Propagation Of Ripples Research Articles

Short Wave Infrared Imaging for Auroral Physics and Aeronomy Studies

Advances in solar-terrestrial physics are generally linked to the development of innovative new sensor technologies, affording us ever better sensitivity, higher resolution, and broader spectral response. Recent advances in low-noise InGaAs sensor technology have enabled the realization of low-light-level scientific imaging within the short-wave infrared (SWIR) region of the electromagnetic spectrum. This paper describes a new and highly sensitive ultra-wide angle imager that offers an expansion of auroral and airglow imaging capabilities into the SWIR spectral range of 900–1,700 nm. The imager has already proven successful in large-area remote sensing of mesospheric temperatures and in providing intensity maps showing the propagation and dissipation of atmospheric gravity waves and ripples. The addition of an automated filter wheel expands the range of applications of an already versatile SWIR detector. Several potential applications are proposed herein, with an emphasis on auroral science. The combined data from this type of instrument and other existing instrumentation holds a strong potential to further enhance our understanding of the geospace environment.

Interneuron switching on and off across memory rhythms

In this issue of Neuron, Szabo etal. uncover a unique subtype of interneurons that is highly active during ripples but largely silent during theta oscillations. The study provides exciting new insights into the regulation and propagation of ripples in CA1 and beyond.

Role of mass redistribution on nanoripple formation and propagation: A molecular dynamics simulation study

Molecular dynamics (MD) is a tool suitable for investigating the formation of nanoscale patterns under low-energy ion beam irradiation. To focus on the role of impact-induced mass redistribution in ripple formation and propagation, an MD method was used in this paper to study the morphological change of a silicon surface under continuous bombardment with 30-eV argon. The atomic-scale dynamics processes of ripple formation and propagation are discussed in detail. The migration rate of surface atoms obtained as a function of an ion incidence angle by analyzing the migration of surface atoms can provide a reasonable explanation for the smooth-to-unstable mode transition of the silicon surface. The propagation of low-energy ion-induced ripples is investigated for the first time, and the ripple propagation velocities associated with four different incidence angles (40°, 50°, 60°, and 70°) are presented. The propagation velocity increases with the incidence angle, and the propagation direction is always in accordance with the incidence direction. This study can help further understand the formation and propagation mechanisms of nanoscale ripples in the experiments and provide a guide for the development and optimization of the theoretical model.

Mapping Propagation of Interictal Spikes, Ripples, and Fast Ripples in Intracranial EEG of Children with Refractory Epilepsy.

Studies on intracranial electroencephalography (icEEG) recordings of patients with drug resistant epilepsy (DRE) show that epilepsy biomarkers propagate in time across brain areas. Here, we propose a novel method that estimates critical features of these propagations for different epilepsy biomarkers (spikes, ripples, and fast ripples), and assess their common onset as a reliable biomarker of the epileptogenic zone (EZ). For each biomarker, an automatic algorithm ranked the icEEG electrodes according to their timing occurrence in propagations and finally dichotomized them as onset or spread. We validated our algorithm on icEEG recordings of 8 children with DRE having a good surgical outcome (Engel score = 1). We estimated the overlap of the onset, spread, and entire zone of propagation with the resection (RZ) and the seizure onset zone (SOZ). Spike and ripple propagations were seen in all patients, whereas fast ripple propagations were seen in 6 patients. Spike, ripple, and fast ripple propagations had a mean duration of 28.3 ± 24.3 ms, 38.7 ± 37 ms, and 25 ± 14 ms respectively. Onset electrodes predicted the RZ and SOZ with higher specificity compared to the entire zone for all three biomarkers (p<0.05). Overlap of spike and ripple onsets presented a higher specificity than each separate biomarker onset: for the SOZ, the onsets overlap was more specific (97.89 ± 2.36) than the ripple onset (p=0.031); for the RZ, the onsets overlap was more specific (98.48 ± 1.5) than the spike onset (p=0.016). Yet, the entire zone for spike and ripple propagations predicted the RZ with higher sensitivity compared to each biomarker's onset (or spread) (p<0.05). We present, for the first time, preliminary evidence from icEEG data that fast ripples propagate in time across large areas of the brain. The onsets overlap of spike and ripple propagations constitutes an extremely specific (but not sensitive) biomarker of the EZ, compared to areas of spread (and entire areas) in propagation.

Differential propagation of ripples along the proximodistal and septotemporal axes of dorsal CA1 of rats.

