1

Abstract

Elderly individuals with reduced sensitivity in their lower limbs and feet often suffer from

impaired postural control resulting in increased risk of falls. Postural control, partially dependent

upon somatosensory feedback signals originating from the soles of the feet, degrades with age in

part due to elevated sensory thresholds of mechanoreceptor neurons. If mechanoreceptor

sensitivity could be artificially augmented in ageing adults, one could increase somatosensory

fidelity, improving postural control and reducing the risk of falls. An emerging approach for

improving somatosensory perception, known as stochastic resonance, involves the addition of an

optimal noise stimulus into the sensory system. The novel application of sub-sensory mechanical

noise about the ankle to target plantar mechanoreceptors was the focus of my research project.

Specifically, my project investigated whether applying stochastic vibration to positions around the

ankle and foot resulted in improved tactile sensation at the sole of the foot. Two different

experimental protocols were implemented on a subject population of young adults. Both protocols

investigated applying noise at similar locations but used different means of quantifying plantar

tactile perception. The first protocol showed no significant enhancement of plantar tactile

perception levels. The second protocol showed a significant improvement in plantar tactile

perception in one subject only, all other subjects showed no significant plantar tactile

enhancement. Many experimental design challenges could have accounted for the lack of

substantial findings and were discussed at length. With the experimental shortcomings in mind,

the feasibility of utilizing stochastic noise applied at the ankle to improve plantar tactile perception

is still in question.

2

Acknowledgements

I would like to thank Professor Darryl Thelen of the University of Wisconsin–Madison

Neuromuscular Biomechanics Laboratory for providing benevolent supervision on the project. I

would also like to thank Samuel Acuña for offering continual guidance and help at all points during

the course of this project.

3

Table of Contents

1 Introduction & background………………………………………………………………...…4

2 Methods…………………………………………………………………………………………9

2.1 Protocol 1……………………………………………………………………………..9

2.2 Protocol 2…………………………………………………………………………….13

3 Results…………………………………………………………………………………………17

3.1 Protocol 1 Results…………………………………………………………………...17

3.2 Protocol 2 Results…………………………………………………………………...19

4 Discussion & Conclusion……………...………………………………………………………21

5 Bibliography…………………………………………………………………………………..26

4

1.1 Introduction

Each year nonfatal falls amongst elderly populations accounts for more than $19 billion in cost on

the U.S. health care system (Stevens et al., 2006). Not only do impairments in balance result in a

burden on our healthcare system, the risk of falling drastically reduces the mobility, independence,

and quality of life of elderly individuals. As such, a therapeutic device that could reduce the risk

of falling for aging adults is of great interest for research. If improved therapies and devices could

be developed to enhance balance and diminish the risk of falling, health care costs could be

significantly reduced and, more importantly, quality of life improved for many older individuals.

Elderly persons are susceptible to falls for a multitude of reasons, one of which is accumulated

sensorimotor system deficits (Rubenstein et al., 2006). Postural control is the product of a complex

sensory feedback system. Integrating sensorimotor, visual, and vestibular information, the cortical

motor control system continually monitors the location of the body in space and initiates motor

corrections as needed. However, the brain’s ability to quickly and correctly provide motor

corrections rests on the accuracy and fidelity of the information collected by afferent sensory

organs.

It is well documented that as individuals age they lose sensation in their peripheral extremities, in

part due to lessened functionality of their mechanoreceptor neurons: the sensory neurons in the

skin that respond to pressure changes and physical distortion (Shaffer et al., 2007). Lessened

sensation in the feet limits the brain’s ability to sense the body’s location in space, disrupting the

motor control feedback system described above and predisposing elderly individuals to be less

stable (Patel et al., 2009). Gait variability analysis conducted on aging subjects at the UW

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Neuromuscular Biomechanics lab suggests that a major consequence of an impaired sensory

feedback system is an overreliance on visual information for maintaining balance (Franz et al.,

2015). The resultant effect is a reduced rate of response to perturbations in balance increasing the

risk of falling when surfaces become irregular, especially when imprecise visual information is

provided.

An emerging method of intervention for increasing somatosensory sensitivity, and in turn

improving postural control, utilizes a phenomenon known as stochastic resonance (Collins et al.,

2003; Magalhães et al., 2012). A somewhat counterintuitive idea, stochastic resonance (SR)

involves the introduction of a white noise signal at an optimal intensity into a nonlinear system.

