Vibrotactile Sensitivity of the Head 

by Kimberly Myles and Joel T. Kalb 

ARL-TR-4696 January 2009 

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Army Research Laboratory 

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ARL-TR-4696 January 2009 

Vibrotactile Sensitivity of the Head 

Kimberly Myles and Joel T. Kalb 

Human Research and Engineering Directorate, ARL 

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14. ABSTRACT 

The brain rarely processes events of the physical world using signals from a single sensory modality. While the visual and 

auditory modalities are considered frequently in communication research, the tactile modality is considered the least as a 

possible mode of communication. In addition, previous studies of tactile sensitivity and solutions in utilizing the tactile 

modality have been focused on torso and limb locations. However, there are currently no tactile sensitivity data for the head. 

The goal of the present study was to investigate tactile sensitivity of the various locations on the head and the effect of signal 

frequency on the tactile threshold. An adaptive psychophysical procedure was used to determine differences in tactile 

sensitivity at various points on the head. Obtained results indicate that the crown of the scalp is less sensitive to vibration than 

the areas near the forehead, temples, and lower part of the back of the head. 

15. SUBJECT TERMS

head tactile display, tactile sensitivity, vibrotactile thresholds 

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Kimberly Myles 

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iii

Contents 

List of Figures iv

List of Tables iv

Acknowledgments v

1. Introduction 1

2. Method 2

2.1 Participants ......................................................................................................................2

2.2 Experimental Design .......................................................................................................2

2.3 Head Locations ................................................................................................................3

2.4 Frequency ........................................................................................................................3

2.5 Vibration Threshold Measurement ..................................................................................4

2.6 Apparatus and Stimuli .....................................................................................................4

2.7 Procedure .........................................................................................................................6

3. Results 6

4. Discussion 10

5. Conclusions 11

6. References 12

Appendix. Head Diagram With Displacement Sensitivity Values (Decibel Referenced 

to 1 μm) 15

Distribution List 17 
iv

List of Figures 

Figure 1. The 10-20 system of electrode placement. (Adapted from D. G. Domenick, 1998

http://members.aol.com /aduial/1020fc.html.) ...........................................................................3

Figure 2. The C-2 tactor. .................................................................................................................5

Figure 3. Headband designed to hold C-2 tactors. ..........................................................................5

Figure 4. Diagram of equipment showing Tucker-Davis Technologies System II (TDT) and 

the seven tactors. ........................................................................................................................5

Figure 5. Diagram of the signal. .....................................................................................................6

Figure 6. Mean threshold by head location. (The error bars indicate standard errors.) .................8

Figure 7. Mean threshold by frequency. (The error bars indicate standard errors.) ......................8

Figure 8. Mean displacement by head location. (The error bars indicate standard errors.) ...........9

Figure 9. Mean displacement by frequency. (The error bars indicate standard errors.) ................9 

Figure A-1. Head diagram with displacement sensitivity values (decibel referenced to 1 μm). ..16

List of Tables 

Table 1. Means and (standard deviations) for vibration thresholds in dB re: 1 Vp. ......................7
v

Acknowledgments 

The authors would like to thank members of the Auditory Research Team for their invaluable 

input, specifically Dr. Tomasz Letowski, Timothy Mermagen, and Phuong Tran. We would also 

like to thank Dr. William Monaco for helping to obtain participants for this study. Lastly, we 

would like to thank the Marines from the U.S. Marine Corps Detachment at Aberdeen Proving 

Ground, MD, for volunteering as participants. 
vi

INTENTIONALLY LEFT BLANK. 
1

1. Introduction 

The brain rarely processes events of the physical world using signals from only one sensory 

modality. Instead, it simultaneously processes signals received by several sensory modalities 

from which it is able to interpret what is going on around us (Moorhead et al., 2004). “Welldesigned multimodal systems integrate complementary modalities to yield a highly synergistic 

blend in which the strengths of each mode are capitalized upon and used to overcome 

weaknesses in the other” (Oviatt, 1999, p. 74). Multimodal systems are also designed to prevent 

information overload from occurring for any one sensory modality, as well as, provide a solution 

for allowing the user to instinctively use the different input modes as needed (Oviatt, 2001). Use 

of the skin as an information channel can be beneficial within a system when the visual and/or 

auditory modalities are overloaded (Raj et al., 2000). However, in the area of system design, 

touch is considered the least as a possible mode of communication for enhancing situational 

awareness (SA) and performance while the visual modality is considered the most (Moorhead 

et al., 2004). 

