The quantification of phase synchrony between neuronal signals is of crucial importance for the study of large-scale interactions in the brain. Two methods have been used to date in neuroscience, ...based on two distinct approaches which permit a direct estimation of the instantaneous phase of a signal Phys. Rev. Lett. 81 (1998) 3291; Human Brain Mapping 8 (1999) 194. The phase is either estimated by using the analytic concept of Hilbert transform or, alternatively, by convolution with a complex wavelet. In both methods the stability of the instantaneous phase over a window of time requires quantification by means of various statistical dependence parameters (standard deviation, Shannon entropy or mutual information). The purpose of this paper is to conduct a direct comparison between these two methods on three signal sets: (1) neural models; (2) intracranial signals from epileptic patients; and (3) scalp EEG recordings. Levels of synchrony that can be considered as reliable are estimated by using the technique of surrogate data. Our results demonstrate that the differences between the methods are minor, and we conclude that they are fundamentally equivalent for the study of neuroelectrical signals. This offers a common language and framework that can be used for future research in the area of synchronization.
Transient periods of synchronization of oscillating neuronal discharges
in the frequency range 30-80 Hz (gamma oscillations) have been
proposed to act as an integrative mechanism that may bring a ...widely distributed
set of neurons together into a coherent ensemble that underlies a cognitive
act. Results of several experiments in animals provide
support for this idea (see, for example, refs 4,5,6,7,8,9,10).
In humans, gamma oscillations have been described both on the scalp
(measured by electroencephalography and magnetoencephalography) and in intracortical
recordings, but no direct participation of synchrony in a
cognitive task has been demonstrated so far. Here we record electrical brain
activity from subjects who are viewing ambiguous visual stimuli (perceived
either as faces or as meaningless shapes). We show for the first time, to
our knowledge, that only face perception induces a long-distance pattern of
synchronization, corresponding to the moment of perception itself and to the
ensuing motor response. A period of strong desynchronization marks the transition
between the moment of perception and the motor response. We suggest that this
desynchronization reflects a process of active uncoupling of the underlying
neural ensembles that is necessary to proceed from one cognitive state to
another.
Even during well-calibrated cognitive tasks, successive brain responses to repeated identical stimulations are highly variable. The source of this variability is believed to reside mainly in ...fluctuations of the subject's cognitive "context" defined by his/her attentive state, spontaneous thought process, strategy to carry out the task, and so on . . . As these factors are hard to manipulate precisely, they are usually not controlled, and the variability is discarded by averaging techniques. We combined first-person data and the analysis of neural processes to reduce such noise. We presented the subjects with a three-dimensional illusion and recorded their electrical brain activity and their own report about their cognitive context. Trials were clustered according to these first-person data, and separate dynamical analyses were conducted for each cluster. We found that (i) characteristic patterns of endogenous synchrony appeared in frontal electrodes before stimulation. These patterns depended on the degree of preparation and the immediacy of perception as verbally reported. (ii) These patterns were stable for several recordings. (iii) Preparatory states modulate both the behavioral performance and the evoked and induced synchronous patterns that follow. (iv) These results indicated that first-person data can be used to detect and interpret neural processes.
A high percentage of hydrocarbon (HC) emissions from gasoline vehicles occur during the cold‐start period. Among the alternatives proposed to reduce these HC emissions, the use of zeolites before the ...three‐way catalyst (TWC) is thought to be very effective. Zeolites are the preferred adsorbents for this application; however, to avoid high pressure drops, supported zeolites are needed. In this work, two coating methods (dip‐coating and in situ crystallization) are optimized to prepare BETA zeolite thin films supported on honeycomb monoliths with tunable properties. The important effect of the density of the thin film in the final performance as a HC trap is demonstrated. A highly effective HC trap is prepared showing 100 % toluene retention, accomplishing the desired performance as a HC trap, desorbing propene at temperatures close to 300 °C, and remaining stable after cycling. The use of this material before the TWC is very promising, and works towards achieving the sustainability and environmental protection goals.
Cautious driving: The optimization of two coating methods allows the preparation of BETA zeolite thin films supported on honeycomb monoliths with tunable properties. A highly effective hydrocarbon trap for the abatement of cold‐start emissions from the gasoline vehicles was obtained. The important effect of the film density on the final performance is demonstrated.
