44.4.2. Why the Tongue? Part I

Electrotactile stimulation supplanted the vibrotactile stimulation because it is simpler, lighter, and consumes less energy, and it is easier to control the stimulus. Various improvements have led to the current system. It is an example of a new generation of sensory substitution devices based on computer-controlled electrical stimulation of the human skin in the most densely innervated tactile areas: the tongue and the fingers. The tongue was preferable because it affords a better environment (constant acidity level [pH], constant temperature and humidity, and low excitability thresholds) in comparison with a fingertip (variable hydration, thickness of the skin, surface contaminants, relatively limited and highly curved surface area available for stimulation, and high excitability thresholds).

44.4.3. Evolution of the PoNs Device

44.4.3.1. Vision Substitution via the Tongue

Initially a stationary, shoebox-sized device, the tongue display unit (TDU, Figure 44.1a), was the basis for our research and development. It was the first electrotactile visual substitution system (ETVSS), which we successfully tested in our laboratory and independently applied in different laboratories around the globe. Using ETVSS as a research tool, Canadian scientist Maurice Ptito verified that stimulation from the tongue was capable of unmasking hidden pathways to visual areas of the human brain in people who were blind. He demonstrated that information delivered through the tongue was capable of reaching the main visual cortical and subcortical areas. The system encoded an electrotactile image on the tongue surface using information provided through a video camera in real time. Using the system, a blind person was capable of orienting themselves in three-dimensional space, navigating in building corridors, avoiding obstacles, recognizing simple objects on tables and walls, or reading simple words (Chebat et al., 2007, 2011; Kupers and Ptito, 2013; Matteau et al., 2010; Ptito et al., 2005). The research demonstrated effective use of brain neuroplasticity to substitute for the missing visual sense.

Figure 44.1

Electrotactile tongue stimulation technology. The TDU (a), developed at the University of Wisconsin-Madison (UW), became the platform technology leading to the BrainPort Vision (b) and Balance (c) devices marketed by Wicab, Inc., as well as the Portable (more...)

The portable version of the ETVSS system, the BrainPort Vision Device (Figure 44.1b, commercialized by Wicab, Inc., under license from the Wisconsin Alumni Research Foundation at the University of Wisconsin-Madison), became our first electrotactile system for the blind. It was intensively tested on blind veterans in the United States and Europe and in 2013 received the CE mark. (As a historical note, the concept for vision substitution via electrotactile stimulation of the tongue was conceived as early as the first part of the 20th century) (Machts, 1920).

44.4.3.2. Vestibular Sensory Substitution Systems

In 2001, we developed the electrotactile vestibular substitution system (EVSS), wherein tactile sensations produced by an electrode matrix placed on the tongue provide head orientation information (relative to gravity) normally obtained from the vestibular complex of the inner ear. We demonstrated that the brain is able to process the head-orientation information presented on the tongue to make appropriate postural corrections and dramatically improve balance in affected individuals.

The first population we tested with EVSS was people who had suffered peripheral bilateral vestibular damage (BVD), typically from ototoxicity. As a result of training with the EVSS, subjects demonstrated significant improvement in balance control. Moreover, we found that training with the EVSS system induced several different levels of balance recovery. The effects of EVSS training were both immediate and retained.

The immediate effect was observed in BVD subjects after 5–10 minutes of familiarization with EVSS and included the ability to control stable vertical posture and body alignment during use. Even for well-compensated BVD subjects, standing on soft or uneven surfaces or stances with a limited base such as tandem Romberg position, was challenging, and unthinkable with closed eyes. To our surprise, using the EVSS, these subjects readily acquired the ability to control balance and body alignment in challenging positions with their eyes closed (Tyler et al., 2003).