1,721,147 research outputs found
Exploring the relationship of phase and peak-frequency EEG alpha-band and beta-band activity to temporal judgments of stimulus duration
No full textPre-stimulus phase has been shown to influence temporal judgments concerning order, causality and simultaneity. One hypothesis is that phase cycles frame discrete perceptual snapshots over time. Yet, existing studies have explored the effect of pre-stimulus phase on fine-grained temporal judgments whereas no study has shown whether pre-stimulus phase influences subsecond temporal judgments lasting several phase cycles. If effects of phase on fine-grainedtemporal judgments reflect perceptual framing, then the perception of longer intervals might show some dependency on the frequency of phase cycles. Higher frequencies should promote increased temporal resolution and discrimination. We tested the relationship between the phase and frequency of oscillations and temporal judgments for longer durations. Participants judged the relative duration of two successive intervals lasting several phase cycles each. Pre-stimulus alpha-band and beta-band phase was associated with subsequent temporal judgments, although not sensitivity, therein providing evidence that pre-stimulus phase is related to temporal judgments that span a longer time-scale than has been previously demonstrated. Although we report evidence that peak-frequency of the alpha-band is related to one measure of task performance, this study does not provide evidence that higher peak frequencies of alpha- or beta-band activity are related to improved duration discrimination of longer intervals.<br/
The phase of pre-stimulus alpha oscillations influences the visual perception of stimulus timing
Full text linkThis study examined the influence of pre-stimulus alpha phase and attention on whether two visual stimuli occurring closely in time were perceived as simultaneous or asynchronous. The results demonstrated that certain phases of alpha in the period immediately preceding stimulus onset were associated with a higher proportion of stimuli judged to be asynchronous. Furthermore, this effect was shown to occur independently of both visuo-spatial attention and alpha amplitude. The findings are compatible with proposals that alpha phase reflects cyclic shifts in neuronal excitability. Importantly, however, the results further suggest that fluctuations in neuronal excitability can create a periodicity in neuronal transfer that can have functional consequences that are decoupled from changes in alpha amplitude. This study therefore provides evidence that perceptual processes fluctuate periodically although it remains uncertain whether this implies the discrete temporal framing of perception
The control of carbonate mineral Mg isotope composition by aqueous speciation: Theoretical and experimental modeling
No full textThe magnesium isotope compositions of sedimentary carbonates are widely used to investigate the geochemical cycling of this element in seawater. Density Functional Theory techniques and experimental data from this study and literature on Mg isotope fractionation between sedimentary minerals and fluids have been used to derive equilibrium fractionation factors and to quantify the impact of aqueous magnesium speciation on the isotopic composition of its aquo ion and Mg in the precipitated carbonates. Although aqueous Mg2 + undergoes hydrolysis to a lesser extent than divalent transition metals, it nevertheless forms relatively strong complexes with inorganic and organic ligands including bicarbonate/carbonate, sulfate and carboxylate. Furthermore, aqueous Mg2 + undergoes a significant contraction of its coordination sphere when it reacts with bicarbonate and carbonate ions to form the MgHCO3+ and MgCO3° complexes which favors the preferential partitioning of heavy Mg into these species. Calculated values of the reduced partition function ratios for Mg2 +(aq) and Mg2 + bound to a number of inorganic and organic ligands show a strong enrichment of 26Mg in MgCO3°, MgHCO3+ and MgSO4° compared to Mg2 + (i.e. 1000ln?26/24MgCO3°-1000ln?26/24Mg2 + = 5.2 at 25 °C), and either a significant enrichment (Mg(oxalate)2 ?, Mg(oxalate)22 ?, Mg(citrate)?) or depletion (Mg(EDTA)2 ?, Mg(citrate)24 ?) of 26Mg in Mg-carboxylate complexes compared to Mg2 +. Analysis of experiments from this study and the literature on Mg isotope fractionation between aqueous solution and Mg carbonate and hydroxide minerals validate the reduced partition functions for Mg2+, Mg bicarbonate/carbonate and carboxylate couples generated in the present work and confirms the significant impact of carbonate and carboxylic ligands on the isotope composition of precipitated Mg-bearing minerals. This study thereby provides new insights into the parameters controlling the isotope composition of aqueous Mg2+ in natural fluids as well as improved tools to reconstruct paleo-environmental conditions from the magnesium isotope compositions recorded in carbonate sediments
Quantitative separation of molybdenum and rhenium from geological materials for isotopic analysis by MC-ICP-MS
No full textWe have developed a new chemical procedure for the quantitative separation of molybdenum (Mo) and rhenium (Re) from a wide variety of geological samples. A single pass anion exchange separation provided complete recovery of pure Mo and Re in a form that was ideal for subsequent isotope and abundance determination by multi-collector inductively coupled plasma-mass spectroscopy (MC-ICP-MS). An enriched 100Mo-97Mo solution, mixed with the sample before digestion, enabled natural mass-dependant isotopic fractionation of Mo to be determined with an external reproducibility of < 0.12‰ (?98Mo/95Mo, 2s). Determination of the concentration of Mo and Re in the same sample was achieved by isotope dilution, with instrumental mass-fractionation of Re being corrected by the simultaneous measurement of the 191Ir/193Ir ratio. We have applied the new procedure to a variety of samples, including seawater, basalt and organic-rich mudrock. The procedure is ideally suited to palaeoredox studies requiring the precise determination of the Mo isotope composition and the Re/Mo ratio from the same sampl
Tracing olivine carbonation and serpentinization in CO<sub>2</sub>-rich fluids via magnesium exchange and isotopic fractionation
Full text linkChemical exchange between seawater and the oceanic crust is thought to play a significant role in the regulation of the global magnesium (Mg) cycle, yet relatively little is known about the rates and mechanisms of Mg exchange in these crustal environments. In this study we experimentally characterize the extent, and nature, of Mg isotope fractionation during the carbonation and serpentinization of olivine (one of the principal minerals found in ultramafic rocks) under hydrothermal conditions. Olivine alteration was found to be incongruent, with the reactant fluid composition varying according to the extent of olivine dissolution and the precipitation of secondary minerals. In mildly acid water (pH ∼ 6.5), olivine dissolved to form Mg-Fe carbonate solid solutions and minor chrysotile. Upon carbonation and a decrease of CO2 in the water, the pH increased to >8, with chrysotile and brucite becoming the dominant alteration minerals. The Mg-rich carbonates preferentially incorporated lighter Mg isotopes, resulting in a ∼0.5‰ increase of the δ26Mg composition of the fluid relative to olivine during the initial carbonation and serpentinization reactions. This was followed by a decrease in δ26Mg under higher pH conditions associated with the formation of brucite. Our experimental and modeling results therefore demonstrate that the δ26Mg composition of fluids involved in olivine alteration reflect the type and quantity of secondary Mg minerals formed, which in turn depend on the pH and CO2 concentration of the water. Comparison of these results with natural groundwaters and geothermal waters from basaltic terrains indicate that the δ26Mg composition of natural waters are likely to also be controlled by mafic rock dissolution and the preferential incorporation of isotopically light Mg into carbonates and isotopically heavy Mg into Mg-Si minerals. Together, these findings improve our understanding of Mg isotope systematics during water-rock interaction, and suggest that δ26Mg may be a useful tool for tracing reactions that are critical to geological CO2 sequestration.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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