PROGRAMME CRYOCONSERVATION OF LYMPHOCYTES IN THE PRACTICE OF IMMUNOLOGICAL DEPARTMENT OF TRANSPLANTATION CENTER

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Radiocarbon Dating of Holocene Archaeological Sites in the Far Northeast of Europe

The paper is devoted to the critical analysis of the radiocarbon dating results of Mesolithic, Neolithic and Chalcolithic complexes of the northeastern part of the East European Plain (Republic of Komi, Arkhangelsk and Vologda Regions and the Nenets Autonomous Area, Russian Federation). The comprehensive evaluation of all available geochronometric data in relation with the studied archaeological events highlighted the following three data sets: reliable, ambiguous and invalid dates. A new chronological model of Far Northeast of Europe colonization and dispersal of innovations over the Holocene is proposed based upon reliable radiocarbon dating results

Аналитическая теория дисперсии оптических волн регулярных микроволноводов

A method for analysis of dispersion characteristics of guided optical modes propagating in the optical waveguides with small cross-sections is proposed. The method is based on introduction of a correction factor for a longitudinal wavenumber of propagating modes. The correction factor arises when a cross-section of the basic rectangular waveguide is subjected to perturbation. The electromagnetic field distributions along with the mode longitudinal wavenumber are found by means of variable separation method. The longitudinal wavenumber correction factor is analytically calculated in terms of coupled mode theory. The combined use of the complete set of equations of electrodynamics together with the concept of effective sources gives rise to the correction factor in the form of an intermodal coupling coefficient. It is pointed out that the coupling coefficient consists of two components, namely bulk and surface, owing to accurate account of the electrodynamics boundary conditions. Using the method proposed, the dispersion characteristics of the fundamental modes propagating in the practically employed optical waveguides having a trapezoidal cross-section are calculated. An impact of the waveguide cross-section shape to cladding dielectric constant ratio on the mode dispersion characteristics is analyzed. The necessity to take into consideration an imperfection of the waveguide cross-section in a wide range of operating wavelengths is demonstrated.Разработан метод анализа дисперсионных характеристик направляемых мод в регулярных оптических микроволноводах малого поперечного сечения. Метод основан на введении поправок к продольному волновому числу мод прямоугольного волновода, выбранного в качестве базового, при искажении формы его поперечного сечения. Распределения электромагнитного поля и продольного волнового числа базового волновода рассчитываются методом разделения переменных. Поправка к продольному волновому числу рассчитывается аналитически в терминах теории связанных мод. Указанная поправка в виде коэффициента межмодовой связи возникает на основании совместного использования полной системы уравнений Максвелла при введении понятия об эффективных источниках. Показано, что последовательный учет граничных условий электродинамики приводит к форме коэффициента связи, включающей объемную и поверхностную составляющие. Разработанный метод применен для расчета дисперсионных характеристик низших волноводных мод, распространяющихся в микроволноводах трапециевидного сечения, применяемых на практике. Продемонстрировано влияние поперечного сечения микроволновода на дисперсионные характеристики мод в зависимости от соотношения сторон, а также от отношения значений диэлектрических проницаемостей сердцевины микроволновода и его оболочки. Показана необходимость учета влияния формы микроволновода на дисперсионные характеристики мод в широком диапазоне значений рабочих длин волн и при различных распределениях диэлектрической проницаемости волноведущей структуры

Fractionation of parietal function in bistable perception probed with concurrent TMS-EEG

When visual input has conflicting interpretations, conscious perception can alternate spontaneously between these possible interpretations. This is called bistable perception. Previous neuroimaging studies have indicated the involvement of two right parietal areas in resolving perceptual ambiguity (ant-SPLr and post-SPLr). Transcranial magnetic stimulation (TMS) studies that selectively interfered with the normal function of these regions suggest that they play opposing roles in this type of perceptual switch. In the present study, we investigated this fractionation of parietal function by use of combined TMS with electroencephalography (EEG). Specifically, while participants viewed either a bistable stimulus, a replay stimulus, or resting-state fixation, we applied single pulse TMS to either location independently while simultaneously recording EEG. Combined with participant’s individual structural magnetic resonance imaging (MRI) scans, this dataset allows for complex analyses of the effect of TMS on neural time series data, which may further elucidate the causal role of the parietal cortex in ambiguous perception

