Influence of amplitude cancellation on the simulated surface electromyogram.

The purpose of the study was to quantify the influence of selected motor unit properties and patterns of activity on amplitude cancellation in the simulated surface electromyogram (EMG). The study involved computer simulations of a motor unit population with physiologically defined recruitment and rate coding characteristics that activated muscle fibers whose potentials were recorded on the skin over the muscle. Amplitude cancellation was quantified as the percent difference in signal amplitude when motor unit potentials were summed before and after rectification. The simulations involved varying the level of activation for the motor unit population, the recording configuration, the upper limit of motor unit recruitment, peak discharge rates, the amount of motor unit synchronization, muscle fiber length, the thickness of the subcutaneous tissue, and the motor unit properties that change with advancing age. The results confirmed a previous experimental report (Day SJ and Hulliger M, J Neurophysiol 86: 2144-2158, 2001) that amplitude cancellation in the surface EMG can reach 62% at maximal activation. A decrease in the range of amplitudes of the motor unit potentials, as can occur during fatiguing contractions, increased amplitude cancellation up to approximately 85%. Differences in the amount of amplitude cancellation were observed across all simulated conditions, and resulted in substantial changes in the absolute magnitude of the EMG signal. The most profound factors influencing amplitude cancellation were the number of active motor units and the duration of the action potentials. The effects of amplitude cancellation were minimal (<5%) when the EMG amplitude was normalized to maximal values, with the exception of variations in peak discharge rate and recruitment range, which resulted in differences up to 17% in the normalized EMG signal across conditions. These results indicate the amount of amplitude cancellation that can occur in various experimental conditions and its influence on absolute and relative measures of EMG amplitude.     

Electrode failure: tissue, electrical, and material responses

The development of invasive, rehabilitative neuroprosthetics for humans requires reliable neural probes that are capable of recording large ensembles of neurons for a long period of time. Recent advances in the development of neuroprosthetics in animals and humans have shown that communication and control can be directly derived from the central nervous system (CNS) for restoring lost motor ability. This proof of concept has opened the possibility of new therapies for the millions of individuals suffering from neurological disorders of the nervous system. The success of these therapies hinges on the ability to reliably access the relevant signals from the brain with high quality for the lifetime of the patient. As a result, research has focused on the cascade of events that follow chronic implantation of microelectrodes and temporal degradation in the signal and electrode quality: signal-to-noise ratio, noise floor, peak amplitude, and neuronal yield. Implanted microelectrodes have been reported to suffer from time-dependent degradation in signal quality due to unknown issues related to tissue interfaces.     

Identification of movement-related cortical potentials with optimized spatial filtering and principal component analysis

We present a method for detecting movement intention from multichannel electroencephalographic (EEG) recordings. Movement intention is expressed as a slow negative deflection in amplitude of the EEG signal recorded above the motor cortex. This deflection is known as a movement-related cortical potential (MRCP). Detection of movement intention implies discrimination between MRCPs and noise. The signal-to-noise ratio of MRCPs was improved by an optimized spatial filter. Features were extracted with principal component analysis or locality preserving projections from the spatially filtered signals and classification between MRCPs and noise was performed with a k-nearest neighbors algorithm, modified by adjusting the decision rule to improve specificity, and a support vector machine approach. In one representative subject the sensitivity and specificity in detection were in the range 80–90% and 98–99.5%, respectively. The method seems promising for the development of asynchronous brain–computer interfaces (BCIs) based on MRCPs.     

The ontogeny of the transepidermal potential difference in frog embryos

The potential difference across the epithelia of embryos and larvae of the frog, Xenopus laevis, has been measured using glass microelectrodes. Blastocoel stage embryos had small potentials of less than 10 mV (inside positive), a value that is within the range of tip potential error. This potential began to increase during neurulation, reaching a peak of about +60 mV by the time that spontaneous movement occurred. The potential then declined as the beginning of metamorphosis approached. A substantial fraction of the peak potential appears to be due to sodium uptake from the external medium, as is the case in the adult frog.     

