Low-frequency Bursts Research Articles

Augmented Ocular Vestibular Evoked Myogenic Potentials to Air-Conducted Sound in Large Vestibular Aqueduct Syndrome

To demonstrate the value of recording air-conducted ocular Vestibular Evoked Myogenic Potentials (oVEMP) in a patient with bilaterally enlarged vestibular aqueducts. Cervical VEMP and oVEMP were recorded from a patient presenting with bilateral hearing loss and imbalance, attributable to large vestibular aqueduct syndrome. The stimuli were air-conducted tone bursts at octave frequencies from 250 to 2000 Hz. Amplitudes and thresholds were measured and compared with the normal response range of 32 healthy control subjects. oVEMP reflexes demonstrated pathologically increased amplitudes and reduced thresholds for low-frequency tone bursts. Cervical VEMP amplitudes and thresholds were within normal limits for both ears across all frequencies of stimulation. This study is the first to describe the augmentation of AC oVEMPs in an adult with large vestibular aqueduct syndrome.

Pulsed Low Intensity Electromagnetic Field (PEMF) Affects Cell Cycle of Human Osteoblast-like Cells <i>in </i><i>Vi</i><i>tro</i>

High frequency electro magnetic field in the range of 4 - 33 kHz given in low frequency bursts of 7-15 Hz (PEMF) is efficient in stimulating osteoblast proliferation and differentiation in vitro. We hypothesize that osteoblast cell cycle changes following this stimulatory PEMF. In order to explore this effect we used primary explant cultures of human osteoblast-like cells. These cells were subjected to low frequency of 20-30 Hz, h igher frequency of 5-15 kHz electro magnetic fields and to PEM F (5-15 kHz basic frequency in pulses of 20-30 Hz) with maximal magnetic flu x of 10-12 Gauss. The maturation state of the cells was estimated by the measurement of cellular alkaline phosphatase activity and the overall cell death by the LDH activity in culture media. The cell cycle was analyzed by cytometry. The PEMF stimulation significantly increased the proportion of cells in the G1 phase (p<.001) and decreased the proportion of apoptotic and necrotic cells (p<.001), with parallel decrease in cellular alkaline phosphatase and media LDH act ivities. Pure 20-30 Hz or 5-15 kHz electro magnetic stimu lations had no effect on these parameters. Therefore we show that PEM F with a basic frequency in the kHz range provided in pulses in the infrasonic range alters human osteoblast cell cycle by reducing cell death and cellular maturation.

Cortical electrical stimulation with varied low frequencies promotes functional recovery and brain remodeling in a rat model of ischemia

In this study, we investigated whether fully implantable CES with low current density and varying low-frequency burst impulse train enhances functional recovery and promotes brain remodeling in both the ipsilesional and contralesional cortex. Adult rats received occlusion of the right middle cerebral artery for 120min. One week after ischemia, electrodes were implanted to rats with CES lasting 2 weeks followed by 4-week observation period. After 2-week stimulation and 4-week observation period, body weight (BW) of the rats in CES group was higher than that in no stimulation (NS) group. Limb placement test, foot-fault test and beam walking test demonstrate that CES significantly enhanced functional recovery. Immunohistochemical study has shown that CES enhanced angiogenesis and dendritic sprouting, and suppressed inflammatory response in the ischemic cortex. CES also promoted dendritic sprouting and suppressed inflammatory response in the contralesional cortex. These results suggest the stimulation protocol is safe, and greatly improves functional recovery and brain remodeling in the 4 weeks following 2 weeks stimulation.

The 17 January 2005 Complex Solar Radio Event Associated with Interacting Fast Coronal Mass Ejections

On 17 January 2005 two fast coronal mass ejections were recorded in close succession during two distinct episodes of a 3B/X3.8 flare. Both were accompanied by metre-to-kilometre type-III groups tracing energetic electrons that escape into the interplanetary space and by decametre-to-hectometre type-II bursts attributed to CME-driven shock waves. A peculiar type-III burst group was observed below 600 kHz 1.5 hours after the second type-III group. It occurred without any simultaneous activity at higher frequencies, around the time when the two CMEs were expected to interact. We associate this emission with the interaction of the CMEs at heliocentric distances of about 25 R ⊙. Near-relativistic electrons observed by the EPAM experiment onboard ACE near 1 AU revealed successive particle releases that can be associated with the two flare/CME events and the low-frequency type-III burst at the time of CME interaction. We compare the pros and cons of shock acceleration and acceleration in the course of magnetic reconnection for the escaping electron beams revealed by the type-III bursts and for the electrons measured in situ.