Previous research studies have shown that touch is an excellent mode of communication and 

designers are exploring a number of opportunities afforded by the tactile modality to enrich the 

connection with the environment. For example, use of the tactile modality has been beneficial in 

improving safety for car drivers (Suzuki and Jansson, 2003; van Erp et al., 2002; van Erp and 

van Veen, 2001). Specifically, Ho et al. (2005) have shown the potential benefit of using the 

tactile modality in cars to impart warning signals that communicate the presence of an impending 

accident. Additionally, Chan et al. (2008) have shown the benefits of using the tactile modality 

to support turn-taking in remote collaboration among groups. Potential applications for the 

military environment include using the tactile modality to distribute information about auditory 

and other events. For example, by using information from mortar or sniper detection systems, 

the Soldier could be cued via tactile signals regarding the direction and location of the event 

source. Furthermore, the tactile modality could be used to indicate movement direction in 

GPS-supported navigation. 

The focus of the authors’ current research program involves evaluating the use of touch as a 

viable input modality for new and existing military equipment rather than exploring 

combinations of visual or auditory modalities to increase soldier performance. There is potential 

for integrating the tactile and auditory modalities in one communication system by using current 

bone conduction (BC) transducers for both speech and tactile signals on a time-sharing basis. 

Bone conduction systems transmit sound energy through bony tissue in the skull, via vibrators 

mounted on the head, which stimulates the cochlea (resulting in an auditory sensation) while 

bypassing the external ear and eardrum (Henry and Letowski, 2007). Because the BC 

transducers are for use on the head, a likely extension of the transducers is the use of the same 
2

transducers in a head tactile communication system. If successful, Soldiers would benefit from a 

head-mounted tactile display (designed in conjunction with BC headgear or as a stand alone 

device) that could be used for directional cueing with very little additional equipment and weight 

to carry. 

However, current research reporting tactile sensitivity measures and practical solutions using the 

tactile modality is exclusively dedicated to the finger (Morioka et al., 2008; Rabinowitz et al., 

1987; Stuart et al., 2003; Wilska, 1954), hand (Verrillo, 1962), arm/forearm (Cholewiak and 

Collins, 2003; Morioka et al., 2008; Piateski and Jones, 2005; Stuart et al., 2003; Verrillo, 1966; 

Wilska, 1954), torso (Piateski and Jones, 2005; van Erp, 2005; van Erp and Werkhoven, 1999; 

Wilska, 1954), wrist (Ferris and Sarter, 2008), shoulder (Stuart et al., 2003; Wilska, 1954), cheek 

(Stuart et al., 2003), thigh/chin/forehead (Wilska, 1954), foot (Morioka et al., 2008; Nurse and 

Nigg, 1999), and toe (Morioka et al., 2008; Wilska, 1954). There are currently no tactile 

threshold data for the head and such data are needed to ensure that tactile systems designed for 

the head are compatible with the sensitivity of the user. The extent to which vibrotactile*

stimulation of the head is viable as a method of communication depends on determining what 

and how much information can be perceived on the head (Lambert, 1990). 

The purpose of this study was to determine (1) vibrotactile thresholds associated with the 

perception of tactile stimuli applied to the head and (2) a relative pattern of vibrotactile 

sensitivity for the head. 

2. Method 

2.1 Participants 

Twelve Marines were recruited for this study. All participants were male with standard military 

haircuts. One participant’s data was identified as an outlier because the participant answered 

“Yes” for each trial which consistently placed the participant’s data at the maximum attenuation 

of the ZEST procedure. Consequently, the data were not usable. For the remaining 

11 participants, the mean age of participants was 21 years (SD = 2.45). 

2.2 Experimental Design 

A 7  3 within-subjects design was used to determine the effect of Head Location and Frequency 

on vibration thresholds for the head. A 2-factor repeated measures analysis of variance 

(ANOVA) was used to analyze differences in vibration thresholds at an alpha level of 0.05. 

 *

Vibrotactile is the sense of touch associated with vibration. 
3

• Independent Variables: 

◦ Head Location (CZ, F3, F8, T4, O2, T3, PZ) 

◦ Frequency (32 Hz, 45 Hz, 63 Hz) 

• Dependent Variable: Vibration Threshold 

2.3 Head Locations 

The locations of the head chosen for this study follow the 10-20 system of electrode placement 

used for high-density electroencephalography (EEG) studies. Figure 1 illustrates the scalp 

topography of the 10-20 system of electrode placement. The vertex (CZ), and temple (T3, T4) 

locations were chosen based on optimal BC vibrator placement (McBride et al., 2005) and F3, 

F8, PZ, and O2 were chosen so that the location sample would be representative of a whole-head 

mapping. The locations were also chosen such that each lobe of the cerebral cortex was 

represented. 

