Experimental Study Methodology

Shown is a high-level overview of the study used to validate the frequency-dependent effects of TTNS predicted by the computational model. Participants were healthy adults (18+) asked to abstain from any nicotine/caffeine for 12 hours, and any fluids for 2 hours before the study. Upon arrival they were asked to ingest 750 ml water after emptying their bladder. A 30 min digestion period was employed before stimulation. Participants were pseudo-randomly allocated between groups, and were blind to the condition. Created with BioRender.com.

Overview of the Computational Bladder Control Model

A. Shown is a block diagram of the simulated neuronal circuit and bladder model. The model used a modified biophysical representation of the bladder produced previously (dashed box, [23]) which used the firing rates of the pelvic (blue), hypogastric (orange), pudendal (green) efferents to calculate bladder state. Tibial input used to modulate the circuit shown in purple. PMC: Pontine Micturition Centre, PAG: Periaquaductal Grey, PGN: Preganglionic Bladder Neurons, ASC: Ascending Interneurons, PUD: Pudendal Afferent, Onuf: Onuf’s Nucleus, Pel: Pelvic Efferent, Hyp: Hypogastric Efferent. Scissors represent possible severed projections. Labels 1 - 5 represent key regions of tibial modulation using opioidergic (1 - 4) or classical (5) inhibitory mechanisms. Created with BioRender.com.

Experimental Study Results

A: effects of low (1 Hz, Group A), high (20 Hz, Group C), and placebo (0Hz, Group B) TTNS stimulation on the time elapsed to first sensation of urge. Shown are raw-data points and Bayesian posteriors for each group.B: effects of the washout period on self-reported urge intensity. Error-bars = ±95% Confidence Interval. Urge intensity self-reported on a scale of 0–4 (see methods), washout period was 10 minutes in duration. C: bivariate and univariate kernel-density estimate plot displaying the relationship between the time elapsed before participants reported the urge to urinate (i.e., the unit reported in panel A), against the self-reported intensity of this urge at (i.e., how strongly participants felt this initial urge, the data reported in panel B before the additional 10-minute washout period).

Behavior of the Simulated Bladder Model.

Shown is the bladder behavior (top trace) and associated efferent neuronal activity (middle trace) recorded from a 1000-second duration simulation. Blowout boxes contain 5 second windows of the full recorded activity (highlighted in red) during filling (left box) and voiding (right box). Each raster trace was obtained from a randomly selected neuron within the Pudendal, Hypogastric, and Pelvic units (Nneurons = 100 in each case).

Computationally Modeling TTNS.

A. Effects of low (1 Hz) and high (20 Hz) frequency TTNS on simulated bladder function compared to unmodulated (control) conditions. B. Frequency dependent effects of TTNS on bladder contraction. Shown is the average total bladder contraction duration for a 500s simulated period under 21 different stimulation frequencies (0-20 Hz, in 1 Hz increments, Nrepeats = 10). Errorbar= ±95%CI. A contraction duration of zero ms indicates that voiding was completely inhibited (i.e., no voiding events occurred during the simulated period of time) C. Effects of disconnecting specific tibial-nerve projections on total contraction duration (for all voids over a 500s simulation period) under low-frequency (1 Hz, i) and high-frequency (20 Hz, ii) conditions. In both cases, baseline behavior represents the unmodulated behavior of the model (i.e., where all tibial-nerve projections remained intact and no tibial-stimulation was applied). In contrast, the “all connected” conditions describe a model configuration where all tibial-nerve projections remained intact and with tibial-stimulation applied at 1 Hz (i) or 20 Hz (ii). Nrepeats = 10 in each condition.

Example urge-intensity survey.

Shown is an example of the survey which was given to participants in printed form.

Parameters Used During Neurostimulation

Shown is the configuration of the DS7A neurostimulator as part of the experimental protcol. Stimulation was monophasic, with a 200µs pulse width, max-voltage of 300V, with a stimulation frequency that was set using a timing module built in-house that could output a trigger signal at 1 or 20 Hz.

Standard Placement of TTNS Electrodes Used During Pilot Study

Pilot Study Time To First Urge ROPE Analysis.

Ndraws = 12, 000. Shown is the difference in time to first urge between groups A and B, in seconds.

Simulated Bladder Volume Under Baseline and Low-Frequency Stimulation

Shown is the raw bladder volume from 10 independent bladder simulations (500s each) under baseline (0 Hz, top) and 1 Hz (bottom) tibial stimulation. All tibial nerve projections remained in-tact in both conditions.

Impact of TNS on Bladder Capacity and Void Onset

Shown is the mean bladder capacity at the time of voiding (left) and the mean simulated time elapsed before voiding commenced (right) from 10 independent bladder simulations (500s each) under baseline (0 Hz) and 1 Hz tibial stimulation. All tibial nerve projections remained in-tact in both conditions. Error bars = 95% CI.

Model Performance During Fitting Process.

Shown is the overlap between ground truth (blue) and simulated (orange) afferent neural activity for a subset of bladder pressure data. Model performance is shown in a random unoptimized state (left), after 200 rounds of Bayesian optimization (center), and after final manual optimization of model weights (right). The magnitude of the overlap between simulated and ground truth data at each stage shown as Normalized Root Mean Square Error (NRMSE). Smoothed firing rate (1st-order Butterworth filter, 1 Hz cutoff frequency) was normalized against the maximum recorded value in each dataset.

Bayesian Optimization Convergence Plot.

Shown is a convergence plot detailing the Normalized Root Mean Square Error (NRMSE) of the ground truth vs. simulated afferent activity data at each phase of the optimization process.

Final Neuronal Model Parameters.

Where possible parameters were matched to the original specifications of the neuronal/synaptic model. Parameters that were altered by the model fitting process are marked as *.