Human sensitivity to vibration declines in mobile contexts. Designers of wearable haptic systems must account for the effects of movement and distraction so that tactile display information is perceived consistently. We compared the sensitivity of seven body sites in simulated mobile contexts, and found that the thigh is least and the wrists the most sensitive of the sites tested.
Many body sites have been considered for wearable tactile
displays, and vibratory information has improved performance of
pilots and drivers. It is well known that some body sites are less
sensitive than other areas, e.g. back versus wrist – a function of
skin type and sensor density and composition. When a body part
is in motion, it becomes less sensitive to stimuli [
Wearable tactile systems have been the focus of many papers in
the last decade due to its variety of applications. Ertan et al.
introduced a wearable navigation system for guidance of blind
users in unfamiliar indoors areas [
16 volunteers (8 male) took part. The participants (counterbalanced for gender and condition) sat in a chair in one condition, and walked on a treadmill in the other. A tall chair maintained a consistent view of the screen between conditions. We attached thirteen vibrotactile displays to seven body sites corresponding to wearable sites based on past studies and potential practicality: chest (left and right, directly below the collar bone), spine, outer thighs, stomach (left and right, halfway between navel and hip bone), feet (on the top surface of the foot), wrists, and upper arms.
During half of the trials in each condition, participants performed a workload task targeting vision, memory, and attention as shown in Figure 1. A screen four meters wide and three meters high displayed twenty-five blocks bouncing slowly around a three-dimensional room; one block was highlighted and participants counted the times the highlighted block hit any of the walls. The task was chosen for its controllable continuous workload typical of normal pedestrian activity, with distraction adjusted so that participants would not fall off the treadmill. Participants reported their collision count at the end of each workload condition.
During all conditions, participants pressed the right-hand button on a modified computer mouse when they detected vibration from any tactor. Vibrations were presented in randomized sites, intensities in randomized order, and the interval between 500 msduration tactor vibrations was randomized between four and six seconds. Reactions later than 3500 ms were discarded. Movement, Body Site (within-subject) and Gender (betweensubjects). Gender, Task, Movement, and Body Site (with spine as a reference point) are categorical variables. The omnibus test of the model coefficients is significant (p < 0.001). The regression results are listed in Table 1, where main effects Gender, Intensity, Movement and Body Sites are seen to be statistically significant.
The six Body Site levels in the table are Foot, Wrist, Stomach, Thigh, Chest, and Arm; Spine is the reference level. Foot, Wrist, Stomach, and Thigh are significantly different than Spine; Figure 2 further shows that the Wrist is more sensitive than the Spine, and the Foot, Stomach, and Thigh are less sensitive.
For Gender, males show a slightly higher detection rate of 65.3% compared to 63.0% for females. The Movement factor showed an important result: participants detected 73.9% of stimuli when sitting, but only 54.4% while walking. As suggested by the regression results, Task did not show significant differences; the detection rate with and without the visual distraction task was 64.4% and 63.8%, respectively. Predictably, there were strong results for Intensity: at intensity 4 (the strongest) almost all stimuli were detected, while at intensity 0 (weakest) only 16.7% were detected.
Interaction effects are difficult to analyze using regression, so we will present these results graphically rather than with tests of significance. There is a strong interaction between movement and intensity, as shown in Figure 3. At the highest intensity there is no difference between movement conditions, while at lower intensities the detection rate is much lower while walking.
All body sites are negatively affected by movement, but some sites more than others, as illustrated in Figure 3. Thighs, and to a lesser extent feet and stomach are particularly strongly affected. These are also areas of motion: the feet can feel heel strikes on the treadmill surface, while the stomach undergoes twisting motions as the arms swing. 5
The results of our experiment confirm the effect of body motion on detection of vibrations. We discovered that movement in a typical mobile context (i.e. walking) affects detection of vibrations on the thighs more than other body sites. Also, reaction times to vibrations are significantly reduced during walking. However, it appears that visual distraction in a mobile context may not have a significant effect on detection of vibration on any body site. In general, the thigh is not suited for applications that require discriminating among vibration patterns in everyday wearable haptics. This may be of interest to cell phone users who typically receive vibration notifications on the site most susceptible to movement effects. On the other hand, the data suggest that the chest, upper arm, and wrist are sufficiently sensitive to lower energy vibrations while the body is in motion.