Excitation Of Fluctuations Research Articles

Temporal framing in apparent motion perception cycles with a 12Hz (alpha) rhythm

Visual perception is periodic and structured in recurrent temporal frames. However, it is unclear whether these cycles simply reflect fluctuations of visual excitability, and/or organize perception into discrete moments of information integration in which two events falling within one frame are more likely to be perceived as causally linked. In 1981, Varela et al. probed temporal framing of perception by presenting two sequential flashes at a constant delay and found these flashes to be perceived as simultaneous or sequential depending on the phase of ongoing alpha (7-13Hz) oscillations. These results are still awaiting replication (VanRullen et al, 2014). We here tested periodicity of visual information integration in two behavioural experiments probing (i) simultaneity and (ii) apparent motion perception in double flash stimuli. We flashed two visual events either sequentially or simultaneously. In experiment 1, two dots were displayed in the left and right visual fields, while participants reported whether the events were perceived as sequential or simultaneous. In experiment 2, the two events consisted of random dot patterns that could be perceived as coherent motion, incoherent motion or glass patterns. These stimulus pairs were presented time-locked to a preceding sound (0-300ms delays), shown to reset the phase of visual brain oscillations (Romei et al., 2012). Spectro-temporal dynamics of behavioural time courses (hits) revealed periodicity in perception. Dots perceived as sequential (Group 1, n=19) and apparent motion perception (Group 2, n=19) showed identical, cyclic patterns in the 12Hz (alpha) frequency band (with a significant phase consistency across participants). In contrast, a cyclic pattern in the 4Hz (theta) band was observed in the control conditions (simultaneously presented stimuli), suggesting that the alpha cycles in visual perception go beyond reflecting fluctuations in visual excitability. These data suggest that perceptual frames in the alpha-band play a key role in spatio-temporal visual information integration.

Nematic fluctuations and semisoft elasticity in swollen liquid-crystal elastomers.

Dynamic light scattering (DLS) experiments were performed on stretched sheets of liquid crystal elastomers (LCEs) swollen with a nematic solvent with different swelling ratios. We show that the obtained stress-strain curve and DLS data can still be explained with the concepts of semisoft elasticity. The stress-strain curve shows a typical semisoft response with a threshold strain and a plateau region where stress increases only a little with the applied strain. The width of the plateau decreases with the increase of the swelling ratio because the polymer backbone anisotropy reduces during the swelling. The relaxation rate of thermally excited director fluctuations, however, still shows a typical response, and our measurements indicate the presence of a soft dynamic director-shear mode, as predicted by the theory of semisoft elasticity.

Neuronal oscillations as a mechanistic substrate of auditory temporal prediction.

Neuronal oscillations are comprised of rhythmic fluctuations of excitability that are synchronized in ensembles of neurons and thus function as temporal filters that dynamically organize sensory processing. When perception relies on anticipatory mechanisms, ongoing oscillations also provide a neurophysiological substrate for temporal prediction. In this article, we review evidence for this account with a focus on auditory perception. We argue that such "oscillatory temporal predictions" can selectively amplify neuronal sensitivity to inputs that occur in a predicted, task-relevant rhythm and optimize temporal selection. We elaborate this argument for a prototypic example, speech processing, where information is present at multiple time scales, with delta, theta, and low-gamma oscillations being specifically and simultaneously engaged, enabling multiplexing. We then consider the origin of temporal predictions, specifically the idea that the motor system is involved in the generation of such prior information. Finally, we place temporal predictions in the general context of internal models, discussing how they interact with feature-based or spatial predictions. We propose that complementary predictions interact synergistically according to a dominance hierarchy, shaping perception in the form of a multidimensional filter mechanism.

