Neural oscillatory signatures of auditory and audiovisual illusions

Questions of the relationship between human perception and brain activity can be approached from different perspectives: in the first, the brain is mainly regarded as a recipient and processor of sensory data. The corresponding research objective is to establish mappings of neural activity patterns and external stimuli. Alternatively, the brain can be regarded as a self-organized dynamical system, whose constantly changing state affects how incoming sensory signals are processed and perceived. The research reported in this thesis can chiefly be located in the second framework, and investigates the relationship between oscillatory brain activity and the perception of ambiguous stimuli. Oscillations are here considered as a mechanism for the formation of transient neural assemblies, which allows efficient information transfer. While the relevance of activity in distinct frequency bands for auditory and audiovisual perception is well established, different functional architectures of sensory integration can be derived from the literature. This dissertation therefore aims to further clarify the role of oscillatory activity in the integration of sensory signals towards unified perceptual objects, using illusion paradigms as tools of study. In study 1, we investigate the role of low frequency power modulations and phase alignment in auditory object formation. We provide evidence that auditory restoration is associated with a power reduction, while the registration of an additional object is reflected by an increase in phase locking. In study 2, we analyze oscillatory power as a predictor of auditory influence on visual perception in the sound-induced flash illusion. We find that increased beta-/ gamma-band power over occipitotemporal electrodes shortly before stimulus onset predicts the illusion, suggesting a facilitation of processing in polymodal circuits. In study 3, we address the question of whether visual influence on auditory perception in the ventriloquist illusion is reflected in primary sensory or higher-order areas. We establish an association between reduced theta-band power in mediofrontal areas and the occurrence of illusion, which indicates a top-down influence on sensory decision-making. These findings broaden our understanding of the functional relevance of neural oscillations by showing that different processing modes, which are reflected in specific spatiotemporal activity patterns, operate in different instances of sensory integration.Fragen nach dem Zusammenhang zwischen menschlicher Wahrnehmung und Hirnaktivität können aus verschiedenen Perspektiven adressiert werden: in der einen wird das Gehirn hauptsächlich als Empfänger und Verarbeiter von sensorischen Daten angesehen. Das entsprechende Forschungsziel wäre eine Zuordnung von neuronalen Aktivitätsmustern zu externen Reizen. Dieser Sichtweise gegenüber steht ein Ansatz, der das Gehirn als selbstorganisiertes dynamisches System begreift, dessen sich ständig verändernder Zustand die Verarbeitung und Wahrnehmung von sensorischen Signalen beeinflusst. Die Arbeiten, die in dieser Dissertation zusammengefasst sind, können vor allem in der zweitgenannten Forschungsrichtung verortet werden, und untersuchen den Zusammenhang zwischen oszillatorischer Hirnaktivität und der Wahrnehmung von mehrdeutigen Stimuli. Oszillationen werden hier als ein Mechanismus für die Formation von transienten neuronalen Zusammenschlüssen angesehen, der effizienten Informationstransfer ermöglicht. Obwohl die Relevanz von Aktivität in verschiedenen Frequenzbändern für auditorische und audiovisuelle Wahrnehmung gut belegt ist, können verschiedene funktionelle Architekturen der sensorischen Integration aus der Literatur abgeleitet werden. Das Ziel dieser Dissertation ist deshalb eine Präzisierung der Rolle oszillatorischer Aktivität bei der Integration von sensorischen Signalen zu einheitlichen Wahrnehmungsobjekten mittels der Nutzung von Illusionsparadigmen. In der ersten Studie untersuchen wir die Rolle von Leistung und Phasenanpassung in niedrigen Frequenzbändern bei der Formation von auditorischen Objekten. Wir zeigen, dass die Wiederherstellung von Tönen mit einer Reduktion der Leistung zusammenhängt, während die Registrierung eines zusätzlichen Objekts durch einen erhöhten Phasenangleich widergespiegelt wird. In der zweiten Studie analysieren wir oszillatorische Leistung als Prädiktor von auditorischem Einfluss auf visuelle Wahrnehmung in der sound-induced flash illusion. Wir stellen fest, dass erhöhte Beta-/Gamma-Band Leistung über occipitotemporalen Elektroden kurz vor der Reizdarbietung das Auftreten der Illusion vorhersagt, was auf eine Begünstigung der Verarbeitung in polymodalen Arealen hinweist. In der dritten Studie widmen wir uns der Frage, ob ein visueller Einfluss auf auditorische Wahrnehmung in der ventriloquist illusion sich in primären sensorischen oder übergeordneten Arealen widerspiegelt. Wir weisen einen Zusammenhang von reduzierter Theta-Band Leistung in mediofrontalen Arealen und dem Auftreten der Illusion nach, was einen top-down Einfluss auf sensorische Entscheidungsprozesse anzeigt. Diese Befunde erweitern unser Verständnis der funktionellen Bedeutung neuronaler Oszillationen, indem sie aufzeigen, dass verschiedene Verarbeitungsmodi, die sich in spezifischen räumlich-zeitlichen Aktivitätsmustern spiegeln, in verschiedenen Phänomenen von sensorischer Integration wirksam sind

