Methods of contemporary physics are increasingly important for biomedical research. For a multitude of diverse reasons there exists a gap between the practitioners of biomedicine and modern physics methodologies. In this work, the technique of surrogate data has been used as a method to test for the linearity or nonlinearity of biomedical functional near-infrared spectroscopy (fNIRS) signals observing brain activities. Throughout three different surrogate tests, the third-order autocovariance, the asymmetry resulting from time reversal, and the delay vector variance, the dynamic response of brain activities through fNIRS biomedical signals is very likely to be a nonlinear system.Key words: nonlinearity, surrogate, functional near-infrared spectroscopy, biomedical, signal analysis.Nonlinear measures such as correlation dimension, Lyapunov exponents, and nonlinear prediction error are often applied to time series with the intention of identifying the presence of nonlinear, or possibly chaotic behavior. Theiler et al. (1992) have introduced the concept of "surrogate data," which has been extensively used in the context of statistical nonlinearity testing [1,2]. The surrogate data method tests for a statistical difference between a test statistic computed for the original time series and for an ensemble of test statistics computed on linearized versions of the data, the so-called "surrogate data," or "surrogates" for short. In other words, a time series is nonlinear if the test statistic for the original data is not drawn from the same distribution as the test statistics for the surrogates. The surrogate data method was also used to test with respect to the nonstationarity of time series by J. Timmer [3].Neurophysiological and neuroimaging technologies have contributed much to our understanding of normative brain function. Functional magnetic resonance imaging (fMRI) is currently considered the "gold standard" for measuring functional brain activation. The limitations of fMRI relative to fNIRS include the requirement that participants must lie within the confines of the magnet bore, which limits its use for many applications. The readout gradients in the imaging pulse sequences also produce a loud noise [4]. fMRI is also highly sensitive to movement artifact; subject movements on the order of a few millimeters can invalidate the data. And fMRI systems are quite expensive [5].In recent years, functional near-infrared spectroscopy (fNIRS) has been introduced as a new neuroimaging modality with which to conduct functional brain-imaging studies. fNIRS technology uses specific wavelengths of light, introduced at the scalp, to enable the noninvasive measurement of changes in the relative ratios of deoxygenated hemoglobin (deoxy-Hb) and oxygenated hemoglobin (oxy-Hb) during brain activity. A wireless fNIRS system consists of personal digital assistant (PDA) software controlling the sensor circuitry, reading, saving, and sending the data via a wireless network. This technology allows the design of portable, safe, ...
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