Zeolite LTA membranes supported on macroporous carbon discs have been modified by means of ion exchange and tested for their H₂ purification performance using a simulated reformer mixture as feed ...stream. As-prepared membranes (i.e. sodium form, Na-LTA/carbon) have been ion-exchanged with CsNO₃ (Cs-LTA/carbon) to tailor the membrane pore size and tested in a Wicke–Kallenbach (WK) cell in order to study their permeation properties. Both membranes have been tested using a simulated reformer mixture in dry conditions (50% H₂, 1.25% CO and n% CO₂ in He), where the CO₂ concentration has been modified (n%=0, 2, 5, 10, 15 and 20%), and their permeation properties studied. In addition, these membranes (Na- and Cs-forms) have been also tested using a simulated reformer mixture on humid conditions (50% H₂, 1.25% CO, 20% CO₂ and 5% H₂O in He). All the experiments using different stream compositions have been carried out at three different temperatures (303, 398 and 423K). Furthermore, in order to analyze and understand the permeation characteristics of the composite materials, commercial zeolite A in powder form has been used to study the interaction between CO₂ and the zeolite. Therefore, thermogravimetric analysis (TGA), in situ diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) experiments and CO₂ adsorption isotherms at different temperatures (303K, 398K and 423K) have been used in order to analyze this interaction. The final composite, Cs-LTA/carbon can separate H₂ from CO under all the experimental conditions studied and possesses high permeance and an excellent H₂/CO separation factor.
We propose a new approach to the neuroscience of consciousness, growing out of the ‘enactive’ viewpoint in cognitive science. This approach aims to map the neural substrates of consciousness at the ...level of large-scale, emergent and transient dynamical patterns of brain activity (rather than at the level of particular circuits or classes of neurons), and it suggests that the processes crucial for consciousness cut across the brain–body–world divisions, rather than being brain-bound neural events. Whereas standard approaches to the neural correlates of consciousness have assumed a one-way causal-explanatory relationship between internal neural representational systems and the contents of consciousness, our approach allows for theories and hypotheses about the two-way or reciprocal relationship between embodied conscious states and local neuronal activity.
We applied a new method of imaging frequency-specific changes in brain activity in humans during a finger brushing task in order to measure changes in cortical rhythms during tactile stimulation. ...Neuromagnetic recordings were conducted in five subjects using a whole-head MEG system during tactile stimulation of the right index finger, with or without visual feedback, and while viewing another individual’s index finger being stimulated. Volumetric images of changes in source power relative to pre-stimulus baseline levels were computed with 2 mm resolution over the entire brain using a minimum-variance beamforming algorithm (synthetic aperture magnetometry). Onset of tactile stimulation produced a brief (200–300 ms) suppression of mu band (8–15 Hz) and beta band (15–30 Hz) cortical activity in the primary somatosensory and primary motor cortex, respectively, followed by a bilateral increase in beta band activity (‘beta rebound’) in motor cortex. This pattern of suppression/rebound was absent when subjects observed finger brushing or brushing motions without receiving stimulation. In contrast, these conditions resulted in bilateral increases in beta band activity in sensorimotor areas and decreased power in the alpha (8–12 Hz) band in primary visual areas. These results show that spatially filtered MEG provides a useful method for directly imaging the temporal sequence of changes in cortical rhythms during transient tactile stimulation, and provide evidence that observation of tactile input to another individual’s hand, or object motion itself, can influence independent rhythmic activity in visual and sensorimotor cortex.
On Becoming Aware Depraz, Nathalie; Varela, Francisco J; Vermersch, Pierre
2000
eBook
This book searches for the sources and means for a disciplined practical approach to exploring human experience. The spirit of this book is pragmatic and relies on a Husserlian phenomenology ...primarily understood as a method of exploring our experience. The authors do not aim at a neo-Kantian a priori 'new theory' of experience but instead they describe a concrete activity: how we examine what we live through, how we become aware of our own mental life. The range of experiences of which we can become aware is vast: all the normal dimensions of human life (perception, motion, memory, imagination, speech, everyday social interactions), cognitive events that can be precisely defined as tasks in laboratory experiments (e.g., a protocol for visual attention), but also manifestations of mental life more fraught with meaning (dreaming, intense emotions, social tensions, altered states of consciousness). The central assertion in this work is that this immanent ability is habitually ignored or at best practiced unsystematically, that is to say, blindly. Exploring human experience amounts to developing and cultivating this basic ability through specific training. Only a hands-on, non-dogmatic approach can lead to progress, and that is what animates this book. (Series B).
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