# The Impact of Stimulation Intensity on Spatial Discrimination with Multi-Pad Finger Electrode
## 2 Methods
### Spatial Discrimination Test  
In this test, the aim was to assess if the intensity of stimulation affects the subject's ability to discriminate between active pads of the electrode. The experiment comprised four phases: ST assessment, familiarization, reinforced learning, and validation. The experimental session started by assessing the ST for each pad within the multi-pad electrode using the same procedure as in the previous test.  
Once the STs for all pads were defined, the familiarization phase began. This phase aimed to familiarize the subject with the electrotactile sensations elicited by each pad and help them associate visual information to the perceived tactile sensation. During the familiarization phase, the pads were activated sequentially from pad number 1 to pad number 6 (following the labeling in Figure 1a) with the current intensity set to 110% of ST for that pad. The stimulation lasted for 2 s, and the active pad was simultaneously highlighted on the electrode drawing on the screen (Figure 1b). The sequence was repeated three times.  
The reinforced learning phase provided further training through the active engagement of the subject. In this phase, the pads 1-6 were activated for 2 s in a pseudo random order with the current intensity set to 110% of ST. The subjects were asked to identify the active pad by pressing the corresponding pad on the electrode drawing in the application. One second after, the correct pad was shown on the screen. If the subject's answer was correct, the pad was framed green (Figure 1b), otherwise, the correct pad was indicated with red. The feedback on the correct answer, which was provided after each trial, allowed the subject to update the mental \"mapping\" between the elicited tactile sensation and the pad position. The six-pad sequence was repeated five times, each time using a new pseudo-random order.  
The validation phase followed a similar protocol as the reinforced learning. The subjects were instructed to identify the active pad on the electrode drawing presented in the application, but this time the visual feedback regarding the correct answer was not provided. Moreover, the pads were activated at a range of stimulation intensities based on the results from the first experimental session (threshold assessment). Eight stimulation levels were used in the amplitude range from 110% ST to 250% ST, in steps of 20% ST. The number of levels was selected to have a good coverage of the range while ensuring that the experiment did not cause significant mental fatigue. The stimulation levels were pseudo randomized, and the subjects were blinded regarding the stimulation level. Within the same level pads were pseudo-randomized within a sequence of six stimuli (as explained before in reinforced learning), and there were five repetitions per level. Therefore, the total number of test stimuli was 8 levels \(\times\) 5 sequences \(\times\) 6 pads = 240. The randomization was chosen to avoid activation of the same pad several times in a row, first with higher amplitudes and then with lower amplitudes, which could lead to diminished perception of the second train of pulses due to habitation, thus favoring higher amplitudes. The subjects needed between 12 and 17 min to finish the whole protocol.