Polarity-dependent Changes Research Articles

Contrasting MEG effects of anodal and cathodal high-definition TDCS on sensorimotor activity during voluntary finger movements.

Protocols for noninvasive brain stimulation (NIBS) are generally categorized as "excitatory" or "inhibitory" based on their ability to produce short-term modulation of motor-evoked potentials (MEPs) in peripheral muscles, when applied to motor cortex. Anodal and cathodal stimulation are widely considered excitatory and inhibitory, respectively, on this basis. However, it is poorly understood whether such polarity-dependent changes apply for neural signals generated during task performance, at rest, or in response to sensory stimulation. To characterize such changes, we measured spontaneous and movement-related neural activity with magnetoencephalography (MEG) before and after high-definition transcranial direct-current stimulation (HD-TDCS) of the left motor cortex (M1), while participants performed simple finger movements with the left and right hands. Anodal HD-TDCS (excitatory) decreased the movement-related cortical fields (MRCF) localized to left M1 during contralateral right finger movements while cathodal HD-TDCS (inhibitory), increased them. In contrast, oscillatory signatures of voluntary motor output were not differentially affected by the two stimulation protocols, and tended to decrease in magnitude over the course of the experiment regardless. Spontaneous resting state oscillations were not affected either. MRCFs are thought to reflect reafferent proprioceptive input to motor cortex following movements. Thus, these results suggest that processing of incoming sensory information may be affected by TDCS in a polarity-dependent manner that is opposite that seen for MEPs-increases in cortical excitability as defined by MEPs may correspond to reduced responses to afferent input, and vice-versa.

Unlocking the potential of solvent polarity in directing ESIPT pathways of HHMB with dual hydrogen bond acceptors: a DFT/TD-DFT study

The excited-state intramolecular proton transfer (ESIPT) process involving dual hydrogen bond acceptors (HBAs) is of great interest in research and has significant potential for applications due to its flexible adjustability. In this work, the solvent polarity-dependent changes in structures, photophysical properties and ESIPT processes of 2-(2-hydroxy-3-hydroxyiminomethyl-5-methylphenyl)-benzimidazole (HHMB) with two asymmetric HBAs are investigated. Firstly, the orientation of intramolecular hydrogen bond (IHB) under different solvent polarities are explored. The computational results indicate that in non-polar solvents, the central hydroxyl group predominantly forms an IHB with the imine of the oxime group, resulting in large Stokes and near-infrared emission peaks. Conversely, in polar solvents, the central hydroxyl group links the imidazole group to form a stronger IHB with improved fluorescence efficiency. Additionally, the quantum chemical calculations are used to simulate the absorption–emission spectra and plot the frontier molecular orbitals, as well as the potential energy curves about ESIPT in different solvents. In conclusion, we elucidate the rationale behind the preferential formation of the biased IHBs and examine its impact on the ESIPT process by manipulating the solvent polarity. Theoretical studies on the dual HBAs molecule, exhibiting specific selectivity, pave the way for the development of higher-performance fluorescence probes and luminescent materials.

Active rejection-enhancement of spectrally tunable liquid crystal geometric phase vortex coronagraphs

Geometric phase optical elements made of space-variant anisotropic media customarily find their optimal operating conditions when a half-wave retardance condition is fulfilled, which allows imparting polarization-dependent changes to an incident wavefront. In practice, intrinsic limitations of a man-made manufacturing process or a finite spectrum of a light source lead to a deviation from the ideal behavior. This implies an implementation of strategies to compensate for the associated efficiency losses. Here, we report on how the intrinsic tunable features of self-engineered liquid crystal topological defects can be used to enhance rejection capabilities of spectrally tunable vector vortex coronagraphs. We also discuss the extent of which current models enable to design efficient devices. The simplicity and decent performance of our approach offer the possibility to an amateur astronomy community to consider the use of vortex coronography.