The functional connectivity of the hippocampus with its primary cortical input, the entorhinal cortex, is organized topographically. In area CA1 of the hippocampus, this leads to different functional gradients along the proximodistal and septotemporal axes of spatial/sensory responsivity and spatial resolution respectively. CA1 ripples, a network phenomenon, allow us to test whether the hippocampal neural network shows corresponding gradients in functional connectivity along the two axes. We studied the occurrence and propagation of ripples across the entire proximodistal axis along with a comparable spatial range of the septotemporal axis of dorsal CA1. We observed that ripples could occur at any location, and their amplitudes were independent of the tetrode location along the proximodistal and septotemporal axes. When a ripple was detected on a particular tetrode ("reference tetrode"), however, the probability of cooccurrence of ripples and ripple amplitude observed on the other tetrodes decreased as a function of distance from the reference tetrode. This reduction was greater along the proximodistal axis than the septotemporal axis. Furthermore, we found that ripples propagate primarily along the proximodistal axis. Thus, over a spatial scale of ∼1.5 mm, the network is anisotropic along the two axes, complementing the topographically organized cortico-hippocampal connections.

Propagation of hippocampal ripples to the neocortex by way of a subiculum-retrosplenial pathway

Bouts of high frequency activity known as sharp wave ripples (SPW-Rs) facilitate communication between the hippocampus and neocortex. However, the paths and mechanisms by which SPW-Rs broadcast their content are not well understood. Due to its anatomical positioning, the granular retrosplenial cortex (gRSC) may be a bridge for this hippocampo-cortical dialogue. Using silicon probe recordings in awake, head-fixed mice, we show the existence of SPW-R analogues in gRSC and demonstrate their coupling to hippocampal SPW-Rs. gRSC neurons reliably distinguished different subclasses of hippocampal SPW-Rs according to ensemble activity patterns in CA1. We demonstrate that this coupling is brain state-dependent, and delineate a topographically-organized anatomical pathway via VGlut2-expressing, bursty neurons in the subiculum. Optogenetic stimulation or inhibition of bursty subicular cells induced or reduced responses in superficial gRSC, respectively. These results identify a specific path and underlying mechanisms by which the hippocampus can convey neuronal content to the neocortex during SPW-Rs.

Wear induced ripplocation during dry sliding wear of TiC-based composite

Monolithic TiC and TiC particulate ceramic composite containing 30 and 50 mol.%SiC were consolidated and synthesized using spark plasma sintering (SPS) without sintering aids. The as-sintered bulk samples microstructural evolution and dry sliding room-temperature tribological properties against Al2O3 ball were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman analysis. The role of coefficient of thermal expansion (CTE) mismatch between TiC and SiC as well as cubic to hexagonal SiC phase transformation on the evolution of residual stresses in the composite was also investigated. The friction and wear properties of the monolithic TiC were superior to that of the composite with frictional heating-induced tribo-oxidation playing a dominant role in the wear mechanism. The increase in friction and wear of the composite is attributed to wear-induced stress-relaxation of the previously trapped residual stresses in the composite leading to extensive ripplocation of the TiC grains and consequent SiC grain pull-outs. Herein, we report for the first time on the wear-induced mechanical exfoliation of carbon, its subsequent decomposition into graphite and eventual deformation by micromechanism involving nucleation and propagation of ripples.

Propagation of Hippocampal Ripples to the Neocortex by Way of a Subiculum-Retrospleanial Pathway

Bouts of high frequency activity known as sharp wave ripples (SPW-Rs) facilitate communication between the hippocampus and neocortex. However, the paths and mechanisms by which SPW-Rs broadcast their content are not well understood. Due to its anatomical positioning, the granular retrosplenial cortex (gRSC) may be a bridge for this hippocampo-cortical dialogue. Using silicon probe recordings in awake, head-fixed mice, we show the existence of SPW-R analogues in gRSC and investigate their coupling to hippocampal SPW-Rs. Ensemble activity across regions was congruent across ripples, as neurons in gRSC reliably distinguished the spike content of different types of SPW-Rs. We demonstrate that this coupling is brain-state-dependent, and delineate a topographically-organized anatomical pathway via VGlut2-expressing, bursty neurons in the subiculum. Optogenetic stimulation or inhibition of bursty subicular cells induced or reduced responses in superficial gRSC, respectively. These results identify a specific path and underlying mechanisms by which the hippocampus can convey neuronal content during SPW-Rs to the neocortex.

