Conventional tDCS Research Articles

The value and cost of complexity in predictive modelling: role of tissue anisotropic conductivity and fibre tracts in neuromodulation

Objectives. Computational methods are increasingly used to optimize transcranial direct current stimulation (tDCS) dose strategies and yet complexities of existing approaches limit their clinical access. Since predictive modelling indicates the relevance of subject/pathology based data and hence the need for subject specific modelling, the incremental clinical value of increasingly complex modelling methods must be balanced against the computational and clinical time and costs. For example, the incorporation of multiple tissue layers and measured diffusion tensor (DTI) based conductivity estimates increase model precision but at the cost of clinical and computational resources. Costs related to such complexities aggregate when considering individual optimization and the myriad of potential montages. Here, rather than considering if additional details change current-flow prediction, we consider when added complexities influence clinical decisions. Approach. Towards developing quantitative and qualitative metrics of value/cost associated with computational model complexity, we considered field distributions generated by two 4 × 1 high-definition montages (m1 = 4 × 1 HD montage with anode at C3 and m2 = 4 × 1 HD montage with anode at C1) and a single conventional (m3 = C3-Fp2) tDCS electrode montage. We evaluated statistical methods, including residual error (RE) and relative difference measure (RDM), to consider the clinical impact and utility of increased complexities, namely the influence of skull, muscle and brain anisotropic conductivities in a volume conductor model. Main results. Anisotropy modulated current-flow in a montage and region dependent manner. However, significant statistical changes, produced within montage by anisotropy, did not change qualitative peak and topographic comparisons across montages. Thus for the examples analysed, clinical decision on which dose to select would not be altered by the omission of anisotropic brain conductivity. Significance. Results illustrate the need to rationally balance the role of model complexity, such as anisotropy in detailed current flow analysis versus value in clinical dose design. However, when extending our analysis to include axonal polarization, the results provide presumably clinically meaningful information. Hence the importance of model complexity may be more relevant with cellular level predictions of neuromodulation.

P 235. Focal EEG effects of High Definition tDCS (HD-tDCS) detected by EEG photic driving

HD-tDCS using a 4 × 1-ring configuration has been proposed as an alternative to conventional tDCS, capable to restrict effective current to an area within the ring perimeter. Previously we observed that HD-tDCS stimulation achieves changes in TMS-evoked motor responses comparable to conventional tDCS. In order to investigate focality predicted by computational models, here we used EEG to detect neuronal changes both inside and outside stimulated region. On 15 normal subjects (5 males, 21–57 years), an EEG cap with special electrode holders arranged according to the 4 × 1-ring configuration was placed with the central electrode holder over central area of the left hemisphere (C3). The peripheral holders were spaced 5 cm radially around the central holder at the corners of a square. Monopolar EEG (linked earlobes reference, eyes closed) was recorded inside stimulation area in the central 4 × 1 (anode-An-C3), post-central (medial cathode-Ct-C3p), pre-central (between the anode and two anterior cathodes-Int-C3a) positions, outside the stimulation area in the left occipital lead (O1) and at the homologous right hemisphere points (C4a, C4, C4p and O2). EEG recordings were performed during intermittent photic stimulation (IPS) of fixed frequencies 3, 5, 10 and 21 Hz with 30s intervals between stimulation runs as well as during 3 min resting states before (1st) and after (2nd background) IPS sessions. After the initial EEG recording of 9.5–10 min duration, stimulation was delivered through a battery-driven constant current stimulator connected to a HD-tDCS adaptor device (Soterix Medical Inc., New York, NY). The current was delivered with a ramp-up time of 10s, held at 2 mA for 20 min, and then ramped down over 10 s. EEG was recorded 2–3 min after HD-tDCS. EEG amplitude spectra before and after HD-tDCS were compared for the background states in the 6 standard frequency bands and for the IPS states at all the EEG frequencies corresponding to those of IPS and its harmonics up to 30 Hz. A generalized increase in amplitude spectra after HD-tDCS was observed for the IPS of 3 Hz at the EEG frequencies of 3, 6, 9 and 24 Hz (fundamental frequency and 2nd, 3rd and 8th harmonics), this effect being significantly lower in the stimulation area of the left hemisphere (An-C3, Ct-C3p and Int-C3a) as compared to the homologous leads of the right hemisphere, with lower increase in the An-C3 as compared to the Ct-C3p at 6 and 24 Hz. No interhemispheric asymmetry was observed between occipital regions (O1 and O2). Similar increase was also observed for the 2nd background state in relation to the same state before HD-tDCS) in An-C3, Int-C3a, C4, and C4a in the theta band, in all the right hemisphere leads and in An-C3 and Ct-C3p in the alpha band with significant right-side prevalence, and in C4, C4a, and C4p in the beta-1 band. The earlier 1st background state after HD-tDCS did not show significant changes. These results are in accordance with HD-tDCS producing focal changes in brain function.