Transcranial direct current stimulation as a preventive treatment in multiple sclerosis? Preclinical evidence

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system, presenting with optic neuritis in about 20–30% of cases. Optic nerve demyelination, associated with delay of visual evoked potentials (VEPs), is also observed prior to motor signs in the preclinical MS model Experimental Autoimmune Encephalomyelitis (EAE). Transcranial direct current stimulation (tDCS), inducing polarity-dependent changes in neuronal excitability, is widely used to promote neuroplasticity in several neurological disorders. However, its potential effects on inflammation and demyelination are largely unknown. We tested the effectiveness of a preventive, 5-day tDCS treatment started 3 days post-immunization, in reducing the severity of VEP delays observed in early EAE. In mice undergoing cathodal tDCS (n = 6/26 eyes) VEPs were significantly less delayed compared with eyes from EAE-Sham (n = 24/32 eyes) and EAE-Anodal (n = 22/32 eyes). Optic nerve immunohistochemistry revealed a significantly lower cell density of microglia/macrophages, and less axonal loss in EAE-Cathodal vs EAE-Sham and EAE-Anodal, while the percent demyelination with Luxol-fast blue staining was comparable among EAE groups. Considering the latter result, immunofluorescence paranodal staining was performed, revealing a significantly higher number of complete paranode domains in EAE-Cathodal, closer to healthy mice, compared with EAE-Sham and EAE-Anodal groups. These results were reflected by the negative correlation between the number of complete paranode domains and VEP latency increase with respect to pre-immunization. Finally, cathodal tDCS was associated with a lower number, closer to healthy, of single paranodes in contrast to EAE-Sham. The effects of cathodal stimulation in preventing VEPs delays and optic nerve myelin damage were already observed in the pre-motor onset EAE stage, and were associated with a lower density of inflammatory cells. These findings suggest that tDCS may exert an anti-inflammatory effect with potential therapeutic application to be further explored in autoimmune demyelinating diseases.

Analysis of pyrene-labelled apolipoprotein A-I oligomerization in solution: Spectra deconvolution and changes in P-value and excimer formation

ApoA-I is the main protein of HDL which has anti-atherogenic properties attributed to reverse cholesterol transport. It shares with other exchangeable apolipoproteins a high level of structural plasticity. In the lipid-free state, the apolipoprotein amphipathic α-helices interact intra- and inter-molecularly, providing structural stabilization by a complex self-association mechanism. In this study, we employed a multi-parametric fluorescent probe to study the self-association of apoA-I. We constructed six single cysteine mutants spanning positions along three helices: F104C, K107C (H4), K133C, L137C (H5), F225C and K226C (H10); and labelled them with N-Maleimide Pyrene. Taking advantage of its spectral properties, namely formation of an excited dimer (excimer) and polarity-dependent changes in its fluorescence fine structure (P-value), we monitored the apoA-I self-association in its lipid-free form as a function of its concentration. Interactions in helices H5 (K133C) and H10 (F225C and K226C) were highlighted by excimer emission; while polarity changes were reported in helix H4 (K107C), as well as in helices H5 and H10. Mathematical models were developed to enrich data analysis and estimate association constants (KA) and oligomeric species distribution. Furthermore, we briefly discuss the usefulness of the multi-parametric fluorescent probe to monitor different equilibria, even at a single labelling position. Results suggest that apoA-I self-association must be considered to fully understand its physiological roles. Particularly, some contacts that stabilize discoidal HDL particles seem to be already present in the lipid-free apoA-I oligomers.

TDCS effects on brain network properties during physiological aging.

Brain neural networks undergo relevant changes during physiological aging, which affect cognitive and behavioral functions. Currently, non-invasive brain stimulation techniques, such as transcranial direct current stimulation (tDCS), are proposed as tools able to modulate cognitive functions in brain aging, acting on networks properties and connectivity. Segregation and integration measures are used and evaluated by means of local clustering (segregation) and path length (integration). Moreover, to assess the balancing between them, the Small World (SW) parameter is employed, evaluating functional coupling in normal brain aging and in pathological conditions including neurodegeneration. The aim of this study was to systematically investigate the tDCS-induced effects on brain network proprieties in physiological aging. In order to reach this aim, cortical activity was acquired from healthy young and elderly subjects by means of EEG recorded before, during, and after anodal, cathodal, and sham tDCS sessions. Specifically, the aim to exploring tDCS polarity-dependent changes in the age-dependent network dynamics was based on a network graph theory application on two groups divided in young and elderly subjects. Eighteen healthy young (9 females; mean age = 24.7, SD = 3.2) and fifteen elderly subjects (9 females; mean = 70.1, SD = 5.1) were enrolled. Each participant received anodal, cathodal, or sham tDCS over the left prefrontal cortex (PFC) in three separate experimental sessions performed 1week apart. SW was computed to evaluate brain network organization. The present study demonstrates that tDCS delivered in PFC can change brain network dynamics, and tDCS-EEG coregistration data can be analyzed using graph theory to understand the induced effects of different tDCS polarities in physiological and pathological brain aging.