Combined experimental and computational investigation of cavitation evolution and excited pressure fluctuation in a convergent–divergent channel

The objective of this paper is to investigate the cavitation evolution and excited pressure fluctuation in unsteady sheet/cloud cavitating flows in a convergent–divergent channel using a combination of experimental and computational studies. In the experiments, a simultaneous sampling technique is used to synchronize the cavitation images with wall-pressure signals. The computations are based on the Navier–Stokes equations solved with a mass transfer cavitation model and a modified RNG k–ε turbulence model. The experimental results indicate that, for non-cavitating flow, neither significant fluctuation nor periodic fluctuation can be observed. However, in the case of sheet/cloud cavitating conditions, significant quasi-periodic pressure fluctuations are captured in the entire cavitation area. The numerically predicted unsteady sheet/cloud cavitation behaviors agree well with the experimental results; both the experimental and computational results indicate that a typical quasi-periodic sheet/cloud cavitation development process can be divided into three main stages: (1) the growth of the attached cavity, (2) shedding of the attached cavity, and (3) growth and collapse of the detached cavities. The quasi-periodic pressure fluctuations in sheet/cloud cavitation are found to be highly related with the time evolution of the cavitation behaviors. The acceleration due to the changes in the cavity volume is found to be the main source of the excited pressure fluctuations. The pressure signal decreases in the middle of the growing process of the attached cavity and the detached cavity. Next, the pressure signal increases and reaches a peak with the collapse of large detached cavity near transducer 4#.

Statistics of non-affine defect precursors: tailoring defect densities in colloidal crystals using external fields.

Coarse-graining atomic displacements in a solid produces both local affine strains and "non-affine" fluctuations. Here we study the equilibrium dynamics of these coarse grained quantities to obtain space-time dependent correlation functions. We show how a subset of these thermally excited, non-affine fluctuations act as precursors for the nucleation of lattice defects and suggest how defect probabilities may be altered by an experimentally realisable "external" field conjugate to the global non-affinity parameter. Our results are amenable to verification in experiments on colloidal crystals using commonly available holographic laser tweezer and video microscopy techniques, and may lead to simple ways of controlling the defect density of a colloidal solid.

Hydrodynamic enhancement of the diffusion rate in the region between two fluctuating membranes in close opposition: a theoretical and computational study.

Periodic variation of the distance between two weakly adhering bodies gives rise to a huge tangential motions of the sandwiched solvent layer (squeezing flow). Oscillations either can be induced by an external applied field or can spontaneously arise from the coupling with the solvent heat bath. First we calculated by the Navier-Stokes equation the components of the fluid velocity near two oscillating juxtaposed plates. Then we evaluated the influence of plate oscillations on the transport properties of a trace diffusant dissolved at t = 0 in the outer medium for both deterministic and stochastic excitations. By employing both analytical (Fokker-Planck) and coarse-grained molecular dynamics (MD) simulations, we proved that the entry and migration rates of the diffusant sharply increases with the oscillation amplitudes. Enhancement was related to relevant parameters like oscillation frequency, fluid layer thickness, fluid viscosity, and temperature. An extension to the case of oscillating multistacked lamellae has been also made. Theoretical and MD results suggest a significant enhancement of the diffusant flux even in the worse situation of thermally excited small amplitude fluctuations. Excitation arising from other sources (e.g., microwave or ultrasound irradiation of solid-fluid layered systems) could have a dramatic effect on the transport phenomena. Possible implications to relevant biological problems have been discussed.

Numerical investigation of three-dimensional cavitation evolution and excited pressure fluctuations around a twisted hydrofoil

Unsteady cavitating turbulent flow around a twisted hydrofoil was analyzed to illustrate the physical mechanism of the cavitygenerated pressure fluctuations. The numerical simulations of cavitating flow were based on the Partially-Averaged Navier-Stokes (PANS) method and a mass transfer cavitation model. The validity of PANS model has been evaluated and confirmed in cavitation simulations by present authors using three different cases, 2D hydrofoil (Ji et al. 2012 [37]), 3D hydrofoil (Ji et al. 2013 [31]) and marine propeller (Ji et al. 2012 [38]), which shows that the PANS model with f k = 0.2 and f e = 1 can obtain more accurate estimates of unsteady cavitating flows with large-scale fluctuations at a reasonable cost. In present paper we intended to shed light on the physical process responsible for the pressure fluctuations excited by cavitation. The cavity volume was analyzed to illustrate the relationship between the cavitation evolution and the pressure fluctuations. The results show that the cavity volumetric acceleration curve tracks remarkably well with the main features of the time-dependent pressure fluctuations except for the high frequency component. Thus, the cavity volumetric acceleration is the main source of the excited pressure fluctuations by cavitation. It is noted that the cavitation induced pressure fluctuations are transmitted along the suction surface of the hydrofoil and are synchronized with those on the pressure surface at the midplane of the twisted hydrofoil. Further, the pressure fluctuations on the pressure surface decrease towards the center from both the leading and trailing edges of the hydrofoil, with a minimum at 60% chord length from the leading edge.