Technique and Considerations in the Use of 4x1 Ring High-definition Transcranial Direct Current Stimulation (HD-tDCS)

High-definition transcranial direct current stimulation (HD-tDCS) has recently been developed as a noninvasive brain stimulation approach that increases the accuracy of current delivery to the brain by using arrays of smaller "high-definition" electrodes, instead of the larger pad-electrodes of conventional tDCS. Targeting is achieved by energizing electrodes placed in predetermined configurations. One of these is the 4x1-ring configuration. In this approach, a center ring electrode (anode or cathode) overlying the target cortical region is surrounded by four return electrodes, which help circumscribe the area of stimulation. Delivery of 4x1-ring HD-tDCS is capable of inducing significant neurophysiological and clinical effects in both healthy subjects and patients. Furthermore, its tolerability is supported by studies using intensities as high as 2.0 milliamperes for up to twenty minutes. Even though 4x1 HD-tDCS is simple to perform, correct electrode positioning is important in order to accurately stimulate target cortical regions and exert its neuromodulatory effects. The use of electrodes and hardware that have specifically been tested for HD-tDCS is critical for safety and tolerability. Given that most published studies on 4x1 HD-tDCS have targeted the primary motor cortex (M1), particularly for pain-related outcomes, the purpose of this article is to systematically describe its use for M1 stimulation, as well as the considerations to be taken for safe and effective stimulation. However, the methods outlined here can be adapted for other HD-tDCS configurations and cortical targets.

Technique and considerations in the use of 4x1 ring high-definition transcranial direct current stimulation (HD-tDCS).

High-definition transcranial direct current stimulation (HD-tDCS) has recently been developed as a noninvasive brain stimulation approach that increases the accuracy of current delivery to the brain by using arrays of smaller "high-definition" electrodes, instead of the larger pad-electrodes of conventional tDCS. Targeting is achieved by energizing electrodes placed in predetermined configurations. One of these is the 4x1-ring configuration. In this approach, a center ring electrode (anode or cathode) overlying the target cortical region is surrounded by four return electrodes, which help circumscribe the area of stimulation. Delivery of 4x1-ring HD-tDCS is capable of inducing significant neurophysiological and clinical effects in both healthy subjects and patients. Furthermore, its tolerability is supported by studies using intensities as high as 2.0 milliamperes for up to twenty minutes.Even though 4x1 HD-tDCS is simple to perform, correct electrode positioning is important in order to accurately stimulate target cortical regions and exert its neuromodulatory effects. The use of electrodes and hardware that have specifically been tested for HD-tDCS is critical for safety and tolerability. Given that most published studies on 4x1 HD-tDCS have targeted the primary motor cortex (M1), particularly for pain-related outcomes, the purpose of this article is to systematically describe its use for M1 stimulation, as well as the considerations to be taken for safe and effective stimulation. However, the methods outlined here can be adapted for other HD-tDCS configurations and cortical targets.

Comparing Cortical Plasticity Induced by Conventional and High-Definition 4 × 1 Ring tDCS: A Neurophysiological Study

