Determination of specific cerebral impedance and cerebral current density during the application of diffuse electrical currents

Non-invasive transcranial electrical brain stimulation guided by functional near-infrared spectroscopy for targeted neuromodulation: A review

One of the primary goals in cognitive neuroscience is to understand the neural mechanisms on which cognition is based. Researchers are trying to find how cognitive mechanisms are related to oscillations generated due to brain activity. The research focused on this topic has been considerably aided by developing non-invasive brain stimulation techniques. The dynamics of brain networks and the resultant behavior can be affected by non-invasive brain stimulation techniques, which make their use a focus of interest in many experiments and clinical fields. One essential non-invasive brain stimulation technique is transcranial electrical stimulation (tES), subdivided into transcranial direct and alternating current stimulation. tES has recently become more well-known because of the effective results achieved in treating chronic conditions. In addition, there has been exceptional progress in the interpretation and feasibility of tES techniques. Summarizing the beneficial effects of tES, this article provides an updated depiction of what has been accomplished to date, brief history, and the open questions that need to be addressed in the future. An essential issue in the field of tES is stimulation duration. This review briefly covers the stimulation durations that have been utilized in the field while monitoring the brain using functional-near infrared spectroscopy-based brain imaging.

Immediate neurophysiological effects of transcranial electrical stimulation

Noninvasive brain stimulation techniques are used in experimental and clinical fields for their potential effects on brain network dynamics and behavior. Transcranial electrical stimulation (TES), including transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS), has gained popularity because of its convenience and potential as a chronic therapy. However, a mechanistic understanding of TES has lagged behind its widespread adoption. Here, we review data and modelling on the immediate neurophysiological effects of TES in vitro as well as in vivo in both humans and other animals. While it remains unclear how typical TES protocols affect neural activity, we propose that validated models of current flow should inform study design and artifacts should be carefully excluded during signal recording and analysis. Potential indirect effects of TES (e.g., peripheral stimulation) should be investigated in more detail and further explored in experimental designs. We also consider how novel technologies may stimulate the next generation of TES experiments and devices, thus enhancing validity, specificity, and reproducibility.

Measurement of electric fields due to time-varying magnetic field gradients using dipole probes

The operation of dipole probes in measuring electric fields in conductive media exposed to temporally varying magnetic fields is discussed. The potential measured by the probe can be thought of as originating from two contributions to the electric field, namely the gradient of the scalar electric potential and the temporal derivative of the magnetic vector potential. Using this analysis, it is shown that the exact form of the wire paths employed when using electric field probes to measure the effects of temporally varying magnetic fields is very important and this prediction is verified via simple experiments carried out using different probe geometries in a cylindrical sample exposed to a temporally varying, uniform magnetic field. Extending this work, a dipole probe has been used to measure the electric field induced in a cylindrical sample by gradient coils as used in magnetic resonance imaging (MRI). Analytic solutions for the electric field in an infinite cylinder are verified by comparison with experimental measurements. Deviations from the analytic solutions of the electric field for the x-gradient coil due to the finite length of the sample cylinder are also demonstrated.

Transistor array with an organotypic brain slice: field potential records and synaptic currents

Linear transistor arrays on a silicon chip were used to map evoked extracellular field potentials in organotypic brain slices of rat hippocampus. The shape and amplitude of the transients were similar to those from records with micropipette electrodes. The spatial resolution was 21 and 4.6 microm. The sampled profile of the field potential showed a wide and shallow trough of transients in the stratum radiatum and a narrow but higher ridge in the stratum pyramidale. Due to the high resolution, the profile could be interpreted quantitatively in terms of synaptic currents. Transistor chips may become a novel tool for neurophysiological and pharmacological studies in brain slices.

Concluding Studies on the Failures of Electrical Lancing of Whales

Electrocution of an animal is inhumane if it is not rendered instantaneously insensible by the application of sufficient current density within vital centres of the brain. Application of electric current which does not achieve this, is likely to cause severe pain. The humane aspects of electrical lancing have aroused widespread concern and debate.

For an electrically lanced whale of the size of those currently hunted, previous research has indicated that the current densities produced in the heart and brain are unlikely to reliably render the animal insensible or stop its heart. This study supports these findings and demonstrates that the presence of salt water/immersion may further reduce current densities. Evidence for the failure of the electric lance includes the necessity for multiple and prolonged applications of electric current.

Reasons for the failure of the electric lance include non-optimal current injection sites, insufficient current injected, the presence of salt water, and the trauma caused by the explosive harpoon. The efficacy of the electric lance may be falsely exaggerated for reasons associated with blood loss and misdiagnosis of death. All evidence clearly indicates that attempts to stop the heart by electrocution will cause severe pain to an already traumatized animal.

We suggest that the use of the electric lance is clearly inhumane, and are pleased to announce that its use in Japanese whaling operations was reportedly discontinued as from 1997.

Assessment of the humane aspects of electric lancing of whales by measurement of current densities in the brain and heart of dead animals

The potential physiological effects of the electric lance are assessed, as used in Japanese whaling operations. Current densities are measured in the brains and hearts of six whales to which a controlled current of 5 A is applied by two electrodes inserted at various sites in the carcasses. The whales vary in size from 1.8 m (22 kg) to 16 m (40 t). The minimum current density in the brain necessary to cause depolarisation of neurones is estimated to be 10 mA cm-2 and to cause ventricular fibrillation is estimated to be 0.5 mA cm-2. No current densities exceeding 4.8 mA cm-2 are recorded in the brain. Very few recordings of current density from the heart are above 0.5 mA cm-2, and they occur only when electrodes are in optimal positions. When electrodes are placed as in whaling operations, no whale over 3 m in length would receive current densities in the heart or brain sufficient to cause permanent dysfunction. It is concluded that electric lancing is ineffective as a secondary method of killing whales and that the current densities recorded could cause pain and suffering to an already distressed animal.

Finite element analysis of current pathways with implanted electrodes

A technique for numerical solution to complex electric field distribution problems has been devised. The specific application for which it was developed is the analysis of current density and isopotential line spacing from implanted neurostimulation electrodes. Three configurations of cerebellar stimulation electrodes in clinical use were studied for current spread to regions distant from the cerebellum using a planar model of the human head, neck, and upper torso in mid-saggital section. It was found that an array of cathodes on the superior cerebellar surface and an array of anodes on the inferior cerebellar surface causes significant current spread to the brainstem, a prediction confirmed by clinical observation in patients with previously implanted electrodes of this configuration. Results modelling other electrode configurations are also presented, along with a study of the effects of possible inaccuracies in available impedance data for neural tissue.