V. Moliadze

Effects of 10 Hz and 20 Hz transcranial alternating current stimulation (tACS) on motor functions and motor cortical excitability.

Synchronized oscillatory activity at alpha (8-12 Hz) and beta (13-30 Hz) frequencies plays a key role in motor control. Nevertheless, its exact functional significance has yet to be solved. Transcranial alternating current stimulation (tACS) allows the frequency-specific modulation of ongoing oscillatory activity. The goal of the present study was to investigate the effect of 10 and 20 Hz tACS over left primary motor cortex (M1) on motor functions and cortical excitability in healthy subjects. To this end, tACS was applied for 10 min. Sham stimulation served as control condition. Movement speed and accuracy of the right hand were assessed in 15 right-handed subjects before and after (0, 30 and 60 min) tACS of M1. Cortical silent period (CSP) and motor evoked potentials (MEPs) were determined as measures of M1 excitability. While 10 Hz tACS particularly increased movement variability, especially in tasks requiring internal pacing, 20 Hz tACS resulted in movement slowing. Behavioural effects occurred in distinct time windows. While 10 Hz effects developed over 30 min after stimulation, 20 Hz tACS effects were found immediately after stimulation. Following 10 Hz tACS these effects were significantly correlated with CSP duration, indicating interference with inhibitory pathways. The present findings suggest differential effects of stimulation frequency on motor behaviour and M1 excitability.

Stimulation intensities of transcranial direct current stimulation have to be adjusted in children and adolescents

OBJECTIVE The aim of the present study was to investigate the effect of the transcranial direct current stimulation (tDCS) on motor cortex excitability in healthy children and adolescents. METHODS We applied 1mA anodal or cathodal tDCS for 10min on the left primary motor cortex of 19 healthy children and adolescents (mean age 13.9±0.4years). In order to prove whether the effects of tDCS may be attributed to the stimulation intensity, 10 children and adolescents were studied again using 0.5mA anodal and cathodal tDCS. Sham stimulation was used as a control. RESULTS Compared with sham stimulation, both 1mA anodal and cathodal tDCS resulted in a significant increase of Motor evoked potentials (MEP) amplitudes which remained to be prominent even one hour after the end of stimulation. Interestingly, the 0.5mA cathodal tDCS decreased cortico-spinal excitability whereas the 0.5mA anodal stimulation did not result in any effect. CONCLUSION For the first time, the study demonstrates age-specific influences of tDCS on cortical excitability of the primary motor cortex. SIGNIFICANCE Thus, the stimulation protocols of the tDCS have to be optimized according to age by planning studies in pediatric population.

The effect of 10 Hz transcranial alternating current stimulation (tACS) on corticomuscular coherence

Synchronous oscillatory activity at alpha (8–12 Hz), beta (13–30 Hz), and gamma (30–90 Hz) frequencies is assumed to play a key role for motor control. Corticomuscular coherence (CMC) represents an established measure of the pyramidal systems integrity. Transcranial alternating current stimulation (tACS) offers the possibility to modulate ongoing oscillatory activity. Behaviorally, 20 Hz tACS in healthy subjects has been shown to result in movement slowing. However, the neurophysiological changes underlying these effects are not entirely understood yet. The present study aimed at ascertaining the effects of tACS at 10 and 20 Hz in healthy subjects on CMC and local power of the primary sensorimotor cortex. Neuromagnetic activity was recorded during isometric contraction before and at two time points (2–10 min and 30–38 min) after tACS of the left primary motor cortex (M1), using a 306 channel whole head magnetoencephalography (MEG) system. Additionally, electromyography (EMG) of the right extensor digitorum communis (EDC) muscle was measured. TACS was applied at 10 and 20 Hz, respectively, for 10 min at 1 mA. Sham stimulation served as control condition. The data suggest that 10 Hz tACS significantly reduced low gamma band CMC during isometric contraction. This implies that tACS does not necessarily cause effects at stimulation frequency. Rather, the findings suggest cross-frequency interplay between alpha and low gamma band activity modulating functional interaction between motor cortex and muscle.