BackgroundTranscranial direct current stimulation (tDCS) induces long-lasting NMDA receptor-dependent cortical plasticity via persistent subthreshold polarization of neuronal membranes. Conventional bipolar tDCS is applied with two large (35 cm2) rectangular electrodes, resulting in directional modulation of neuronal excitability. Recently a newly designed 4 × 1 high-definition (HD) tDCS protocol was proposed for more focal stimulation according to the results of computational modeling. HD tDCS utilizes small disc electrodes deployed in 4 × 1 ring configuration whereby the physiological effects of the induced electric field are thought to be grossly constrained to the cortical area circumscribed by the ring. ObjectiveWe aim to compare the physiological effects of both tDCS electrode arrangements on motor cortex excitability. MethodstDCS was applied with 2 mA for 10 min. Fourteen healthy subjects participated, and motor cortex excitability was monitored by transcranial magnetic stimulation (TMS) before and after tDCS. ResultsExcitability enhancement following anodal and a respective reduction after cathodal stimulation occurred in both, conventional and HD tDCS. However, the plastic changes showed a more delayed peak at 30 min and longer lasting after-effects for more than 2 h after HD tDCS for both polarities, as compared to conventional tDCS. ConclusionThe results show that this new electrode arrangement is efficient for the induction of neuroplasticity in the primary motor cortex. The pattern of aftereffects might be compatible with the concept of GABA-mediated surround inhibition, which should be explored in future studies directly.

Lack of clinically detectable acute changes on autonomic or thermoregulatory functions in healthy subjects after transcranial direct current stimulation (tDCS)

Neuromodulatory techniques, such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), have been increasingly studied as possible treatments for many neurological and psychiatric disorders. tDCS is capable of inducing changes in regional cerebral blood flow in both cortical and subcortical structures, as shown by positron emission tomography studies, and might conceivably affect hypothalamic and autonomic nervous system functions. However, it remains unknown whether acute changes in autonomic or hypothalamic functions may be triggered by conventional tDCS protocols. To verify whether tDCS, when performed with a bipolar cephalic montage, is capable of inducing acute changes in autonomic or hypothalamic functions in healthy subjects. Fifty healthy volunteers were studied. tDCS was performed with the anode over the C3 position and the cathode over the right supraorbital region. Subjects received either real or sham tDCS. Parameters assessed before and after a 20-minute session included blood pressure, tympanic thermometry, hand skin temperature, heart rate and ventilatory rate. Plasma concentrations of cortisol were also measured in a sub-set of 10 participants. A repeated-measures, mixed-design ANOVA showed significant changes in hand skin temperature (P= .005) and cortisol levels (P < .001) after both real and sham stimulation. There were no statistically significant changes in any of the other measurements. The changes in hand temperature and cortisol levels, having occurred in both the sham and experimental groups, probably reflect a non-specific stress response to a new procedure. There were no significant changes in autonomic functions, ventilation rate or core body temperature that can be attributed to conventional tDCS applied to healthy volunteers.

A high resolution highly linear low spur fractional time-to-digital converter (FTDC) for ADPLL

A high resolution highly linear low spur fractional time-to-digital converter (FTDC) for All Digital PLL (ADPLL) is presented. This FTDC employs a linear high gain time amplifier (TAMP) and a spur reduction digital filter to eliminate the spurs at the output. Unlike conventional TDCs, no delay line is utilized in the new FTDC, and hence no mismatch error cancelation technique is required. The FTDC structure is verified in theory and via simulation using an 180nm CMOS technology. The results illustrate a time resolution of 5psec, differential nonlinearity (DNL) free dynamic range of about 350psec, and the total power consumption, apart from the clock generator, of nearly 3mW.

A proton tagging detector for the NA48 experiment

The concept of KS proton tagging and first prototype test results are reported from the NA48 experiment. Thin scintillator foils were arranged as two staircases intersecting in space. A geometric alignment to 0.4 mrad, a light yield of 300–400 photoelectrons and a time resolution of 70–90 ps with conventional TDCs were achieved. A FADC system operating at 450 megasamples/s performed at 50 ps intrinsic time resolution. Overlapping pulses could be resolved down to 7 ns.

A proton tagging detector for the NA48 experiment

Prototype test results are reported from the NA48 K S tagging system. Thin scintillator foils were arranged as two staircases intersecting in space. A geometric alignment to 0.4 mrad, a light yield of 300–400 photoelectrons and a time resolution of 70–90 ps with conventional TDCs were achieved. A FADC system operating at 450 megasamples/s performed at 50 ps intrinsic time resolution. Overlapping pulses could be resolved down to 7 ns.

Operational experience with a 2.5 × 1.5 meter delay line chamber

We have completed a fixed target hadron experiment at Fermilab which used a large delay line drift chamber in the tracking spectrometer. Such a chamber delivers a 2-dimensional coordinate for each hit, uses conventional TDCs, and requires no special low noise electronics. The chamber's performance was quite satisfactory, but some improvement might be realized with better construction and operating procedures.