Modulation of the Photophysical Properties of β-substituted BODIPY Dyes.

Photophysical properties of BODIPY dyes containing acetyl acetone and benzoyl acetone BF2 unit as an electron accepting substituent at beta position linked via double bond have been investigated using a wide range of solvents of different polarities. The substitution effect at beta position of the BODIPY dyes on their absorption, emission and quantum yield of fluorescence have been the aim of present study. For the synthesized BODIPY dyes fluorescence quantum yields and lifetimes show very sharp decrease with an increase in the solvent polarity, suggesting the involvement of highly polar ICT state de-excitation mechanism along with the local excitation process. The polarity dependent changes in average fluorescence life time and quantum yield values rationalize the formation of ICT states.

Effect of Polarization Reversal in Ferroelectric TiN/Hf0.5Zr0.5O2/TiN Devices on Electronic Conditions at Interfaces Studied in Operando by Hard X-ray Photoemission Spectroscopy.

Because of their compatibility with modern Si-based technology, HfO2-based ferroelectric films have recently attracted attention as strong candidates for applications in memory devices, in particular, ferroelectric field-effect transistors or ferroelectric tunnel junctions. A key property defining the functionality of these devices is the polarization dependent change of the electronic band alignment at the metal/ferroelectric interface. Here, we report on the effect of polarization reversal in functional ferroelectric TiN/Hf0.5Zr0.5O2/TiN capacitors on the potential distribution across the stack and the electronic band line-up at the interfaces studied in operando by hard X-ray photoemission spectroscopy. By tracking changes in the position of Hf0.5Zr0.5O2 core-level lines with respect to those of the TiN electrode in both short- and open-circuit configurations following in situ polarization reversal, we derive the conduction band offset to be 0.7 (1.0) eV at the top and 1.7 (1.0) eV at the bottom interfaces for polarization, pointing up (down), respectively. Energy dispersive X-ray spectroscopy profiling of the sample cross-section in combination with the laboratory X-ray photoelectron spectroscopy reveal the presence of a TiOx/TiON layer at both interfaces. The observed asymmetry in the band line-up changes in the TiN/Hf0.5Zr0.5O2/TiN memory stack is explained by different origin of these oxidized layers and effective pinning of polarization at the top interface. The described methodology and first experimental results are useful for the optimization of HfO2-based ferroelectric memory devices under development.

A Combined Experimental and Computational Investigation on Spectroscopic and Photophysical Properties of a Coumarinyl Chalcone.

Here, we synthesized a new coumarinyl chalcone derivative 3-[3-(3-Methyl-thiophen-2-yl)-acryloyl]-chromen-2-one (MTC) by simple and proficient method. A comprehensive study on the photophysics of a coumarinyl chalcone derivative having pi-conjugated potential chromophore system 3-[3-(3-Methyl-thiophen-2-yl)-acryloyl]-chromen-2-one (MTC) has been carried out spectroscopically. The electronic absorption and emission characteristic of MTC were studied in different protic and aprotic solvents using absorption and steady-state fluorescence techniques. The spectral behavior of this compound is found to be extremely sensitive to the polarity and hydrogen bonding nature of the solvent. The compound shows very strong solvent polarity dependent changes in their photophysical characteristics, namely, remarkable red shift in the emission spectra with increasing solvent polarity, change in Stokes shift, significant reduction in the fluorescence quantum yield; indicating that the fluorescence states of these compounds are of intramolecular charge transfer (ICT) character. The solvent effect on the photophysical parameters such as singlet absorption, molar absorptivity, oscillator strength, dipole moment, fluorescence spectra, and fluorescence quantum yield of the compound has been investigated in detail. The difference between the excited and ground state dipole moments (Δμ) were estimated from solvatochromic methods using Lippert-Mataga and Reichardt's correlations. The prepared compound was also studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results revealed that it could be easily reproduce by computational means.

