Guidelines for Head Tactile Communication 

by Kimberly Myles and Joel T. Kalb 

ARL-TR-5116 March 2010 

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5425 

ARL-TR-5116 March 2010 

Guidelines for Head Tactile Communication 

Kimberly Myles and Joel T. Kalb 

Human Research and Engineering Directorate, ARL

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Guidelines for Head Tactile Communication 

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Kimberly Myles and Joel T. Kalb 

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14. ABSTRACT 

A helmet-mounted tactile display is desired for military applications such as cueing to alert Soldiers to the direction and 

location of an event or indicating movement direction in GPS-supported navigation. However, the system must be compatible 

with the head sensitivity of the user to ensure the user’s optimal perception of the provided information. The purpose of this

report is to document the overall findings of the basic research program regarding head tactile sensitivity and to provide 

answers to two basic questions: (1) What locations on the head are most sensitive to vibration stimulation? (2) What is the 

optimal frequency for tactile signals to be applied to the head? The overall findings are discussed and are considered as initial 

guidelines for head tactile communication and for using vibration stimulation on the head. 

15. SUBJECT TERMS

head-mounted tactile display, tactile sensitivity, vibration thresholds, head tactile communication 

16. SECURITY CLASSIFICATION OF: 

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Kimberly Myles 

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iii

Contents 

List of Figures iv

Acknowledgments v

1. Introduction 1

2. Experiment 1 2

2.1 Method.............................................................................................................................2

2.1.1 Participants ..........................................................................................................2

2.1.2 Apparatus and Stimuli .........................................................................................2

2.1.3 Procedure .............................................................................................................3

2.2 Results .............................................................................................................................5

2.2.1 Tactile Sensitivity by Head Region .....................................................................5

3. Experiment 2 7

3.1 Method.............................................................................................................................7

3.1.1 Participants ..........................................................................................................7

3.1.2 Apparatus and Stimuli .........................................................................................8

3.1.3 Procedure .............................................................................................................8

3.2 Results .............................................................................................................................8

3.2.1 Tactile Sensitivity by Head Region ...................................................................10

3.2.2 Head Tactile Sensitivity Compared With Other Body Locations .....................12

4. General Discussion and Guidelines 12

4.1 Head Location ...............................................................................................................12

4.2 Frequency ......................................................................................................................13

4.2.1 Vibration Signals ...............................................................................................13

4.2.2 Frequency and Sensory Receptors ....................................................................14

4.2.3 Frequency of the Signal and User Discomfort ..................................................15

4.3 Background Noise .........................................................................................................15

4.4 Head Tactile Communication and Bone Conduction ....................................................15

5. Conclusions 16

6. References 17 

Appendix. Diagram of the Proposed HMTD: Four-Tactor, Circular Head Array 21 

Distribution List 23 
iv

List of Figures 

Figure 1. Diagram of the signal. .....................................................................................................3

Figure 2. Diagram of equipment showing Tucker-Davis Technologies System II and the 

seven tactors. ..............................................................................................................................3

Figure 3. A 10-20 system of electrode placement (adapted from D. G. Domenick [2006]) 

and the seven locations chosen for experiment 1. ......................................................................4

Figure 4. Mean displacement by head location for experiment 1. (The error bars indicate 

standard errors.) .........................................................................................................................6

Figure 5. Noise condition by frequency interaction for experiment 1. ...........................................6

Figure 6. Mean displacement by head region for experiment 1. (The error bars indicate 

standard errors.) .........................................................................................................................7

Figure 7. A 10-20 system of electrode placement (adapted from D. G. Domenick [2006]) 

and the eight locations chosen for experiment 2. .......................................................................8

Figure 8. Mean displacement by head location for experiment 2. (The error bars indicate 

standard errors.) .........................................................................................................................9

Figure 9. Mean displacement by frequency for experiment 2. (The error bars indicate 

standard errors.) .........................................................................................................................9

Figure 10. Noise condition × frequency interaction for experiment 2. .........................................10

Figure 11. Mean displacement by head region for experiment 2. (The error bars indicate 

standard errors.) .......................................................................................................................11

Figure 12. Mean displacement by head region for experiments 1 and 2. (The error bars 

indicate standard errors.) ..........................................................................................................11

