Figure 44.3

The PoNS stimulation waveform. This incorporates pulse timing to achieve a comfortable neuromodulation stimulus to the tongue. Sixteen individual stimulation channels each activate nine electrodes on the electrode array.

44.6.3.3. Electrode Array and Pulse Sequencing

The PoNS electrode array, irregularly shaped to take advantage of the most sensitive regions of the tongue, comprises 143 electrodes nominally organized into nine 16-electrode sectors. Within each sector, one electrode is active at any moment (pulse beginnings staggered by 312.5 µs), with unstimulated electrodes serving as the return current path. The nine sectors present simultaneous stimulation, with the intensity of each sector adjusted to compensate for the variability of tongue sensitivity to electrotactile stimuli (Tyler et al., 2009). The sensation produced by the array has been described as similar to the feeling of drinking a carbonated beverage.

The impedance of the electrode–skin interface presents as a resistive component of approximately 1 kΩ, in series with a resistive–capacitive network of 4–6 kΩ in parallel with 0.5 nF (Kaczmarek, 2011). The pulse current therefore contains a brief leading spike followed by an exponential decay to a plateau current of approximately 3 mA. Voltage control is used rather than the current control of typical electrotactile systems because the tongue electrode impedance is relatively stable compared with cutaneous loci otherwise used (abdomen, fingertip). Use of voltage control affords circuit simplicity and therefore component, space, and battery economy.

The electrode size and geometry were chosen to achieve a reasonable balance between number of electrodes that may be packed into the array area and the comfortability and controllability of the electrotactile percept (Kaczmarek and Tyler, 2000). The overall result of this stimulation is the comfortable and convenient presentation of almost 26 million stimulation pulses to the tongue during a typical 20-minute therapy session. How many action potentials are propagated to the brain as a result of this surface stimulation is at this point unknown.

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44.7. How it works

In brief, CN-NINM uses sequenced patterns of electrical stimulation on the anterior dorsal surface of the tongue to stimulate the trigeminal and facial nerves.

44.7.1. Why the Tongue? Part II

The anterior dorsal surface of the tongue is a unique patch of the human skin with a unique innervation pattern. The relatively thin (in comparison to other areas) epithelium (300–400 µm) is saturated by different kinds of mechano, thermo, and taste receptors in addition to free nerve endings, stratified in its depth. It is the area with maximal density of mechanoreceptors, and (e.g., the fovea in the retina) have the minimal two-point discrimination threshold: 0.5–1 mm for mechanical stimulation (Vallbo and Johansson, 1984; Vallbo et al., 1984) and 0.25–0.5 mm for electrotactile stimulation (unpublished data). The physical density, spatial distribution, size of the receptive fields and their overlapping coefficient, and spatial and temporal summation properties are largely unknown, especially for electrotactile stimulation (Johansson and Vallbo, 1979a, 1979b).

The two major nerves from the tip of the tongue deliver information streams directly to the brainstem—the lingual nerve (texture of food) and chorda tympani (taste of food). According to our approximation, approximately 20–25 thousand neural fibers are delivering neural impulses from this area (about 7.5 cm²) covered by our electrode array (Heasman and Beynon, 1986).