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Vargas L, Musselman ED, Grill WM, Hu X. Asynchronous axonal firing patterns evoked via continuous subthreshold kilohertz stimulation. J Neural Eng 2023; 20:10.1088/1741-2552/acc20f. [PMID: 36881885 PMCID: PMC10433012 DOI: 10.1088/1741-2552/acc20f] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/2022] [Accepted: 03/07/2023] [Indexed: 03/09/2023]
Abstract
Objective.Transcutaneous electrical stimulation of peripheral nerves is a common technique to assist or rehabilitate impaired muscle activation. However, conventional stimulation paradigms activate nerve fibers synchronously with action potentials time-locked with stimulation pulses. Such synchronous activation limits fine control of muscle force due to synchronized force twitches. Accordingly, we developed a subthreshold high-frequency stimulation waveform with the goal of activating axons asynchronously.Approach.We evaluated our waveform experimentally and through model simulations. During the experiment, we delivered continuous subthreshold pulses at frequencies of 16.67, 12.5, or 10 kHz transcutaneously to the median and ulnar nerves. We obtained high-density electromyographic (EMG) signals and fingertip forces to quantify the axonal activation patterns. We used a conventional 30 Hz stimulation waveform and the associated voluntary muscle activation for comparison. We modeled stimulation of biophysically realistic myelinated mammalian axons using a simplified volume conductor model to solve for extracellular electric potentials. We compared the firing properties under kHz and conventional 30 Hz stimulation.Main results.EMG activity evoked by kHz stimulation showed high entropy values similar to voluntary EMG activity, indicating asynchronous axon firing activity. In contrast, we observed low entropy values in EMG evoked by conventional 30 Hz stimulation. The muscle forces evoked by kHz stimulation also showed more stable force profiles across repeated trials compared with 30 Hz stimulation. Our simulation results provide direct evidence of asynchronous firing patterns across a population of axons in response to kHz frequency stimulation, while 30 Hz stimulation elicited synchronized time-locked responses across the population.Significance.We demonstrate that the continuous subthreshold high-frequency stimulation waveform can elicit asynchronous axon firing patterns, which can lead to finer control of muscle forces.
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Affiliation(s)
- Luis Vargas
- Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, United States of America
| | - Eric D Musselman
- Department of Biomedical Engineering, Duke University, Durham, NC, United States of America
| | - Warren M Grill
- Department of Biomedical Engineering, Duke University, Durham, NC, United States of America
- Department of Electrical and Computer Engineering, Duke University, Durham, NC, United States of America
- Department of Neurobiology, Duke University, Durham, NC, United States of America
- Department of Neurosurgery, Duke University, Durham, NC, United States of America
| | - Xiaogang Hu
- Department of Mechanical Engineering, Pennsylvania State University, University Park, PA, United States of America
- Department of Kinesiology, Pennsylvania State University, University Park, PA, United States of America
- Department of Physical Medicine & Rehabilitation, Pennsylvania State Hershey College of Medicine, Hershey, PA, United States of America
- Huck Institutes of the Life Sciences, Pennsylvania State University, University Park, PA, United States of America
- Center for Neural Engineering, Pennsylvania State University, University Park, PA, United States of America
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Wang B, Aberra AS, Grill WM, Peterchev AV. Responses of model cortical neurons to temporal interference stimulation and related transcranial alternating current stimulation modalities. J Neural Eng 2023; 19:10.1088/1741-2552/acab30. [PMID: 36594634 PMCID: PMC9942661 DOI: 10.1088/1741-2552/acab30] [Citation(s) in RCA: 6] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/23/2022] [Accepted: 12/13/2022] [Indexed: 12/15/2022]
Abstract
Objective.Temporal interference stimulation (TIS) was proposed as a non-invasive, focal, and steerable deep brain stimulation method. However, the mechanisms underlying experimentally-observed suprathreshold TIS effects are unknown, and prior simulation studies had limitations in the representations of the TIS electric field (E-field) and cerebral neurons. We examined the E-field and neural response characteristics for TIS and related transcranial alternating current stimulation modalities.Approach.Using the uniform-field approximation, we simulated a range of stimulation parameters in biophysically realistic model cortical neurons, including different orientations, frequencies, amplitude ratios, amplitude modulation, and phase difference of the E-fields, and obtained thresholds for both activation and conduction block.Main results. For two E-fields with similar amplitudes (representative of E-field distributions at the target region), TIS generated an amplitude-modulated (AM) total E-field. Due to the phase difference of the individual E-fields, the total TIS E-field vector also exhibited rotation where the orientations of the two E-fields were not aligned (generally also at the target region). TIS activation thresholds (75-230 V m-1) were similar to those of high-frequency stimulation with or without modulation and/or rotation. For E-field dominated by the high-frequency carrier and with minimal amplitude modulation and/or rotation (typically outside the target region), TIS was less effective at activation and more effective at block. Unlike AM high-frequency stimulation, TIS generated conduction block with some orientations and amplitude ratios of individual E-fields at very high amplitudes of the total E-field (>1700 V m-1).Significance. The complex 3D properties of the TIS E-fields should be accounted for in computational and experimental studies. The mechanisms of suprathreshold cortical TIS appear to involve neural activity block and periodic activation or onset response, consistent with computational studies of peripheral axons. These phenomena occur at E-field strengths too high to be delivered tolerably through scalp electrodes and may inhibit endogenous activity in off-target regions, suggesting limited significance of suprathreshold TIS.
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Affiliation(s)
- Boshuo Wang
- Department of Psychiatry and Behavioral Sciences, School of Medicine, Duke University, Durham, NC 27710, USA
| | - Aman S. Aberra
- Department of Biomedical Engineering, School of Engineering, Duke University, Durham, NC 27708, USA
| | - Warren M. Grill
- Department of Biomedical Engineering, School of Engineering, Duke University, Durham, NC 27708, USA
- Department of Electrical and Computer Engineering, School of Engineering, Duke University, Durham, NC 27708, USA
- Department of Neurobiology, School of Medicine, Duke University, Durham, NC 27710, USA
- Department of Neurosurgery, School of Medicine, Duke University, Durham, NC 27710, USA
| | - Angel V. Peterchev
- Department of Psychiatry and Behavioral Sciences, School of Medicine, Duke University, Durham, NC 27710, USA
- Department of Biomedical Engineering, School of Engineering, Duke University, Durham, NC 27708, USA
- Department of Electrical and Computer Engineering, School of Engineering, Duke University, Durham, NC 27708, USA
- Department of Neurosurgery, School of Medicine, Duke University, Durham, NC 27710, USA
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