Figure 13. Vibratory stimulation threshold displacements measured on the head (mean of 

experiments 1 and 2) compared with other sites on the body. The area of the contactor 

that was used to measure threshold for each body site is given. (Adapted from Jones 

and Sarter [2008]). ...................................................................................................................12 

Figure A-1. Diagram of the proposed HMTD: four-tactor, circular head array. .........................22
v

Acknowledgments 

The authors would like to thank the U.S. Marines from the Marine Corps Detachment at 

Aberdeen Proving Ground, MD, for volunteering as participants. 
vi

INTENTIONALLY LEFT BLANK. 
1

1. Introduction 

A few years ago, the authors were tasked to answer the question, Can the head be used as a 

communication tool for providing tactile alerts to the Soldier in hopes of alleviating sensory 

overload on the Soldier’s visual and auditory channels? To answer the question, the authors 

initially intended to design a prototype head-mounted tactile display (HMTD) to test Soldier 

performance. While searching for the best parameters to build the HMTD, the authors 

discovered that very little research had been done on head tactile stimulation. The lack of 

research regarding head tactile sensitivity and stimulation produced minimal background data 

from which to design a head device. 

For example, current research regarding basic tactile sensitivity measures or applicable 

equipment design solutions that utilize the tactile modality is largely 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). The list clearly shows a gap in the body of research regarding tactile sensitivity 

measures for the head. 

An HMTD is envisioned for use with military applications such as alerting Soldiers to the 

direction and location of an event or indicating movement direction in GPS-supported 

navigation. However, the system must be compatible with the head sensitivity of the user to 

ensure the user’s optimal perception of the information transmitted. The purpose of the 

experiment was to evaluate the effect of background noise on vibration thresholds for the head. 

The authors wanted to know if background noise (i.e., noise on the battlefield) would affect the 

Soldier’s ability to detect tactile signals on the head. The purpose of this report was to determine 

head tactile sensitivity for the basic research program and to provide answers to two basic 

questions (Myles and Binseel, 2007, 2009): (1) What locations on the head are most sensitive to 

vibration stimulation? (2) What is the optimal frequency for tactile signals that will be applied to 

the head? The overall findings are discussed and are considered as initial guidelines for head 

tactile communication and using vibration stimulation on the head.
2

2. Experiment 1 

The purpose of experiment 1 was to obtain vibration thresholds for the head (seven head 

locations) and evaluate if thresholds differed as a function of head location, frequency, and 

background noise. 

2.1 Method

2.1.1 Participants

Twenty-two U.S. Marines (all males) were recruited for this study, and a portable audiometer 

(Maico AC-40) was used to conduct hearing screenings to ensure that each participant’s hearing 

threshold was in the normal range. The screenings were conducted at a 20-dB hearing level 

(HL) at octave audiometric frequencies between 250 and 4000 Hz by presentation of tones 

through supra-aural headphones. The mean age of participants was 20 years (SD = 1.74), with a 

range of 18 to 24 years. Previously reported data from Myles and Kalb (2009), consisting of 11 

participants, was used for the quiet noise condition in this experiment. 

2.1.2 Apparatus and Stimuli 

The C-2 Tactor designed by Engineering Acoustics, Inc. (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. Its moving 

contactor surface is 0.3 inch in diameter. 

A computer-generated, modulated sinusoidal carrier wave with a 10-V peak amplitude was used 

as the vibration stimulus. 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 1). 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. Each signal was attenuated, sent to one of seven 

multiplexer channels, and amplified by a unity-voltage-gain power driver that was connected to a 

tactor (figure 2). 

The noises used in this experiment consisted of pink and High-Mobility Multipurpose Wheeled 

Vehicle (HMMWV) (25 mph at the driver’s ear) recordings that were played through four, 

8-ohm Bose surface mount speakers placed around the perimeter of the testing station. They 

were played at a level of 85 dB(A) as measured at the listener location. Pink noise represented 

uniform masking of the listener’s critical band of hearing, and HMMWV noise represented a 

real-world noise that Soldiers are likely to encounter on the battlefield. The sounds were stored 

on a computer and sent to multiplexed power amplifiers.
3

Figure 1. Diagram of the signal. 

