Gabriel Alfredo Ruiz

Discriminability measures and time-frequency features: an application to vibrissal tactile discrimination.

BACKGROUND Often, the first problem that the neuroscientist must face is to determine if a specific stimulus set applied to biological system produces specific, precise and well differentiated responses. NEW METHOD In the present study we have proposed four discriminability measures to evaluate the feasibility of differentiating experimental conditions: information measures based on information theory, percentage overlap based on Linacre method, Bhattacharyya distance and univariate standard distance. All discriminability measures were evaluated on experimental protocols related to vibrissal tactile discrimination. RESULTS Time-frequency features were extracted from afferent discharges and then, pairwise comparisons were realized by using the proposed discriminability measures. Our results reveal the existence of time-frequency patterns which allows differentiating of sweep conditions from multifiber recordings. COMPARISON WITH EXISTING METHODS Currently, statistical methods used to justify significant differences in experimental conditions have rigorous criteria that must be met for correct validation of results. Discriminability measures proposed here are robust and can be adjusted to different experimental conditions (time series, repeated measures, specific variables and other). CONCLUSIONS Discriminability measures allowed determining the time intervals where two sweep situations have the highest probability to be differentiated from each other. High discriminability percentages were observed into protraction phase, although to a lesser degree, it was also observed in retraction phase. It was demonstrated that sensibility of discriminability measures are different. This revealing a greater ability to highlight percentage changes of pairwise comparisons. Finally, the methods here proposed can be adapted to other features of biological responses.

Impedance spectroscopy of yeast cells attached to gold electrodes

Abstract In this paper, we used impedance spectroscopy and gold electrodes to detect the presence of yeast cells and monitor the attachment of these cells to the electrodes. We analyzed the effect of conductivity changes of the medium and the attachment on the electrode-electrolyte interface impedance. A three-electrode cell was designed to produce a uniform electric field distribution on the working electrode and to minimize the counter electrode impedance. Moreover, we used a small AC overpotential (10 mV) to keep the system within the linear impedance limits of the electrode-electrolyte interface. This study proposes a new method to differentiate the impedance changes due to the attachment of yeast cells from those due to conductivity changes of the medium. The experiments showed that when the difference between the cell suspension and base solution conductivities is within the experimental error, the impedance changes are only due to the attachment of yeast cells to the electrodes. The experiments also showed a strong dependence (decrease) of the parallel capacity of the electrode electrolyte interface with the yeast cell concentration of suspension. We suggest that this decrease is due to an asymmetrical redistribution of surface charges on both sides of cell, which can be modeled as a biologic capacity connected in series with the double layer capacity of the interface. Our results could help to explain the rate of biofilm formation through the determination of the rate of cell adhesion.

Period doubling bifurcations in cardiac systems

Abstract Adams et al. (1981), observed in their experiences with dogs, a decrease in the Electrical Fibrillation Threshold (EFT) by hypothermal conditions and coronary arterial ligament. In this paper, it is proved that there appears also a decrease in the EFT, produced by the intravenous injection of Ketalar in concentrations larger than 9 mg/Kg of weight of the animal. Under these conditions, it is observed that the cardiac system presents a period doubling bifurcations dynamics, that is, the first “chaos” onsets represented by Ventricular Fibrillation (VF). Frequency spectra show this behavior by the appearance of the components of frequency one half and one fourth. This quantitative method is proposed to determine the proximity to chaos (VF) by measuring the Alternance Degree (AD).

Fast nonlinear region localisation for nonlinear dielectric spectroscopy of biological suspensions.

The nonlinear properties of biological suspensions have been previously presented as a bulk phenomenon without the influences of the electrodes. However, some authors have showed that the behaviour of a biological suspension is due to the nonlinear characteristics of the electrode-electrolyte interface (EEI), which is modulated by the presence of yeast cells. We have developed a method, complementary to the nonlinear dielectric spectroscopy (NLDS) which is used for the study of the behaviour of EEI with resting cell suspensions of Saccharomyces cerevisiae. The method allows researchers to detect simply and quickly the voltage and frequency ranges where the metabolic activity of yeasts is detectable. This method does not replace NLDS, and aims to reduce the time during which the electrodes are exposed to corrosion by high voltages. In this paper we applied AC overpotentials (10-630 mV) with frequencies in the range from 1 to 1000 Hz. Also, we measured current harmonic distortion produced by the nonlinearity of the interface. Changes in the transfer function were observed when yeast suspension was used. Apart from the nonlinear response typical of the EEI, we also observed the biological nonlinear behaviour. The changes in the transfer functions were assessed using the overlapping index which was defined in terms of the conditional probability. The methodology was contrasted favourably with Fourier analysis. This novel strategy has the advantages of simplicity, sensitivity, reproducibility and involves basic tools such as the usual measurement of current.

Recognition of Movements Through Dynamic Electromyographic Signals

The recognition of movements through electromyographic (EMG) signals is critical for myoelectric control systems. Performance of these systems depend on processing methods and protocols used to extract the EMG signals. The aim of this study is to evaluate the performance of classification of a kinematic recognition system based on dynamic EMG signals. For this, a correlation analysis between dynamic EMG signals and kinematic features of movements is realized, and then, a kinematic recognition system based on dynamic EMG signals is implemented. Dynamic EMG signals from forearm muscles during finger flexion movements were recorded and analyzed by using an amplitude estimator. Linear and no-linear correlations between EMG amplitudes and kinematic features were found. Then, a step of classification based on discriminant analysis was implemented to categorize the finger movements in multiple kinematic states. The accuracy of classifications were 95%, 88%, 81% and 76% for two, three, four and five states respectively, and by using a simple-channel recording and an EMG amplitude estimator. The results of this study demonstrate that it is possible to improve aspects of “intuitiveness” through dynamic EMG evoked by natural and more intuitive movements. Keywords—EMG; root mean square; kinematic features; discriminant analysis.

Evaluation of sugar yeast consumption by measuring electrical medium resistance

Abstract The real-time monitoring of alcoholic fermentation (sugar consumption) is very important in industrial processes. Several techniques (i.e., using a biosensor) have been proposed to realize this goal. In this work, we propose a new method to follow sugar yeast consumption. This novel method is based on the changes in the medium resistance (Rm) that are induced by the CO2 bubbles produced during a fermentative process. We applied a 50-mV and 700-Hz signal to 75 ml of a yeast suspension in a tripolar cell. A gold electrode was used as the working electrode, whereas an Ag/AgCl electrode and a stainless-steel electrode served as the reference and counter electrodes, respectively. We then added glucose to the yeast suspension and obtained a 700% increase in the Rm after 8 minutes. The addition of sucrose instead of glucose as the carbon source resulted in a 1200% increase in the Rm. To confirm that these changes are the result of CO2 bubbles in the fermentation medium, we designed a tetrapolar cell in which CO2 gas was insufflated at the bottom of the cell and concluded that the changes were due to CO2 bubbles produced during the fermentation. Consequently, this new method is a low-cost and rapid technology to follow the sugar consumption in yeast.