Modulation of frontal effective connectivity during speech

Noninvasive neurostimulation methods such as transcranial direct current stimulation (tDCS) can elicit long-lasting, polarity-dependent changes in neocortical excitability. In a previous concurrent tDCS-fMRI study of overt picture naming, we reported significant behavioural and regionally specific neural facilitation effects in left inferior frontal cortex (IFC) with anodal tDCS applied to left frontal cortex (Holland et al., 2011). Although distributed connectivity effects of anodal tDCS have been modelled at rest, the mechanism by which ‘on-line’ tDCS may modulate neuronal connectivity during a task-state remains unclear. Here, we used Dynamic Causal Modelling (DCM) to determine: (i) how neural connectivity within the frontal speech network is modulated during anodal tDCS; and, (ii) how individual variability in behavioural response to anodal tDCS relates to changes in effective connectivity strength. Results showed that compared to sham, anodal tDCS elicited stronger feedback from inferior frontal sulcus (IFS) to ventral premotor (VPM) accompanied by weaker self-connections within VPM, consistent with processes of neuronal adaptation. During anodal tDCS individual variability in the feedforward connection strength from IFS to VPM positively correlated with the degree of facilitation in naming behaviour. These results provide an essential step towards understanding the mechanism of ‘online’ tDCS paired with a cognitive task. They also identify left IFS as a ‘top-down’ hub and driver for speech change.

Photo-generated metamaterials induce modulation of CW terahertz quantum cascade lasers.

Periodic patterns of photo-excited carriers on a semiconductor surface profoundly modifies its effective permittivity, creating a stationary all-optical quasi-metallic metamaterial. Intriguingly, one can tailor its artificial birefringence to modulate with unprecedented degrees of freedom both the amplitude and phase of a quantum cascade laser (QCL) subject to optical feedback from such an anisotropic reflector. Here, we conceive and devise a reconfigurable photo-designed Terahertz (THz) modulator and exploit it in a proof-of-concept experiment to control the emission properties of THz QCLs. Photo-exciting sub-wavelength metastructures on silicon, we induce polarization-dependent changes in the intra-cavity THz field, that can be probed by monitoring the voltage across the QCL terminals. This inherently flexible approach promises groundbreaking impact on THz photonics applications, including THz phase modulators, fast switches, and active hyperbolic media.

Different Response of Carbonyl Carotenoids to Solvent Proticity Helps To Estimate Structure of the Unknown Carotenoid from Chromera velia.

In order to estimate the possible structure of the unknown carbonyl carotenoid related to isofucoxanthin from Chromera velia denoted as isofucoxanthin-like carotenoid (Ifx-l), we employed steady-state and ultrafast time-resolved spectroscopic techniques to investigate spectroscopic properties of Ifx-l in various solvents. The results were compared with those measured for related carotenoids with known structure: fucoxanthin (Fx) and isofucoxanthin (Ifx). The experimental data were complemented by quantum chemistry calculations and molecular modeling. The data show that Ifx-l must have longer effective conjugation length than Ifx. Yet, the magnitude of polarity-dependent changes in Ifx-l is larger than for Ifx, suggesting significant differences in structure of these two carotenoids. The most interesting spectroscopic feature of Ifx-l is its response to solvent proticity. The transient absorption data show that (1) the magnitude of the ICT-like band of Ifx-l in acetonitrile is larger than in methanol and (2) the S1/ICT lifetime of Ifx-l in acetonitrile, 4 ps, is markedly shorter than in methanol (10 ps). This is opposite behavior than for Fx and Ifx whose S1/ICT lifetimes are always shorter in protic solvent methanol (20 and 13 ps) than in aprotic acetonitrile (30 and 17 ps). Comparison with other carbonyl carotenoids reported earlier showed that proticity response of Ifx-l is consistent with presence of a conjugated lactone ring. Combining the experimental data and quantum chemistry calculations, we estimated a possible structure of Ifx-l.