The effect is an ability

of the system to detect

stimuli that were

previously below the

system’s detection

threshold. Noise is

usually considered undesirable in

most engineering applications, and indeed too much

added noise will washout the stimulus signal.

However, if the noise is of the proper intensity the

noise signal “thickens” the stimulus signal by means of superposition, artificially shifting the

stimulus signal to the detection threshold (Figure 1).

Figure1

This diagram illustrates the “thickening” superposition mechanism whereby noise enhances signal detection.

(Zeng et al., 2000)

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In the context of the human body activation of sensory neurons elicits a discrete nonlinear electrical

response known as an action potential, thus forming a basis for the application of stochastic

resonance phenomena. A mechanoreceptor neuron generates action potentials when physical

pressure applied to the nerve endings is sufficiently large. The specific magnitude of pressure

required to trigger action potential firing is referred to as the sensory threshold. If the magnitude

of pressure applied to the nerve falls below the sensory threshold, no action potentials will be

generated and the central nervous system will not perceive the stimulus. As individuals age

physiological changes in afferent sensory organs reduces the sensitivity of the somatosensory

system, increasing sensory thresholds (Shaffer, 2007). As consequence, the level of pressure

required to trigger a mechanoreceptor action potential, and thereby perceive the pressure sensation,

is increased. It is thought that if stochastic resonance phenomena could be elicited in

mechanoreceptor neurons, deficits in the postural control feedback system created by lessened

sensory perception could be compensated for and improvements in balance should then ensue

through natural postural control.

Supporting this theory is evidence suggesting that noise introduced through the soles of the feet

exhibits SR enhanced elevations of plantar tactile sensitivity and leads to improvements in balance

as predicted (Dettmer et al., 2015; Harry et al., 2005; Priplata et al., 2003). However, for a device

that introduces noise directly to the sole of the foot to be a clinically viable means of treatment,

many major design challenges must be overcome. Currently constructed devices resemble bulky

shoe insoles with exterior power supply wires protruding, and are incompatible with most

consumer footwear, limiting the practical application (Hijmans et al., 2007; Lipsitz et al., 2015).

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Research conducted on stroke survivors at the UW–Milwaukee uncovered a novel finding that

mechanical noise applied to various locations around the wrist lowered sensory thresholds in the

fingertips (Enders et al., 2013). Research has also shown that electrical noise introduced through

the placement of electrodes at the ankles has been found to exhibit SR like phenomena in the feet

resulting in improved plantar sensation (Breen et al., 2014). The physiological mechanism

underpinning why SR phenomena can be exhibited when noise is applied away from the location

of signal input is not well understood. However, researchers have determined that mechanical

noise applied to the wrist modulates activity in cortical regions responsible for processing fingertip

somatosensation. Specifically peak-to-peak potentials in the somatosensory cortex were found to

increase with the application of sub-sensory mechanical vibration at the wrist (Seo et al., 2015).

This forms a neurological basis for the observation that noise applied away from the site of sensory

input can improve sensory perception.

The finding that noise applied at a distance from the site of stimulus input can improve tactile

sensory perception provides the impetus for the research described herein. Can the same

interaction that was found between noise applied at the wrist and improved tactile sensation in the

fingertip be replicated using regions around the ankle and the plantar aspect of the foot

respectively? If so, could a vibrotactile device worn around the ankle invoke the same

improvements in plantar tactile sensitivity and demonstrate the same improvements in balance as

insoles that deliver mechanical noise directly to the soles of the feet? If such improvements in

plantar sensation and balance could be demonstrated with a device worn around the ankle the

design challenges associated with vibrating insoles could be avoided. However, the first necessary

step in the development of a wearable vibrotactile device would be validating the presence of an

8

interaction between stochastic vibration applied about the ankle and tactile sensory perception at

the sole of the foot.

The validation of such an interaction was the focus of my undergraduate research project. In

particular, I investigated what affect the application of mechanical noise to specific locations about

the ankle had on tactile sensory levels at the plantar of the foot. Two different experimental

protocols were followed to answer this question, the second improving upon the short comings of

the first. Due to subject population availability, healthy adults were recruited as participants for

the study. It was hypothesized that imperceptible (sub-sensory) stochastic mechanical vibration

applied to I.) the medial calcaneal tunnel, II.) the Achilles tendon, or III.) the dorsal surface of the

first metatarsal would elicit improvements in tactile perception thresholds at 1.) The plantar surface

of the great toe, 2.) The plantar surface of the 3rd metatarsal, or 3.) the plantar surface of the 5th

metatarsal. Those three plantar locations were selected because of the location of the Lateral

Plantar nerve, the Medial Plantar

nerve, and the Calcaneal nerve

(figure 2). The first and last noise

input locations (1 & 3 respectively)

were selected because of the location

of the Tibial and Medial Plantar

nerves, and the Achilles tendon was

selected because of stretch receptors

in the tendon (Figure 2).