Kinetics of oxygen consumption after a single flash of light in photoreceptors of the drone (Apis mellifera)

The time course of the rate of oxygen consumption (QO2) after a single flash of light has been measured in 300-micrometers slices of drone retina at 22 degrees C. To measure delta QO2(t), the change in QO2 from its level in darkness, the transients of the partial pressure of O2 (PO2) were recorded with O2 microelectrodes simultaneously in two sites in the slice and delta QO2 was calculated by a computer using Fourier transforms. After a 40-ms flash of intense light, delta QO2, reached a peak of 40 microliters O2/g.min and then declined exponentially to the baseline with a time constant tau 1 = 4.96 +/- 0.49 s (SD, n = 10). The rising phase was characterized by a time constant tau 2 = 1.90 +/- 0.35 s (SD, n = 10). The peak amplitude of delta QO2 increased linearly with the log of the light intensity. Replacement of Na+ by choline, known to decrease greatly the light-induced transmembrane current, caused a 63% decrease of delta QO2. With these changes, however, the kinetics of delta QO2 (t) were unchanged. This suggest that the recovery phase is rate-limited by a single reaction with apparent first-order kinetics. Evidence is provided that suggests that this reaction may be the working of the sodium pump. Exposure of the retina to high concentrations of ouabain or strophanthidin (inhibitors of the sodium pump) reduced the peak amplitude of delta QO2 by approximately 80% and increased tau 1. The increase of tau 1 was an exponential function of the time of exposure to the cardioactive steroids. Hence, it seems likely that the greatest part of delta QO2 is used for the working of the pump, whose activity is the mechanism underlying the rate constant of the descending limb of delta QO2 (t).     

Depolarization-contraction coupling in short frog muscle fibers. A voltage clamp study

Short muscle fibers (1.5 mm) were dissected from hindlimb muscles of frogs and voltage clamped with two microelectrodes to study phenomena related to depolarization-contraction coupling. Isometric myograms obtained in response to depolarizing pulses of durations between 10 and 500 ms and amplitudes up to 140 mV had the following properties. For suprathreshold pulses of fixed duration (in the range of 20-100 ms), the peak tension achieved, the time to peak tension, and contraction duration increased as the internal potential was made progressively more positive. Peak tension eventually saturates with increasing internal potentials. For pulse durations of greater than or equal to 50 ms, the rate of tension development becomes constant for increasing internal potentials when peak tensions become greater than one-third of the maximum tension possible. Both threshold and maximum steepness of the relation between internal potential and peak tension depend on pulse duration. The relation between the tension-time integral and the stimulus amplitude-duration product was examined. The utility of this relation for excitation-contraction studies is based on the observation that once a depolarizing pulse configuration has elicited maximum tension, further increases in either stimulus duration or amplitude only prolong the contractile response, while the major portion of the relaxation phase after the end of a pulse is exponential, with a time constant that is not significantly affected by either the amplitude or the duration of the pulse. Hence, the area under the tension-response curve provides a measure of the availability to troponin of the calcium released from the sarcoplasmic reticulum in response to membrane depolarization. The results from this work complement those obtained in experiments in which intramembrane charge movements related to contractile activation were studied and those in which intracellular Ca++ transients were measured.     

Effect of forward masking on evoked potentials of auditory cortex in guinea pigs

Thresholds of the event-related potentials (ERPs) appearance were measured for one stationary and four moving auditory images presented in silence or under forward masking conditions. The difference between thresholds in silence and after noise masker was considered as masking level. Under the forward masking, the amplitude of the ERP to the first click in the test series decreased in guinea pig auditory cortex. Masking level decreased with the time lag between signal and masker and didn't depend on the fused auditory image localization that corresponded to the first click in different test signals. This fact can support the hypothesis that for the long test signals the initial part can be masked more than the final one. The ERPs amplitude to next clicks in test series depended on interaction of two factors: forward masking in the "masker-signal" system and interaction of separate ERPs in the series evoked by the test signal.     

