R. Barraco

Analysis of the human a-wave ERG component

The a-wave is one of the main issues of research in the field of ocular electrophysiology, since it is strictly connected with early photoreceptoral activities. The present study proposes mathematical methods that analyse this component in human subjects, and supports experimental evidence relating to possible correlations among the responses of photoreceptoral units under a light stimulus. The investigation is organized in two parts: the first part concerns the onset and the initial slope, up to the first minimum (about 10-15 ms), the second part deals with the main portion of the wave, up to about 30 ms. In both cases, the a-waves, recorded at various levels of luminance, have been fitted with a set of appropriate functions representing possible models of physiological behaviour which would take place in the early stages of phototransduction. The statistical nature of the underlying processes is also discussed. The results indicate that correlations occur in the early stages, whereas random processes are set up later.

Empirical mode decomposition and neural network for the classification of electroretinographic data.

Abstract The processing of biosignals is increasingly being utilized in ambulatory situations in order to extract significant signals’ features that can help in clinical diagnosis. However, this task is hampered by the fact that biomedical signals exhibit a complex behavior characterized by strong nonlinear and non-stationary properties that cannot always be perceived by simple visual examination. New processing methods need be considered. In this context, we propose a signal processing method, based on empirical mode decomposition and artificial neural networks, to analyze electroretinograms, i.e., the retinal response to a light flash, with the aim to detect and classify retinal diseases. The present application focuses on two retinal pathologies: achromatopsia, which is a cone disease, and congenital stationary night blindness, which affects the photoreceptoral signal transmission. The results indicate that, under suitable conditions, the method proposed here has the potential to provide a powerful tool for routine clinical examinations, since it is able to recognize with high level of confidence the eventual presence of one of the two pathologies.

A study of the human rod and cone electroretinogram a-wave component

The study of the electrical response of the retina to a luminous stimulus is one of the main fields of research in ocular electrophysiology. The features of the first component (a-wave) of the retinal response reflect the functional integrity of the two populations of photoreceptors: rods and cones. We fit the a-wave for pathological subjects with functions that account for possible mechanisms governing the kinetics of the photoreceptors. The paper extends a previous analysis, carried out for normal subjects, in which both populations are active, to patients affected by two particular diseases that reduce the working populations to only one. The pathologies investigated are Achromatopsia, a cone disease, and Congenital Stationary Night Blindness, a rod problem. We present evidence that the analysis of a pathological a-wave can be employed to quantitatively measure either cone or rod activities and to test hypotheses about their responses. The results show that the photoreceptoral responses differ in the two cases and functions implying a different number of photocascade stages are necessary to achieve a correct modeling of the early phototransduction process. Numerical values of the parameters characterizing the best-fit functions are given and discussed.

An approach based on wavelet analysis for feature extraction in the a-wave of the electroretinogram

Most biomedical signals are non-stationary. The knowledge of their frequency content and temporal distribution is then useful in a clinical context. The wavelet analysis is appropriate to achieve this task. The present paper uses this method to reveal hidden characteristics and anomalies of the human a-wave, an important component of the electroretinogram since it is a measure of the functional integrity of the photoreceptors. We here analyse the time-frequency features of the a-wave both in normal subjects and in patients affected by Achromatopsia, a pathology disturbing the functionality of the cones. The results indicate the presence of two or three stable frequencies that, in the pathological case, shift toward lower values and change their times of occurrence. The present findings are a first step toward a deeper understanding of the features of the a-wave and possible applications to diagnostic procedures in order to recognise incipient photoreceptoral pathologies.

LIBS and XRF analysis for a stratigraphic study of pictorial multilayer surfaces

The study of painting materials is a primary goal for the historical and artistic characterization of works of artbecause it provides useful information about the original materials and their degradation products, the paintingstechnique and any conservation treatments. However, the archaeometric investigations are very complex because theneed to safeguard the Cultural Heritage makes necessary the use of non or micro invasive techniques. Among thepossible analytical approaches, Laser-Induced Breakdown Spectroscopy, supported by X-ray Fluorescencespectroscopy for a faster elements identification, allows stratigraphic analysis with high spatial resolution enough todiscriminate different layers. Moreover, an integrated approach based on XRF and LIBS investigation is particularlyuseful for the complementarities of the outcomes in terms, both of investigated thicknesses and detectable elementalcontent.This paper aims to validate, in the archaeometry field, the use of LIBS for stratigraphic investigation of paintedsurfaces/objects by means of analytical results obtained from original fragments of mural paintings originating fromdifferent sites and artistic eras. Indeed the focus of this work is to determine the optimum parameters for bothtechniques in order to acquire the maximum information with the least damage to the works of art.The results have demonstrated the usefulness of the LIBS investigation, through that it has been possible toreveal chemical elements undetectable by XRF and to analyse the stratigraphic sequence of paint layers from thesurface up to the support layer. Furthermore they have provided good insight into the different painting layers byconfirming the potential of the LIBS to carried out stratigraphic analyses in a micro destructive way.