Figure 2. Diagram of equipment showing Tucker-Davis Technologies System II and the seven tactors. 

2.1.3 Procedure 

Participants were seated in a standard office chair in an Industrial Acoustics Company soundattenuating chamber and provided a consent form to review and sign if they agreed to participate. 

The seven head locations chosen for this experiment follow the 10-20 system of electrode 

placement used for high-density electroencephalography (EEG) studies (figure 3). 

TDT 

MultiYes No plexer 
4

Figure 3. A 10-20 system of electrode placement (adapted from D. G. Domenick [2006]) and the seven 

locations chosen for experiment 1. 

Therefore, seven tactors were placed in a headband designed to hold one tactor at each chosen 

head location. The experimenter placed the stretchable headband on the participant’s head 

(secured with Velcro fasteners), examined the headband to ensure that all tactors made proper 

contact with the scalp, and measured the static force (using a Digital Force Gauge) created by the 

tension of the headband. 

Vibration threshold measurements were obtained using the psychophysical adaptive procedure 

ZEST (Zippy Estimation by Sequential Testing). For an in-depth review regarding how this 

procedure was used to obtain threshold measurements, see Myles and Kalb (2009). 

Each participant was assigned to either the pink or HMMWV noise condition.* The appropriate 

background noise was turned on and measured at 85 dB(A) using a sound level meter. Each 

head location was tested one at a time, and the presentation order of head location was 

randomized. The presentation order of frequency (32, 45, and 63 Hz) within head location was 

also randomized. Each treatment was presented 10 times for a total of 210 vibration stimuli 

presented to the head of each participant. The last response obtained on trial 10 was recorded as 

the vibration threshold for that treatment within 1 dB error. 

For each stimulus, participants used 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 verified that each tactor was emitting a vibration stimulus, which 

also served as practice trials to familiarize the participants with the task and stimuli. Verification 

consisted of the experimenter identifying each tactor individually, sending a vibration stimulus to 

the identified location, and asking the participant to report if the stimulus was felt. 

 *

Data from Myles and Kalb (2009) was used for the quiet condition in this experiment. 

 Nose 

F8 F3 

CZ 

O2 

PZ 

T4 T3 
5

2.2 Results 

A 3 × 7 × 3 mixed design was used to determine the effect of noise condition (quiet, pink, 

HMMWV), head location (F8, F3, T4, CZ, T3, PZ, O2), and frequency (32, 45, and 63 Hz) on 

vibration threshold. Noise was used as a between-subjects variable, and head location and 

frequency were within-subjects variables. Vibration thresholds were measured as the skin 

displacement level required for the perception of a vibration stimulus. An analysis of variance 

(ANOVA) was used to compare the differences in thresholds attributable to head location, 

frequency, and noise condition, and significant effects were explored using least significant 

difference (LSD). The mean static force created on the head by the headband was 0.47 N. Due 

to potential confounds for the noise condition variable resulting from the combination of two 

different experiments, interpretations involving the variable were done with care. 

An ANOVA revealed a significant main effect for head location, F (6, 180) = 12.07, p < 0.01, 

and a significant noise condition × frequency interaction, F (4, 60) = 4.48, p < 0.01. The main 

effects of noise condition, F (2, 30) = 0.32, p = 0.73, and frequency, F (2, 60) = 2.33, p = 0.11, 

were not significant. In figure 4, head locations are arranged from least to most sensitive. Post 

hoc comparisons of the seven head locations revealed that vibration thresholds for T4, O2, PZ, 

and T3 were not significantly different. Also, thresholds for head locations CZ, F3, and F8 were 

not significantly different. However, head locations T4, O2, PZ, and T3 were significantly more 

sensitive (p < 0.01) to vibration stimulation than locations CZ, F3, and F8. To further evaluate 

the noise condition × frequency interaction (figure 5), the frequency data were collapsed across 

head location at each level of noise, and for each level of noise, comparisons were performed 

between each possible pair of frequency levels. A paired sample t-test revealed that vibration 

thresholds in quiet were lower at 32 Hz than at 45 [t(10) = 2.24; p = 0.05] and 63 Hz [t(10) 

= 2.60; p = < 0.05]. 