Chaotic decorrelation of Globus Pallidus by periodic forcing: a possible mechanism for the therapeutic effects of deep brain stimulation

High frequency (~100 Hz) stimulation of the subthalamic nucleus is an effective therapy for the symptoms of Parkinson's disease. Presumably, stimulation partly normalizes a pathological change in rate or pattern of neuronal activity in the output neurons of the globus pallidus. The rate effects of subthalamic stimulation are paradoxical, because firing rate in the globus pallidus is increased in Parkinson's disease, and subthalamic input is excitatory. Parkinson's disease is also accompanied by changes in globus pallidus cell firing pattern. Pallidal neurons are autonomous single-spiking oscillators that fire continuously and usually show no periodic bursting. They fire independently of each other, having flat cross-correlations. In Parkinson's disease the cells burst at 5-10/s and are highly correlated [1]. Rubin and Terman [2] have proposed that subthalamic stimulation entrains globus pallidus cells interfering with the low frequency bursting. In their model, the synchrony of globus pallidus cells should be high, because all cells are entrained by the same stimulus. The periodic nature of deep brain stimulation is inconsequential in their model. A recent clinical study has shown that the periodicity of deep brain stimulation is critical to its effectiveness [3]. Poisson stimulation at the same rate was much less beneficial. This observation, coupled with the frequency dependence of DBS effectiveness, suggests that disruption of low frequency bursting may not be the only mechanism of action. Globus pallidus cells are oscillators, so it is possible that periodic stimulation may not only suppress low frequency bursting, but also disrupt synchrony by driving the cells into chaotic firing patterns. We examined the range of stimulus frequencies and amplitudes required to chaotically desynchronize pallidal neurons. Oscillating pallidal cells were represented by their phase resetting curves. We used a type 1 phase resetting curve similar to that measured from model pallidal neurons [4]. The variance of the phase resetting curve was estimated as in Ermentrout et al [5] and used to form a stochastic phase map. The stationary distribution of latencies and the phase resetting curve were used to calculate Lyapunov exponents for both hyperpolarizing and depolarizing stimulation, and compared to the synchronizing or desynchronizing effect of stimulation on a population of simulated pallidal neurons that were weakly synchronized by common noisy input. Powerful synchronization and desynchronization were evoked at different frequencies as predicted by the sign of the Lyapunov exponents. Chaotic decorrelation in response to depolarizing stimulation occurred at frequencies similar to those known to be most effective for deep brain stimulation clinically. We suggest that in addition to its effects on low frequency oscillations in the pallido-thalamic projection, deep brain stimulation decorrelates the activity of pallidal neurons by driving them into a chaotic firing pattern. When the stimulus is randomized the chaotic response is destroyed, explaining why it is less effective. This chaotic effect may be essential to the clinical benefit of stimulation, being responsible for its frequency sensitivity and the requirement for periodicity in the stimulus pattern.

Direct and indirect generation of Jovian quasiperiodic radio bursts by relativistic electron beams in the polar magnetosphere

