becomes even more important when signaling stress. Another two mentioned the advantage that other feedback channels, namely audio or vibration are already occupied by other devices and therefore thermal feedback has a unique value making it easy to identify its source and meaning. One additional remark was referring to the quality of unobtrusiveness of thermal feedback; one participant said that it felt like more "direct" coming from his own body and not being induced externally.
values, it can be obtained that the user’s stress level greatly increases as soon as vibrotactile feedback is presented. This observation is in line with findings from related work [92] showing that vibrotactile feedback had been perceived more mentally demanding. In contrast, the results suggest that pressure-based tactile stimulation is particularly useful in situations where the overall stress level is rather low. However, although the pressure-based feedback indicated lower ascents of the physiologically traceable stress level, the difference has not been shown to be significant. This finding is also supported by prior research [211]
revealing that both stimulation patterns are perceived similarly when asking for their annoyance and comfort. Further, the subjective ratings of the participants’
stress level (cf., Table 5.1) suggest that the individual perception of one’s stress is not significantly affected by the presentation of tactile stimulation. When asking the participants informally about their opinion on the two distinct feedback patterns, they reported that the study setting as such had been perceived very stressful. Therefore, the physiologically indicated stress could be evoked by the experimental setting additional to the stress-inducing mental arithmetic task. This explains why the subjectively perceived stress level had been high in general, even when no stimulation pattern has been presented (cf., Table 5.1).
Referring to how the feedback stimulus should be designed, the preliminarily conducted study revealed that participants perceived stimuli presented at a lower rate more comfortable. In practice, 50% of the resting heart rate had been rated more comfortable (M=4.0) than 75% (M=3.4) and 100% (M=3.1) of the resting heart rate at average. Thus, feedback notifying the user about one’s stress level, should follow a slow rhythm not imitating the heart beat when wanting to consider the perceived comfort. Nevertheless, comfort is being sensed highly subjectively. As addressed in Chapter 4, it is a broad term which implies different perspectives and what makes it difficult to find generalizable guidelines.
According to comfort, Pohl et al. [211] investigated the suitability of tactile stimulation with respect to a longer usage period, namely in an 1 hour-experiment.
During that time of mobile usage, participants reported that they had not been
"feeling inhibited or annoyed by the device".
Properties of a Thermal Stimulus for Design Suitable Feedback Based on the findings from the first study, I focused on thermal sensation for notifying users about their stress level, since the pressure-based feedback seemed to not provide a huge benefit in comparison to classic vibrotactile feedback.
As known from prior work [279, 281] thermal feedback perception is highly subjective and includes various variables, such as thetemperature level,rate of changeas well as thebody locationwhere the feedback is being presented.
Hence, the aim of the latter work was to identify users’ preferences for these main influencing factors building a basis for the design of a suitable feedback
stimulus. As can be obtained from the results, the perception of thermal is highly subjective and prone to personal preferences. For instance, one participant rated all temperature levels as unpleasant and another did vice versa. This highlights that thermal feedback is a personalized feedback channel that does not follow
"one size fits all" configuration, which is in line with previous work. Further, prior work shows that thermal cues are conveying emotions [281] and are rated in dependency to context factors [85]. The findings validate the influence of the variables of thermal feedback on both the comfort and interference.
When inquiring the participants about their reasons for choosing the parameters as they did, it became obvious that there needs to be ensured a balance of feedback intensity which is not distracting but can still be noticed in a stressful situations.
Accordingly, for choosing therate of change, three participants preferred a low rate to avoid being distracted by a faster change. In contrast, five participants worried that having a less intensetemperature levelor a lowrate of change, they would not notice the thermal feedback under stress and consequently be irritated because they expected it then.
Interestingly, the qualitative results are not necessarily reflecting how people rated the different stimuli in the questionnaires. Most striking is the preference on the body location. Although the mean ratings are very similar for each of them and did not show any significant difference, in the interviews, participants stated that the wrist or upper arm would be a suitable location despite the upper arm was rated to have the most discomfort (M=0.48) and the highest interference with the ability to work (M=0.62). In this context, the named advantage of preserving privacy as explained by eight participants, played an important role. While on the one hand users appreciated the unobtrusiveness of thermal feedback which can be also found in related work [157], on the other hand they would appreciate to have such a thermal stimulus incorporated in existing technology, such as wrist-worn devices like smartwatches. Speaking of the integration into prevalent interactive systems the specific purpose of signaling stress through thermal feedback has to be considered. Given that another two participants mentioned the advantage that other feedback channels, namely audio or vibration are already occupied by other devices and therefore thermal feedback has a unique value making it easy to identify its source and meaning, leads to the question whether integrating thermal feedback in existing wrist-worn devices would not take away this advantage due to its close proximity to already used feedback.
Despite the positive comments on the quality of unobtrusiveness of thermal feedback and one participant saying that it felt like more "direct" coming from his own body and not being induced externally, the efficiency of such feedback was questioned by three participants. This concern emphasizes the problem that
possibly a notifier about stress could intensify the feeling of being stressed, what is also strengthened by the subjective data on the perceived stress obtained from the first user study. Accordingly, there can be seen in 5.1, that there is almost no difference in the participants’ perception of their own stress level regardless if a tactile stimulation is provided or not. Additionally, the high subjectivity in what how the feedback stimulus should be designed has been shown to require individual knowledge about a person’s preferences, particularly when it comes to the preferred location. For this, the exploration of the concept of notifying about stress has been shown to be an important step revealing the challenges when wanting to design such feedback. In the following chapter another approach will be investigated which neglects the subjectivity of tactile perceptions and rather relies on a more generic concept, namely to manipulate the potential source of stress or so-called stressor.
Limitations Both research studies targeted to examine the suitability of tactile stimulation, and particularly thermal feedback for notifying the user about one’s stress state. Although this concept seems promising since for example biofeedback has been shown to improve effectively stress coping [258], the it is still not fully discovered in which demanding situations exactly such feedback would be appreciated taking into account the complexity of stress. While in the qualitative data obtained in the later study it has been found that participants had ambiguous opinions about the utility of such a system, a detailed investigation evaluating its effects in the wild is still missing. Therefore, the concept which has been focused on in this chapter provides a theoretical foundation for the implementation of such a stress notifier considering the stimulus design for its potential integration in an interactive system.