# The Impact of Stimulation Intensity on Spatial Discrimination with Multi-Pad Finger Electrode
## 2 Methods
### Threshold Assessment  
The aims of this test were threefold: (1) to determine four amplitude thresholds, namely, sensation, localization (LT), smearing (SMT) and discomfort (DT), for each pad within the electrode, (2) relate LT, SMT and DT to ST and (3) define amplitude ranges to be used in the spatial discrimination test. ST and DT are well-established psychometric  
Figure 1: (a) Technical drawing of the multi-pad electrode for electrotactile stimulation of a finger phalanx with labelled dimensions in millimeters and pad numeration for active electrodes; (b) A subject performing the experiment. The image shows two electrodes positioned on the proximal and distal phalanx of the index finger (non-dominant hand). The electrodes were secured with a transparent silicone cover. The dominant hand was used to control the custom-designed LabView application on the touch-screen tablet PC.  
measures [1; 36] defined as the minimum current intensity resulting in stimuli that can be perceived (ST), and the current intensity that elicits an unpleasant sensation (DT). Therefore, for the stimulation to be comfortable during an actual application (e.g., restoration of touch in VR/AR), the intensity should always be maintained between these two threshold values [46]. We introduced two additional subjective measures that related sensation intensity to the quality of spatial localization. LT was defined as the current intensity that resulted in a clear sensation that was localized under the active pad, while SMT denotes the threshold at which the subjects perceived that the sensation started shifting and/or radiating outside of the active pad area. LT is therefore a good candidate for the stimulation intensity that will be used in practical applications of the electrotactile interface. Contrary to ST, which corresponds to a barely perceptible sensation, LT elicits a clear perception that, at the same time, does not jeopardize the localization of the electrotactile stimulus. Contrarily, SMT denotes the threshold where, based on subjects' own perception, the intensity of the stimuli could potentially reduce their ability to correctly identify an active pad.  
Starting from pad number 1 (Figure 1a), the subjects were instructed to adjust the current amplitude in steps of 0.1 mA to determine ST, LT, SMT and DT sequentially. The subjects started from 0 mA and then increased the current amplitude until they reached ST. Then, the subjects continued increasing the amplitude until LT, etc. For LT, SMT and DT, they were allowed to modulate the intensity up and down around the potential threshold value to cross check that the elicited sensation indeed corresponded to the threshold definition. After the DT was determined, the next electrode pad was automatically activated, the current intensity was set to 0 mA, and the procedure was repeated. If SMT and/or DT were not achieved even after reaching the maximum current (9 mA) the device could deliver, the thresholds were set to 9 mA (this, however, happened only in approximately 4% of cases). The session was completed when all the thresholds were determined for all electrode pads. Each subject repeated the test 12 times (6 per tested phalanx) in individual experimental sessions separated by at least 15 min. The electrode was carefully positioned to the target location at the beginning and removed at the end of each session.