Browsing by Author "Figueiredo, N."
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- Clustering evoked potential signals using subspace methodsPublication . Tomé, A. M.; Teixeira, Ana; Figueiredo, N.; Georgieva, P.; Santos, I.M.; Lang, E.This work proposes a clustering technique to analyze evoked potential signals. The proposed method uses an orthogonal subspace model to enhance the single-trial signals of a session and simultaneously a subspace measure to group the trials into clusters. The ensemble averages of the signals of the different clusters are compared with ensemble averages of visually selected trials which are free of any artifact. Preliminary results consider recordings from an occipital channel where evoked response P100 wave is most pronounced.
- SSA of biomedical signals: A linear invariant systems approachPublication . Figueiredo, N.; Georgieva, P.; Lang, E.W.; Santos, I.M.; Teixeira, A.R.; Tomé, A.M.Singular spectrum analysis (SSA) is considered from a linear invariant systems perspective. In this terminology, the extracted components are considered as outputs of a linear invariant system which corresponds to finite impulse response (FIR) filters. The number of filters is determined by the embedding dimension.We propose to explicitly define the frequency response of each filter responsible for the selection of informative components. We also introduce a subspace distance measure for clustering subspace models. We illustrate the methodology by analyzing Electroencephalograms (EEG).
- The corrosion resistance of Wiron88 in the presence of S. mutans and S. sobrinus bacteriaPublication . Proença, L.; Barroso, H.; Figueiredo, N.; Lino, A. R.; Capelo, S.; Fonseca, I. T. E."The corrosion resistance of Wiron®88, a Ni–Cr–Mo alloy, was evaluated in liquid growth media in the absence and presence of the Streptococcussobrinus and Streptococcus mutans strains. Open circuit potential measurements, cyclic voltammetry, linear sweep voltammetry, as well as electronic microscopy coupled to electron diffraction spectroscopy (SEM/EDS), were the main techniques used in this study. It was concluded that the presence of S. sobrinus and S. mutans have only a slight effect on the corrosion resistance of the Wiron®88 alloy, with the S. mutans being slightly more aggressive. For both strains the corrosion resistance Rp is of the same order (kΩ cm2). After 24 h immersion the S. sobrinus lead to and Rp of 11.02, while the S. mutans lead to of 5.59 kΩ cm2. SEM/EDS studies on the Wiron®88 samples, with 24 days of immersion, at 37 °C, have confirmed bio-corrosion of the alloy occurring through the dissolution of Ni as Ni2+ and formation of chromium and molybdenum oxides. The bacterial adhesion to the surface is not uniform."