Medial Plantar Nerve

Lateral Plantar Nerve

Calcaneal Nerve

Tibial Nerve

Figure 2 The innervation of the foot and ankle

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2 Methods

2.1 Experimental Protocol 1

Subjects:

Subjects were recruited from the mechanical engineering department at the University of

Wisconsin–Madison College of Engineering. Subjects were comprised of 4 healthy young adults

mean age 23±3 years old. Subjects possessed no known

lower extremity neuropathies or gait

disorders.

Experimental Setup:

Data collection occurred at the UW-

Madison Mechanical Engineering

building in the UW Neuromuscular

Biomechanics Lab space. Stochastic

mechanical vibration was supplied using a piezo

electric actuator (Thor Labs model PK4JQP2)

located in a custom manufactured housing (Figure

3). A 0-500Hz white noise signal was generated

using LabView software (Version 15.0) and amplified with a piezo controller

(Thor Labs model MDT 694). The vibratory devices were secured to subjects’

ankles and feet using Velcro strapping (Figure 4. Tactile sensation thresholds

were quantified using a clinical measure of tactile sensitivity know as a Semmes

Figure1

CustomdesignedvibratorydevicesFigure 3 Custom designed housing holding the piezo electric actuator (pictured right) used to provide vibrotactile simulation

Figure 4 Velcro strapping used to attach the vibrotactile device to the subject’s foot

10

Weinstein monofilament test (Aesthesio 20 piece

kit). The 0.016g monofilament was selected for

the duration of protocol 1 (Figure 5).

Methodology:

Subjects were positioned sitting with their legs

extend and right feet exposed. Noise input

locations and sensory evaluation areas were

located and marked for repeatability. Initially the

vibration perception thresholds of the noise input

locations were determined to allow for sub-sensory tuning of the noise signal. The noise input

locations were I.) the medial calcaneal tunnel, II.)

the Achilles tendon, and III.) the dorsal surface of

the first metatarsal (Figure 6). To determine noise

vibration perception thresholds an ascending and

descending sensation threshold test was

performed. For each noise location, the voltage

(this translated into amplitude of vibration) was

increased (or decreased) until the subject could

(or could not) consciously perceive the vibrations.

The mean of the ascending and descending values

Figure 5 The Semmes Weinstein monofilament test as used at the sole (plantar) of the foot

I

III

II

Figure 6 Noise input locations: I.) Medial Calcaneal Tunnel II.) Achilles Tendon III.) Dorsal surface of the first metatarsal

11

was then computed, and 90% of that value was used as the

amplitude of the noise signal for each respective noise

location.

Once noise signal amplitudes were determined,

quantification of plantar tactile sensitivity could ensue.

Plantar tactile sensation was measured using a monofilament

test at 1.) the plantar surface of the great toe, 2.) the plantar

surface of the 3rd metatarsal, and 3.) the plantar surface of the

5th metatarsal (Figure 7). The monofilament device was

depressed into the skin at each location and would bend under

a constant amount of force (Figure 5). Subjects were

prompted as to when the test would be administered but not

the exact location. Subjects had to respond with a binary

“Yes” or “No” and indicate the correct position of

administration for a correct identification to be counted.

Catch trials, consisting of no actual monofilament test administered, were also interspersed

throughout the collection process. Subjects were blind to the condition of each test and as such a

proper identification of a catch trial was counted if the subject identified no perceived

monofilament test.

For each noise location 8 total monofilament tests were administered per plantar location. The 8

monofilament tests were divided into 4 tests (3 actual monofilament tests and 1 catch) with the

1

2

3

Figure 7 Areas of evaluation for plantar tactile perception

1.) Great Toe 2.) 3rd Metatarsal 3.) 5th metatarsal

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stochastic vibration on (active), and 4 tests (3 actual monofilament tests and 1 catch) with the

stochastic vibration off (control). A custom written MATLAB script randomized the order of noise

locations evaluated, and randomized all trial conditions within each noise location. The script

would then prompt the data collector as to which trial condition was being evaluated. Once a noise

location was selected all plantar monofilament tests would be evaluated before the vibratory device

would be repositioned at the next randomly chosen noise location. Trial conditions are summarized

in Figure 8 below.