2.2.1 Tactile Sensitivity by Head Region 

A relative comparison of the head locations provides little information for the design of a head 

tactile display, so the data were organized into five head regions: central (CZ), frontal (F3, F8), 

temple (T3, T4), occipital (O2), and parietal (PZ). Head location data were collapsed across 

frequency and noise condition. A repeated measures one-way ANOVA was used to evaluate 

vibration thresholds by head region, and the analysis revealed a significant main effect for head 

region, F (4, 128) = 16.31, p < 0.01.
6

Figure 4. Mean displacement by head location for experiment 1. (The error bars indicate standard errors.) 

Figure 5. Noise condition by frequency interaction for experiment 1.

12.5

11.8 11.8

7.4

6.6

6.2 6.1

0

3

6

9

12

15

CZ F3 F8 T4 O2 PZ T3

HEAD LOCATION

DISPLACEMENT IN DECIBELS 

 (RE 1 MICRON PEAK)

0

2

4

6

8

10

12

Quiet HMMWV Pink

DISPLACEMENT IN DECIBELS

(RE 1 MICRON PEAK)

NOISE CONDITION

32 Hz

45 Hz

63 Hz
7

In figure 6, the head regions are arranged from least to most sensitive to vibration stimulation, 

and post hoc comparisons of the five head regions revealed that the central and frontal regions of 

the head were least sensitive to vibration while the parietal, occipital, and temple regions of the 

head were most sensitive to vibration. All comparisons were significant at p < 0.01. 

Figure 6. Mean displacement by head region for experiment 1. (The error bars indicate standard errors.) 

3. Experiment 2 

The purpose of experiment 2 was to obtain vibration thresholds for the head (eight additional 

head locations) and evaluate if thresholds differed as a function of head location, frequency, and 

background noise. 

3.1 Method 

3.1.1 Participants 

Thirty-three Marines (32 males, 1 female) were recruited for this study, and a portable 

audiometer (Maico AC-40) was used to conduct hearing screenings to ensure that each 

participant’s hearing threshold was in the normal range of hearing. The screenings were 

conducted at a 20-dB HL at octave audiometric frequencies between 250 and 4000 Hz 

by presentation of tones through supra-aural headphones. The mean age of participants was 

20 years (SD = 2.95), with a range of 18 to 30 years. 

12.5

11.8

6.8 6.6

6.2

0

3

6

9

12

15

Central (CZ) Frontal (F3, F8) Temple (T3, T4) Occipital (O2) Parietal (PZ)

HEAD REGION

DISPLACEMENT IN DECIBELS 

(RE 1 MICRON PEAK)
8

3.1.2 Apparatus and Stimuli 

The apparatus and stimuli used in this experiment were the same as used in experiment 1. 

3.1.3 Procedure 

The procedures were the same as in experiment 1, except vibration threshold measurements were 

obtained for eight different head locations (figure 7), participants were assigned to either the 

quiet, pink, or HMMWV noise level, and participants received a total of 240 vibration stimuli. 

Figure 7. A 10-20 system of electrode placement (adapted from D. G. Domenick [2006]) and the eight 

locations chosen for experiment 2. 

3.2 Results 

A 3 × 8 × 3 mixed design was used to determine the effect of noise condition (quiet, pink, 

HMMWV), head location (Fp2, FZ, F7, C4, C3, P4, P3, O1), and frequency (32, 45, and 63 Hz) 

on vibration threshold. Noise was used as a between-subjects variable, and head location and 

frequency were within-subjects variables. An ANOVA was used to compare the differences in 

thresholds attributable to head location, frequency, and noise condition, and significant effects 

were explored using LSD. The mean static force created on the head by the headband was 

0.55 N. 

An ANOVA revealed a significant main effect for head location, F (7, 210) = 3.10, p < 0.01, and 

a significant noise condition × frequency interaction, F (4, 60) = 3.30, p < 0.05. The main effect 

of noise condition, F (2, 30) = 1.22, p = 0.31, was not significant, but frequency, F (2, 60) 

