A slice through the tragus with a stimulating electrode on each side. Each pulse drives current from one pad to the other through skin, subcutis and cartilage. Change the pulse, the pads or the skin contact and watch where the current goes, what voltage it takes, and which nerve fibres feel it.
Drag across the trace to scrub through the pulse.
| Fibre | Strongest depolarisation |
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A slice across the tragus, the small flap in front of the ear canal, as if cut horizontally through the head. Skin wraps a thin layer of loose subcutis and a plate of cartilage that runs on into the face. The clip puts one pad on the outer face and one on the inner face, the way a tragus ear clip grips, so every pulse has to cross the tragus to get from one pad to the other.
The slice is divided into 0.1 mm squares, each given the conductivity of its tissue. The solver then finds the voltage everywhere such that no current is lost or created in any square, which is the same quasi-static approach used in finite-element models of ear stimulation. The default conductivities for skin (0.465 S/m) and cartilage (0.2 S/m) follow Kreisberg and colleagues; the subcutis value of 0.08 S/m is an assumption between their skin and fat values. The two-dimensional slice is extruded to the depth that gives each pad the area of a round pad of the chosen diameter.
Two field patterns are solved. In the first, current enters evenly across each pad, which is what happens once the outer skin barrier has charged. In the second, the pad metal is one uniform voltage with only the gel in series, which is the situation for the first few microseconds of every change in current, when the barrier is uncharged and current crowds towards the rims of the pads. The display blends from the second pattern to the first with the barrier's charging time.
The stratum corneum behaves like a resistor and a capacitor side by side, and most of the electrode voltage ends up across it rather than across the tissue. The stimulator is treated as a current source with a voltage limit. When the voltage needed goes past the limit, the current falls short. The barrier is still charged from the first phase when the second phase starts, which gives the second phase extra headroom, so a nominally symmetric pulse can deliver unequal charge and leave a small net DC current. The model assumes the stimulator has no DC-blocking capacitor and leaves the electrodes open between pulses; many devices do one or both, which would change this.
The gold lines are straight fibres at two depths on each face, placed schematically. Where the tissue voltage along a fibre curves upwards (a positive activating function), a fibre running there is pushed towards firing; where it curves downwards it is hyperpolarised. Whether a fibre actually fires also depends on its diameter, the pulse width and its true three-dimensional course, none of which is modelled. The optional contour marks where the field exceeds 6.15, 12.3 or 24.6 V/m, the nerve activation thresholds assumed by Kreisberg and colleagues.
This is a two-dimensional sketch with smooth, uniform layers. Real skin carries much of its current through sweat ducts and hair follicles, tissue has capacitance of its own, and the gel and pad contact vary. How much of the tragus the vagus nerve supplies is also debated: the one dissection study usually cited for it reported the auricular branch of the vagus in under half of tragus specimens, and later authors have pointed out inconsistencies in how those results were reported. The model is for building intuition. It does not control or represent any particular stimulator and is not medical advice.
Tragus current-flow simulator, version 1.3. An educational physics model: it is not connected to any device and is not medical advice.