# The Impact of Stimulation Intensity on Spatial Discrimination with Multi-Pad Finger Electrode
## 4 Discussion
The subject with the lowest success rate for 10 out of 16 combinations of stimulation level and finger phalanx was the oldest (57 years) while the remaining subjects were all younger than 35 years suggesting that an additional factor could be age. Spatial acuity of touch is significantly affected by aging with middle-aged subjects (41-63 years) having two times larger grating orientation discrimination threshold than the young adults (18-33 years) [48] and the difference being even larger for elderly subjects [49]. However, with strong evidence that discrimination of electrotactile patterns can be significantlyimproved through training [23; 50], we hypothesize that the performance could increase for all subjects, stimulation levels and finger phalanxes. The overall success rate of spatial discrimination was significantly higher when the electrode was positioned on the distal phalanx (Figure 4b). This result can be explained by the higher relative number of tactile sensory units innervating the distal phalanx of the index finger, especially SA1 and RA units [51]. Higher tactile spatial acuity of the distal phalanx has been demonstrated with both two-point discrimination and two-point orientation tasks resulting in a lower mean 95%-correct threshold for the distal than for the proximal phalanx of the index finger [52]. The required minimum clearance between two touch effectors is shown to be larger for the proximal and middle phalanx of the index finger than for the distal [53; 54]. It should be noted that contact between the electrode and the skin plays a significant role in defining the current flow and resulting sensations. Many uncontrollable parameters contribute to this including inter-subject variability in skin type, sweat formation and composition (where the existence of sweat glands under the electrodes is a factor of relevance as well), and mechanical disturbances [2]. Although the position of the electrode was precisely defined, small shifts in placement and differences evoked by these factors were expected. Considering that these variations are impossible to avoid in the practical use of the system, these factors were not considered in the data analysis. Another limitation of the study is that we focused only on one stimulation parameter, current amplitude, as changes in the pulse width and frequency also influence the sensation thresholds and perception of the stimuli. As in terms of elicited sensation, this set of stimulation parameters presents dependent variables that are nonlinear but ascending. We therefore decided to focus only on the amplitude calibration to determine the adequate amplitude level in which all three parameters can potentially be modulated to provoke various sensations. How the stimuli modulation can be used to provoke specific haptic effects in respect to the underlying position of the nerves and mechanoreceptors was out of the scope of the presented study but will be a theme of interest for our future research. One of the motivations for this study was to establish an appropriate stimulation amplitude range, which in turn could simplify calibration procedure for electrotactile feedback interfaces. Setting the current amplitude in systems that rely on multi-channel electrotactile stimulation normally requires intensity adjustments for each individual channel. The tendency for future multi-pad electrotactile systems is to increase the number of pads in order to improve the fidelity of the feedback and the bandwidth of information that can be transmitted through the interface. This will further complicate the setup if the simplified procedures for multi-electrode intensity calibration are not established. In the present study, there was no significant difference for any of the determined thresholds when they were compared across the pads within the same phalanx, as well as between phalanges (Figure 2b). This means that the ST could be determined for a single pad, and this value could then be used as an initial guess for all other pads.