Ten minutes of 1 mA transcranial direct current stimulation was well tolerated by children and adolescents: Self-reports and resting state EEG analysis

Transcranial direct current stimulation (tDCS) is a promising and well-tolerated method of non-invasive brain stimulation, by which cortical excitability can be modulated. However, the effects of tDCS on the developing brain are still unknown, and knowledge about its tolerability in children and adolescents is still lacking. Safety and tolerability of tDCS was assessed in children and adolescents by self-reports and spectral characteristics of electroencephalogram (EEG) recordings. Nineteen typically developing children and adolescents aged 11-16 years participated in the study. Anodal and cathodal tDCS as well as sham stimulation were applied for a duration of 10 min over the left primary motor cortex (M1), each with an intensity of 1 mA. Subjects were unable to identify whether they had received active or sham stimulation, and all participants tolerated the stimulation well with a low rate of adverse events in both groups and no serious adverse events. No pathological oscillations, in particular, no markers of epileptiform activity after 1mA tDCS were detected in any of the EEG analyses. In summary, our study demonstrates that tDCS with 1mA intensity over 10 min is well tolerated, and thus may be used as an experimental and treatment method in the pediatric population.

EEG-MEG Integration Enhances the Characterization of Functional and Effective Connectivity in the Resting State Network

At the sensor level many aspects, such as spectral power, functional and effective connectivity as well as relative-power-ratio ratio (RPR) and spatial resolution have been comprehensively investigated through both electroencephalography (EEG) and magnetoencephalography (MEG). Despite this, differences between both modalities have not yet been systematically studied by direct comparison. It remains an open question as to whether the integration of EEG and MEG data would improve the information obtained from the above mentioned parameters. Here, EEG (64-channel system) and MEG (275 sensor system) were recorded simultaneously in conditions with eyes open (EO) and eyes closed (EC) in 29 healthy adults. Spectral power, functional and effective connectivity, RPR, and spatial resolution were analyzed at five different frequency bands (delta, theta, alpha, beta and gamma). Networks of functional and effective connectivity were described using a spatial filter approach called the dynamic imaging of coherent sources (DICS) followed by the renormalized partial directed coherence (RPDC). Absolute mean power at the sensor level was significantly higher in EEG than in MEG data in both EO and EC conditions. At the source level, there was a trend towards a better performance of the combined EEG+MEG analysis compared with separate EEG or MEG analyses for the source mean power, functional correlation, effective connectivity for both EO and EC. The network of coherent sources and the spatial resolution were similar for both the EEG and MEG data if they were analyzed separately. Results indicate that the combined approach has several advantages over the separate analyses of both EEG and MEG. Moreover, by a direct comparison of EEG and MEG, EEG was characterized by significantly higher values in all measured parameters in both sensor and source level. All the above conclusions are specific to the resting state task and the specific analysis used in this study to have general conclusion multi-center studies would be helpful.

Comparison of three ICA algorithms for ocular artifact removal from TMS-EEG recordings

The combination of transcranial magnetic stimulation (TMS) and electroencephalography (EEG) is a powerful tool to investigate brain excitability and information processing in brain networks. However, EEG-TMS recordings are challenging because EEG is contaminated by powerful TMS-related artifacts. Because of these artifacts, different EEG-driven analyses (for instance, source analysis and analysis of information flow on the sensors and source level) reveal incorrect results. The aim of this study was to remove ocular artifacts from TMS-EEG recordings following stimulation of motor cortex using three independent component analysis (ICA) algorithms and to evaluate the effectiveness of these algorithms. We showed that the temporal ICA algorithm better separates those components that contain time-locked eye blink artifacts.