Figure 8 Experimental trial conditions for protocol 1

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2.2 Experimental Protocol 2

Subjects:

Subjects were recruited from the mechanical engineering department at the University of

Wisconsin–Madison College of Engineering. Subjects comprised of 5 healthy young adults mean

age 24±3 years old. Subjects possessed no known lower extremity neuropathies or gait disorders.

Experimental Setup:

Data collection occurred at the UW-Madison Mechanical Engineering

building in the UW Neuromuscular Biomechanics Lab space.

Stochastic mechanical vibration was supplied using an Engineering

Acoustics C-3 Tactor (Figure 9). A 100-300Hz band passed white

noise signal was generated using LabView software (Version 15.0) and

amplified with a

20-watt stereo

amplifier (Lepy LP-2020A) (Figure 10).

Tactile sensation thresholds were quantified

using a second Engineering Acoustics C-3

tactor secured to the foot with medical tape.

The stimulus signal, a 200Hz or 50Hz sine

wave of variable amplitude, was also generated

using LabView software and amplified using a

Figure 9 Engineering Acoustics C-3 Tactor

Figure 10 Tactor amplifier setup for controlling tactor signal amplification

14

second 20-watt stereo amplifier. The vibratory

devices were secured to subjects’ ankles and

feet using medical wrap (Figure 11).

Methodology:

Both the calcaneal tunnel and the plantar tactile

perception thresholds were determined using a

technique known as the Cornsweet Staircase

Method (Cornsweet, 1962). Designed to

provide a more robust means of quantifying

psychophysical perception thresholds, the

staircase method replaced the Semmes Weinstein monofilament and ascending/descending limits

tests employed in the first protocol. The generic staircase protocol was as follows: to begin the

tactile stimulus intensity was set to an arbitrarily low value that was likely sub-threshold, this value

was the same across subjects. The stimulus was administered and subjects were asked to verbally

indicate “Yes” or “No” as to whether they perceived the stimulus. Depending on the subject’s

response the stimulus was then either increased or decreased by a step size. If the subject responded

with a “Yes” to perceiving the stimulus, the stimulus intensity was decreased by a step size. If the

subject responded with a “No” to perceiving the stimulus, the stimulus was then increased by a

step size.

In this stepwise manner, the Cornsweet staircase progressed towards a stimulus vibration

perception threshold. After three reversals in step direction were accomplished, the following five

Figure 11 C-3 tactors secured to the foot and ankle with medical wrap

15

steps where then used to determine the vibration perception threshold. The mean value of the

stimuli magnitude for each of the final five values was calculated and determined to be the stimulus

perception threshold. Figure 12 depicts a prototypical Cornsweet staircase collection.

The noise input locations for the second protocol were the medial calcaneal tunnel and the plantar

surface of the 3rd metatarsal (Figure 6 & 7). The only plantar sensory location evaluated was the

plantar surface of the 3rd metatarsal (Figure 7). An explanation as to why the number of noise and

plantar locations was reduced in the second protocol can be found in the discussions section. Of

note was that the stimulus signal changed depending on the noise location selected. When the noise

After three reversals

Perception Threshold

(Cornsweet, 1962)

Start by increasing stimulus intensity

Figure 12 Data points from a prototypical Cornsweet staircase protocol. On the Y-axis is stimulus intensity or magnitude, on the X-axis is each trial. Note that for the protocol followed in this experiment, only five trials were conducted after three reversals were reached.

16

was positioned away from the stimulus site, at the medial calcaneal tunnel, the stimulus signal was

a 200Hz sine wave. When the noise was positioned adjacent to the stimulus signal, at the plantar

surface of the 3rd metatarsal, the stimulus signal was a 50Hz sine wave. This was done to ensure

that the noise signal, a 100-300Hz white noise, did not overlap with the stimulus signal when both

were applied adjacent to one another.