Oscillatory brain state predicts variability in working memory.

Our capacity to remember and manipulate objects in working memory (WM) is severely limited. However, this capacity limitation is unlikely to be fixed because behavioral models indicate variability from trial to trial. We investigated whether fluctuations in neural excitability at stimulus encoding, as indexed by low-frequency oscillations (in the alpha band, 8-14 Hz), contribute to this variability. Specifically, we hypothesized that the spontaneous state of alpha band activity would correlate with trial-by-trial fluctuations in visual WM. Electroencephalography recorded from human observers during a visual WM task revealed that the prestimulus desynchronization of alpha oscillations predicts the accuracy of memory recall on a trial-by-trial basis. A model-based analysis indicated that this effect arises from a modulation in the precision of memorized items, but not the likelihood of remembering them (the recall rate). The phase of posterior alpha oscillations preceding the memorized item also predicted memory accuracy. Based on correlations between prestimulus alpha levels and stimulus-related visual evoked responses, we speculate that the prestimulus state of the visual system prefigures a cascade of state-dependent processes, ultimately affecting WM-guided behavior. Overall, our results indicate that spontaneous changes in cortical excitability can have profound consequences for higher visual cognition.

Is there signal in the noise?

A study now shows that variability in neuronal responses in the visual system mainly arises from slow fluctuations in excitability, presumably caused by factors of nonsensory origin, such as arousal, attention or anesthesia.

Partitioning neuronal variability.

Responses of sensory neurons differ across repeated measurements. This variability is usually treated as stochasticity arising within neurons or neural circuits. However, some portion of the variability arises from fluctuations in excitability due to factors that are not purely sensory, such as arousal, attention, and adaptation. To isolate these fluctuations, we developed a model in which spikes are generated by a Poisson process whose rate is the product of a drive that is sensory in origin, and a gain summarizing stimulus-independent modulatory influences on excitability. This model provides an accurate account of response distributions of visual neurons in macaque LGN, V1, V2, and MT, revealing that variability originates in large part from excitability fluctuations which are correlated over time and between neurons, and which increase in strength along the visual pathway. The model provides a parsimonious explanation for observed systematic dependencies of response variability and covariability on firing rate.

EEG oscillatory phase-dependent markers of corticospinal excitability in the resting brain.

Functional meaning of oscillatory brain activity in various frequency bands in the human electroencephalogram (EEG) is increasingly researched. While most research focuses on event-related changes of brain activity in response to external events there is also increasing interest in internal brain states influencing information processing. Several studies suggest amplitude changes of EEG oscillatory activity selectively influencing cortical excitability, and more recently it was shown that phase of EEG activity (instantaneous phase) conveys additional meaning. Here we review this field with many conflicting findings and further investigate whether corticospinal excitability in the resting brain is dependent on a specific spontaneously occurring brain state reflected by amplitude and instantaneous phase of EEG oscillations. We applied single pulse transcranial magnetic stimulation (TMS) over the left sensorimotor cortex, while simultaneously recording ongoing oscillatory activity with EEG. Results indicate that brain oscillations reflect rapid, spontaneous fluctuations of cortical excitability. Instantaneous phase but not amplitude of oscillations at various frequency bands at stimulation site at the time of TMS-pulse is indicative for brain states associated with different levels of excitability (defined by size of the elicited motor evoked potential). These results are further evidence that ongoing brain oscillations directly influence neural excitability which puts further emphasis on their role in orchestrating neuronal firing in the brain.