= 3.12, p = 0.051, did approach significance. In figure 8, head locations are arranged from least 

to most sensitive. Post hoc comparisons revealed that head location Fp2 was significantly more 

sensitive to vibration stimulation than locations P4 and C3, p < 0.01. The remaining 

comparisons revealed that head location P4 was also significantly less sensitive to vibration than 

 Nose 

FZ F7 

C4 

P3 P4 

C3 

Fp2 

O1 
9

Figure 8. Mean displacement by head location for experiment 2. (The error bars indicate standard errors.) 

locations C4, O1, and P3, p < 0.01. Head location FZ was neutral and was not significantly 

different from any of the other head locations, and head locations C4, O1, P3, and Fp2 were not 

significantly different from one another. The frequency approached significance, which strongly 

suggested that vibration thresholds may be lower at 32 Hz than at 45 or 63 Hz, p = 0.04 

(figure 9). To further evaluate the noise condition × frequency interaction (figure 10), the 

frequency data were collapsed across head location at each level of noise, and for each level of 

noise, comparisons were performed between each possible pair of frequency levels. A paired 

sample t-test revealed that vibration thresholds in the pink noise condition were significantly 

lower at 32 Hz than at 45 Hz, t(10) = 3.28, p < 0.01. 

Figure 9. Mean displacement by frequency for experiment 2. (The error bars indicate standard errors.)

9.1

7.6

7.1 7.0 6.7 6.5

5.4

4.9

0

3

6

9

12

P4 C3 F7 FZ C4 O1 P3 Fp2

HEAD LOCATION

DISPLACEMENT IN DECIBELS

(RE 1 MIRCON PEAK)

6.7 6.9 6.9

0

2

4

6

8

10

32 Hz 45 Hz 63 Hz

FREQUENCY (HZ)

DISPLACEMENT IN DECIBELS

(RE 1 MICRON PEAK)
10

Figure 10. Noise condition × frequency interaction for experiment 2. 

3.2.1 Tactile Sensitivity by Head Region 

Again, the data were organized into five head regions: central (C3, C4), frontal (FZ, F7), 

occipital (O1), parietal (P3, P4), and forehead (Fp2). Head location data were collapsed across 

frequency and noise condition. A repeated measures one-way ANOVA was used to evaluate 

vibration thresholds by head region, and the analysis revealed a significant main effect for head 

region, F (4, 128) = 2.45, p = 0.05. 

In figure 11, the head regions are arranged from least to most sensitive to vibration stimulation 

and post hoc comparisons of the five head regions revealed that the forehead (Fp2) was 

significantly more sensitive to vibration than the central, frontal, and parietal regions of the head, 

p < 0.05. The occipital and forehead regions of the head demonstrated equal levels of sensitivity 

to vibration stimulation. Figure 12 shows displacement thresholds as a function of head region 

for experiments 1 and 2. 

0

2

4

6

8

10

Quiet HMMWV Pink

DISPLACEMENT IN DECIBELS

(RE 1 MICRON PEAK)

NOISE CONDITION

32 Hz

45 Hz

63 Hz
11

Figure 11. Mean displacement by head region for experiment 2. (The error bars indicate standard errors.) 

Figure 12. Mean displacement by head region for experiments 1 and 2. (The error bars indicate standard 

errors.) 

7.3 7.2 7.0

6.5

4.9

0

2

4

6

8

10

Parietal (P3, P4) Central (C3, C4) Frontal (FZ, F7) Occipital (O1) Forehead (Fp2)

HEAD REGION

DISPLACEMENT IN DECIBELS

 (RE 1 MICRON PEAK)

0

3

6

9

12

15

Parietal Frontal Central Occipital Temple Forehead

HEAD REGION

DISPLACEMENT IN DECIBELS 

(RE 1 MICRON PEAK)

Experiment 1

Experiment 2
12

3.2.2 Head Tactile Sensitivity Compared With Other Body Locations 

Figure 13 presents displacement measured as a function of frequency for the head (from 

experiments 1 and 2) compared with tactile displacement for the abdomen, forearm, and 

fingertip. A relative comparison of the body sites shows that the head is more sensitive to 

vibration stimulation than the forearm and abdomen between 32–63 Hz. Also, the head is 

relatively equal to the fingertip in sensitivity to vibration stimulation at 32 Hz. 