Surgical resection of ripple onset predicts outcome in pediatric epilepsy

In patients with medically refractory epilepsy (MRE), interictal ripples (80-250Hz) are observed in large brain areas whose resection may be unnecessary for seizure freedom. This limits their utility as epilepsy biomarkers for surgery. We assessed the spatiotemporal propagation of interictal ripples on intracranial electroencephalography (iEEG) in children with MRE, compared it with the propagation of spikes, identified ripples that initiated propagation (onset-ripples), and evaluated their clinical value as epilepsy biomarkers. Twenty-seven children who underwent epilepsy surgery were studied. We identified propagation sequences of ripples and spikes across multiple iEEG contacts and calculated each ripple or spike latency from the propagation onset. We classified ripples and spikes into categories (ie, onset, spread, and isolated) based on their spatiotemporal characteristics and correlated their mean rate inside and outside resection with outcome (good outcome, Engel 1 versus poor outcome, Engel≥2). We determined, as onset-zone, spread-zone, and isolated-zone, the areas generating the corresponding ripple or spike category and evaluated the predictive value of their resection. We observed ripple propagation in all patients and spike propagation in 25 patients. Mean rate of onset-ripples inside resection predicted the outcome (odds ratio = 5.37; p = 0.02) and correlated with Engel class (rho = -0.55; p = 0.003). Resection of the onset-ripple-zone was associated with good outcome (p = 0.047). No association was found for the spread-ripple-zone, isolated-ripple-zone, or any spike-zone. Interictal ripples propagate across iEEG contacts in children with MRE. The association between the onset-ripple-zone resection and good outcome indicates that onset-ripples are promising epilepsy biomarkers, which estimate the epileptogenic tissue better than spread-ripples or onset-spikes. Ann Neurol 2018;84:331-346.

Platform Session – Electroencephalography/Epilepsy: Onset of interictal ripple high-frequency oscillations in intracranial electroencephalography predicts outcome in children undergoing epilepsy surgery

Introduction Approximately 20–30% of children with epilepsy develop medically refractory epilepsy (MRE), and require surgical resection of the epileptogenic zone for seizure-freedom. High-frequency oscillations (HFOs), recorded with intracranial electroencephalography (iEEG) and separated into ripples (>80 Hz) and fast-ripples (>250 Hz) are new promising interictal biomarkers of the epileptogenic zone. Yet, ripple HFOs (80–250 Hz) are seen in large brain areas whose resection may be unnecessary for seizure-freedom, limiting their utility as biomarkers for surgical planning. The goal of this study was to: (i) assess the spatiotemporal propagation of ripples on iEEG, (ii) identify the ripples that initiate interictal propagation (onset-ripples), and (iii) evaluate the clinical value of onset-ripples as new epilepsy biomarkers that estimate the epileptogenic zone better than the entirety of ripples observed on iEEG. Methods We analyzed interictal iEEG data from children with MRE who underwent surgical resection. Postsurgical outcome was evaluated with Engel scale at more than 12 months from surgery, and was classified as good (Engel class = 1) or poor (Engel class ⩾ 2) outcome. We developed an automated method to detect ripples and to evaluate their spatiotemporal propagation across iEEG contacts. We identified sequences of ripples propagating across neighboring contacts, and determined latency and distance of each ripple from the propagation onset. We defined three ripple categories based on their spatiotemporal properties, i.e. onset-ripples, spread-ripples, and isolated-ripples, and correlated their mean rates inside and outside resection with postsurgical outcome. We determined the areas that most frequently generated these ripples as onset-ripple-zone, spread-ripple-zone andisolated-ripple-zone, and evaluated the positive and negative predictive value (PPV and NPV) of their resection. Results Twenty-seven patients were included in the study (16 had good postsurgical outcome). We observed propagation sequences of ripples in all patients. The mean rate of onset-ripples inside resection correlated with Engel class (Spearman rho = −0.55, p = 0.003) and predicted good vs. poor outcome (odds ratio = 5.37, confidence interval = 1.34–21.56, p = 0.02). Resection of onset-ripple-zone was associated with good-outcome (p = 0.047) with a PPV of 80% and NPV of 60%. No association was found for spread-ripple-zone or isolated-ripple-zone. Conclusion This study presented for the first time propagation of interictal ripples across multiple iEEG contacts in children with MRE. Resecting the areas that initiate ripple propagation predicts good postsurgical outcome. Onset-ripples are potential biomarkers of epileptogenicity and may help surgical treatment of patients with MRE.

