ORIGINAL RESEARCH article
Front. Neurosci.
Sec. Neuroscience Methods and Techniques
Volume 19 - 2025 | doi: 10.3389/fnins.2025.1627705
This article is part of the Research TopicNeuroengineering for health and disease: a multi-scale approachView all 10 articles
Neuronal effects of epicranial current stimulation in macaque cortex
Provisionally accepted
- Leuven Brain Institute, KU Leuven, Leuven, Belgium
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Transcranial electrical stimulation (TES) using scalp electrodes is noninvasive, safe and inexpensive. However, because the scalp shunts most of the current, electric fields(E-fields) in the brain are relatively weak. Conversely, invasive neuromodulation methods such as deep brain stimulation (DBS) and invasive cortical stimulation (ICS) successfully treat many brain diseases. However, the expensive and risky surgery limits the reach of these approaches. Epicranial current stimulation (ECS), where electrodes are implanted on the skull, is a novel approach which can bridge the gap between these two extremes. In current study we investigated the effects of ECS on neural activity. In two macaque monkeys we implanted two concentric ring electrodes directly on the skull. Each electrode targeted one area PFG (PFG is not an acronym; rather it is the full name of a particular part of the parietal cortex) of the parietal convexity. Furthermore, a craniotomy was drilled in the skull to access the same area PFG. While recording (2 minutes) we stimulated (during the second recording minute) with a 10 or 40 Hz sinewave using an unfocused montage (between two electrodes on each side of the head) or a focused (through the concentric electrodes) over an intensity range of 0.25 to 4 mA. These two montages allowed us to investigate neural responses to targeted and broad brain stimulation. Furthermore, in a functional magnetic resonance imaging (fMRI) experiment we stimulated, at only 10Hz, through an unfocused montage. Our results show E-field strengths depended on a combination of montage and stimulation intensity. For focused and unfocused stimulation at lower amplitudes neural activity became entrained to the stimulation (similar to TES). For the unfocused stimulation, as stimulation amplitude increased, spike-rates also increased (similar to ICS and DBS). The fMRI study showed a distributed pattern of activations which is suggestive of a network response caused by ECS. Here we show that as a standalone technique it can be applied to a larger and more complex brain. This makes it a promising neuromodulation approach with clinical applications in patients who do not respond to TES but are not yet candidates for ICS or DBS.
Keywords: epicranial stimulation, transcranial electric stimulation, functional magnetic resonance imaging, entrainment, 40 Hz stimulation
Received: 13 May 2025; Accepted: 02 Sep 2025.
Copyright: © 2025 Asamoah, Khatoun, Romero, Premereur, Janssen and Mc Laughlin. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Boateng Asamoah, Leuven Brain Institute, KU Leuven, Leuven, 3000, Belgium
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