[1] The Jovian polar magnetosphere has relativistic particle accelerations with quasiperiodicity (QP accelerations), which are accompanied by periodic auroral emissions and low-frequency radio bursts called QP bursts. Although there have been some observations, the generation process of QP bursts by relativistic electrons from QP accelerations has not been revealed yet. This paper presents calculated wave growth rates for the discussion of the QP radio burst generation processes based on wave generation theories. Linear growth rates were computed for free space mode waves and plasma waves in cold plasma dispersion relations, assuming that these waves are generated by relativistic electron beams in two kinds of polar source regions, as suggested by wave observations and by the ray-tracing results reported in our previous studies. One of the source regions is at high altitudes where emission frequency f is close to local right-handed extraordinary (RX) mode cutoff frequency fRX and the other is at low altitudes where f is close to local plasma frequency fp. We found that ordinary (O) mode free space waves are sufficiently amplified, with broad beaming at both of the sources in the duration of the relativistic electron populations when they have an unstable velocity distribution like a ring beam structure. This means that O mode free space waves can be generated directly from energetic electrons via the “cyclotron maser instability” (CMI) process. We also confirmed that extraordinary (X) mode free space waves are not sufficiently amplified at both of the sources in the beam duration but Z mode waves propagating along field lines from the sources toward the Jovian polar ionosphere are significantly excited. Z mode waves propagating toward the planet could be converted to free space O mode waves at a steep plasma density gradient via the “mode conversion” (MC) process. We conclude that both direct (CMI) and indirect (MC) process can generate O mode QP radio bursts with radiation characteristics consistent with those observed by spacecraft. This suggests that relativistic electrons with unstable velocity distributions are generated by the QP acceleration and that Z mode and O mode QP radio bursts are excited by these particles.

Periodicity analysis of Jovian quasi-periodic radio bursts based on Lomb-Scargle periodograms

[1] The Jovian polar magnetosphere has relativistic particle accelerations with quasi-periodicity (hereafter QP accelerations) that are accompanied by periodic auroral emissions and low-frequency radio bursts called quasi-periodic (QP) bursts. Some previous observations suggested a possible physical relationship between the QP accelerations and QP radio bursts. However, the cause of the QP accelerations has not been revealed yet. This study investigated the generation process of QP radio bursts that constrain the QP acceleration process. The statistical features of QP bursts' periodicity were investigated by applying Lomb-Scargle periodogram analysis to the variations of the QP bursts' spectral densities observed by the Galileo and Ulysses spacecraft. The Lomb-Scargle analysis revealed remarkable characteristics: QP bursts have statistically large amplitudes with periods of 30–50 min at all latitudes. This result suggests that 30–50 min is an “eigenfrequency” of the QP accelerations which is close to the 45 min periodicity of the pulsating X-ray hot spot in the polar cap region. In addition, it was also revealed that successive pulses sometimes exhibit periodicity transition. We discussed one possible scenario which links Jovian periodic accelerations to those in the terrestrial magnetosphere. The scenario is that particles are energized within the period of the dispersive Alfven waves with field-aligned electric fields that obliquely propagate between the northern and southern ionospheres. The observed eigenfrequency and periodicity transition of QP bursts are consistent with the Alfvenic acceleration scenario.

Intragastric administration of capsiate, a transient receptor potential channel agonist, triggers thermogenic sympathetic responses

The sympathetic thermoregulatory system controls the magnitude of adaptive thermogenesis in correspondence with the environmental temperature or the state of energy intake and plays a key role in determining the resultant energy storage. However, the nature of the trigger initiating this reflex arc remains to be determined. Here, using capsiate, a digestion-vulnerable capsaicin analog, we examined the involvement of specific activation of transient receptor potential (TRP) channels within the gastrointestinal tract in the thermogenic sympathetic system by measuring the efferent activity of the postganglionic sympathetic nerve innervating brown adipose tissue (BAT) in anesthetized rats. Intragastric administration of capsiate resulted in a time- and dose-dependent increase in integrated BAT sympathetic nerve activity (SNA) over 180 min, which was characterized by an emergence of sporadic high-activity phases composed of low-frequency bursts. This increase in BAT SNA was abolished by blockade of TRP channels as well as of sympathetic ganglionic transmission and was inhibited by ablation of the gastrointestinal vagus nerve. The activation of SNA was delimited to BAT and did not occur in the heart or pancreas. These results point to a neural pathway enabling the selective activation of the central network regulating the BAT SNA in response to a specific stimulation of gastrointestinal TRP channels and offer important implications for understanding the dietary-dependent regulation of energy metabolism and control of obesity.