Resting-state fMRI reveals enhanced functional connectivity in spatial navigation networks after transcranial direct current stimulation

A number of studies have established that transcranial direct current stimulation (tDCS) modulates cortical excitability. We previously demonstrated polarity dependent changes in parietal lobe blood oxygen level dependent (BOLD) fMRI in a group of young adults during a spatial navigation task [15]. Here we used resting state functional connectivity (rsFC) to examine whether analogous changes were also evident during the resting state. Participants were randomized to either a parietal-anodal, frontal-cathodal (P+F−) or the opposite montage (P−F+) and received 20min of tDCS (2mA) before undergoing resting-state fMRI. rsFC was evaluated between the groups by placing a seed in the medial superior parietal lobule (mSPL), which was under the target electrode. rsFC between the mSPL and a number of other areas involved in spatial navigation, scene processing, and sensorimotor processing was significantly higher in the P+F− than the P−F+ group. Thus, the modulatory effects of tDCS were evident during rest and suggest that stimulation primes not just the underlying neocortex but an extended network that can be recruited as necessary during active task performance.

Synthesis, spectral behaviour and photophysics of donor–acceptor kind of chalcones: Excited state intramolecular charge transfer and fluorescence quenching studies

The spectral and photophysical properties of two chalcones containing electron donating and accepting groups with intramolecular charge transfer characteristics were synthesized and characterized by 1H NMR, 13C NMR and X-ray crystallography. Both compounds show very strong solvent polarity dependent changes in their photophysical characteristics, namely, remarkable red shift in the emission spectra with increasing solvent polarity, large change in Stokes shift, significant reduction in the fluorescence quantum yield; indicating that the fluorescence states of these compounds are of intramolecular charge transfer (ICT) character. The solvent effect on the photophysical parameters such as singlet absorption, molar absorptivity, oscillator strength, dipole moment, fluorescence spectra, and fluorescence quantum yield of both compounds have been investigated comprehensively. For both dyes, Lippert–Mataga and Reichardt’s correlations were used to estimate the difference between the excited and ground state dipole moments (Δμ). The interactions of dyes with colloidal silver nanoparticles (Ag NPs) were also studied in ethanol using steady state fluorescence quenching measurements. The fluorescence quenching data reveal that dynamic quenching and energy transfer play a major role in the fluorescence quenching of dyes by Ag NPs.

Zn(II)-bisthienylethynylbipyridine complex: Preparation, characterization and vapochromic behaviour in polymer films

In the present work, we report the vapochromic behaviour of PMMA films doped with small amounts of 1a:Zn fluorophore, a Zn2+ adduct of the bisthienylethynylbipyridine ligand.1a:Zn exhibited polarity-dependent changes in the fluorescence spectral properties when dissolved in solvents with different polarity index, indicating that its fluorescence states are of intramolecular charge transfer character. When 1a:Zn was dispersed in the PMMA matrix, its emission resulted barely influenced by the polymer environment. By contrast, 1a:Zn/PMMA films showed a significant variation of its emission with a 40 nm red-shift when exposed to a saturated atmosphere of dichloromethane. This vapochromic behaviour was much less evident when progressive amounts of methanol were added to dichloromethane due to both lower vapour pressure and limited interaction of the former with PMMA. The experiments revealed also that the rate constants associated with the vapochromic behaviour were affected by solvent composition and easily determined by luminescent experiments.

Transcranial Direct Current Stimulation Modulates Activation and Effective Connectivity During Spatial Navigation

BackgroundAllocentric navigation declines with age and neurologic disease whereas egocentric navigation does not; differences that likely arise from maladaptive changes in brain regions mediating spatial (parietal cortex; hippocampus) but not procedural processing (caudate nucleus). Transcranial direct current stimulation (tDCS) holds promise for treating such decline given its ability to modulate neuronal excitability, but its effects have yet to be examined on spatial navigation. Objectives/hypothesesUsing healthy young adults as a model, Study 1 intended to validate a novel spatial navigation paradigm using functional magnetic resonance imaging (fMRI). Using these data to determine targets for tDCS, Study 2 aimed to determine if 1) stimulation modulates activation in a polarity-specific manner; 2) stimulation results in global and/or task-specific activation changes; 3) activation changes are accompanied by changes in effective connectivity. MethodsAll participants underwent fMRI while learning allocentric and egocentric environments. Twelve participants completed Study 1. In Study 2, 16 participants were randomized to 20 min of tDCS (2 mA) using a montage with the anode over PZ and cathode over AF4 (P+F−) or the reverse montage (P−F+). ResultsStudy 1 revealed that distinct networks preferentially mediate allocentric and egocentric navigation. Study 2 revealed polarity-dependent changes in activation and connectivity. The P+F− montage increased these measures in spatial regions, especially during allocentric navigation, and the caudate nucleus. Conversely, the P−F+ montage increased activation and connectivity in lateral prefrontal cortices and posterior hippocampus. ConclusionsThese findings support the neuromodulatory effects of tDCS in non-motor areas and demonstrate proof-of-principle for ameliorating age- and disease-related decline in navigational abilities.

