# The Impact of Stimulation Intensity on Spatial Discrimination with Multi-Pad Finger Electrode
## 1 Introduction  
Electrotactile systems deliver low-intensity electrical currents to the surface of the skin, which depolarizes skin afferents and produces tactile sensations [1]. This technology has been traditionally used for sensory communication in different areas of rehabilitation engineering [1, 2], for instance, to restore somatosensory feedback in upper [3, 4] and lower [5] limb myoelectric prostheses, to provide vestibular substitution for patients with balance dysfunction [6], and to implement auditory prostheses [7, 8] and assistive devices for the visually impaired [9, 10, 11, 12, 13]. Recently, electrotactile interfaces have penetrated the emerging applications [1] that have so far been dominated by other haptic modalities, such as provision of tactile sensations in augmented/virtual reality [14, 15, 16], design of multi-touch panels [17], and conveying feedback to the human operator in teleoperation [18]. Favorable physical characteristics (low power consumption, simple fabrication, fast response) combined with the possibility of modulating multiple stimulation parameters (e.g., pulse width, amplitude, frequency, and location) independently and simultaneously, allow thedesigning of compact and high-resolution electrotactile displays. Such displays, based on multi-pad electrode technology, can be leveraged to successfully deliver high-fidelity information on multiple variables of interest in an intuitive manner, as recently exemplified in the context of the restoration of proprioceptive and grasping force feedback in myoelectric prostheses [19; 20; 21; 22; 23]. However, a clear drawback of electrotactile stimulation, which is even more pronounced in multi-pad systems, is inter- and intra-subject variability of psychometric parameters (e.g., sensation and discomfort thresholds) and qualities of elicited sensations [24]. When stimulation parameters are not properly adjusted, stimuli can be unclear, uncomfortable, or even painful. In addition, the same stimulation parameters can produce substantially different sensations when applied to a different location. Therefore, electrotactile interfaces require intensity to be calibrated before each application. This takes time, especially when using a multi-channel interface, as the calibration needs to be performed for each channel individually. The intensity of the elicited sensations depends on the quantity of electrical charge, which is a product of pulse width and amplitude. Hence, the intensity can be controlled by modulating either pulse width [25; 26], or amplitude [18]. In many applications, the stimulation is applied using a constant intensity, for instance, when conveying contact information by activating a specific pad. In this case, the stimulation intensity is adjusted to produce localized, clear, and comfortable sensations [19; 27; 28], which requires determining the sensation threshold using the method of limits [29] or a staircase procedure [30]. The method of limits is a simpler procedure, in which the intensity is increased incrementally until the subject indicates that he/she has felt the sensation, whereas in the staircase approach, the intensity is adapted in each trial based on subject answers. The latter determines the parameters more accurately but is also more time-consuming. If the application requires modulating the intensity to convey continuous feedback variables (e.g., contact force in addition to contact event) [31], then discomfort thresholds need to be determined as well, which further increases the calibration time. Nevertheless, the calibration procedure can be simplified by adopting an informed choice of the starting amplitudes in the calibration algorithm [24]. Many researchers set the stimulation intensity based on sensation threshold (ST) [32], i.e., by multiplying ST by a predefined factor to obtain clearly perceivable sensation.