Observing high redshift galaxy clusters through lensing of the Ostriker-Vishniac effect

In this paper we study the possibility of detecting lensing signals in high-resolution and high-sensitivity CMB experiments. At scales below 1 arcmin, the CMB background is dominated by the Sunyaev-Zel'dovich effect in clusters and by Ostriker-Vishniac effect distortions elsewhere. Assuming the Sunyaev-Zel'dovich component in clusters can be removed, we focus on the Ostriker-Vishniac effect and study the possibility of its detection while paying special attention to contaminants, such as instrumental noise and point sources. After designing an optimal filter for this particular lensing signal we explore the signal-to-noise ratio for different scenarios varying the resolution of the experiment, its sensitivity, and the level of contamination due to point sources. Our results show that the next generation of experiments should be able to do new and exciting science through the lensing effect of the Ostriker-Vishniac background.Comment: Submiteed to MNRA

Cosmic shear statistics in cosmologies with non-Gaussian initial conditions

We computed the power spectrum of weak cosmic shear in models with non-Gaussian primordial density fluctuations. Cosmological initial conditions deviating from Gaussianity have recently attracted much attention in the literature, especially with respect to their effect on the formation of non-linear structures and because of the bounds that they can put on the inflationary epoch. The fully non-linear matter power spectrum was evaluated with the use of the physically motivated, semi-analytic halo model, where the mass function and linear halo bias were suitably corrected for non-Gaussian cosmologies. In agreement with previous work, we found that a level of non-Gaussianity compatible with CMB bounds and with positive skewness produces an increase in power of the order of a few percent at intermediate scales. We then used the matter power spectrum, together with observationally motivated background source redshift distributions in order to compute the cosmological weak lensing power spectrum. We found that the degree of deviation from the power spectrum of the reference Gaussian model is small compared to the statistical error expected from even future weak lensing surveys. However, summing the signal over a large range of multipoles can beat down the noise, bringing to a significant detection of non-Gaussianity at the level of $|f_\mathrm{NL}| \simeq$ few tens, when all other cosmological parameters are held fixed. Finally, we have shown that the constraints on the level of non-Gaussianity can be improved by $\sim 20$ with the use of weak lensing tomography.Comment: 15 pages, 10 figures. Accepted by MNRA

