Iron impregnation on the amorphous shell of vapor grown carbon fibers and the catalytic growth of secondary nanofibers

Vapor grown carbon fibers (VGCFs) with diameters of several microns were synthesized and investigated by high resolution transmission electron microscopy. It was found that the shell of the VGCFs consisted of densely-packed domains embedded in loosely-packed matrix, and both were highly amorphous. Regular edge planes as observed on the surface of fishbone nanofibers do not exist on VGCFs. Hence, surface treatment is more important for the deposition of catalysts. Ammonium ferric citrate (AFC) was employed for the impregnation of iron, where the high viscosity of the aqueous solution of AFC is beneficial. Calcination was found to be a key step to improve the dispersion of the iron particles, which can be attributed to enhanced interactions between iron and carbon due to the gasification of carbon occurring at the iron-carbon interface. Quantitative analysis by X-ray photoelectron spectroscopy showed that the calcination of the supported AFC led to a higher atomic concentration of iron on the surface, indicating smaller particle size and higher dispersion. Secondary carbon nanofibers were grown subsequently on the VGCFs from cyclohexane. The specific surface area was enhanced considerably, from less than 1 m2 g-1 to 106 m2 g-1 after the growth of the secondary nanofibers. The obtained composites are promising materials as structured support in heterogeneous catalysis

Detecting event-related recurrences by symbolic analysis: Applications to human language processing

Quasistationarity is ubiquitous in complex dynamical systems. In brain dynamics there is ample evidence that event-related potentials reflect such quasistationary states. In order to detect them from time series, several segmentation techniques have been proposed. In this study we elaborate a recent approach for detecting quasistationary states as recurrence domains by means of recurrence analysis and subsequent symbolisation methods. As a result, recurrence domains are obtained as partition cells that can be further aligned and unified for different realisations. We address two pertinent problems of contemporary recurrence analysis and present possible solutions for them.Comment: 24 pages, 6 figures. Draft version to appear in Proc Royal Soc

Long-term coding of personal and universal associations underlying the memory web in the human brain

Neurons in the medial temporal lobe (MTL), a critical area for declarative memory, have been shown to change their tuning in associative learning tasks. Yet, it is unclear how durable these neuronal representations are and if they outlast the execution of the task. To address this issue, we studied the responses of MTL neurons in neurosurgical patients to known concepts (people and places). Using association scores provided by the patients and a web-based metric, here we show that whenever MTL neurons respond to more than one concept, these concepts are typically related. Furthermore, the degree of association between concepts could be successfully predicted based on the neurons’ response patterns. These results provide evidence for a long-term involvement of MTL neurons in the representation of durable associations, a hallmark of human declarative memory

Seizure characterisation using frequency-dependent multivariate dynamics

The characterisation of epileptic seizures assists in the design of targeted pharmaceutical seizure prevention techniques and pre-surgical evaluations. In this paper, we expand on recent use of multivariate techniques to study the crosscorrelation dynamics between electroencephalographic (EEG) channels. The Maximum Overlap Discrete Wavelet Transform (MODWT) is applied in order to separate the EEG channels into their underlying frequencies. The dynamics of the cross-correlation matrix between channels, at each frequency, are then analysed in terms of the eigenspectrum. By examination of the eigenspectrum, we show that it is possible to identify frequency dependent changes in the correlation structure between channels which may be indicative of seizure activity. The technique is applied to EEG epileptiform data and the results indicate that the correlation dynamics vary over time and frequency, with larger correlations between channels at high frequencies. Additionally, a redistribution of wavelet energy is found, with increased fractional energy demonstrating the relative importance of high frequencies during seizures. Dynamical changes also occur in both correlation and energy at lower frequencies during seizures, suggesting that monitoring frequency dependent correlation structure can characterise changes in EEG signals during these. Future work will involve the study of other large eigenvalues and inter-frequency correlations to determine additional seizure characteristics