Figure 1. The 10-20 system of electrode placement. (Adapted 

from D. G. Domenick, 1998 http://members.aol.com 

/aduial/1020fc.html.) 

2.4 Frequency 

Pilot testing was conducted to determine the optimal frequency ranges to use in this study. Pilot 

results showed evidence of both tactile and auditory perception of the signal for tactile signals 

that contained frequencies above 64 Hz. For tactile signals containing frequencies below 64 Hz, 

only tactile perception of the signal occurred. Thus, to report vibration thresholds that did not 

contain auditory interference, signals needed to be below 64 Hz. 
4

2.5 Vibration Threshold Measurement 

To determine vibration threshold levels (i.e., the smallest amplitude at which an observer felt 

vibration), the Zippy Estimation by Sequential Testing (ZEST) psychophysical adaptive 

procedure was used (King-Smith et al., 1994; Watson and Pelli, 1983). Thresholds obtained 

using ZEST is based on a participant’s past performance and the use of this history makes it 

possible to estimate thresholds quicker and with a smaller number of trials than other traditional 

psychophysical methods. Thus, for this reason, ZEST is considered an efficient method for 

obtaining sensitivity thresholds. 

The ZEST procedure was used to find threshold values for each of the treatment conditions. 

Each of the 21 treatment conditions (Head Location  Frequency) was presented 10 times. A 

starting attenuation in the middle of the range (8–40 dB) was presented on the first trial. Based 

on the participant’s response for the first trial, ZEST estimated a new attenuation. If the 

participant’s response was “No” (I do not feel the stimulus), the attenuation was reduced on the 

second trial. In contrast, if the participant’s response was “YES” (I feel the stimulus), the 

attenuation increased on the second trial. The participant’s responses from both trials 1 and 2 

were again considered for a new attenuation for trial 3. This sequence was repeated for every 

trial within a treatment until the last response for that treatment was collected. The last response 

obtained on trial 10 was recorded as the vibration threshold for that treatment within a 1-dB 

error. In addition, the range of signal attenuation and the choice of using 10 trials per treatment 

for the ZEST procedure were determined based on pilot test data. 

2.6 Apparatus and Stimuli 

The C-2 tactor designed by Engineering Acoustics, Inc. (EAI) (www.eaiinfo.com) was used for 

this study. The C-2 tactor is 1.2 in (diameter) × 0.31 in (height) and weighs 17 g. The tactor is 

designed with a moving contactor surface (0.3 inch in diameter). When electrical energy is 

applied (i.e., sine wave) the contactor oscillates perpendicular to the skin (Mortimer et al., 2007; 

van Erp, 2002) (figure 2). Seven tactors were placed in a headband (figure 3) designed to hold 

one tactor at each chosen head location and the static force was uniform across all tactors. The 

tactors were measured to confirm that each tactor was capable of producing similar tactile output 

and each tactor was found to be within plus or minus 2dB of the mean (see figure 4 in Kalb et al., 

2008). 

Figure 4 shows a diagram of the equipment used for this study in which the computer generated 

a modulated sinusoidal carrier wave with a 10-V peak amplitude. Each stimulus contained a 

cycle period of 250 ms, a duty cycle fraction of 0.25, and three repetitions extending the 

stimulation period of the signal to 750 ms (figure 5). Furthermore, the signal was divided into 

three equal phases during the on time of the duty cycle, namely the rise, sustain, and fall. Rise 

and fall both followed a half cosine response while sustain was at full modulation. The signal 

was then attenuated by the ZEST procedure, sent to one of seven multiplexer channels, and 

amplified by a unity-voltage-gain power-driver which was connected to a tactor. 
5

Figure 2. The C-2 tactor. 

Figure 3. Headband designed to 

hold C-2 tactors. 

Figure 4. Diagram of equipment showing Tucker-Davis Technologies System II (TDT) and the 

seven tactors. 

TDT 

MultiYes No plexer 
6

Figure 5. Diagram of the signal. 

2.7 Procedure 

Participants were seated in a standard office chair in an Industrial Acoustics Company (IAC) 

sound attenuating chamber and provided a consent form to review and sign if they agreed to 

participate. Next, the experimenter placed a stretchable headband on the participant’s head 

(secured with Velcro fasteners) and examined the headband to ensure that the tactors made 

proper contact with the scalp. Participants received a total of 210 vibrating stimuli on the head. 