Low-frequency bursts of horizontally polarized waves in the Arctic sea-ice cover

AbstractWe report the detection of bursts of low-frequency waves, typicallyf= 0.025 Hz, on horizontal channels of broadband seismometers deployed on the Arctic sea-ice cover during the DAMOCLES (Developing Arctic Modeling and Observing Capabilities for Long-term Environmental Studies) experiment in spring 2007. These bursts have amplitudes well above the ambient ice swell and a lower frequency content. Their typical duration is of the order of minutes. They occur at irregular times, with periods of relative quietness alternating with periods of strong activity. A significant correlation between the rate of burst occurrences and the ice-cover deformation at the ∼400 km scale centered on the seismic network suggests that these bursts are caused by remote, episodic deformation involving shearing across regional-scale leads. This observation opens the possibility of complementing satellite measurements of ice-cover deformation, by providing a much more precise temporal sampling, hence a better characterization of the processes involved during these deformation events.

Anticholinesterase-induced epileptiform activity in immature rat piriform cortex slices, in vitro

Purpose: Acute in vitro brain slice models are commonly used to study epileptiform seizure generation and to test anti-epileptic drug action. Seizure-like activity can be readily induced by manipulating external ionic concentrations or by adding convulsant agents to the bathing medium. We previously showed that epileptiform bursting was induced in slices of immature (P14–28) rat piriform cortex (PC) by applying oxotremorine-M, a potent muscarinic receptor agonist. Here, we examined whether raising levels of endogenous acetylcholine (ACh) by exposure to anticholinesterases, could also induce epileptiform events in immature (P12–14) or early postnatal (P7–9) rat PC brain slices. Methods: The effects of anticholinesterases were investigated in rat PC neurons using both extracellular MEA (P7–9 slices) and intracellular (P12–14 slices) recording methods. Results: In P7–9 slices, eserine (20 μM) or neostigmine (20 μM) induced low amplitude, low frequency bursting activity in all three PC cell layers (I–III), particularly layer III, where neuronal muscarinic responsiveness is known to predominate. In P12–14 neurons, neostigmine produced a slow depolarization together with an increase in input resistance and evoked cell firing. Depolarizing postsynaptic potentials evoked by intrinsic fibre stimulation were selectively depressed although spontaneous bursting was not observed. Neostigmine effects were blocked by atropine (1 μM), confirming their muscarinic nature. We conclude that elevation of endogenous ACh by anticholinesterases can induce bursting in early postnatal PC brain slices, further highlighting the epileptogenic capacity of this brain region. However, this tendency declines with further development, possibly as local inhibitory circuit mechanisms become more dominant.

Ionic conditions modulate stimulus-induced capacitance changes in isolated neurohypophysial terminals of the rat

Peptidergic nerve terminals of the neurohypophysis (NH) secrete both oxytocin and vasopressin upon stimulation with peptide-specific bursts of action potentials from magnocellular neurons. These bursts vary in both frequency and action potential duration and also induce in situ ionic changes both inside and outside the terminals in the NH. These temporary effects include the increase of external potassium and decrease of external calcium, as well as the increase in internal sodium and chloride concentrations. In order to determine any mechanism of action that these ionic changes might have on secretion, stimulus-induced capacitance recordings were performed on isolated terminals of the NH using action potential burst patterns of varying frequency and action potential width. The results indicate that in NH terminals: (1) increased internal chloride concentration improves the efficiency of action potential-induced capacitance changes, (2) increasing external potassium increases stimulus-induced capacitance changes, (3) decreasing external calcium decreases the capacitance induced by low frequency broadened action potentials, while no capacitance change is observed with high frequency un-broadened action potentials, and (4) increasing internal sodium increases the capacitance change induced by low frequency bursts of broadened action potentials, more than for high frequency bursts of narrow action potentials. These results are consistent with previous models of stimulus-induced secretion, where optimal secretory efficacy is determined by particular characteristics of action potentials within a burst. Our results suggest that positive effects of increased internal sodium and external potassium during a burst may serve as a compensatory mechanism for secretion, counterbalancing the negative effects of reduced external calcium. In this view, high frequency un-broadened action potentials (initial burst phase) would condition the terminals by increasing internal sodium for optimal secretion by the physiological later phase of broadened action potentials. Thus, ionic changes occurring during a burst may help to make such stimulation more efficient at inducing secretion. Furthermore, these effects are thought to occur within the initial few seconds of incoming burst activity at both oxytocin and vasopressin types of NH nerve terminals.