Separated and overlapping neural coding of face and body identity

Recognising a person's identity often relies on face and body information, and is tolerant to changes in low-level visual input (e.g., viewpoint changes). Previous studies have suggested that face identity is disentangled from low-level visual input in the anterior face-responsive regions. It remains unclear which regions disentangle body identity from variations in viewpoint, and whether face and body identity are encoded separately or combined into a coherent person identity representation. We trained participants to recognise three identities, and then recorded their brain activity using fMRI while they viewed face and body images of these three identities from different viewpoints. Participants' task was to respond to either the stimulus identity or viewpoint. We found consistent decoding of body identity across viewpoint in the fusiform body area, right anterior temporal cortex, middle frontal gyrus and right insula. This finding demonstrates a similar function of fusiform and anterior temporal cortex for bodies as has previously been shown for faces, suggesting these regions may play a general role in extracting high-level identity information. Moreover, we could decode identity across fMRI activity evoked by faces and bodies in the early visual cortex, right inferior occipital cortex, right parahippocampal cortex and right superior parietal cortex, revealing a distributed network that encodes person identity abstractly. Lastly, identity decoding was consistently better when participants attended to identity, indicating that attention to identity enhances its neural representation. These results offer new insights into how the brain develops an abstract neural coding of person identity, shared by faces and bodies

The effects of TMS over the parietal cortex on binocular rivalry

Human fMRI studies of binocular rivalry and other bistable phenomena suggest that a network of frontal and parietal areas, predominantly in the right hemisphere, is particularly involved during switches between the two conflicting percepts. However, these studies do not provide information about causality, i.e. whether fMRI activity is a consequence or a cause of the perceptual change. In the current study we localized areas that were activated during perceptual switches in individual subjects using fMRI. We then tested the effect of disturbing neural processing in two distinct parietal regions along the ventral-dorsal axis in both hemispheres using 2 Hz repetitive transcranial magnetic stimulation (TMS). Our results show that on the group level, TMS over the right intraparietal sulcus (IPS) prolonged the periods of stable percepts. In individual subjects, the IPS in the hemisphere with higher fMRI activation also showed a stronger TMS effect, as reflected in the positive correlation between the lateralization of TMS effects and that of fMRI activations. Our results thus demonstrate a causal, de-stabilizing effect of the IPS on perceptual continuity and provide a direct link between correlational and causal measures of cortical function during conscious perception

Conscious perception of global motion is related to higher-level motion regions

The processing of motion in the primate brain is distributed across multiple regions of the cerebral cortex. The two well-studied visual areas MT and MST have been linked to conscious perception and decision-making related to simple flow stimuli as well as to integration of component plaid motion into a coherently moving pattern. However, it is unclear whether processing and perception of other types of global motion, which require large-scale integration of the local signals, is related to activity of the same areas. In the current study we used fMRI to investigate neural responses to a bi-stable visual motion stimulus. The stimulus consisted of four pairs of dots, each pair coherently moving on a circular path. Perception alternated spontaneously between two states: local dot motion in each of the four quadrants of the visual field or global planar motion of two illusory squares spanning all four visual quadrants [1]. Importantly, these alternations were purely perceptual and involved no stimulus manipulation. We localized visual areas that are known to respond to visual movement (V3a, V6, V7, MT, MST, IPS1-4, and the recently described cingulate sulcus visual area (CSv) [2,3]) individually in each subject. We then investigated responses of these areas to global and local perceptual states of our subjects, while they viewed the bistable stimulus and reported their perception. We found that activity of two of the areas, CSv and IPS4, specifically correlated with global, but not the local perceptual states, while V6 showed a trend in the opposite direction. Interestingly, neither V5/MT, nor MST, nor any other motion-responsive region differentiated between global and local perceptual states. Our results suggest that CSv and IPS4 may be involved in the computation of global motion by large-scale integration of similar motion directions, or by spatial binding between distant loci in the visual field, respectively. Importantly, these results imply a certain 'blindness' of V5/MT and of MST to vivid changes in the conscious perception of large-scale motion stimuli. The perception of global, large-scale motion may therefore be mediated by higher-level motion-processing regions with larger receptive fields, such as by areas CSv and IPS4