Figure 13. Vibratory stimulation threshold displacements measured on the head (mean of experiments 1 and 

2) compared with other sites on the body. The area of the contactor that was used to measure 

threshold for each body site is given. (Adapted from Jones and Sarter [2008]). 

4. General Discussion and Guidelines 

4.1 Head Location 

The data from experiments 1 and 2 provide a complete vibration sensitivity map of the head via 

head location positions in the 10-20 system of electrode placement. Vibration thresholds were 

measured for 15 of the 21 head locations in the system that represented all lobes of the cerebral 

CONTACTOR AREA (CM^2)

 HEAD 0.38 

 ABDOMEN 0.39 

 FINGERTIP 0.72 

 FOREARM 2.9

HEAD

ABDOMEN 

FINGERTIP

FOREARM
13

cortex. 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 the crown is 

less sensitive than the skin near the forehead, temples, and lower part of the back of the head. In 

Myles and Kalb (2009), the authors confirmed the general pattern of sensitivity for the scalp 

reported by Weber (1834/1978) using just seven head locations in the 10-20 system. In this 

study, using a more extensive array of head locations in the system, the authors again confirm 

Weber’s (1834/1978) pattern of head sensitivity via the head region data. 

In experiment 1, the parietal (PZ), occipital (O2), and temple (T3, T4) regions of the head were 

found to be most sensitive to vibration stimulation, and the central (CZ) and frontal (F3, F8) 

regions were found to be least sensitive to vibration stimulation. In experiment 2, the occipital 

(O1) and forehead (Fp2) regions of the head were found to be most sensitive to vibration 

stimulation, and the central (C3, C4), frontal (F7, FZ), and parietal regions (P3, P4) were found 

to be least sensitive to vibration stimulation. Because the parietal region of the head was not 

found to be highly sensitive in both experiments, it was eliminated as a viable region for use for 

head tactile communication. The authors conclude that the forehead, occipital, and temple 

regions are the most sensitive to vibration stimulation. Therefore, these regions of the head 

should be considered for purposes related to head tactile communication. 

Based on the sensitivity data for the six head regions (forehead, occipital, temple, parietal, 

frontal, central) evaluated in this study, a four-tactor, circular head array is recommended for the 

design of an HMTD (see appendix). This design makes use of the most sensitive regions of the 

head and should be most intuitive for directional cueing and navigation applications. One tactile 

transducer should be placed in the middle of the forehead, one on each of the left and right 

temples, and one in the middle of the back of the head. 

4.2 Frequency 

4.2.1 Vibration Signals 

Based on the results of a pilot test conducted in the beginning of this research program, the 

optimal frequency range for vibratory tactile signals used on the head is 32–64 Hz. It was 

reported that for frequencies above 64 Hz, both tactile and auditory perception of a signal could 

occur, but only tactile perception (no auditory perception) seems to occur for signals below 64 

Hz (Myles and Kalb, 2009). This information is especially important for laboratory settings to 

ensure that tactile perception, not auditory perception, is truly being measured. As evidence, in 

most laboratory studies measuring vibration thresholds on the skin, participants don earphones, 

or background noise is used without earphones to mask the noise thought to be produced by a 

vibrating signal. The conveyance of vibration stimulation to the head/face is more susceptible to 

this phenomenon (than any other body site) because these locations are closer to the ear canal 

opening. Adherence to the optimal frequency range for vibration signals on the head will 

eliminate or significantly reduce sound transmission into the ear canal or cochlea from vibration 

signals. 
14

4.2.2 Frequency and Sensory Receptors 

Verrillo (1966) reported similar thresholds between 25–40 Hz for hairy skin. However, in 

experiment 2 the effect of frequency approached significance and may suggest that vibration 

thresholds were lower at 32 Hz than at 45 or 63 Hz. The thresholds used in the quiet 

environment in experiment 1 are re-reported from Myles and Kalb (2009) and exhibited the trend 

as well but were not produced in the HMMWV and pink noise environments. Even within the 

small range of 32–63 Hz, this finding may suggest that individuals are more sensitive to head 

vibration at lower frequencies. 