Dynamics of acoustical evoked potentials on a background of habituation to verbal stimulus in persons with different individual anxiety levels

Changes in the amplitude and the latencies of peak P300 of acoustical evoked potential on a background of the verbal stimulus novelty loss were used for determination of the brain activity specifity in individuals with various anxiety levels. The increase of the peak P300 latency of acoustical evoked potential on a background of trace processes arising after habituation was determined. This parameter did not depend on an anxiety level. However, the latency change depended on the attentional level to a submitted signal. The peak P300 amplitude in the right temporal cortex field was connected with the anxiety level--there was a growth of amplitude on a habituation background with the low anxious subjects, and with the highly anxious people the changes of this parameter were not observed. The anxiety level has rendered influence to dehabituation reaction. Dehabituation is observed with the low-anxiety subjects.     

A simple exploratory algorithm for the accurate and fast detection of spontaneous synaptic events

We have developed a program for the fast and accurate detection of spontaneous synaptic events. The algorithm identifies each event of which the slope and amplitude which meet criteria. The significant feature of this algorithm is its stepwise and exploratory search for the onset and the peak points. During the first step, the program employing the algorithm makes a rough estimate of the candidate for a synaptic event, and determines a 'temporary' onset data point. The next step is the detection of the true onset data point and 'temporary' peak data point, which probably exist several points after the temporary onset data point. The third step is a backward search to detect the true peak data point. The final step is to check whether the amplitude of the detected event exceeds the threshold. This stepwise and shuttlewise search allows for the accurate detection of the peak points. Using this program, we succeeded in detecting an increased frequency and amplitude of spontaneous excitatory postsynaptic currents in chick cerebral neurons following the application of 12-O-tetradecanoyl-phorbol-13-acetate (TPA). In addition, we demonstrated that the program employing the algorithm was able to be used for the detection of extracellular action potentials.     

Computer-assisted evaluation of surface electromyograms of 8 different arm muscles]

The aim of the present study was, via digitization of the EMG signals and processing them in the computer, to make possible effective evaluation of electromyographically detectable modifications of the activity of the forearm muscles developing during wrist movements under defined conditions of loading. The search for the optimal measuring parameter was of particular importance. In groups of test subjects the electric voltage potentials generated by 8 arm muscles were measured simultaneously on the skin and evaluated using suitable variables (integral, RMS [root mean square] value, mean amplitude, summed amplitude, amplitude peak count [peaks], zero crossings) with the aid of a PC and a computer program specially developed for the task. On evaluation it was found that the parameters reacting sensitively mainly to amplitude size--integral, RMS value, mean amplitude height and summed amplitude--reflected, with gradually decreasing clarity, the activities of each muscle. The correlation among the first three parameters was almost linear. The parameters sensitive primarily to frequency modulations, such as peaks and number of zero crossings, yielded little information under the load applied. They did not adequately reflect the extension and flexion of the hand.     

Electrophysiological analysis of retrieval of conditioned taste aversion. Physiological techniques and data processing.

New techniques suitable for electrophysiological investigation of retrieval of conditioned taste aversion (CTA) were developed. The gustatory discrimination apparatus consisted of two parallel drinking spouts (20 mm apart) equipped with photoelectric lick sensors. The spouts contained water or the aversive fluid (0.15 M LiCl), respectively, and their position could be rapidly (50 ms) interchanged with a reversive electromotor. Licking at either spout and the position of the fluids were recorded on one channel of a tape recorder. Unit activity was picked up with tungsten microelectrodes (40 micrometers) inserted into the brain of a freely moving rat with a head-mounted microdrive system (weight 2 g), fixed to an implanted guiding cannula. The electrode was connected through a head-carried FET signal follower to a wide band integrated circuit amplifier and the unit activity was recorded in the other channel of the tape recorder. The records were evaluated using an off-line computer program (LINC 8) consisting of a spike detection subroutine followed by amplitude histogram analysis. The mean (M) and SD values were computed for uni- or bimodal amplitude distributions of the principal spike components. Spike falling within the M+/-2 SD range of the selected parameter were identified as single units and used for construction of post stimulus histograms triggered by licking or by spout switching.     

