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Yinghua Liu, Kohitij Kar, Jacob Duijnhouwer, Pierre-Olivier Polack, Bart Krekelberg; Mechanisms of Neuromodulation by Transcranial Current Stimulation. Journal of Vision 2018;18(10):434. doi: https://doi.org/10.1167/18.10.434.
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© ARVO (1962-2015); The Authors (2016-present)
In transcranial current stimulation (TCS), low amplitude currents are applied to the head with the goal to modulate neural activity. Cognitive neuroscience studies use this technique to gain insight into the functional roles of brain areas (by modulating excitability) or brain oscillations (by entraining specific rhythms). TCS is also gaining acceptance in applications, for instance to alleviate the symptoms of depression, treat epileptic seizures, or to increase learning and cognition in healthy volunteers. However, little is known about the nature of the intracranial changes induced by transcranial currents. To address this gap in understanding and to improve TCS approaches we are developing awake animal models (mice and nonhuman primates (NHP)) that allow us to study the consequences of TCS using two-photon imaging and multi-electrode recordings in primary visual cortex. At current amplitudes that were well tolerated by the animals, TCS generated intracranial fields that were theoretically sufficient to modulate neural activity (< 1 V/m). Surprisingly, these fields were substantially inhomogeneous even at sub millimeter spatial scales. We speculate that similar inhomogeneities could contribute to the variability of behavioral effects observed in humans. tACS at 10 Hz induced robust and consistent changes in the gain or offset of visual responses of individual V1 neurons in mice, but there was substantial variability across neurons. In mice, anodal tDCS increased excitability while cathodal tDCS had little if any influence. In NHP the consequences of tDCS were also highly variable across neurons. Notably, the same polarity of stimulation could result in opposite effects on excitability. These findings cast doubt on universal statements such as "anodal tDCS increases neuronal excitability while cathodal tDCS decreases excitability". Understanding the origin of the variability and developing a more refined view of the neural consequences of TCS is critical to improve the reliability of the TCS method.
Meeting abstract presented at VSS 2018
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