Motor imagery (MI) is a commonly used cognitive task in brain-computer interface (BCI) applications because it produces reliable activity in motor-planning regions. However, a number of functional near-infrared spectroscopy (fNIRS) studies have reported the unexpected finding of inverse oxygenation: increased deoxyhemoglobin and decreased oxyhemoglobin during task periods. This finding questions the reliability of fNIRS for BCI applications given that MI activation should result in a focal increase in blood oxygenation. In an attempt to elucidate this phenomenon, fMRI and fNIRS data were acquired on 15 healthy participants performing a MI task. The fMRI data provided global coverage of brain activity, thus allowing visualization of all potential brain regions activated and deactivated during task periods. Indeed, fMRI results from seven subjects included activation in the primary motor cortex and/or the pre-supplementary motor area during the rest periods in addition to the expected activation in the supplementary motor and premotor areas. Of these seven subjects, two showed inverse oxygenation with fNIRS. The proximity of the regions showing inverse oxygenation to the motor planning regions suggests that inverse activation detected by fNIRS may likely be a consequence of partial volume errors due to the sensitivity of the optodes to both primary motor and motor planning regions.
Time-resolved (TR) spectroscopy is well-suited to address the challenges of quantifying light absorbers in highly scattering media such as living tissue; however, current TR spectrometers are either based on expensive array detectors or rely on wavelength scanning. Here, we introduce a TR spectrometer architecture based on compressed sensing (CS) and time-correlated single-photon counting. Using both CS and basis scanning, we demonstrate that—in homogeneous and two-layer tissue-mimicking phantoms made of Intralipid and Indocyanine Green—the CS method agrees with or outperforms uncompressed approaches. Further, we illustrate the superior depth sensitivity of TR spectroscopy and highlight the potential of the device to quantify absorption changes in deeper (>1 cm) tissue layers.
A comparison is presented of two different methods for polarized radiative transfer in coupled media consisting of two adjacent slabs with different refractive indices, each slab being a stratified medium with no change in optical properties except in the direction of stratification. One of the methods is based on solving the integro-differential radiative transfer equation for the two coupled slabs using the discrete ordinate approximation. The other method is based on probabilistic and statistical concepts and simulates the propagation of polarized light using the Monte Carlo approach. The emphasis is on non-Rayleigh scattering for particles in the Mie regime. Comparisons with benchmark results available for a slab with constant refractive index show that both methods reproduce these benchmark results when the refractive index is set to be the same in the two slabs. Computed results for test cases with coupling (different refractive indices in the two slabs) show that the two methods produce essentially identical results for identical input in terms of absorption and scattering coefficients and scattering phase matrices.
The vibrational modes and sound spectra of some pre-1929 archtop mandolins and pre-1922 Neapolitan (‘‘bowlback’’) mandolins have been studied. The results have been compared with those obtained previously on archtop mandolins constructed more recently [D. Cohen and T. D. Rossing, CASJ 4(2), 48–54 (2000), D. Cohen and T. D. Rossing, Acoust. Sci. Tech. 24, 1–6 (2003)]. Some obvious and predictable differences between the Neapolitan mandolins and the archtop mandolins were found. The very stiff bowls of the Neapolitans do not contribute to corpus vibrations below about 1.2 kHz. The ladder-braced top plates of the Neapolitans are also quite stiff, with the (0,0) mode first occurring at or above 500 Hz. The (0,0) modes in archtop mandolins generally occur at lower frequencies. Archtop mandolins with f-holes generally have either longitudinal bracing or X-bracing, with the results that the modes involving cross-grain bending [e.g., (1,0), (2,0), etc.] occur at lower frequencies than the modes involving bending along the grain [e.g., (0,1), (0,2), etc.]. In the ladder-braced Neapolitans, the modes involving cross-grain bending occur at higher frequencies than the modes involving bending along the grain.
Monte-Carlo experiments were conducted to investigate the sensitivity of late-photons hyperspectral near-infrared spectroscopy (Lp-hNIRS) for assessing cerebral oxygenation (ScO2) in adults. Results suggest that Lp-hNIRS provides more accurate estimates of ScO2 than multispectral time-resolved NIRS.
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