# Rendering Strategy to Counter Mutual Masking Effect in Multiple Tactile Feedback
## 2 The Masking Function
### Stimuli and Procedure  
In both experiments, the participants were exposed to the test and reference stimuli when they slide their fingertip across the touch panel. The target stimulus (variable intensity) was presented only for the test stimulus, whereas the masking stimulus (or masker) had a constant frequency and intensity and was presented for both the test and reference stimuli. In experiment I, EV was the target stimulus and MV was the masker. The reverse was done in experiment II. Table 1 summarizes the test stimulus sets. For both experiments, a frequency of 270 Hz was selected for EV, and 120, 180, and 270 Hz for MV. The masker's intensities varied between 5 and 25 dB SL (sensation level) in increments of 5 dB SL. These signal amplitudes for EV and MV in dB SL units were determined based on the results of our previous work [14]. Each stimulus lasted 1.8 s.  
We measured the AL for each stimulus set to estimate the masking function. A well-known three-interval forced-choice (3 IFC) paradigm with an one-up three-down adaptive procedure was employed in both experiments [14; 15; 16]. Figure 2 shows the stimulus sequence within a 3 IFC paradigm, and the experimental procedure. The participants were instructed to slide their dominant fingertip from left to right along with the graphic object, which was programmed to move at a constant  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline  & & **Set 1–5** & **Set 6–10** & **Set 11–15** \\ \hline \multirow{3}{*}{Experiment I} & Frequency of EV (Hz) & 270 & 270 & 270 \\  & Frequency of masker, MV (Hz) & 120 & 180 & 270 \\  & Intensity of masker, MV (dB SL) & 5, 10, 15, 20, 25 & 5, 10, 15, 20, 25 & 5, 10, 15, 20, 25 \\ \hline \multirow{3}{*}{Experiment II} & Frequency of MV (Hz) & 120 & 180 & 270 \\  & Frequency of masker, EV (Hz) & 270 & 270 & 270 \\  & Intensity of masker, EV (dB SL) & 5, 10, 15, 20, 25 & 5, 10, 15, 20, 25 & 5, 10, 15, 20, 25 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Stimulus sets presented in Experiment I and II.  
Figure 1: Custom-built hardware set-up for the experiments.  
speed. The sliding speed was set to 51 mm/s based on the typical speed range at which users would normally move their fingertip to recognize objects [17]. All participants naturally performed the experiment, without restrictions in other parameters such as the contact force and the area. For each trial, we presented the participants with three consecutive stimuli. The test stimulus was applied randomly at one of these three intervals. The participants' task was to indicate or speculate which was the odd interval, i.e., the one that presented the test stimulus. For each session (or set), the initial intensity of the target stimulus was set at a level higher than the expected masked AL. If a participant answered three consecutive correct answers, the intensity of the target stimulus was decreased in the subsequent trial. If a participant gave a wrong answer, the intensity of the target stimulus was increased in the following trial. A session was terminated after eleven reversals, and the AL was then estimated by averaging the intensities of the last eight reversals.  
Each session lasted approximately 9-15 min, and the order of the sessions was random for each participant. The participants rested for at least 5 min between sessions. During the experiments, all participants wore a wristband (ECWS61M-1, 3M Inc.), which was used for electrical grounding. Headphones with pink noise of \(\sim\)80 dB SPL (sound pressure level) were used to prevent possible auditory interference. As some participants reported sweaty fingertip, we allowed the use of a small amount of talcum powder.