Spectrally-resolved UV photodesorption of CH4 in pure and layered ices

Context. Methane is among the main components of the ice mantles of insterstellar dust grains, where it is at the start of a rich solid-phase chemical network. Quantification of the photon-induced desorption yield of these frozen molecules and understanding of the underlying processes is necessary to accurately model the observations and the chemical evolution of various regions of the interstellar medium. Aims. This study aims at experimentally determining absolute photodesorption yields for the CH4 molecule as a function of photon energy. The influence of the ice composition is also investigated. By studying the methane desorption from layered CH4:CO ice, indirect desorption processes triggered by the excitation of the CO molecules is monitored and quantified. Methods. Tunable monochromatic VUV light from the DESIRS beamline of the SOLEIL synchrotron is used in the 7 - 13.6 eV (177 - 91 nm) range to irradiate pure CH4 or layers of CH4 deposited on top of CO ice samples. The release of species in the gas phase is monitored by quadrupole mass spectrometry and absolute photodesorption yields of intact CH4 are deduced. Results. CH4 photodesorbs for photon energies higher than ~9.1 eV (~136 nm). The photodesorption spectrum follows the absorption spectrum of CH4, which confirms a desorption mechanism mediated by electronic transitions in the ice. When it is deposited on top of CO, CH4 desorbs between 8 and 9 eV with a pattern characteristic of CO absorption, indicating desorption induced by energy transfer from CO molecules. Conclusions. The photodesorption of CH4 from the pure ice in various interstellar environments is around 2.0 x 10^-3 molecules per incident photon. Results on CO-induced indirect desorption of CH4 provide useful insights for the generalization of this process to other molecules co-existing with CO in ice mantles

The Sunyaev-Zel'dovich effect in WMAP data

Using WMAP 5 year data, we look for the average Sunyaev-Zel'dovich effect (SZE) signal from clusters of galaxies by stacking the regions around hundreds of known X-ray clusters. We detect the average SZE at a very high significance level. The average cluster signal is spatially resolved in the W band. This mean signal is compared with the expected signal from the same clusters calculated on the basis of archival ROSAT data. From the comparison we conclude that the observed SZE seems to be less than the expected signal derived from X-ray measurements when a standard beta-model is assumed for the gas distribution. This conclusion is model dependent. Our predictions depend mostly on the assumptions made about the core radius of clusters and the slope of the gas density profile. Models with steeper profiles are able to simultaneously fit both X-ray and WMAP data better than a beta-model. However, the agreement is not perfect and we find that it is still difficult to make the X-ray and SZE results agree. A model assuming point source contamination in SZE clusters renders a better fit to the one-dimensional SZE profiles thus suggesting that contamination from point sources could be contributing to a diminution of the SZE signal. Selecting a model that better fits both X-ray and WMAP data away from the very central region, we estimate the level of contamination and find that on average, the point source contamination is on the level of 16 mJy (at 41 GHz), 26 mJy (at 61 GHz) and 18 mJy (at 94 GHz). These estimated fluxes are marginally consistent with the estimated contamination derived from radio and infrared surveys thus suggesting that the combination of a steeper gas profile and the contribution from point sources allows us to consistently explain the X-ray emission and SZE in galaxy clusters as measured by ROSAT and WMAP.Comment: 17 pages and 17 figures. Submited to MNRA

How Low Can You Go?: Widespread Challenges in Measuring Low Stream Discharge and a Path Forward

Low flows pose unique challenges for accurately quantifying streamflow. Current field methods are not optimized to measure these conditions, which in turn, limits research and management. In this essay, we argue that the lack of methods for measuring low streamflow is a fundamental challenge that must be addressed to ensure sustainable water management now and into the future, particularly as climate change shifts more streams to increasingly frequent low flows. We demonstrate the pervasive challenge of measuring low flows, present a decision support tool (DST) for navigating best practices in measuring low flows, and highlight important method developmental needs

Large Einstein Radii: A Problem for LambdaCDM

The Einstein radius of a cluster provides a relatively model-independent measure of the mass density of a cluster within a projected radius of ~ 150 kpc, large enough to be relatively unaffected by gas physics. We show that the observed Einstein radii of four well-studied massive clusters, for which reliable virial masses are measured, lie well beyond the predicted distribution of Einstein radii in the standard LambdaCDM model. Based on large samples of numerically simulated cluster-sized objects with virial masses ~ 1e15 solar, the predicted Einstein radii are only 15-25'', a factor of two below the observed Einstein radii of these four clusters. This is because the predicted mass profile is too shallow to exceed the critical surface density for lensing at a sizable projected radius. After carefully accounting for measurement errors as well as the biases inherent in the selection of clusters and the projection of mass measured by lensing, we find that the theoretical predictions are excluded at a 4-sigma significance. Since most of the free parameters of the LambdaCDM model now rest on firm empirical ground, this discrepancy may point to an additional mechanism that promotes the collapse of clusters at an earlier time thereby enhancing their central mass density.Comment: 9 pages, 5 figures, accepted by MNRA