Partial Fourier imaging anisotropically reduces spatial independence of BOLD signal time courses

Introduction: Partial Fourier (pF) imaging is a common method used to accelerate image acquisition or shorten the minimum TE in 2D EPI. The technique takes advantage of conjugate symmetry in k-space, which allows acquiring a subset (at least half) of k-space and estimates the skipped data from the acquired data [1], [2]. Despite its wide use for BOLD fMRI and the theoretically predicted blurring effects, the actual impact of pF on spatial independence of the BOLD signal time courses-and hence on the spatial resolution of fMRI experiments-is not known. In this study we quantify the effect of pF on temporal correlation of resting-state BOLD signal time courses along each of the three image encoding dimensions. We find that with increasing pF spatial correlations between neighboring voxels increase anisotropically, with most blurring occurring along the phase-encoding direction. Methods: Resting-state BOLD fMRI data were acquired from four healthy volunteers on a Siemens TIM Trio 3T MR scanner using a 32-channel head coil and either a GE (TR=3.3 s, TE=30 ms, FA=90°, 2.5 mm3 resolution, 48 axial slices with A–P phase encoding, 120 volumes per measurement) or SE (same parameters as GE but 30 slices and TE=70ms) single-shot 2D EPI sequence. The partial Fourier factor was varied from run to run between 8/8 (i.e. no pF) to 5/8 in a pseudorandom order to avoid confounding effects of fatigue, motion, etc. EPI data were reconstructed using zero-padding algorithm [3]. After standard preprocessing we employed an analysis procedure illustrated in Fig. 1. This procedure generated three maps representing the amount of local temporal correlation along phase-encoding, readout, and slice dimensions. The resulting values were then averaged across all voxels within the brain to yield one global value per run. To rule out effects of brain tissue asymmetry or physiological noise correlations along different dimensions we conducted separate measurements where the EPI matrix was rotated by ±30° relative to the A–P axis. To exclude the effect of a specific image reconstruction algorithm used, we compared the results obtained using default zero-filling reconstruction with those obtained using the POCS algorithm [4]. Results: We observed the expected tSNR increase with increasing pF due to implicit spatial smoothing caused by omitting the high spatial frequencies and interpolation during image reconstruction (Fig. 2). The only exception was observed for pF 5/8 in GE sequence, presumably because the signal loss due to strong dephasing (as the echo becomes closer to the edge of the acquisition window) outweighs the smoothing effect. Importantly, we also observed an anisotropic increase in temporal signal correlation in neighboring voxels, with strongest increase occurring along the phase-encoding dimension (again with an exception of pF 5/8; see Fig. 3A). The observed effect of pF is similar to applying a 1D Gaussian smoothing kernel along the same dimension (Fig. 3B), where pF 6/8 is roughly equivalent to a smoothing of 1.1 mm FWHM (as determined by linear interpolation of the results in Fig. 3B). Additional control experiments show that the correlation anisotropy is unlikely to be due to differences in tissue or noise properties along different dimensions (Fig. 4A) or the specific reconstruction algorithm used (4B). Conclusions: Using partial Fourier for fMRI introduces an asymmetric loss of resolution along the phase-encoding dimension comparable to the effect of smoothing single volumes with a 1D Gaussian kernel during standard preprocessing. This can not only cause bias in high-resolution fMRI analyses where explicit smoothing is avoided [5], but also can complicate interpretation of resting-state functional connectivity including promising new measures of local connectivity anisotropy [6]. This effect is expected to increase with field strength since increases in B0 inhomogeneity cause assumptions of conjugate symmetry and smoothly-varying phase in the image domain to be less justified