Pacinian corpuscle (PC) receptors have a large receptive field, are excited by higher frequencies 

(>80 Hz), are numerous in distal joints, and are located in deep tissues of the skin (Mahns et al., 

2006; Sherrick and Cholewiak, 1986; Sherrick et al., 1990). Non-PC receptors have a small 

receptive field, are excited by lower frequencies (<80 Hz), and are located in superficial skin 

(Mahns et al., 2006; Sherrick et al., 1990). According to Mahns et al. (2006), the detection of 

vibration stimulation in hairy skin is dependent upon sensory fibers found in hair follicles called 

hair follicle afferent (HFA) fibers (populous in non-PC receptors) as opposed to rapidly adapting 

(RA) fibers (populous in PC receptors) found in glabrous (i.e., non-hairy) skin. 

The characteristic makeup of the head or scalp, which is populated with HFA fibers, may help to 

explain why the head might be more sensitive to vibration stimulation at 32 Hz. A body site that 

is heavily populated with HFA fibers is more likely to respond to vibration stimulation at the 

lower frequencies; one would imagine this would enhance perception at the lowest frequency 

within a given range. The authors are not asserting that the head will exhibit an optimal response 

to vibration at perhaps 5 Hz because that particular frequency was not part of the scope of this 

project and would need to be investigated further. Also, the head, unlike other body sites such as 

the forearm, is not surrounded by distal joints that contain RA fibers that respond to vibration 

stimulation at the higher frequencies. Vibration seldom stimulates one fiber/receptor but moves 

throughout nearby skin tissues (Sherrick and Cholewiak, 1986; van Erp and van den 

Dobbelsteen, 1998) that can excite fibers that respond to both the lower and higher vibration 

frequencies. This is seen in figure 13, where the hairy forearm and abdomen exhibit similar 

sensitivity thresholds (often seen for non-hairy skin) that begin to decrease at ~100 Hz with 

maximum sensitivity occurring between 250–300 Hz (Bolanowski et al., 1988; Lamoŕe and 

Keemink, 1988; Verrillo, 1962, 1966) where the trend for the head should begin to show an 

increase in thresholds at 100 Hz. For hairy skin to show similar sensitivity trends like that of 

non-hairy skin, vibration on the forearm and abdomen recruit nearby RA fibers at the higher 

frequencies. Because the head/scalp has much more moderate access to RA fibers, the authors 

speculate that increasing thresholds are likely to be seen at 100 Hz. 

No general statements can be made regarding vibration sensitivity for frequencies lower than 

32 Hz, but of the frequency range used in this study, 32 Hz appears to be the reasonable 

frequency choice for the optimal perception of vibration on the head.
15

4.2.3 Frequency of the Signal and User Discomfort 

For tactile communication on the head, vibration signals also need to be evaluated for user 

comfort. This characteristic is demonstrated from the user’s perspective (Brill and Gilson, 2006; 

Hawes and Kumagai, 2005; Redden et al., 2007). Hawes and Kumagai (2005) reported a 

preference for a chest tactile system vs. a head tactile system because participants thought the 

head tactile system applied too much energy to the head, causing them discomfort and 

headaches. Brill and Gilson (2006) also reported user aversion to a head tactile system for 

similar reasons such as dizziness, disorientation, and a general feeling of unsettledness. While 

Brill and Gilson (2006) did not report the frequency of their signal, Hawes and Kumagai (2005) 

reported that they used a 260-Hz signal for both the chest and head displays. The authors agree 

with the perspectives of the user because during the pilot test, vibration signals in the 5- to 

300-Hz range were created and tested. It was discovered that above 150 Hz, user discomfort 

becomes a serious issue, and these frequencies should not be used for vibrating the head. During 

debriefing in Myles and Kalb (2009) and in this study, no participants reported discomfort or 

pain due to the vibration signals. 

4.3 Background Noise 

Noise condition by frequency interactions suggests that at 32 Hz the type of background noise 

may increase one’s sensitivity to vibration stimulation on the head. Specifically, in the quiet 

condition in experiment 1, thresholds were lowest at 32 Hz, and in the pink condition in 

experiment 2, thresholds were lowest at 32 Hz. Because each noise by frequency interaction was 

not found in both experiments, a repeat study using the current methodology (or at least the quiet 

and pink noise condition levels) is advocated before general recommendations are made 

regarding the effect of background noise on head tactile sensitivity. For both experiments, tactile 

sensitivity in the HMMWV condition was not affected by frequency. This finding is promising 

for real-world applications of Soldier performance on the battlefield because it shows the 

potential to detect tactile signals on the head regardless of exposure to HMMWV noise in the 

background. 

