# Sensing Performance of a Vibrotactile Glove for Deaf-Blind People
## 1 Introduction  
Human-environment interaction is carried out by means of the five senses: sight, ear, taste, smell, and touch. In some cases, these senses--understood as information channels--become overloaded, weakened, or even lost. This is an issue for individuals with deaf-blindness, due to the fact that their principal information channels to interact with their environment are noticeably weakened or, in many cases, lost. Therefore, these subjects need different channels to manage external information and a specific approach to communicate with other individuals and express themselves. Communication is achieved through one of the unimpaired senses; in this case, touch. Haptic communication has been one of the most common strategies applied to improve deaf-blind individuals' interaction with the environment [1]. The solutions include the Braille system [2] and related devices such as the finger-Braille [3, 4] or the body-Braille [5], the Tadoma method [6], the tactile sign language, the Malossi alphabet [7], spectral displays [1], tactile displays [8], electro-tactile displays [9], or the stimulation of the mechano-receptive systems of the skin, among others. In the case of residual sight, sign language or lipreading [10] can be used. On the other hand, for some deaf individuals, acoustic nerve stimulating devices can be implanted when suitable. Commercial products for deaf-blind people adapted to the telephone are available, based on teletype writer (TTY) systems adapted with a telephone device for the deaf (TDD). Some examples are the PortaView 20 Plus TTY system from Krown manufacturing, the FSTTY (Freedom Scientific's TTY) adapted to the deaf-blind users with the FaceToFace proprietary software, or the more recent Interpretype Deaf-Blind Communication System [11, 12]. These systems rely on the Braille code by means of tactile displays or input devices. In order to design a communication channel based on touch, it is necessary to know the basic physiology of the skin [8]. Skin is classified either as non-hairy (glabrous) or hairy. This classification is relevant for haptic stimulation because each type of skin possesses different sensory receptor systems and, thus, different perception mechanisms [13]. Also, it is important to consider that each mechano-receptive fibre plays a specific role in the perception of external stimuli [8]. Although there are several methods [14], tactile stimulation is usually based on either a moving coil or a direct current (DC) motor with an eccentric weight mounted on it [15]. Therefore, the stimuli generate a vibrotactile sensation that is related to the stimulation frequency and its amplitude [16]. The literature on vibrotactile methods describes two physiological systems, according to the fibre grouping: the Pacinian system and the non-Pacinian system. The Pacinian system involves a large receptive field that can be stimulated by higher frequencies (40-500 Hz). On the contrary, the non-Pacinian system has a small receptive field that can be stimulated by lower frequencies: 0.4-3 Hz and 3-40 Hz [8; 17]. Vibration is highly suitable because the user tends to adapt rapidly to stationary touch stimuli [18], in such a way that a repeated stimulus produces a sensation that remains in conscious awareness. Tactile stimulation through vibration has been widely studied. These studies include the evaluation of different place and space effects on the abdomen [19], torso [20], upper leg [21], arm [22; 23], and palm and fingers [13]. In addition, other parameters were taken into account when evaluating vibrotactile sensitivity: age [24], skin temperature [25], menstrual cycle [26], body fat [27], contactor area [28], several spatial parameters involved in the stimulation of mechano-receptors [29; 30], or the influence of having yet another sense weakened or impaired, e.g., sight or hearing [31]. Blind individuals are familiar with the Braille code, but deaf-blindness is often degenerative, as in those diagnosed with [PERSON]'s syndrome, and visual loss becomes progressive.