# The Impact of Stimulation Intensity on Spatial Discrimination with Multi-Pad Finger Electrode
## 4 Discussion  
The aim of the present study was to investigate how stimulation intensity affects the subject's ability to localize the sensation induced by electrotactile stimulation in proximal and distal phalanges of the index finger. This question was addressed from two perspectives, namely, by testing the subjective experience regarding the relation between stimulation intensity and perceived spreading of the elicited sensations (LT and SMT), and by objectively assessing the performance in the spatial discrimination test across a range of intensities. The threshold assessment has indeed demonstrated that the subjects could identify the current amplitude values that produced clear but localized sensations (LT) and that they could also feel when these sensations started radiating around the active pad (SMT). The established thresholds, LT and SMT, were clearly separated (significantly different) between each other, as well as compared to ST and DT. In general, threshold values depend on several factors (e.g., stimulator properties, electrode surface and configuration, stimulation parameters, skin interface and contact), which makes it challenging to compare the results of different studies. However, the obtained STs are comparable with those reported by [PERSON] et al. [38] as using a 2 \(\times\) 2 array of electrodes positioned on the fingertips and of a similar surface (9.6 mm\({}^{2}\)) as the one used in the present study. The threshold assessment showed that the subjects could indeed feel the spreading of sensations when increasing the stimulation intensity, which was also verbally reported by the participants. Therefore, our hypothesis was that this will impact their ability to discriminate between active pads. More specifically, it was expected that the highest performance would be achieved for the three stimulation levels that were within the confidence interval of LT (130%, 150% and 170% of ST). For lower values, the spatial discrimination might be challenged by the low intensity (close to ST), while for the higher levels (above SMT), the spatial discrimination might be impaired due to the increased area of the perceived stimuli. The possibility of achieving better spatial discrimination in the fingertip when increasing amplitudes above the ST was previously suggested by [PERSON] et al. [38]. Contrary to our expectations, the spatial discrimination test demonstrated that the stimulation intensity did not significantly affect the subjects' ability to recognize an active pad. There is no visible trend in the performance across the tested intensities (see medians in Figure 4a,c) and there is a substantial overlap between the distribution of success rates, as shown by the box plots. Although there is a slight drop for the lowest intensity (110% ST), the difference was not statistically significant. Importantly, more localized sensations are likely to assist in spatial discrimination, but the subjects can also rely on other cues to identify an active pad. During the training (familiarization and reinforced learning phases), the subjects could have learned to recognize the pads using differences in the quality or intensity of sensations elicited when activating specific pads. Such salient cues might explain why the performance did not decrease for the higher intensities as initially expected. Despite being contrary to our hypothesis, these results are encouraging as they imply that the full range of current intensities, between ST and DT, can be used in an application of the electrotactile interface without compromising spatial discrimination. In general, there is a notable variability in spatial discrimination performance across subjects for all stimulation levels and both phalanxes with the success rate ranging from as low as 30% to up to 100% (Figure 4a). One of the possible reasons is the difference in the finger size since there is evidence that tactile spatial acuity is better for smaller finger sizes [47].