Laser flow cytometric light scatter and fluorescence pulse width and pulse rise-time sizing of mammalian cells.

In laser flow cytometry, an increasingly popular technique of analytical cytology, quantitative measurements of interest include cell and nuclear diameters. Electronic circuitry for a new cell sizing technique has been developed which measured the time that signal pulses from either fluorescence or light scatter sensors exceed a preset constant fraction of the peak signal amplitude (pulse width) or the time that it takes a signal to rise between constant fractions of the peak signal amplitude on the rising side of the pulse (pulse rise-time). These pulse width or pulse rise-time measurements were related to cell or nuclear diameters and were used in combination to determine nuclear size to cell size ratios. This method of sizing was found to be independent of fluorescent or light-absorbing stain intensity, linearly related to cell or nuclear diameter, and capable of resolving small diameter differences.     

Streaming potentials maps are spatially resolved indicators of amplitude, frequency and ionic strength dependant responses of articular cartilage to load.

Streaming potential distributions were measured on the surface of articular cartilage in uniaxial unconfined compression using a linear array of microelectrodes. Potential profiles were obtained for sinusoidal and ramp/stress-relaxation displacements and exhibited dependencies on radial position, sinusoidal amplitude and frequency, time during stress relaxation, and on ionic strength. The measurements agreed with trends predicted by biphasic and related models. In particular, the absolute potential amplitude was maximal at the disk center, as was the predicted fluid pressure and the potential gradient (the electric field) was seen to be maximal at the disk periphery, as was the predicted fluid velocity. We also observed a similarity between non-linear behavior of streaming potential amplitude and load amplitude with respect to sinusoidal displacement amplitude. Taken together, these results support many of the phenomena concerning relative fluid-solid movement and fluid pressurization predicted by biphasic and related models, and they indicate the general utility of spatially resolved measurements of streaming potentials for the investigation of electromechanical phenomena in tissues. For example, these streaming potential maps could be used to non-destructively diagnose cartilage extracellular matrix composition and function, as well as to quantify spatially and temporally varying physical signals in cartilage that can induce cellular and extracellular biological responses to load.     

Single-unit pH-sensitive double-barreled microelectrodes for extracellular use

The purpose of this study is to systematically describe the construction of pH-sensitive double-barreled microelectrodes for extracellular use. The most important advantages of these microelectrodes are as follows: the reference and the pH barrels are next to each other, and therefore the measured pH is not affected by asymmetric or slowly spreading direct current potential. The diameter of the tip of the microelectrodes is between 7 and 35 micron. These pH-sensitive microelectrodes are generally stable and Nernstian. They can be used repeatedly both in vivo and in vitro to measure tissue extracellular fluid pH. Some applications are described.     

Computer Dependence of An Improvement of The Cell Neighbour-Table Method For Short-Range Potentials

Abstract

A comparative test is presented for molecular dynamics, MD, computer simulation between the original Cell Neighbour-Table method, CNT, and a later development, the Link-Cell Neighbour-Table, LCNT. The test was simultaneously carried out on a vector Convex 210 computer and on a scalar Pentium 120. The comparison of the two methods for very large systems (up to 100000 particles) and for two short-range pair potentials (Weeks-Chandler-Andersen, WCA, and Lennard-Jones, LJ) showed that on the Convex, in vectorial mode, the LCNT method is about 25% more efficient in time than the CNT method for the shorter-range WCA potential, the difference being unnoticeable (less than 5%) for the LJ potential. However, when the scalar mode is on in the Convex, the difference disappears. In the Pentium, which was found to be systematically faster than the Convex even in vectorial mode, no significant difference of any kind between the both methods was found, with the either of the two potentials.     