4.4 Head Tactile Communication and Bone Conduction 

The issue concerning whether individuals perceive vibration signals through auditory or tactile 

perception is critical to the design of an HMTD. Since the tactile display will be head-mounted, 

most would agree that the logical placement for the tactile transducers would be on the helmet. 

However, with so many other systems mounted to the helmet, designing the HMTD as part of an 

existing system would be most beneficial to the Soldier in that the Soldier would benefit from 

novel technology that would help to alleviate overload on the visual and auditory channels, with 

very little additional equipment and weight to carry. Distinguishing between tactile vs. auditory 

perception as a function of frequency is important if the HMTD will be integrated with existing 

bone conduction (BC) systems to improve Soldier performance. The three important parameters 
16

to consider for integrating the two systems are type of transducer, head location placement, and 

frequency of the stimuli. The tactile and BC transducers share similar properties, and the chosen 

transducer must offer characteristics where both speech and tactile signals are conveyed. Either 

the same BC transducers could be used for both speech or tactile communications on a timesharing basis or a system of two independent transducers could perform these functions. 

Additional research is needed to make an informed recommendation regarding the dual use of 

tactile or BC transducer equipment for both tactile and BC communication. According to 

McBride et al. (2005), the vertex (CZ) and the temples (T3, T4) are head locations that are most 

suitable for BC vibrator placement. Based on the results from this study, the temples (T3, T4) 

are also suitable head locations for vibration stimulation or head tactile communication. 

5. Conclusions 

Eriksson et al. (2008) reported Soldier preference for a directional cueing tactile display over 

similar visual and auditory displays. Dorneich et al. (2006) found that a tactile cueing system 

mitigated the cognitive challenges associated with navigation in an unfamiliar environment. 

Elliott et al. (2006) demonstrated that tactile vs. compass and GPS navigation allowed greater 

accuracy for reaching waypoints, and tactile vs. compass navigation facilitated faster navigation 

and walking speeds. Savick et al. (2008) reported a lower response time and lower ratings for 

frustration and effort with the use of a tactile vs. visual and auditory displays for target 

acquisition and robot navigation tasks. These tactile applications have been used successfully on 

the torso, and the authors envision success conveying similar information via the head. 

Therefore, the guidelines for head tactile communication summarized in this report will be 

influential in the design of an HMTD to support a directional cueing or covert communication 

system for Soldiers. Guidelines and recommendations for HMTDs are listed as follows: 

1. The forehead, occipital, and temple regions of the head are most sensitive to vibration 

stimulation. Therefore, these regions should be considered for purposes related to head 

tactile communication. 

2. A four-tactor, circular head array is recommended for an HMTD design (see appendix). 

3. The reasonable frequency for the optimal perception of vibration on the head appears to be 

32 Hz. 

4. Above 150 Hz, user discomfort becomes a serious issue; these frequencies should not be 

used for vibrating the head. 

5. The vertex (CZ) and the temples (T3, T4) are head locations that are most suitable for BC 

vibrator placement. The temples (T3, T4) are also suitable head locations for vibration 

stimulation or head tactile communication.
17

6. References 

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Mahns, D. A.; Perkins, N. M.; Sahai, V.; Robinson, L.; Rowe, M. J. Vibrotactile Frequency 

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Fort Rucker, AL, 2009; pp 849–876. 

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Laboratory: Aberdeen Proving Ground, MD, January 2009. 

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Research Laboratory: Aberdeen Proving Ground, MD, May, 2007; pp 12–15. 

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20

INTENTIONALLY LEFT BLANK. 
21

Appendix. Diagram of the Proposed HMTD: Four-Tactor, 

Circular Head Array 
22

Figure A-1. Diagram of the proposed HMTD: four-tactor, circular head array. 

Forehead Region 

Occipital Region 

Left Temple Region Right Temple Region 
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