Assessment of different induction protocols to elicit long-term depression (LTD) in the rat visual cortex in vivo

Changes in synaptic efficacy, including long-term potentiation (LTP) and long-term depression (LTD), provide mechanisms for experience-induced plasticity of cortical and subcortical circuits. LTP is readily induced under drastically different experimental conditions (e.g., in vitro and in vivo). However, few studies have compared the effectiveness of different induction protocols to elicit synaptic depression, especially under in vivo conditions. Here, we assessed the effectiveness of four different low frequency stimulation (LFS) protocols, applied to the lateral geniculate nucleus, to induce LTD-like changes of local field postsynaptic potentials (fPSPs) recorded on the surface of the primary visual cortex (V1) of urethane-anesthetized rats. Three LFS protocols (900 pulses at 1 Hz; 1800 pulses at 1 Hz, 1800 pulses at 1 Hz, repeated three times), known to induce LTD in neocortical and hippocampal slice preparations, failed to induce synaptic depression. In contrast, strong low frequency burst stimulation (3 pulses/burst at 20 Hz, 900 bursts repeated at 1 Hz) resulted in significant, but transient (∼20 min) depression of fPSPs in V1. This effect was resistant to systemic treatment with MK 801 (0.5 mg/kg) or local, cortical application of either APV (10 mM) or MCPG (10 mM), indicative of non-essential roles of N-methyl- d-aspartate and metabotropic glutamate receptors. A similar depressant effect was also observed under sodium pentobarbital anesthesia. These experiments emphasize the resistance of the in vivo neocortex to express the long-lasting down-regulation of synaptic strength, observations that require integration into current models and theories regarding the functions of LTD as a homeostatic and experience-dependent plasticity mechanism.

Cortical electrical stimulation alone enhances functional recovery and dendritic structures after focal cerebral ischemia in rats

Using a fully implanted cortical electrical stimulation (CES) device with low-frequency burst impulse train, we investigated the effects of CES alone on behavioral recovery and surface density of dendritic structure in a rat model of middle cerebral artery occlusion (MCAO). After MCAO in rats, magnetic resonance imaging (MRI) was used to confirm cortex infarction and to identify a location for implantation of stimulating electrode over the peri-infarct cortex. The device was implanted on the 6th day after MCAO with CES then lasting for 16 days. The stimulation program consisted of two sessions lasting half an hour in the morning (0.65 mA, 0.13 μC/phase) and in the afternoon (0.5 mA, 0.1 μC/phase). The stimulator delivered biphasic charge balanced pulses (pulse width = 200 μs) with various frequencies of 50 Hz, 20 Hz and 5 Hz in repeated 10-s blocks. Rats in the CES group ( n = 12) spend a much shorter time to regain preoperative levels of body weight (BW) than those in the no stimulation (NS) group ( n = 9). In behavioral tests, the rats in the CES group showed greater functional recovery compared to the NS group. Moreover, the functional improvement coincided with an increase in surface density of dendritic processes immunoreactive to microtubule-associated protein 2 (MAP2) in peri-infarct cortex. These results suggest the feasibility of the fully implanted CES device and the efficacy of the new stimulation protocol alone to improve functional outcome and cortical neuronal structural plasticity following focal cerebral ischemia in rats.

Individual differences and left/right asymmetries in auditory space perception. I. Localization of low-frequency sounds in free field

The number of subjects in studies on human spatial hearing is generally small. Therefore, individual differences and the factors underlying variability are unknown. In this study, we investigated across-listener variability in auditory localization abilities in a group of 50 naïve adults with normal hearing. Targets were trains of low-frequency noise bursts presented to 1 of 12 hidden speakers in the azimuthal plane. We observed less across-listener variability in the variance of individual responses but more in the root-mean-square and signed errors, which tended to increase with target angle. One third of the listeners demonstrated systematically smaller signed errors with left-sided targets than with right-sided ones. These asymmetries were observed less frequently in left-handers and females than in right-handers and males. Performance was not correlated with age. About 4 of 6 listeners trained with sensory feedback showed no reduction of asymmetries with training but rather showed a reduction in errors on their “best” side. Across-listener variability in the asymmetry of brain organization, notably linked to handedness or gender, is discussed.

