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. 2020 Sep 22;8(9):e20488. doi: 10.2196/20488

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©Asim H Gazi, Nil Z Gurel, Kristine L S Richardson, Matthew T Wittbrodt, Amit J Shah, Viola Vaccarino, J Douglas Bremner, Omer T Inan. Originally published in JMIR mHealth and uHealth (http://mhealth.jmir.org), 22.09.2020.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work, first published in JMIR mHealth and uHealth, is properly cited. The complete bibliographic information, a link to the original publication on http://mhealth.jmir.org/, as well as this copyright and license information must be included.

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High-level illustration of the modeling task investigated in this study. Transcutaneous cervical vagus nerve stimulation (tcVNS) is treated as the input to the underlying physiological system, while the observed cardiovascular responses found pertinent to tcVNS effects in previous work are treated as the output signals. This work focuses on the dynamics described by linear time-invariant difference equations, formulated as discrete-time state-space equations. The latent state is depicted here as variable x, while f and g are nonlinear functions. HR: heart rate; PPG: photoplethysmogram; ECG: electrocardiogram.