Pulsed Interleaved Excitation Fluctuation Imaging: Method and Application to HIV-1 Assembly

Pulsed interleaved excitation (PIE) is the method of nanosecond alternating excitation with time-resolved detection and allows accurate, independent and quasi-simultaneous determination of fluorescence intensities and lifetimes of different fluorophores. We have combined PIE with fluctuation imaging methods (PIE-FI) such as raster image correlation spectroscopy (RICS) and number and brightness analysis (N&B) on a home-built multicolor laser/sample scanning microscope. PIE-FI is a novel method that allows robust and accurate measurements of concentrations (down to pM), diffusion and stoichiometry, crosstalk-free dual-color measurements to probe protein-protein interactions, dual-color fluorescence lifetime measurements to quantify Forster resonance energy transfer and even allows resolving different species in images on the basis of their fluorescence lifetime rather than their color. We have applied PIE-FI to study the HIV-1 Gag protein inside live cells. During de novo synthesis of HIV particles in infected cells, this protein assembles at the plasma membrane and creates the shell that ultimately defines the size and shape of budded virions. Little is known about interactions of HIV Gag with cellular and/or viral components inside the cytosol of infected cells. We investigated the wild-type HIV Gag as well as some critical site-specific mutants that influence membrane and/or RNA and/or auto-interactions of Gag to gain more insight into these early stages of HIV assembly.

Frequency-specific mechanism links human brain networks for spatial attention

Selective attention allows us to filter out irrelevant information in the environment and focus neural resources on information relevant to our current goals. Functional brain-imaging studies have identified networks of broadly distributed brain regions that are recruited during different attention processes; however, the dynamics by which these networks enable selection are not well understood. Here, we first used functional MRI to localize dorsal and ventral attention networks in human epileptic subjects undergoing seizure monitoring. We subsequently recorded cortical physiology using subdural electrocorticography during a spatial-attention task to study network dynamics. Attention networks become selectively phase-modulated at low frequencies (δ, θ) during the same task epochs in which they are recruited in functional MRI. This mechanism may alter the excitability of task-relevant regions or their effective connectivity. Furthermore, different attention processes (holding vs. shifting attention) are associated with synchrony at different frequencies, which may minimize unnecessary cross-talk between separate neuronal processes.

Revisiting the entropic force between fluctuating biological membranes

The complex interplay between the various attractive and repulsive forces that mediate between biological membranes governs an astounding array of biological functions: cell adhesion, membrane fusion, self-assembly, binding-unbinding transition among others. In this work, the entropic repulsive force between membranes—which originates due to thermally excited fluctuations—is critically reexamined both analytically and through systematic Monte Carlo simulations. A recent work by Freund (2013) has questioned the validity of a well-accepted result derived by Helfrich (1978). We find that, in agreement with Freund, for small inter-membrane separations (d), the entropic pressure scales as p~1/d, in contrast to Helfrich's result: p~1/d3. For intermediate separations, our calculations agree with that of Helfrich and finally, for large inter-membrane separations, we observe an exponentially decaying behavior.

Binding constants of membrane-anchored receptors and ligands depend strongly on the nanoscale roughness of membranes

Cell adhesion and the adhesion of vesicles to the membranes of cells or organelles are pivotal for immune responses, tissue formation, and cell signaling. The adhesion processes depend sensitively on the binding constant of the membrane-anchored receptor and ligand proteins that mediate adhesion, but this constant is difficult to measure in experiments. We have investigated the binding of membrane-anchored receptor and ligand proteins with molecular dynamics simulations. We find that the binding constant of the anchored proteins strongly decreases with the membrane roughness caused by thermally excited membrane shape fluctuations on nanoscales. We present a theory that explains the roughness dependence of the binding constant for the anchored proteins from membrane confinement and that relates this constant to the binding constant of soluble proteins without membrane anchors. Because the binding constant of soluble proteins is readily accessible in experiments, our results provide a useful route to compute the binding constant of membrane-anchored receptor and ligand proteins.

P 166. Absence of delayed facilitation of cortical excitability at early stage of multiple sclerosis: Evidence of altered cortical plasticity?

