Persons with disabilities as a result of various causes, from traumatic brain injury and spinal cord injury (SCI) to amyotrophic lateral sclerosis and stroke, generally find performing everyday tasks extremely difficult without continuous help [1-3]. In the United States alone, an estimated 11,000 new cases of SCI are added every year to a population of a quarter of a million as a result of acts of violence, falls, and accidents . Fifty-five percent of SCI patients are between 16 and 30 years old and will need lifelong special care that currently costs about $4 billion each year . With the help of assistive technologies (ATs), people with severe disabilities can lead self-supportive, independent, and high-quality lives. ATs can not only ease these individuals' need to receive continuous help, thus releasing a family member or dedicated caregiver and reducing their healthcare costs, but may also provide them with opportunities to return to full, active, and productive lives within society by helping them to be employed.
Although many devices are available to assist people with lower levels of disabilities, people who have minimal or no movement ability (e.g., individuals with tetraplegia) and who probably need assistance the most have very limited options. Even the existing ATs for this group of people have limitations such that only a small number have become popular among their intended users. The sip-n-puff switch, for example, is a simple, easy-to-learn, and relatively low-cost AT. However, it is slow, cumbersome, and inflexible, with only 2 to ~4 direct commands . * It also requires its users to have airflow and diaphragm control, which patients who use ventilators do not have.
Another group of ATs tracks eye movements from corneal reflection and pupil position [5-6]. Electrooculo-graphic potentials have also been used to detect eye movements [7-8]. An inherent drawback of these methods is that they interfere with the users' vision by requiring extra eye movements for eye control. In many cases, whether the user is issuing a command or simply gazing at an object is not clear; this is also known as the "Midas touch" problem . Head pointers, another group of ATs, require a certain level of head movement ability that may not exist in many patients with high-level SCI . These devices also require the user to always be in a sitting position while using them.
Some ATs, such as electroencephalogram (EEG) systems, directly use brain waves . These devices require user concentration, a long procedure for electrode attachment, and daily removal. EEG systems are also prone to external interference and motion artifacts due to the small magnitude of the EEG signals. More recently, invasive brain-computer interfaces (BCIs) have emerged based on subdural electrocorticograms or intracortical neural recording [12-15]. These procedures are highly invasive, costly, and involve risks associated with brain surgeries. Finally, voice-activated ATs are quite popular for computer access and operate well in quiet settings. However, they are unreliable in noisy and outdoor environments. They also require diaphragm control, similar to the sip-n-puff, and functional vocal cords .
The tongue and mouth occupy an amount of sensory and motor cortex in the human brain that rivals that of the fingers and the hand. Hence, they are inherently capable of sophisticated motor control and manipulation tasks with many degrees of freedom . The tongue is connected to the brain by the hypoglossal nerve, which generally escapes severe damage in SCI. The tongue muscle is similar to the heart muscle in that it does not fatigue easily . Further, the tongue is noninvasively accessible and not influenced by the position of the rest of the body, which can be adjusted for maximum comfort.
The just-named reasons have resulted in the development of a few tongue-operated ATs, such as the Tongue Touch Keypad (TTK). …