44.6.1. Purposeful Neurostimulation

The PoNS device was designed to provide optimized neurostimulation via the tongue specifically to induce neuromodulation as part of CN-NINM therapy. In this sense it belongs to two broad categories of technologies. The first category includes devices that electrically activate the nervous system. For example, cochlear implants induce action potentials that propagate to the end of the nerve fiber, and that once there, are indistinguishable from APs of natural origin, resulting in auditory sensations. Similarly, transcutaneous electrical nerve stimulation introduces stimulation for pain relief. The PoNS electrical stimulation functions in the same manner in that it too is applied to a body surface, the tongue.

The PoNS device also belongs to a category of technologies intended to induce broad modulation of brain activity. For example, transcranial magnetic stimulation (TMS) can activate, upregulate, or downregulate limited regions of brain tissue with volumes ranging from mm³ to cm³. Based on the principal of electromagnetic induction, TMS actively evokes action potentials in the underlying neural tissue, mainly in axons oriented orthogonal to the magnetic field. Transcranial direct current stimulation (TDCS) applies an external electric current to the head surface, in this case spreading stimulation across the entire skull. TDCS does not cause neurons to produce spikes, but rather modulates the spontaneous firing rate by changing the membrane potentiation level. In both cases, the direct neural activation (or suppression) occurs under the influence of external electromagnetic factors, changing the excitability of neuronal membranes.

Unlike both TMS and TDCS, however, the PoNS device achieves changes through natural neural pathways. The electrode array of the PoNS device induces an electric field in the tongue epithelia that we believe, based on the relevant anatomy and sensory percept, activates sensory fibers (predominantly mechano, thermo, and free nerve endings) to a depth of approximately 300–400 μm. This creates a massive flow of action potentials that are perceived by the subject as a “buzzing” or “champagne bubble” sensation. Here, the stimulation is a flow of neural impulses filling the brainstem nuclei through the trigeminal and facial nerve fibers. Activation of primary targets—these brainstem nuclei neurons—happens through existing synaptic connections, initiating a cascade of activation through multiple neural circuitries.

44.6.2. Electrotactile Stimulation

Controlled electrotactile stimulation, a particular form of purposeful neurostimulation (others include, for example, mechanical, thermal, chemical, and electromagnetic), was first described as early as 1879 (Tschiriew and Watteville, 1879) and has since been applied to a wide variety of applications, most of which involve information display to replace lost senses or to create entirely new ones (Kaczmarek et al., 1985, 1991). Electrotactile stimulation became the technological basis for Bach-y-Rita’s collaborative efforts to make the ETVSS more portable as well as a number of related applications, including feedback of touch information from artificial pressure sensors mounted on an insensate or prosthetic hand, auditory sensory substitution, and aircraft cockpit augmentative information displays (Collins and Madey, 1974).

In the 1990s, Bach-y-Rita continued this line of research at the University of Wisconsin-Madison, this time in collaboration with the present authors, discovering that the fingertips (Haase and Kaczmarek, 2005; Kaczmarek and Haase, 2003a, 2003b; Kaczmarek et al., 1994) and tongue (Bach-y-Rita and Kaczmarek, 2002; Bach-y-Rita et al., 1998) were also suitable sites for high-resolution electrotactile information display. These studies resulted in the TDU, an experimental tool for programmable electrical stimulation of the tongue (Figure 44.1a). The principles of operation of this device have been described in detail (Kaczmarek, 2011), and were subsequently instantiated commercially as the BrainPort Vision and Balance Devices described in Section 44.3. With the subsequent discovery that information-free electrotactile stimulation of the tongue enhanced neurorehabilitation, the concept for the PoNS device was born, likewise derived from the TDU.