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system, characterized by demyelination and axonal degeneration. By means of Transcranial magnetic stimulation (TMS) MS has been studied. Disabilities correlate with alterations of TMS parameters such as prolonged central motor conduction time, prolonged motor evoked potentials (MEP) latencies and increased motor threshold (MT). TMS has also been used to explore movement-related cortical plasticity. In healthy subjects if exercise is non-exhausting, an early transient facilitation of MEPs can occur immediately after exercise (post-exercise facilitation) and also after 15 min of rest (delayed facilitation). Neural mechanisms underlying post-exercise facilitation seem to reside within motor cortex. The delayed facilitation may be functional evidence of intracortical synaptic reorganization consequent to repetitive motor tasks. We aimed to compare cortical excitability in patients at early stage of MS versus healthy controls. In each subject TMS of non-dominant hemisphere was used to define MEP amplitude and motor threshold (MT). Then subjects performed 3 blocks (30-s, 60-s, and 90-s duration) of a bimanual motor task (exercise condition). Amplitude of MEPs elicited immediately after each block, and then after a 15-min rest period were compared with baseline to evaluate the presence of post-exercise facilitation and delayed facilitation. Patients were not receiving any immunomodulatory or immunosuppressive therapy and their Expanded Disability Status Scale score was between 1 and 2.5 MT resulted significantly lower in patients compared to controls. Compared with baseline, controls had larger MEP amplitudes after 30 and SMP 60 s of exercise (post-exercise facilitation) and also after the rest period (delayed facilitation). In contrast, MEP amplitudes in patients were not significantly different from baseline after any of the exercise conditions or following the rest period. Patients do not show the normal fluctuations of cortical excitability usually found after a bimanual finger motor task. These results suggest a modification in the central circuits and suppose a reduction or alteration in the cortical plasticity. The absence of postexercise facilitation and delayed facilitation in patients suggests a reduction or an alteration in neural plasticity that mediates functions such as learning and memory. We speculate that in SM patients capability to recover from functional impairments caused by demyelination is compromised. In patients at early stage of MS cortical excitability changes can occur before motor performances appear significantly altered. So, when patients present clinical symptoms and the disease becomes evident, cerebral mechanisms of compensation are already totally exploited, and possibility of further increasing performances are already lost. Like in chronic fatigue patients, and maybe in the majority of patients affected by central fatigue, impaired motor cortex plasticity may affect recovery of function in the motor system.

Pulsed Interleaved Excitation Fluctuation Imaging

Fluorescence fluctuation imaging is a powerful means to investigate dynamics, interactions, and stoichiometry of proteins inside living cells. Pulsed interleaved excitation (PIE) is the method of nanosecond alternating excitation with time-resolved detection and allows accurate, independent, and quasi-simultaneous determination of fluorescence intensities and lifetimes of different fluorophores. In this work, we combine pulsed interleaved excitation with fluctuation imaging methods (PIE-FI) such as raster image correlation spectroscopy (RICS) or number and brightness analysis (N&B). More specifically, we show that quantitative measurements of diffusion and molecular brightness of Venus fluorescent protein (FP) can be performed in solution with PIE-RICS and compare PIE-RICS with single-point PIE-FCS measurements. We discuss the advantages of cross-talk free dual-color PIE-RICS and illustrate its proficiency by quantitatively comparing two commonly used FP pairs for dual-color microscopy, eGFP/mCherry and mVenus/mCherry. For N&B analysis, we implement dead-time correction to the PIE-FI data analysis to allow accurate molecular brightness determination with PIE-NB. We then use PIE-NB to investigate the effect of eGFP tandem oligomerization on the intracellular maturation efficiency of the fluorophore. Finally, we explore the possibilities of using the available fluorescence lifetime information in PIE-FI experiments. We perform lifetime-based weighting of confocal images, allowing us to quantitatively determine molecular concentrations from 100 nM down to <30 pM with PIE-raster lifetime image correlation spectroscopy (RLICS). We use the fluorescence lifetime information to perform a robust dual-color lifetime-based FRET analysis of tandem fluorescent protein dimers. Lastly, we investigate the use of dual-color RLICS to resolve codiffusing FRET species from non-FRET species in cells. The enhanced capabilities and quantitative results provided by PIE-FI make it a powerful method that is broadly applicable to a large number of interesting biophysical studies.