Intra-Individual Variability of Surface Electromyography in Front Crawl Swimming

The variability of electromyographic (EMG) recordings between and within participants is a complex problem, rarely studied in swimming. The importance of signal normalization has long been recognized, but the method used might influence variability. The aims of this study were to: (i) assess the intra-individual variability of the EMG signal in highly skilled front crawl swimmers, (ii) determine the influence of two methods of both amplitude and time normalization of the EMG signal on intra-individual variability and of time normalization on muscle activity level and (iii) describe the muscle activity, normalized using MVIC, in relation to upper limb crawl stroke movements. Muscle activity of rectus abdominis and deltoideus medialis was recorded using wireless surface EMG in 15 adult male competitive swimmers during three trials of 12.5 m front crawl at maximal speed without breathing. Two full upper limb cycles were analyzed from each of the swimming trials, resulting in six full cycles used for the intra-individual variability assessment, quantified with the coefficient of variation (CV), coefficient of quartile variation (CQV) and the variance ratio (VR). The results of this study support previous findings on EMG patterns of deltoideus medialis and rectus abdominis as prime mover during the recovery (45% activity relative to MVIC), and stabilizer of the trunk during the pull (14.5% activity) respectively. The intra-individual variability was lower (VR of 0.34–0.47) when compared to other cyclic movements. No meaningful differences were found between variability measures CV or VR when applying either of the amplitude or the time normalization methods. In addition to reporting the mean amplitude and standard deviation, future EMG studies in swimming should also report the intra-individual variability, preferably using VR as it is independent of peak amplitude, provides a good measure of repeatability and is insensitive to mean EMG amplitude and the degree of smoothing applied.     

Analysis of fetal breathing in real time using a microprocessor

A system has been designed using an inexpensive microprocessor to analyze fetal breathing movements on a breath-by-breath basis in real time. An algorithm was developed which would recognize fetal breathing from the changes in tracheal pressure and which was capable of rejecting the artifactual changes caused by fetal or maternal movements. The tracheal pressure signal was digitized (at 51 samples/s), differentiated, and the start and peak of each breath was recognized from the zero-crossing points of the differentiated signal. Each breath was validated for size and shape according to a set of criteria incorporated into the breath recognition algorithm. On acceptance of the pressure change as a valid breath, the inspiratory time, breath amplitude, breath-to-breath interval, and inspiratory effort were calculated and stored in memory. The program was structured so that the microprocessor was able to accept new data and output summarizes of previous data concurrently. More than 95% of breaths in records contaminated with movement artifacts were recognized.     

Seasonal abundance, biting cycle and parity of the mosquito Haemagogus leucocelaenus in Trinidad, West Indies

Adult female populations of Haemagogus leucocelaenus (Dyar and Shannon), the sylvan vector of yellow fever, were monitored weekly during 1981–82 by human collectors on the ground at Point Gourde in Chaguaramas Forest, 16 km west of Port of Spain, Trinidad.
Hg.leucocelaenus showed only diurnal landing activity, from 06.00 to 18.00 hours (sunrise to sunset, universal time), with a single peak of activity between 10.00 and 14.00 hours.
Densities of Hg.leucocelaenus during the wet season (May–November) were about double the level recorded during the dry season (December–April).
Monthly parous rates averaged 53.9% (range 25–90%) and some females were up to five-pars. Retained eggs (range 2–6, mean 4/female) were found in the ovaries of 0.34% of landing females, all of which had stage 1 ovarian follicles for the next gonotrophic cycle. Therefore blood-feeding is not inhibited by egg retention. Hg. leucocelaenus vector potential is reappraised in the light of these findings.     

The Electrical Response of the Planarian Ocellus

The planarian ocellar potential (OP), an action potential evoked from the planarian ocellus by a light flash, was recorded with microelectrodes. OP amplitude, latency, and peak delay varied as a function of stimulus intensity and state of adaptation in a manner similar to the responses of other photoreceptors. Changes in the OP that occurred with different directions of incident light are described and attributed to screening effects of the ocellar pigment cells. The temperature coefficient (Q₁₀) of OP latency was 1.5; latency decreased continuously as temperature was increased to destructive levels. The energy of activation of the rate of OP formation was calculated to approximate 10 kcal. These findings suggest dependence of OP latency on ionic diffusion and of OP formation on a biocatalytic process.