Double laser LIBS and micro-XRF spectroscopy applied to characterize materials coming from the Greek-Roman theater of Taormina

The laser-induced breakdown spectroscopy (LIBS) is an applied physical technique that has shown, in recent years, its great potential for rapid qualitative analysis of materials. The possibility to implement a portable instrument that perform LIBS analysis makes this technique particularly useful for in situ analysis in the field of cultural heritages. The aim of this work is to compare the results, obtained by LIBS measurements with X-Ray Fluorescence (XRF) ones, on calcareous and refractory materials coming from the Greek-Roman theater of Taormina. Calibration curves for LIBS and XRF were obtained by measuring certified reference materials and using them as standards. LIBS measurements were performed with a new mobile instrument Modì (Mobile Double pulse Instrument for LIBS Analysis) that use an innovative experimental set-up, based on the use of two suitably retarded laser pulses that greatly improves the potential of this technique for accurate quantitative analysis. XRF analyses were performed with a portable instrument ArtTAX that use a collimated X-Ray beam equipped with a fluxing system of helium gas on the detection path to the aim to detect the lighter elements with a good accuracy. LIBS results have shown that this technique is highly subjected to the sample chemical and physical characteristics.

Functional analysis of Normal and CSNB a-wave ERG component

The features of a-wave of the human electroretinogram are one of the more debated problems in electrophysiology since the a-wave reflects the functional integrity of the two photoreceptoral populations (rods and cones). Although different models concerning the contributions of the early photoreceptoral response are available in current literature, a fully comprehensive theory is difficult to formulate because of the large amount of individual photoreceptors. We study the kinetics of the photoreceptoral response through the analysis of the a-wave shape both in healthy and in patients affected by the Congenital Stationary Night Blindness, that interests the rod population only. The physiological behavior has been explored by modeling the a-waves with a set of appropriate statistical functions, representing all possible mechanisms governing the interactions occurring in the phototransduction process. The results indicate that in healthy subjects, correlations between photoreceptoral populations take place, as a consequence of direct cell-to-cell interactions among rods and/or cones and a sum of Gaussian-Lorentzian functions is appropriate. In pathological subjects the best fit is obtained using the Symon-Vavilov function, that is suitable to simulate coherent responses in situations affected by fluctuations.

Time-Frequency behaviour of the a-wave of the human electroretinogram

The electroretinogram is the record of the electrical response of the retina to a light stimulus. The two main components are the a-wave and the b-wave, the former is related to the early photoreceptoral activity. Aim of this paper is to acquire useful information about the time-frequency features of the human a-wave, by means of the wavelet analysis. This represents a proper approach in dealing with nonstationary signals. We have used the Mexican Hat as mother wavelet. The analysis, carried out for four representative values of the luminance, comprehends the frequency dependence of the variance and the skeleton. The results indicate a predominance of low frequency components, their time distribution depends on the luminance whereas that of the high frequency components is little affected by the luminance.

Wavelet analysis of human photoreceptoral response

Feature detection of biomedical signals is crucial for deepening our knowledge of the physiological phenomena giving rise to them. To achieve this aim, even if many analytic approaches have been suggested only few are able to deal with signals whose features are time dependent, and to provide useful clinical information. In this work we use the wavelet analysis to extract peculiarities of the early response of the photoreceptoral human system, known as a-wave ERG-component. The analysis of the a-wave features is important since this component reflects the functional integrity of the two populations of photoreceptors, rods and cones whose activation dynamics are not well known. Moreover, in incipient photoreceptoral pathologies the eventual anomalies in a-wave are not always detectable with a naked eye analysis of the traces. We here propose the possibility to discriminate the pathologic from the healthy traces throughout the differentiation of their time-frequency characteristics, revealed by the wavelet analysis. The investigated pathologies are the Achromatopsia, a cone disease and the Congenital Stationary Night Blindness, a rod trouble. The results show that the number of stable frequencies present and their times of occurrence are indicative of the status of the retinal photoreceptors. In particular, in the pathological cases, the frequency components shift toward lower values and change their times of occurrence, with respect to healthy traces.

X-ray CT imaging as a scientific tool to study the capillary water absorption in sedimentary rocks used in cultural heritages

This paper proposes the X-Ray CT imaging as appropriate tool for investigating the capillary water absorption in sedimentary rocks. This technique, in fact, provides information useful for deeping the knowledge about of the porosity and the kinetics of the water capillary absorption in porous materials. The possibility to improve in non invasive manner, the understanding of this phenomenon, constitutes a fundamental aspect to take actions in the restoration and conservation of lapideous artifact and monuments from cultural heritages. The investigated sedimentary rocks come from different Sicilian quarries and were used for the building of the Greek temples in the archaeological areas of Agrigento, Selinunte and in the baroque monuments of the Val di Noto area. The image acquisition was carried out at different times after contact with water. The wetting front progression along the height of the sample was evaluated on the basis of the estimated CT attenuation value maps. An average of the CT attenuation values measured by the ROI (Region of Interest) was used to evaluate the mean height of the wetting front, with different CT scans at fixed time intervals after sample wetting, in order to describe the dynamic behaviour of the imbibition process. CT imaging results have shown that the water absorption is highly subject to the sample porous morphology.