Anticipatory alpha phase influences visual working memory performance

Alpha band (8–12Hz) phase dynamics in the visual cortex are thought to reflect fluctuations in cortical excitability that influences perceptual processing. As such, visual stimuli are better detected when their onset is concurrent with specific phases of the alpha cycle. However, it is unclear whether alpha phase differentially influences cognitive performance at specific times relative to stimulus onset (i.e., is the influence of phase maximal before, at, or after stimulus onset?). To address this, participants performed a delayed-recognition, working memory (WM) task for visual motion direction during two separate visits. The first visit utilized functional magnetic resonance (fMRI) imaging to identify neural regions associated with task performance. Replicating previous studies, fMRI data showed engagement of visual cortical area V5, as well as a prefrontal cortical region, the inferior frontal junction (IFJ). During the second visit, transcranial magnetic stimulation (TMS) was applied separately to both the right IFJ and right V5 (with the vertex as a control region) while electroencephalography (EEG) was simultaneously recorded. During each trial, a single pulse of TMS (spTMS) was applied at one of six time points (−200, −100, −50, 0, 80, 160ms) relative to the encoded stimulus onset. Results demonstrated a relationship between the phase of the posterior alpha signal prior to stimulus encoding and subsequent response times to the memory probe two seconds later. Specifically, spTMS to V5, and not the IFJ or vertex, yielded faster response times, indicating improved WM performance, when delivered during the peak, compared to the trough, of the alpha cycle, but only when spTMS was applied 100ms prior to stimulus onset. These faster responses to the probe correlated with decreased early event related potential (ERP) amplitudes (i.e., P1) to the probe stimuli. Moreover, participants that were least affected by spTMS exhibited greater functional connectivity between V5 and fronto-parietal regions. These results suggest that posterior alpha phase indexes a critical time period for motion processing in the context of WM encoding goals, which occurs in anticipation of stimulus onset.

Time‐varying coupling of EEG oscillations predicts excitability fluctuations in the primary motor cortex as reflected by motor evoked potentials amplitude: An EEG‐TMS study

Motor evoked potentials (MEPs) elicited by a train of consecutive, individual transcranial magnetic stimuli demonstrate fluctuations in amplitude with respect to time when recorded from a relaxed muscle. The influence of time-varying, instantaneous modifications of the electroencephalography (EEG) properties immediately preceding the transcranial magnetic stimulation (TMS) has rarely been explored. The aim of this study was to investigate the influence of the pre-TMS motor cortex and related areas EEG profile on time variants of the MEPs amplitude. MRI-navigated TMS and multichannel TMS-compatible EEG devices were used. For each experimental subject, post-hoc analysis of the MEPs amplitude that was based on the 50th percentile of the MEPs amplitude distribution provided two subgroups corresponding to "high" (large amplitude) and "low" (small amplitude). The pre-stimulus EEG characteristics (coherence and spectral profile) from the motor cortex and related areas were analyzed separately for the "high" and "low" MEPs and were then compared. On the stimulated hemisphere, EEG coupling was observed more often in the high compared to the low MEP trials. Moreover, a paradigmatic pattern in which TMS was able to lead to significantly larger MEPs was found when the EEG of the stimulated motor cortex was coupled in the beta 2 band with the ipsilateral prefrontal cortex and in the delta band with the bilateral centro-parietal-occipital cortices. This data provide evidence for a statistically significant influence of time-varying and spatially patterned synchronization of EEG rhythms in determining cortical excitability, namely motor cortex excitability in response to TMS.

Stimulus detection rate and latency, firing rates and 1–40 Hz oscillatory power are modulated by infra-slow fluctuations in a bistable attractor network model

Recordings of membrane and field potentials, firing rates, and oscillation amplitude dynamics show that neuronal activity levels in cortical and subcortical structures exhibit infra-slow fluctuations (ISFs) on time scales from seconds to hundreds of seconds. Similar ISFs are salient also in blood-oxygenation-level dependent (BOLD) signals as well as in psychophysical time series. Functional consequences of ISFs are not fully understood. Here, they were investigated along with dynamical implications of ISFs in large-scale simulations of cortical network activity. For this purpose, a biophysically detailed hierarchical attractor network model displaying bistability and operating in an oscillatory regime was used. ISFs were imposed as slow fluctuations in either the amplitude or frequency of fast synaptic noise. We found that both mechanisms produced an ISF component in the synthetic local field potentials (LFPs) and modulated the power of 1–40Hz oscillations. Crucially, in a simulated threshold-stimulus detection task (TSDT), these ISFs were strongly correlated with stimulus detection probabilities and latencies. The results thus show that several phenomena observed in many empirical studies emerge concurrently in the model dynamics, which yields mechanistic insight into how infra-slow excitability fluctuations in large-scale neuronal networks may modulate fast oscillations and perceptual processing. The model also makes several novel predictions that can be experimentally tested in future studies.