proposed for synchronization of the individual movement-related cerebral potentials on the basis of the first derivative of the mechanogram. All procedures are computer performed using neither trigger device nor time-reverse averaging later on. The main advantages of the technique over the trigger approach are demonstrated when determining onset of both commencement and termination of the sustained contraction. Besides the objectivity and high accuracy, the proposed technique gives an additional possibility to determine movement parameters together with cerebral potential characteristics and to sort out the movement-related EEG records in greater details.

Brain↗
PubMed1981

Influence of the visual feedback for some parameters of the body oscillations on their stability.

In accordance with the hypothesis that in addition to the generalized information from the visual feedback about the body oscillations, the information from the different parameters of the oscillations may also play a stabilizing role for the vertical posture of the body and its balance, studies were carried out on the influence of the parameters of body position, velocity of the body oscillations and accelerations on body stability. The results have shown that the visual feedback about the position of the body and about the velocity of the oscillations leads to stabilization of its balance. Visual feedback about the accelerations of the body oscillations also has a stabilizing action on the vertical posture and equilibrium. Its influence did not prove greater than the visual feedback about the velocity of body oscillations. Repeated recordings of the body oscillations under the experimental conditions manifested a slight initial training effect, expressed in a decrease of the total way of the body oscillations.

Biofeedback, Psychology↗