Acció dels andrògens en el testicle: un paper per a la meiosi

La funció que duen a terme els andrògens en l'espermatogènesi és, encara en certa mesura, enigmàtica: mentre que llur implicació és absolutament vital en la iniciació i en el manteniment del procés espermatogènic normal, la seva funció específica encara no està definida de manera precisa. Els andrògens, com les altres hormones esteroïdals, actuen a través del seu corresponent receptor anomenat receptor d'andrògens (AR). Fins avui, no hi ha gaire evidència que recolzi l'existència de diverses isoformes de l'AR com en el cas del sistema estrògensreceptor d'estrògens. Per tant, la pregunta de com els andrògens duen a terme la seva acció en l'espermatogènesi s'ha d'abordar definint dos processos: en primer lloc, s'han d'identifi- car amb total certesa els tipus cell. ulars testiculars capaços de respondre directament a l'estimulació androgènica. De manera específica, la qüestió per resoldre és quins són els tipus cellulars que expressen l'AR en el testicle. En segon lloc, sabent també que el complex del lligand unit a l'AR actua com a factor de transcripció, caldrà determinar quins són els gens que estaran activats o reprimits en les cèll. ules que tenen AR en resposta a l'estimulació androgènica. Fins que aquestes dues preguntes no estiguin contestades amb tota certesa, el mecanisme pel qual els andrògens regulen l'espermatogènesi serà, en el millor dels casos, especulatiu. En aquesta revisió presentem evidència que els andrògens actuen únicament a les cèll. ules somàtiques del testicle, com són les cèll. ules de Sertoli, les de Leydig, les mioides peritubulars i les cèll. ules del múscul llis que envolten els vasos sanguinis. A més a més, també discutim la possibilitat que els andrògens siguin indispensables per a l'inici de la meiosi, encara que continua essent desconegut el mecanisme pel qual els andrògens actuen en aquest procés.The role that androgens play in spermatogenesis still remains enigmatic: whereas their involvement is absolutely vital to the initiation and maintenance of the normal spermatogenic process, their specific role is yet to be defined. Androgens, like other steroid hormones, act via their corresponding receptor termed the androgen receptor (AR). To date, there is little evidence to support the notion that there are multiple forms of AR as is the case for the estrogen-estrogen receptor system. Thus, the question of how androgens manifest their action on spermatogenesis becomes one of defining two processes: First, the cell types within the testis that are capable of responding directly to androgen stimulation must be identified with absolute certainty. Specifically, this question can be stated as what cell types in the testis express AR. Second, given that the ligand-bound AR serves as a transcription factor, the question then becomes what are the genes turned on or off in AR positive cells in response to androgen stimulation? Until these two questions are unequivocally answered, the mechanism of how androgens regulate spermatogenesis will remain speculative at best. In this review we present evidence that androgens act solely at the level of the somatic cells of the testis, including Sertoli cells, Leydig cells, peritubular myoid cells and smooth muscle cells surrounding blood vessels. In addition, we discuss the likely possibility that androgens are indispensable for the onset of meiosis, albeit how they accomplish this remains a mystery

Complex lithium ion dynamics in simulated LiPO3 glass studied by means of multi-time correlation functions

Molecular dynamics simulations are performed to study the lithium jumps in LiPO3 glass. In particular, we calculate higher-order correlation functions that probe the positions of single lithium ions at several times. Three-time correlation functions show that the non-exponential relaxation of the lithium ions results from both correlated back-and-forth jumps and the existence of dynamical heterogeneities, i.e., the presence of a broad distribution of jump rates. A quantitative analysis yields that the contribution of the dynamical heterogeneities to the non-exponential depopulation of the lithium sites increases upon cooling. Further, correlated back-and-forth jumps between neighboring sites are observed for the fast ions of the distribution, but not for the slow ions and, hence, the back-jump probability depends on the dynamical state. Four-time correlation functions indicate that an exchange between fast and slow ions takes place on the timescale of the jumps themselves, i.e., the dynamical heterogeneities are short-lived. Hence, sites featuring fast and slow lithium dynamics, respectively, are intimately mixed. In addition, a backward correlation beyond the first neighbor shell for highly mobile ions and the presence of long-range dynamical heterogeneities suggest that fast ion migration occurs along preferential pathways in the glassy matrix. In the melt, we find no evidence for correlated back-and-forth motions and dynamical heterogeneities on the length scale of the next-neighbor distance.Comment: 12 pages, 13 figure

Data driven optimal filtering for phase and frequency of noisy oscillations: application to vortex flowmetering

A new method for extracting the phase of oscillations from noisy time series is proposed. To obtain the phase, the signal is filtered in such a way that the filter output has minimal relative variation in the amplitude (MIRVA) over all filters with complex-valued impulse response. The argument of the filter output yields the phase. Implementation of the algorithm and interpretation of the result are discussed. We argue that the phase obtained by the proposed method has a low susceptibility to measurement noise and a low rate of artificial phase slips. The method is applied for the detection and classification of mode locking in vortex flowmeters. A novel measure for the strength of mode locking is proposed.Comment: 12 pages, 10 figure