# The Impact of Stimulation Intensity on Spatial Discrimination with Multi-Pad Finger Electrode
## 1 Introduction
However, there is no consensus in literature and different multiplication values have been used, e.g., 1.2 \(\times\) ST [21], 1.5 \(\times\) ST [23; 33], 2 \(\times\) ST [30], 3 \(\times\) ST [34], as well as adding a constant value to ST (e.g., ST + 1.5 mA [35]) or some percentage of amplitude range between the sensation and discomfort thresholds (DT), e.g., using the midpoint amplitude [36] or three intermediate levels [31]. In addition to obtaining a clear and comfortable sensation, an important aspect of calibrating the intensity is related to the subjects' ability to localize delivered stimuli. When the stimulation amplitude is increased, the sensation intensity is stronger but the localization and area where the sensation is felt can increase due to a larger spread of the current injected into the skin. This can potentially impact the subject's ability to localize the delivered electrotactile stimulus. Good localization is particularly important in the case of multi-channel interfaces where the information is often conveyed through the activation of different pads (e.g., contact location). In this case, if the pad activations elicit dispersed sensations, it can be difficult for the subject to identify the active pad and therefore interpret the stimuli. This can be especially pronounced when the stimulation is delivered to a confined area, such as the tip of a finger. Only a few studies compared the impact of different levels of stimulation intensity on spatial localization. The study tested subjects' ability to detect the boundary of a small electrotactile display with 10 \(\times\) 18 pads using three different amplitude levels: ST + 10%, 2 \(\times\) ST, and a level manually set by the subject; it showed inconclusive results [37]. [PERSON] et al. [38] explored spatial discrimination of 2 \(\times\) 2 electrode matrix positioned on the fingertip using amplitudes adjusted in two different ways. In both cases, ST was determined and slightly increased for each of four electrodes, and then additionally adjusted (1) so that subjects felt all points had similar intensity or (2) all points had the same pulse amplitude (highest value of the four sites). The findings showed that the latter condition resulted in better localization, but a drawback of this approach is that discomfort might be evoked in points for which the current amplitude was increased. The present study systematically investigated the interaction between the stimulation intensity and the subject's ability to localize the stimuli delivered using a multi-pad electrotactile interface. The assessment was conducted on the distal and proximal phalange of the index finger to investigate the potential impact of the stimulation location. First, in addition to commonly used sensation and discomfort thresholds, we aimed to establish the intensity levels that, as reported by the subjects, lead to a clear and comfortable sensation that is felt as localized below the active pad versus smeared sensations that spread outside of the pads. The determined intensities are then expressed relative to ST to potentially simplify the calibration procedure for the multi-pad electrode. Finally, we have compared the subjective impressions about the relation between the intensity and localization to the actual (objective) performance of the subjects during spatial discrimination tests. To this aim, spatial discrimination tests were conducted at several stimulation intensities covering the full range of amplitudes between sensation and discomfort threshold. We hypothesized that spatial discrimination performance would decrease for higher levels of intensity due to the spreading of the current in the tissue and the corresponding blurring of sensation area and location. We also expected that a similar decrease would be observed for low intensities, where the sensations are barely perceivable.