Human Sensation of Transcranial Electric Stimulation.

Noninvasive transcranial electric stimulation is increasingly being used as an advantageous therapy alternative that may activate deep tissues while avoiding drug side-effects. However, not only is there limited evidence for activation of deep tissues by transcranial electric stimulation, its evoked human sensation is understudied and often dismissed as a placebo or secondary effect. By systematically characterizing the human sensation evoked by transcranial alternating-current stimulation, we observed not only stimulus frequency and electrode position dependencies specific for auditory and visual sensation but also a broader presence of somatic sensation ranging from touch and vibration to pain and pressure. We found generally monotonic input-output functions at suprathreshold levels, and often multiple types of sensation occurring simultaneously in response to the same electric stimulation. We further used a recording circuit embedded in a cochlear implant to directly and objectively measure the amount of transcranial electric stimulation reaching the auditory nerve, a deep intercranial target located in the densest bone of the skull. We found an optimal configuration using an ear canal electrode and low-frequency (<300 Hz) sinusoids that delivered maximally ~1% of the transcranial current to the auditory nerve, which was sufficient to produce sound sensation even in deafened ears. Our results suggest that frequency resonance due to neuronal intrinsic electric properties need to be explored for targeted deep brain stimulation and novel brain-computer interfaces

Exposure and neuronal excitation by wireless power transfer for auricular vagus nerve stimulation

Inductive wireless power transfer (WPT) can be used to power implanted as well as wearable medical devices, such as a percutaneous auricular vagus nerve stimulation device. This device is placed on the neck of the patient and is connected to needle electrodes in the auricle. With regard to WPT, limitations on exposure to electric and magnetic fields should not be exceeded. Furthermore, these fields should not interfere with the therapeutic goal of stimulation, i.e., with unintended peripheral nerve stimulation in the auricle. These effects are investigated by numerical simulation of induced internal fields in the head and neck and, for the first time, subsequent neuronal simulations, quantifying the potential of neuronal excitation by the fields in the auricle in particular. Internal electric field values were in the range of 1\%-5\% of the ICNIRP 2010 basic restrictions, and current densities were in the range of 30\%-45\% of the ICNIRP 1998 basic restrictions, indicating that all tested configurations are conform the guidelines. Basic restrictions on heating of tissue turned out not to be of relevance for this application. Thresholds for neuronal stimulation were two orders of magnitude higher than the induced fields, suggesting that there is almost no risk for unintended stimulation

Bioelectrical activity of limb muscles during cold shivering of stimulation of the vestibular apparatus

The effects of caloric and electric stimulation of the vestibular receptors on the EMG activity of limb muslces in anesthetized cats during cold induced shivering involved flexor muscles alone. Both types of stimulation suppressed bioelectrical activity more effectively in the ipsilateral muscles. The suppression of shivering activity seems to be due to the increased inhibitory effect of descending labyrinth pathways on the function of flexor motoneurons

Restoring the encoding properties of a stochastic neuron model by an exogenous noise

Here we evaluate the possibility of improving the encoding properties of an impaired neuronal system by superimposing an exogenous noise to an external electric stimulation signal. The approach is based on the use of mathematical neuron models consisting of stochastic HH-like circuit, where the impairment of the endogenous presynaptic inputs is described as a subthreshold injected current and the exogenous stimulation signal is a sinusoidal voltage perturbation across the membrane. Our results indicate that a correlated Gaussian noise, added to the sinusoidal signal can significantly increase the encoding properties of the impaired system, through the Stochastic Resonance (SR) phenomenon. These results suggest that an exogenous noise, suitably tailored, could improve the efficacy of those stimulation techniques used in neuronal systems, where the presynaptic sensory neurons are impaired and have to be artificially bypassed

Transcranial electric stimulation and cognitive training improves face perception

Recently, there has been much interest the effectiveness of cognitive training programmes across a variety of cognitive and perceptual domains. Some evidence suggests that combining training programmes with noninvasive brain stimulation techniques such as transcranial random noise stimulation (tRNS) can enhance training gains, but to date this has only been examined in numerosity and arithmetic tasks. In this study, we examined whether tRNS modulated the effects of a face recognition training programme. Participants completed a face discrimination training task for an hour per day over five days. Each day, training was preceded by twenty minutes of active high frequency tRNS or sham stimulation, targeted at the posterior temporal cortices or the inferior frontal gyri (IFG). Participants who received active stimulation to the posterior temporal cortices showed significant improvement on a facial identity discrimination task (the Cambridge Face Perception Test) after training, whereas those receiving sham or IFG stimulation showed no performance change. There was no evidence of an effect of stimulation on a face memory task (the Cambridge Face Memory Test). These results suggest that tRNS can enhance the effectiveness of cognitive training programmes, but further work is needed to establish whether perceptual gains can be generalised to face memory

Transcranial Electric Stimulation Entrains Cortical Neuronal Populations in Rats

Low intensity electric fields have been suggested to affect the ongoing neuronal activity in vitro and in human studies. However, the physiological mechanism of how weak electrical fields affect and interact with intact brain activity is not well understood. We performed in vivo extracellular and intracellular recordings from the neocortex and hippocampus of anesthetized rats and extracellular recordings in behaving rats. Electric fields were generated by sinusoid patterns at slow frequency (0.8, 1.25 or 1.7 Hz) via electrodes placed on the surface of the skull or the dura. Transcranial electric stimulation (TES) reliably entrained neurons in widespread cortical areas, including the hippocampus. The percentage of TES phase-locked neurons increased with stimulus intensity and depended on the behavioral state of the animal. TES-induced voltage gradient, as low as 1 mV/mm at the recording sites, was sufficient to phase-bias neuronal spiking. Intracellular recordings showed that both spiking and subthreshold activity were under the combined influence of TES forced fields and network activity. We suggest that TES in chronic preparations may be used for experimental and therapeutic control of brain activity

TiO2 surfaces support neuron growth during electric field stimulation

The authors are grateful to Francisco Almendros and Ismael Santamaría for help in preparation of the TiO2 substrates. We acknowledge the European Project NERBIOS (NEST/STREP (FP6), 028473-2) for financial support. Maria Canillas acknowledges the JAE-CSIC program of her PhD scholarship. Berta Moreno acknowledges the Fondo Social Europeo and the CSIC for the funding of her JAE Doc contract. Ann Rajnicek acknowledges financial support from The Development Trust at the University of Aberdeen to the Aberdeen Spinal Research Group, including support from the Scottish Rugby Union.Peer reviewedPostprin