Image SNR requirements for cortical surface reconstructions from sub-millimeter anatomical data

Introduction: Brain morphometry studies typically utilize anatomical data with 1 mm3 isotropic resolution. However, the widespread availability of high-field MRI scanners and receive coil arrays have led to increased interest in submillimeter resolution data. While higher resolution may have an advantage (Bazin et al., 2014; Lüsebrink et al., 2013; Zaretskaya et al., 2017), lifting this constraint on voxel size opens the door to a variety of acquisition protocols with a wide range of image contrast and SNR. Here we aim to systematically investigate the effects of image SNR on surface reconstruction quality based on sub-millimeter MPRAGE data acquired at 3T. Methods: To systematically vary image SNR without changing voxel size, we acquired multiple repetitions of high-resolution (0.6 mm isotropic) MPRAGE data from 9 participants, and for each subject generated multiple image volumes with progressively higher SNR by averaging together increasing number of repetitions (i.e. averaging 1 to 6 (n=5) or 1 to 8 (n=4) repetitions, depending on the total number of repetitions acquired within the session). Images were acquired on a 3T Siemens MAGNETOM Trio Tim system using a 32 channel coil and a 0.6 mm isotropic multiecho MPRAGE (van der Kouwe et al., 2008) protocol (TR/TE1/TE2/TI/FA/BW/ESP/matrix=2510 ms/2.88 ms/ 5.6 ms/1200 ms/7°/420 Hz/px/8.4 ms/400×400). To minimize blurring during the inversion recovery, we used acceleration in the partition direction (R=2) without partial Fourier in any direction and a slab-selective axial acquisition (slices per slab=224) to minimize the number of partition encoding steps. Each average volume was used to generate cortical surfaces using FreeSurfer's native "hires" sub-millimeter reconstruction stream (Zaretskaya et al., 2017). To assess surface quality, for each set of surfaces (the gray-white and gray-CSF interface surfaces of each hemisphere) we computed the following parameters: (1) image SNR (the mean divided by the standard deviation of voxel intensities within the FreeSurfer white matter mask), (2) surface smoothness (defined as the median value of per-vertex local mean curvature), (3) number of topological defects in the initial surface, identified by FreeSurfer (Segonne et al., 2007), (4) median defect size. We also report gray-white matter contrast, derived from the reconstruction with 6 repetitions, and defined as the median of vertex-wise contrast values expressed as (white matter intensity – gray matter intensity)/(white matter intensity + gray matter intensity)⋅100. Results: Automatic reconstruction completed successfully in all but 2 cases. Both failed cases were comprised of a single repetition and could not complete automatically due to low SNR. The gray-white matter contrast of our data, based on averaging 6 repetitions for each subject, was 26.8 ± 0.4 (mean across subjects ± S.E.M). We observed a consistent increase in image SNR - accompanied by an increase in surface smoothness and by a decrease in the number of topological defects - with increasing number of repetitions included in the average. There was no change in the median defect size. While surface quality clearly improved with increasing SNR, gray-white surface smoothness and the number of defects did not reach a clear plateau even after 7 repetitions, suggesting that more repetitions may be needed to determine the SNR beyond which there is no noticeable improvement in surface quality. However, for this protocol and scanner, and across this small group of subjects, we find that qualitatively the performance of the surface reconstruction is adequate after averaging 5–6 repetitions. Conclusions: Overall we show that averaging multiple repetitions can compensate for the corresponding loss of image SNR due to increase in image resolution. High-quality reconstructions can hence be generated from voxels far smaller than conventional acquisitions, even at standard field strength, provided sufficient data quality