A NEW MINIMALLY INVASIVE TREATMENT FOR MALE STRESS URINARY INCONTINENCE: THE ADVANCE® MALE SLING, ONE YEAR DATA FROM A MULTICENTER PROSPECTIVE STUDY

The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H2 band systems and H Lyman-α from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2–4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5–16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassini Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325–326 (November 20–21). The global brightening events on November 20–21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for ∼2 min is associated with auroral flares inside the main auroral ovals. On January 8, 2001, from 18:45–19:35 UT UVIS H2 band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to auroral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6–5 kHz Galileo PWS data.

A large-scale model of thalamocortical dysrhythmia

Event Abstract Back to Event A large-scale model of thalamocortical dysrhythmia Henning Proske1*, Daniel Jeanmonod2, Daniel Kiper3 and Paul Verschure4 1 Uni/ETH, Switzerland 2 Universitaetsspital, Switzerland 3 Institute of Neuroinformatics, Switzerland 4 Universitat Pompeu Fabra, Spain Several neuropathologies such as Parkinson's Disease, neuropathic pain and major depression have shown to correlate with the existence of 2-5 Hz bursting discharges in thalamic neurons [1] and characteristic shifts in the power-spectrum of EEG/MEG measures [2,3,4]. These shifts are coherent with the disease associated bursting activity in the thalamus. It has been suggested that clinical symptoms arise from the abnormal interaction that the repetitive and synchronized bursting in thalamic cells has on the dynamical stability of neocortical circuits (thalamocortical dysrhythmia - TCD). Indeed, these symptoms can be significantly ameliorated by focal lesions to specific locations in the thalamus. The original description of TCD as proposed by Llinas and others provides an abstract model to explain the above pattern of physiological data that is observed in patients. Two open question pertain to how exactly hyperpolarisation in the thalamic relays leads to low-frequency bursting and, secondly, how the low-frequency activity in turn spreads from specific locations to entrain large parts of the thalamocortical system. In order to answer these questions we have built a large-scale computational model of the thalamocortical system that shows the generation, persistence, and spread of low-frequency activity through the thalamocortical system. Our model also shows how low-frequency dysrhythmia originates in single neurons, slowly invades individual thalamocortical modules and subsequently spreads to large-scale cell ensembles. In our model persistent TCD can result from an interaction between recurrent feedback within the thalamus and cell-intrinsic mechanisms. Furthermore, divergent thalamocortical and corticothalamic excitatory connections provide a substrate for the synchronization and spread of low-frequency activation through the whole thalamocortical system. Starting with replicating the known physiology of TCD our model generates a number of testable predictions: 1. Excitatory noise leads to a decrease of the probability of bursting in thalamic cells by interfering with the intrinsic hyperpolarisation-activated current (Ih). 2. Recurrent bursting in the thalamus depends largely on the time-scale of the thalamic reticular inhibition on the thalamic relay neurons. 3. Higher-order thalamic nuclei will be particularly affected by the bursting activity due to their divergent projections. Hence, our model of TCD shows that the specific symptomatology of neuropathologies result from the impact of local perturbations on the system level organization of the brain.

Influence of Nocturnal Low-level Jets on Eddy-covariance Fluxes over a Tall Forest Canopy

Observations of low-level jets (LLJs) at the Howland AmeriFlux site in the USA and the jet's impact on nocturnal turbulent exchange and scalar fluxes over a tall forest canopy are discussed. Low-frequency motions and turbulent bursts characterize moderately strong LLJs, whereas low-frequency motions are suppressed during periods with strong LLJs and enhanced shear. An analysis based on the shear-sheltering hypothesis seeks to elucidate the effect of LLJs on flux measurements. In the absence of shear sheltering, large eddies penetrate the roughness sublayer causing enhanced mixing while during periods with shear sheltering, mixing is reduced. In the absence of the latter, 'upside-down' eddies are primarily responsible for the enhanced velocity variances, scalar and momentum fluxes. The integral length scales over the canopy are greater than the canopy height. The variance spectra and cospectra from the wavelet analysis indicate that large eddies (spatial scale greater than the low-level jet height) interact with active canopy-scale turbulence, contributing to counter-gradient scalar fluxes.