P 99. Effects of transcranial direct-current stimulation on diaphragm corticospinal pathway excitability

Background Transcranial direct-current stimulation (tDCS) applied over the primary motor cortex induces polarity-dependent changes in excitability: anodal tDCS enhances cortical excitability as assessed using transcranial magnetic stimulation (TMS), whereas cathodal tDCS has the opposite effect. Objective In this randomized study, we assessed effects of anodal, cathodal, and sham tDCS on the diaphragmatic primary motor cortex. Methods Twelve healthy right-handed men received anodal, cathodal, and sham tDCS applied to the left diaphragmatic motor cortex. Motor-evoked-potential (MEP) recruitment curves were obtained by recording the surface electromyogram at the right 8th intercostal space during TMS at intensities ranging from the resting motor threshold (MT) to 130% of MT. For each condition, MEPs were recorded before (Pre) tDCS then after 10 min (Post1, at tDCS discontinuation in the anodal and cathodal sessions) and 20 (Post2) minutes. Results Both anodal and cathodal tDCS significantly decreased MEP amplitude at the right hemidiaphragm at both Post1 and Post2, versus Pre. MEP amplitude was unchanged versus Pre during the sham condition. Conclusion The effects of cathodal and anodal tDCS applied to the diaphragm motor cortex differ from those observed during tDCS of the limb motor cortex. This difference may be related either to the mechanisms involved in the effects of tDCS or to the diaphragm contractions during tDCS.

P 193. Excitability modulation of the motor system induced by transcranial direct current stimulation

Question Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that modulates cortical excitability and activity in a polarity-dependent way. In the human motor system, such cortical modulations are inferred through changes in the amplitude of motor evoked potentials (MEPs). To directly evaluate tDCS-induced changes at the cortical level, we investigated polarity-dependent tDCS-induced effects on the motor system, evaluating changes in MEPs, TMS-evoked potentials (TEPs) and in the EEG oscillatory activity. Methods Sixteen young healthy right-handed subjects participated in this study. Two experimental sessions were performed for each subject in randomized order: anodal and cathodal tDCS (a- and c-tDCS). The EEG activity was recorded from 10 scalp electrodes while EMG activity was recorded from the right FDI. Corticospinal excitability and cortical reactivity were investigated through the recording of MEPs and TEPs, whereas the cortical state was evaluated through the acquisition of the EEG activity. All the measures were collected before tDCS, immediately and 30 min after tDCS, to evaluate short and long-lasting tDCS effects. The tDCS was applied for 13 min (1 mA) over the left primary motor cortex. The TEP-MEP block consisted of 100 TMS pulses (intensity of 110% of the RMT), delivered with a random inter stimulus interval of 2–4 s. The EEG block consisted of 3 min of recording during a resting state. To determine the tDCS induced changes in the cortical evoked potentials, a local mean field power analysis was computed. To characterize tDCS-induced changes in cortical oscillatory activity, the EEG power density was estimated by means of the Fast Fourier transform. Results The application of a- and c-tDCS over M1 induced respectively a short-term increase and decrease of MEPs amplitude. The MEPs changes persisted 30 min after a-tDCS but not after c-tDCS. The TEPs analysis highlighted short term changes induced by tDCS polarities. Particularly, we found a significant pattern of topographically specific and current-dependent changes. a- and c-tDCS induced consistent differences in cortical reactivity only on the stimulated area. The long lasting changes partially overlapped those observed in the short-term analyses. Finally, the EEG frequency analysis revealed a significant main effect only in the theta and alpha bands, suggesting a general increase in their power density after tDCS. These changes were reduced 30 min after stimulation. Conclusions a-tDCS over primary motor cortex induced an enhancement of corticospinal excitability, whereas c-tDCS produced an excitability reduction. More interestingly, the cortical reactivity resulted increased after anodal stimulation whereas cathodal stimulation produced a decrease over the stimulated area. These cortical reactivity changes lasted for at least30 min. Moreover, a general increase in the power density of theta and alpha frequencies was also present over all scalp sites for both the stimulation polarities. These results shows direct evidence that tDCS induces polarity-dependent changes on brain activity at cortical level.