Each treatment was presented ten times and each head location was tested one at a time. For 

each stimulus, participants indicated if they felt or did not feel a vibrating sensation on the head 

by using a mouse to click a “YES” button if they felt a vibrating sensation and a “NO” button if 

they did not feel a vibrating sensation. Prior to starting the experiment, the experimenter 

checked that each tactor was emitting a vibration stimulus which also served as practice trials to 

familiarize the participants with the task. 

3. Results 

Expressed as a voltage level referenced to a 1-V peak, the full stimulus of a 10-V peak was 

equivalent to a level of 20 dB. This level was then reduced by ZEST attenuations in response to 

the participant action and recorded accordingly. For example, a 30-dB attenuation yielded a 

–10 dB level at the tactor. Table 1 shows the means and (standard deviations) of these voltage 

levels at all locations and frequencies. An insensitive location required significantly more 

stimulation for detection than a more sensitive location, thus, more voltage was required at the 

tactor to produce an increase in displacement (i.e., less attenuation). 
7

Table 1. Means and (standard deviations) for vibration thresholds in dB re: 1 Vp. 

Frequency 

Head Location 32 Hz 45 Hz 63 Hz 

CZ 0.84 (7.60) 1.70 (7.32) 2.00 (6.86) 

F3 0.67 (5.85) 0.86 (6.08) 0.81 (5.81) 

F8 –1.85 (8.59) –0.79 (7.71) –0.61 (7.53) 

T4 –3.69 (4.83) –3.30 (5.18) –1.99 (5.73) 

O2 –6.48 (4.95) –5.06 (4.13) –4.62 (4.10) 

T3 –6.36 (4.18) –5.59 (3.71) –5.29 (3.69) 

PZ –8.04 (6.18) –8.22 (6.00) –7.85 (6.68) 

An ANOVA revealed a significant main effect for Head Location, F (6, 60) = 4.35, p = .001 and 

Frequency, F (1.21, 12.11) = 6.02, p = 0.03 (figures 6 and 7, respectively).* No significant main 

effect was found for the Head Location  Frequency interaction, F (12, 120) = .82, p = .63. Post 

hoc comparisons (LSD) revealed that PZ (M = –8.04 dB) was significantly more sensitive to 

vibrotactile stimulation than CZ (M = 1.51 dB), p <0.01. In figure 6, head locations are arranged 

from least to most sensitive and the remaining significant comparisons revealed that the three 

least sensitive head locations (CZ, F3, and F8) were significantly different from the three most 

sensitive head locations (O2, T3, and PZ). Location T4 was the neutral location which was not 

significantly different from any of the other head locations. Post hoc comparisons (LSD) also 

revealed that thresholds at 45 and 63 Hz were significantly higher than thresholds at 32 Hz 

(M = –3.56), p <0.05 (figure 7). Thresholds at 45 Hz (M = –2.92 dB) and 63 Hz (M = –2.51 dB) 

were not significantly different from one another. 

For level conversion from tactor voltage to displacement, we combined known fingertip 

displacement thresholds (Lamoŕe and Keemink, 1988) with measured tactor voltage levels at the 

finger to obtain a transfer relationship shown in Kalb et al. (2008). This revealed a second order 

low-pass response with an 11.5 dB (referenced to 1 /V) gain below 100 Hz. Assuming that the 

tactor displacement for a given voltage is the same regardless if the tactor is touching the 

fingertip or the head†

, we converted our voltage thresholds to displacement levels by adding 11.5 

dB. Figures 8 and 9 show these thresholds. 

 *

Mauchly’s Test of Sphericity (p = .009) revealed that frequency violated the sphericity assumption; thus, the GreenhouseGeisser correction was used to adjust the df. 

†

This is true if the skin input impedance is at least ten times higher than the tactor output impedance (Mortimer et al., 2007).
8

Figure 6. Mean threshold by head location. (The error bars indicate standard errors.) 

Figure 7. Mean threshold by frequency. (The error bars indicate standard errors.)

1.5

0.8

-1.1

-3.0

-5.4 -5.8

-8.0

-10.0

-5.0

0.0

5.0

CZ F3 F8 T4 O2 T3 PZ

HEAD LOCATION

VOLTAGE THRESHOLD IN DECIBELS 

(RE 1V PEAK)

-3.6

-2.9

-2.5

-5

0

32 Hz 45 Hz 63 Hz

FREQUENCY (HZ)

VOLTAGE THRESHOLD IN DECIBELS 

(RE 1V PEAK)
9

Figure 8. Mean displacement by head location. (The error bars indicate standard errors.) 