P263: Effects of transcranial direct current stimulation in children and adolescents: TMS/EEG study

were assessed before, during, immediately after and 30 minutes after the DC offset. CMAP and F-wave from MN, CMAP and SAP from RN were assessed at the same time intervals as a measurement of nerve excitability (NE). Results: C-tsDCS induced a progressive leftward shift of the HR-RC during stimulation and the effects persisted after stimulation offset. Indeed, C-tsDCS induced a significant reduction of RI at all the times points. Both A-tsDCS and SHAM had no significant effects on HR-RC and RI. None of the stimulation polarities showed an effect on PI and NE. Conclusions: tsDCS can modulate cervical SC excitability. The orientation of the spinal interneurons and motorneurons with respect to the electrical field (EF) seems to be important in establishing the final effects. Further studies are needed to elucidate mechanisms of tsDCS, with respect to polarity and geometry of EF. References: [1] Cogiamanian F. et al. Transcutaneous spinal direct current stimulation. Front Psychiatry 2012; 3:63. [2] Lim CY, Shin HI. Noninvasive DC stimulation on neck changes MEP. Neuroreport 2011 Nov 16; 22(16):819-23.

Predictable information in neural signals during resting state is reduced in autism spectrum disorder

The neurophysiological underpinnings of the nonsocial symptoms of autism spectrum disorder (ASD) which include sensory and perceptual atypicalities remain poorly understood. Well‐known accounts of less dominant top–down influences and more dominant bottom–up processes compete to explain these characteristics. These accounts have been recently embedded in the popular framework of predictive coding theory. To differentiate between competing accounts, we studied altered information dynamics in ASD by quantifying predictable information in neural signals. Predictable information in neural signals measures the amount of stored information that is used for the next time step of a neural process. Thus, predictable information limits the (prior) information which might be available for other brain areas, for example, to build predictions for upcoming sensory information. We studied predictable information in neural signals based on resting‐state magnetoencephalography (MEG) recordings of 19 ASD patients and 19 neurotypical controls aged between 14 and 27 years. Using whole‐brain beamformer source analysis, we found reduced predictable information in ASD patients across the whole brain, but in particular in posterior regions of the default mode network. In these regions, epoch‐by‐epoch predictable information was positively correlated with source power in the alpha and beta frequency range as well as autocorrelation decay time. Predictable information in precuneus and cerebellum was negatively associated with nonsocial symptom severity, indicating a relevance of the analysis of predictable information for clinical research in ASD. Our findings are compatible with the assumption that use or precision of prior knowledge is reduced in ASD patients.

P530: Functional and directed coherence on simultaneous recorded EEG and MEG data during resting state

Question: Modulation of the 20-Hz rolandic rhythm to somatosensory input has been proposed to reflect alterations in the motor cortex excitability. In stroke patients, the strength of this modulation is associated with recovery of hand function. In this study we compared the suppression and rebound amplitudes of the 20-Hz (15-25 Hz) rhythm to tactile stimulation and passive movement. Methods: We recorded rhythmic brain activity in 22 healthy subjects (11 males, mean 59 years) with a 306-channel MEG system during tactile stimulation and passive movement. Tactile stimuli were delivered using pneumatic diaphragms to both index fingers alternately with an interstimulus interval of 1.5 s. For passive movements, the subjects’ index fingers were lifted by a nurse every 3 s. Suppression and rebound of the 20-Hz rhythm were analyzed using Temporal Spectral Evolution (TSE) and their amplitudes were quantified from the MEG channel displaying the strongest rebound/suppression. Results: The peak amplitudes of the rebound in both ipsiand contralateral hemispheres were significantly (p<0.05) stronger after passive movement than after tactile stimulation. In contrast, the peak amplitudes of the suppression did not significantly differ between the stimuli. Conclusions: According to our results, passive movements are strong modulators of the motor cortex excitability. Thus, passive movement might be a more robust and feasible tool than tactile stimulation to study motor cortex alterations in stroke patients.

P22.2 Modulation of motor functions by transcranial alternating current stimulation (tACS)

T. Bocci1,2, M. Caleo3, E. Giorli1,2, D. Barloscio3, S. Tognazzi4, L. Murri1, S. Rossi2, A. Rossi2, F. Sartucci1,3 1Department of Neuroscience, Unit of Neurology, Pisa University Medical School, Pisa, Italy, 2Department of Neuroscience, Neurology and Clinical Neurophysiology Section, Siena University Medical School, Siena, Italy, 3Institute of Neuroscience, CNR, Pisa, Italy, 4Institute of Neuroscience, CNR; Department of Neuroscience, Unit Outpatients Neurological Activity, Pisa University Medical School, Pisa, Italy