Polarity- and valence-dependent effects of prefrontal transcranial direct current stimulation on heart rate variability and salivary cortisol

Recent evidence has supported the notion that the hypothalamic-pituitary-adrenal (HPA) and the sympatho-adreno-medullary (SAM) systems are modulated by cortical structures such as the prefrontal cortex. This top-down modulation may play a major role in the neuroendocrine changes associated with stressful events. We aimed to investigate further this hypothesis by modulating directly prefrontal cortex excitability using transcranial direct current stimulation (tDCS) - a non-invasive, neuromodulatory tool that induces polarity-dependent changes in cortical excitability - and measuring effects on salivary cortisol and heart rate variability as proxies of the HPA and SAM systems. Twenty healthy participants with no clinical and neuropsychiatric conditions were randomized to receive bifrontal tDCS (left anodal/right cathodal or left cathodal/right anodal) or sham stimulation, in a within-subject design. During each stimulation session, after a resting period, subjects were shown images with neutral or negative valence. Our findings showed that excitability enhancing left anodal tDCS induced a decrease in cortisol levels. This effect is more pronounced during emotional negative stimuli. Moreover, vagal activity was higher during left anodal tDCS and emotional negative stimuli, as compared to sham stimulation and neutral images. We also observed an association between higher mood scores, higher vagal activation and lower cortisol levels for anodal stimulation. Subjective mood and anxiety evaluation revealed no specific changes after stimulation. Our findings suggest that tDCS induced transient, polarity specific modulatory top-down effects with anodal tDCS leading to a down-regulation of HPA and SAM systems. Further research using tDCS and neuroendocrine markers should explore the mechanisms of stress regulation in healthy and clinical samples.

Contrasting Solvent Polarity Effect on the Photophysical Properties of Two Newly Synthesized Aminostyryl Dyes in the Lower and in the Higher Solvent Polarity Regions

Solvent polarity effect on the photophysical properties of two newly synthesized aminostyryl-thiazoloquinoxaline dyes, one with a flexible diphenylamino group, namely, N,N-diphenyl-4-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]aniline (TQ1), and the other with a rigid julolidinylamino group, namely, (9-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]julolidine) (TQ2), have been investigated in different aprotic solvents and solvent mixtures. From the polarity dependent changes in the absorption and fluorescence spectral properties, it is indicated that the fluorescent states of the dyes are of intramolecular charge transfer (ICT) character. For both the dyes, the photophysical properties like fluorescence quantum yields (Phi(f)), fluorescence lifetimes (tau(f)), radiative rate constants (k(f) = Phi(f)/tau(f)), and nonradiative rate constants (k(nr) = 1/tau(f) - Phi(f)/tau(f)) show clearly contrasting solvent polarity effects in the lower and in the higher solvent polarity region, causing an interesting reversal in the properties below and above an intermediate solvent polarity. It is inferred that the domination of the cis-trans isomerization in the lower solvent polarity region and that of the twisted intramolecular charge transfer (TICT) state formation in the higher solvent polarity region are responsible for the observed contrasting solvent polarity effects on the photophysical properties of the two dyes. As both isomerization and TICT state formation causes an enhancement in the nonradiative decay rate of the excited dyes and both the processes become less significant at the intermediate solvent polarity region, the two dyes show their largest Phi(f) and tau(f) values at intermediate solvent polarities. Suitable mechanistic schemes have been proposed and qualitative potential energy diagrams have been presented to explain the observed results with the changes in the polarity of the solvents used.