Genetic Reconstruction of Protozoan rRNA Decoding Sites Provides a Rationale for Paromomycin Activity against Leishmania and Trypanosoma

Aminoglycoside antibiotics target the ribosomal decoding A-site and are active against a broad spectrum of bacteria. These compounds bind to a highly conserved stem-loop-stem structure in helix 44 of bacterial 16S rRNA. One particular aminoglycoside, paromomycin, also shows potent antiprotozoal activity and is used for the treatment of parasitic infections, e.g. by Leishmania spp. The precise drug target is, however, unclear; in particular whether aminoglycoside antibiotics target the cytosolic and/or the mitochondrial protozoan ribosome. To establish an experimental model for the study of protozoan decoding-site function, we constructed bacterial chimeric ribosomes where the central part of bacterial 16S rRNA helix 44 has been replaced by the corresponding Leishmania and Trypanosoma rRNA sequences. Relating the results from in-vitro ribosomal assays to that of in-vivo aminoglycoside activity against Trypanosoma brucei, as assessed in cell cultures and in a mouse model of infection, we conclude that aminoglycosides affect cytosolic translation while the mitochondrial ribosome of trypanosomes is not a target for aminoglycoside antibiotics

The impact of dust on the scaling properties of galaxy clusters

We investigate the effect of dust on the scaling properties of galaxy clusters based on hydrodynamic N-body simulations of structure formation. We have simulated five dust models plus a radiative cooling and adiabatic models using the same initial conditions for all runs. The numerical implementation of dust was based on the analytical computations of Montier and Giard (2004). We set up dust simulations to cover different combinations of dust parameters that put in evidence the effects of size and abundance of dust grains. Comparing our radiative plus dust cooling runs to a purely radiative cooling simulation we find that dust has an impact on cluster scaling relations. It mainly affects the normalisation of the scalings (and their evolution), whereas it introduces no significant differences on their slopes. The strength of the effect depends critically on the dust abundance and grain size parameters as well as on the cluster scaling. Indeed, cooling due to dust is effective at the cluster regime and has a stronger effect on the "baryon driven" statistical properties of clusters such as $L_{\rm X}-M$, $Y- M$, $S-M$ scaling relations. Major differences, relative to the radiative cooling model, are as high as 25% for the $L_{\rm X}-M$ normalisation, and about 10% for the $Y-M$ and $S-M$ normalisations at redshift zero. On the other hand, we find that dust has almost no impact on the "dark matter driven" $T_{\rm mw}-M$ scaling relation. The effects are found to be dependent in equal parts on both dust abundances and grain sizes distributions for the scalings investigated in this paper. Higher dust abundances and smaller grain sizes cause larger departures from the radiative cooling (i.e. with no dust) model.Comment: 12 pages, 6 figures, submitted to MNRA

Mid-Infrared observations of GRS 1915+105 during plateau and flaring states

We present mid-infrared (4-18 micron) observations of the microquasar GRS 1915+105 obtained with ISOCAM, the camera on board the Infrared Space Observatory (ISO), in 1996 April and 1997 October. The first observation probably occurred during a flaring event with oscillating synchrotron emission. The 1997 observation occurred a few days before a major relativistic ejection, during a plateau state of inverted-spectrum radio emission and hard quasi-stable X-ray emission. The K-M giant donor star in GRS 1915+105 cannot account for the mid-IR emission and we discuss the possible additional components depending on two absorption laws. Thermal emission from dust seems unlikely. The flat mid-IR spectrum obtained during the plateau state is likely to be synchrotron emission. It would be the first evidence of the infrared extension of the radio synchrotron emission from the compact jets, although optically thin free-free emission from an X-ray driven-wind from the accretion disc cannot be excluded.Comment: 12 pages, 5 figures, accepted by A&