Stress vs sputtering effects in the propagation of surface ripples produced by ion-beam sputtering

Under low energy ion irradiation, periodic features (ripples) can develop on the surfaces of semiconductor materials, with typical sizes in the nanometric range. Recently, a theory of pattern formation has been able to account for the variability with the ion/target combination of the critical angle value separating conditions on ion incidence that induce the presence or the absence of ripples. Such a theory is based in the accumulation of stress in the damaged irradiated layer and its relaxation via surface-confined viscous flow. Here we explore the role of stress, and its competition with purely erosive mechanisms, to determine the sign of the velocity with which the ripple pattern moves across the target plane. Based on this theory, we discuss different situations and make specific testable predictions for the change of sign in that velocity.

A theoretical study of ripple propagation in defective graphene

The propagation of dynamic ripples induced by a C60 molecule is observed in pristine and defective graphene by molecular dynamics simulations. The ripples centered in the impact point propagate outward through the graphene, accompanied by a strain energy transfer. The noticeable diffraction and interference of the dynamic ripples are also observed. The Stone–Wales (SW) defect or vacancies greatly affect the propagation of the ripples near the defect. Both the SW defect and vacancies might be more capable of absorbing energy from the ripples, but have less capability to spread the gained energy again. The ripple propagation is also affected by the temperature of the system. It is expected that applications of graphene could be promoted in shock absorbers and protectors, and the theoretical support is provided for detecting defects in the graphene sheet.

Local Generation and Propagation of Ripples along the Septotemporal Axis of the Hippocampus

A topographical relationship exists between the septotemporal segments of the hippocampus and their entorhinal-neocortical targets, but the physiological organization of activity along the septotemporal axis is poorly understood. We recorded sharp-wave ripple patterns in rats during sleep from the entire septotemporal axis of the CA1 pyramidal layer. Qualitatively similar ripples emerged at all levels. From the local seed, ripples traveled septally or temporally at a speed of ∼0.35 m/s, and the spatial spread depended on ripple magnitude. Ripples propagated smoothly across the septal and intermediate segments of the hippocampus, but ripples in the temporal segment often remained isolated. These findings show that ripples can combine information from the septal and intermediate hippocampus and transfer integrated signals downstream. In contrast, ripples that emerged in the temporal pole broadcast largely independent information to their cortical and subcortical targets.

Propagation of ripple patterns on Si during ion bombardment

The lateral propagation of surface ripples on Si, generated by Xe ion irradiation with and without codeposition of Fe surfactant atoms, was investigated by scanning electron microscopy with the help of micron-sized marker structures prepared by focused ion beam milling. For 10-keV Xe ion irradiation of Si at oblique incidence between 62 and 70\ifmmode^\circ\else\textdegree\fi{}, we determine lateral ripple propagation velocities varying from \ensuremath{-}1.9 to $+$2.9 nm per 10${}^{15}$ Xe ions/cm${}^{2}$. The propagation direction changes from opposite to the projected direction of the incident ion beam to along the projected beam direction within a narrow angular regime. At 67\ifmmode^\circ\else\textdegree\fi{}, the pattern is almost static. The result is in good agreement with predictions from the theoretical model of Bradley and Harper. For perpendicular incident 5-keV Xe ions and oblique codeposition of Fe surfactant atoms, we find that ripple patterns propagate across the surface with a negative ripple propagation velocity of about \ensuremath{-}0.7 nm per 10${}^{15}$ Xe ions/cm${}^{2}$, i.e., opposite to the projected deposition direction of Fe surfactant atoms. The novel experimental method to determine the lateral ripple propagation based on markers set with a focused ion beam system does not require an in situ analysis and can therefore be applied in general to analyze the dynamics of ion beam-induced patterns.

Propagation of nanoscale ripples on ion-irradiated surfaces

The irradiation of glass surfaces by 30keV Ga+ ions under oblique incidence angles results in the development of “ripple”-like nanostructures. The evolution of these ripples was monitored in situ in the ion fluence range from 3×1016 to 1×1018 Ga+ions/cm2 using a focused ion beam system which incorporates a scanning electron microscope. With increasing fluence, the ripples were found to propagate across the surface in a direction which coincides with the projection of the ion beam’s incident direction onto the surface. The propagation velocity was determined to be constant over the investigated fluence regime and to amount to 77±1.8nm/1017ionscm−2. Ex situ atomic force microscopy was employed to derive the post-mortem wavelength and amplitude of the ripple structures and their values were 450±43 and 127±23nm, respectively.

