*3.5. ColorVibrotactile*

Research on diversifying cognitive patterns is being actively conducted to deliver meaningful information using vibrational tactile sensation [117,118]. Among the vibrational and tactile studies, Cappelletti et al. [90] compared two solutions. A firstly devised solution consisted in the most similar setting to the one of color: the superposition of three signals (vibrations) at different frequencies. Single frequencies—well in the 50 to 300 Hz range of skin sensibility—were not distinguished. It turned out that amplitude could be much better discriminated than frequency. The second solution was for all to have the same frequency, but each can independently be modulated in amplitude. Just three levels of amplitude are admitted at the present stage of the research: Low (L), Medium (M), and High (H), corresponding to signals of 0 v, 0 mA; 0.7 v, 30 mA; and 1.4 v, 60 mA, respectively, all at 75 Hz. The RGB color model is an additive color model in which red, green, and blue light are added together in various ways to reproduce a broad array of colors. Thus, with the RGB color model, color is encoded as a triple of vibrations. The corresponding color codes are black (L, L, L); dark red (M, L, L); red (H, L, L); orange (H, M, L); yellow (H, H, L), dark green (L, M, L); green (L, H, H); sky-blue (L, H, H); blue (L, L, H); dark-blue (L, L, M); violet (H, L, H); grey (M, M, M); white (H, H, H). The choice was made in order to have a palette of well-distinguishable colors with definite names. The experiments, performed on subjects with different sight histories, are satisfying [90].

Brewster's Tacton [119] links specific tactile signal patterns and specific meanings to deliver specific information to users by playing the corresponding tactile pattern during interaction. The more the recognition rate of feeling, the change of vibration is getting better. In the range of 100–160 Hz, the change of vibration was most sensitively recognized, and in the range of 160–315 Hz, it was found that the perception of change gradually became dull. However, simply distinguishing colors by varying the intensity of vibrations is likely to prevent users from finding a correlation between colors and vibrations. All of the subjects answered that it was difficult to semantically connect color and vibration. If instead of converting colors into emotions, they were converted in other ways to create vibrating tactile sensations, it could be possible to realize more clear tactile sensations of colors [119].

Maclean and Enriquez [120] studied semantic tactile messages, called haptic icons, created by changing signals in the dimensions of frequency, amplitude, and waveform (sine, square, and triangle). Subjects were able to consistently distinguish between the two dimensions of the data: frequency and waveform. The frequency range of 10–20 Hz was optimal for the user to recognize the signal. The initial experiment used 36 stimuli, combining three wave shapes (sine, square, and sawtooth), four frequencies (0.5, 5, 20, and 100 Hz) and three amplitudes (12.3, 19.6, and 29.4 mNm), each with duration of 2 s. All force magnitudes are scaled as torque values in peak-to-peak mNm. Effect of shape– smooth vs. jerky: there is a clear separation between the sine and the square/sawtooth wave shapes. This is likely due to the discontinuity of the square and sawtooth waves relative to the smooth derivatives of the sine wave. However, separations between smooth and discontinuous shapes diminish with frequency; experience with the stimuli confirms that these shape differences were indeed less perceptible at higher frequencies. While still most important, frequency does not dominate other parameters to the same extent [120].

Another potential issue is the extent to which this kind of coding is intuitive, given the perceptual tendency to link vibrotactile frequency to luminance rather than hue in sighted individuals [121].

Saket et al. [122] conducted an experiment to understand how mobile phone users perceive the urgency of ten simple vibration alerts that were created from four basic signals: short on, short off, long on, and long off. The short and long signals correspond to 200 and 600 ms, respectively. To convey the level of urgency of notifications and help users prioritize them, the design of mobile phone vibration alerts should consider that the gap length preceding or succeeding a signal, the number of gaps in the vibration pattern, and the vibration's duration affect an alert's perceived level of urgency. Their study specifically shows that shorter gap lengths between vibrations (200 vs. 600 ms), a vibration pattern with one gap instead of two, and shorter vibration all contribute to making the user perceive the alert as more urgent. A vibration pattern is defined as an arrangemen<sup>t</sup> of the simplest repeatable alternating sequence of an actuator's on and off state, with specific lengths (short and long) assigned to each state. They limited the variable of short to 200 ms and long to 600 ms, without any median values due to its susceptibility to detection errors. Participants could differentiate vibrotactile signals with extreme values well but were less able to do so with median values. We used four basic types of signal to form distinguishable vibration patterns: short on, short off, long on, and long off. In order to produce unique repeatable sequences, an equal number of on and off signals must be alternated in arrangements that do not replicate any other sequence. Two pairs of such signals form additional 16 patterns. An initial test confirmed that the ten patterns were distinguishable, while those that were comprised of three pairs of on–off signals were hard to distinguish. Thus, they did not consider patterns consisting of three or more pairs of on–off signals in the study. Three underlying factors contribute to users' perceived urgency of vibration alerts: gap length is the strongest factor, followed by number of gaps, and finally vibration length. Experiment results and qualitative analysis reveal that the short on signal is highly susceptible to varying perceptions of its level of urgency, depending on the length of the gaps that precede and succeed it. A gap length of 200 ms between short on signals heightens perceived urgency, because the short and sharp pulse is delivered in a stronger manner. On the other hand, a 600 ms gap length preceding or succeeding a short on signal diminishes its strength, making the pulse feel weaker [122].