Figure 9. Mean displacement by frequency. (The error bars indicate standard errors.) 

13.0

12.3

10.4

8.5

6.1 5.8

3.5

0

3

6

9

12

15

18

CZ F3 F8 T4 O2 T3 PZ

HEAD LOCATION

DISPLACEMENT IN DECIBELS 

(RE 1 MICRON PEAK)

7.9

8.6

9.0

0

2

4

6

8

10

32 Hz 45 Hz 63 Hz

FREQUENCY (HZ)

DISPLACEMENT IN DECIBELS 

(RE 1 MICRON PEAK)
10

4. Discussion 

Due to the very limited information available regarding tactile sensitivity of the head, the authors 

made no prior hypothesized assumptions about the outcome of the data. The most revealing 

information in the literature regarding the pattern of tactile sensitivity for the head was published 

by Weber (1834/1978) which suggests that all areas of the scalp are not equally sensitive and the 

crown is less sensitive than the skin near the forehead, temples, and lower part of the back of the 

head. The thresholds found in this study are in agreement with the pattern of scalp sensitivity 

proposed by Weber (1834/1978). A significant finding for the main effect of Head Location 

suggests that vibration sensitivity is different for the seven head locations in this study. 

Furthermore, the crown (CZ) was found to be the least sensitive to vibration stimuli relative to 

T3, T4 (the temples) and PZ, O2 (back of the head). 

The back of the head (O2) was found to be more sensitive than the front of the head (F3, F8)*

. 

This difference in sensitivity was 5 dB. PZ was found to be the most sensitive to vibration 

stimuli relative to the other head locations included in this study. The difference in sensitivity 

between PZ and the back of the head (O2), the sides of the head (T3, T4), and the front of the 

head (F3, F8) was 3 dB, 4 dB, and 8 dB, respectively. Gilliland and Schlegel (1994) found that 

participants were able to detect 100% of low- and high-intensity vibration stimuli presented to 

the parietal region of the scalp. Thus, it may be true that the other locations in the parietal region 

of the head (P4 and P3 found in figure 1) are just as sensitive as PZ but further studies would be 

required to confirm such a claim. Weinstein (1968) reported that skin sensitivity is generally the 

same for both the left and right sides of the body. The same may be true for sensitivity on the 

head as the vibration threshold for the left temple (T3) was not significantly different from the 

vibration threshold for the right temple (T4). Also, vibration thresholds for F3 and F8 were not 

significantly different as F3 is located within the left hemisphere of the scalp and F8 is located 

within the right hemisphere of the scalp. In addition, the difference in sensitivity between the 

left side of the head (F3, T3) and the right side of the head (F8, T4, O2) was 0.8 dB. 

There are no other studies that report tactile or vibration thresholds for different locations on the 

head. In general, the thresholds reported in this study are lower than those reported by Wilska 

(1954) at similar frequencies on the forehead, however it is known that tactile sensitivity 

thresholds are affected by a number of tactor characteristics, specifically contactor size and the 

presence or absence of a surround (Lamoŕe and Keemink, 1988; Stuart et al., 2003). Although 

we found significance for the main effect of Frequency, the differences between the thresholds at 

 *

General sensitivity for the front of the head was calculated as the average sensitivity of F3 and F8. 
11

the tested frequencies is so small that the data is still in agreement with Verrillo (1966) who 

found that thresholds were essentially the same for frequencies between 25 and 40 Hz for hairy 

skin. 

5. Conclusions 

The measures found for vibration sensitivity in this study confirm the general pattern of 

sensitivity for the scalp reported by Weber (1834/1978). In addition, this study extends the 

general pattern of sensitivity for the scalp by providing objective measures that demonstrate the 

pattern. Of the seven chosen locations, PZ appears to be the best location for presenting 

vibrotactile information on the head because it is the most sensitive to vibration stimuli, followed 

by the temples (T3, T4) and O2. In fact, the entire parietal region may be the most sensitive 

region of the scalp however additional studies are needed to confirm this hypothesis. These 

findings may be beneficial in designing a head-mounted tactile display to support a directional 

cueing system to alert Soldiers to important events in the environment. 
12

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15

Appendix. Head Diagram With Displacement Sensitivity Values 

(Decibel Referenced to 1 μm) 
16

Figure A-1. Head diagram with displacement sensitivity values (decibel referenced to 1 μm). 

13.0 

3.5 

5.8 

10.4 

8.5 

12.3 

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