Focal disruption of the blood–brain barrier due to 260-kHz ultrasound bursts: a method for molecular imaging and targeted drug delivery

The goal of this study was to explore the feasibility of using low-frequency magnetic resonance (MR) image-guided focused ultrasound as a noninvasive method for the temporary disruption of the blood-brain barrier (BBB) at targeted locations. Rabbits were placed inside a clinical 1.5-tesla MR imaging unit, and sites in their brains were targeted for 20-second burst sonications (frequency 260 kHz). The peak pressure amplitude during the burst varied between 0.1 and 0.9 MPa. Each sonication was performed after an intravenous injection of an ultrasound contrast agent (Optison). The disruption of the BBB was evaluated with the aid of an injection of an MR imaging contrast agent (MAG-NEVIST). Additional tests involving the use of MION-47, a 20-nm magnetic nanoparticle contrast agent, were also performed. The animals were killed at different time points between 3 minutes and 5 weeks postsonication, after which light or electron microscopic evaluation was performed. The threshold for BBB disruption was approximately 0.2 MPa. More than 80% of the brain sites sonicated showed BBB disruption when the pressure amplitude was 0.3 MPa; at 0.4 MPa, this percentage was greater than 90%. Tissue necrosis, ischemia, and apoptosis were not found in tissue in which the pressure amplitude was less than 0.4 MPa; however, in a few areas of brain tissue erythrocytes were identified outside blood vessels following exposures of 0.4 MPa or higher. Survival experiments did not show any long-term adverse events. These results demonstrate that low-frequency ultrasound bursts can induce local, reversible disruption of the BBB without undesired long-term effects. This technique offers a potential noninvasive method for targeted drug delivery in the brain aided by a relatively simple low-frequency device.

Plasma rotation evolution near the peripheral transport barrier in the presence of low-frequency MHD bursts in TUMAN-3M tokamak

The experiments described in the paper are aimed at investigating the possible influence of the low frequency magnetohydrodynamic (MHD) activity burst on the Ohmic H-mode in the TUMAN-3M tokamak. During the MHD burst a transient deterioration of improved confinement was observed. The study has been focused on the measurements of plasma fluctuation poloidal velocity performed by microwave Doppler reflectometry. The plasma fluctuation rotation observed before the MHD burst in the vicinity of the edge transport barrier was in the direction of plasma drift in the negative radial electric field. During the MHD activity the measured poloidal velocity was drastically decreased and even changed its sign. Radial profiles of the poloidal velocity measured in a set of reproducible tokamak shots exhibited the plasma fluctuation rotation in the ion diamagnetic drift direction at the location of the peripheral transport barrier. The possible reasons for this phenomenon are discussed.

Pattern Playback revisited: Unvoiced stop consonant perception

Among the most influential publications in speech perception is Liberman, Delattre, and Cooper's [Am. J. Phys. 65, 497-516 (1952)] report on the identification of synthetic, voiceless stops generated by the Pattern Playback. Their map of stop consonant identification shows a highly complex relationship between acoustics and perception. This complex mapping poses a challenge to many classes of relatively simple pattern recognition models which are unable to capture the original finding of Liberman et al. that identification of /k/ was bimodal for bursts preceding front vowels but otherwise unimodal. A replication of this experiment was conducted in an attempt to reproduce these identification patterns using a simulation of the Pattern Playback device. Examination of spectrographic data from stimuli generated by the Pattern Playback revealed additional spectral peaks that are consistent with harmonic distortion characteristic of tube amplifiers of that era. Only when harmonic distortion was introduced did bimodal /k/ responses in front-vowel context emerge. The acoustic consequence of this distortion is to add, e.g., a high-frequency peak to midfrequency bursts or a midfrequency peak to a low-frequency burst. This likely resulted in additional /k/ responses when the second peak approximated the second formant of front vowels. Although these results do not challenge the main observations made by Liberman et al. that perception of stop bursts is context dependent, they do show that the mapping from acoustics to perception is much less complex without these additional distortion products.