Wavelike deformation traveling on a carbon nanotube

The propagation of wavelike ripples on a carbon nanotube (CNT) induced by the radial impact of a C(60) molecule is investigated by molecular dynamics simulations. The ripples start at the impact point and spreads through the tube, accompanied by energy transfer. The ripples would effectively reduce the local energy concentration around the impact point. The propagation of ripples is clearly affected by the diameter but is independent of the chirality of the CNT. Noticeable diffraction occurs when the deformation ripples encounter obstacles or narrow slits, which indicates that the propagation of ripples can be used to detect defects in the CNT. This work provides new and exciting possibilities for CNTs to serve as energy buffers, sensors and new nanoelectromechanical devices.

Extending the time scale in molecular dynamics simulations: Propagation of ripples in graphene

We show that it gives an exact solution of the temporal part of the MD equations for up to quadratic terms in atomic displacements in the interatomic potential. Higher-order terms are then included by iteration. We will refer to our technique as Green’s function in molecular dynamics GFMD. This technique should be applicable to all physical, chemical, and biological systems where MD is used. In certain class of problems in which the atoms vibrate about an equilibrium site, GFMD gives exact results in the harmonic approximation. Examples of such class of problems are phonon transport, thermal conduction etc., in disordered or finite solids and low-dimensional material systems for which MD has to be used because analytical solutions are not available even in the harmonic approximation. For nonlinear vibration problems, depending upon the anharmonicity, GFMD can accelerate MD by about 8 orders of magnitude, and model processes at microseconds. In other classes of problems in which the atoms are itinerant, such as diffusion or crystal growth, GFMD can be used iteratively and should still accelerate MD by a significant amount. Consider a set of N interacting atoms. We label the atoms by indices L and L. We assume a Cartesian frame of reference and denote the position vector of atom L by rL at t = 0. We denote the displacement and velocity of atom L at time t by uL ,t and cL ,t, respectively. As in classical MD, we need to solve the following equation for u:

Propagation of ripples on pyrochlore induced by ion beam bombardment

The morphological evolution of ripples formed on the surface of ${\text{Cd}}_{2}{\text{Nb}}_{2}{\text{O}}_{7}$ pyrochlore single crystals by focused ion beam bombardment was investigated using in situ electron microscopy. At high ion fluences and off-normal bombardment angles, faceted surface ripples with a ``terracelike'' structure were observed. The ripple propagation direction was oriented along the projected ion beam direction at incident angles ranging from 35 to $65\ifmmode^\circ\else\textdegree\fi{}$ under high-dose ion bombardment. One side of the terrace was found to be perpendicular to the incident ion beam direction, while the other side was parallel to the ion beam. The terrace propagation velocity and direction were determined and interpreted on the basis of this asymmetric structure. A model based on the propagation of a shock wave that effectively ``self-selects'' a stable slope was developed in order to explain the observed faceted ripple formation.

Patterning polycrystalline thin films by defocused ion beam: The influence of initial morphology on the evolution of self-organized nanostructures

We report on self-organized patterning of polycrystalline noble metal films, supported on dielectric substrates, by defocused Ar+ ion beam irradiation. The initial surface morphology affects the formation of nanostructures, forcing the growth of ripples with a lateral periodicity imposed by the pristine polycrystalline grain distribution. At the early stages, the self-organization process is dominated by the shadowing of taller grains, while a crossover to the conventional erosive regime, observed for single crystals, sets in at longer sputtering times. The grain boundaries, although providing an additional bias for diffusion of mobile defects, do not affect the propagation of nanoscale ripples across individual grains.

Propagation of specific network patterns through the mouse hippocampus

Network oscillations bind neurons into transient assemblies with coherent activity, enabling temporal coding. In the mammalian hippocampus, spatial relationships are represented by sequences of action potentials of place cells. Such patterns are established during memory acquisition and are re-played during sharp wave-ripple complexes in CA1 in subsequent sleep episodes. These events originate in CA3 and travel towards CA1 and downstream cortical areas. It is unclear, however, whether specific sequences of ripple-associated firing are solely defined within the CA1 network or whether these patterns are directly entrained by preceding activities of neurons within CA3. Using a model of sharp wave-ripple oscillations (SPW-R) in mouse hippocampal slices we analyzed the propagation of these signals between CA3 and CA1. We found tight coupling between high-frequency network activity in CA3 and CA1. Propagation of ripples through the hippocampal loop maintained precise temporal relationships at the network and cellular level, as indicated by coupling of field potentials, multiunit and single cell activity between major portions of CA3 and CA1. Moreover, SPW-R-like activity in CA1 could be elicited by electrical stimulation within area CA3 while antidromic activation of CA1 failed to induce organized high-frequency oscillations. Our data show that the specificity of neuronal assemblies is maintained with cell-to-cell precision while SPW-R propagate along the hippocampal loop.