Subjects were positioned sitting horizontally with their legs extend and right feet exposed. The

stimulus and noise tactors were positioned accordingly and secured with medical tape. It should

be noted that only one of the noise locations, either the medial calcaneal tunnel or the plantar

surface of the 3rd metatarsal, was selected during a collection. Initially the calcaneal tunnel was

selected for the first four collections, the remaining five collections placed the noise adjacent to

the stimulus at the 3rd metatarsal. Using the staircase method described previously the vibration

perception threshold for the noise location was determined first. Once determined, 70% of the

noise perception threshold was used to achieve sub-sensory noise vibration for the active, vibration

on, conditions. Next attention turned to quantifying plantar perception thresholds. Two staircases

were performed, one for the control condition with noise vibration off, and one for the active

condition with noise vibration on. The two staircases were interspersed and the order was randomly

chosen to alternate between them via a MATLAB script. The MATLAB program would prompt

the data collector of the stimulus intensity and the condition, either noise on or noise off. Once the

staircases for each stimulus condition were completed the data collection was finished.

17

3 Results

3.1 Protocol 1 Results

A subject population consisting of 4 healthy young adults, mean age 23±3 years old, was sampled.

A two-sample T-test was used to test for statistical significance between the percent of correctly

identified monofilament scores between the control (stochastic vibration off) and active (stochastic

vibration on) conditions. For all 4 subjects, all p-values were greater than 0.05 indicating no

statistical significance was found. Statistical results are displayed in Table 1; a graphical summary

of individual subject trials is found in figure 13.

Subject

Number

P-value

1 NotSignificant

2 NotSignificant

3 NotSignificant

4 NotSignificant

Table1Resultssummaryforexperimentalprotocol1

18

Figure 13 Graphs show percent of correctly identified monofilament tests at each location for all four subjects tested during the first experimental protocol. The left side of each graph is with noise off. The right side of each graph is with noise on. We excepted to see an upward slope for each line indicating an increased percentage of monofilament tests currently identified when noise was added. However, the trend was not observed.

19

3.2 Protocol 2 Results

A subject population consisting of 4 healthy adults, 24±3 years old, was sampled with the

stochastic vibration applied at the medial calcaneal tunnel and a 200Hz sine wave used for the

plantar stimulus. A two-sample T-test was used to determine statistical significance between the

control (stochastic vibration off) and active (stochastic vibration on) conditions. All 4 subjects had

p-values that were greater than 0.05 indicating no statistical significance. Results are summarized

in Table 2a. A subject population of 5 healthy adults, 24±3 years old, was sampled with the

stochastic vibration and the stimulus signal both applied to the plantar surface of the 3rd metatarsal.

Out of 5 subjects tested at this condition 4 subjects had a p-value greater than 0.05 indicating that

no statistical significance was found. One subject however had a p-

value lower than 0.05, indicating that statistical significance was

determined. The data for the

statistically significant subject is

show in figure 14, Table 2b

summarizes the statistical data for

all subjects.

Subject

Number

P-value

1 Not Significant

2 Not Significant

3 Not Significant

4 Not Significant

5 P = 0.000493

Subject

Number

P-value

1 Not Significant

2 Not Significant

3 Not Significant

4 Not Significant

Table2bResultssummaryforexperimentalprotocol2withnoiseappliedattheplantarsurfaceofthe3rdmetatarsal

Table2aResultssummaryforexperimentalprotocol2withnoiseappliedatthemedialcalcanealtunnel

20

Figure 14 The graph of the plantar perception staircase for the subject with statistical significance from protocol 2

21

4 Discussion & Conclusion

The relationship between stochastic mechanical white noise applied about the ankle and plantar

tactile perception thresholds was examined as the scope of this project. It was theorized that

stochastic resonance would induce improvements in plantar tactile sensory perception.

Specifically, mechanical vibratory white noise was applied at I.) the medial calcaneal tunnel, II.)

the Achilles tendon, and III.) the dorsal surface of the first metatarsal. Tactile vibration perception

thresholds were evaluated at 1.) the plantar surface of the great toe, 2.) the plantar surface of the

3rd metatarsal, and 3.) the plantar surface of the 5th metatarsal.

Two distinct protocols were developed over the duration of the project, both implemented on

similar subject populations. In the first experimental protocol noise, 0-500Hz, was generated using

a piezo actuator and applied at all three noise input locations outlined. Plantar tactile perception

was evaluated using Semmes Weinstein monofilament tests at all three plantar locations outlined.

A two-sample T-test conducted on the data collected for each subject revealed no statistically

significant differences in monofilament scores between the control (no vibration applied) and

active (vibration applied) conditions. Several factors could explain the lack of findings from the

first experimental procedure and were considered when designing the second protocol. The factors

included minimal displacement of the piezo actuator, aspects relating to the use of monofilament

tests to quantify perception thresholds, and the way the piezo housing interfaced with the ankle.

The piezo actuator used, manufactured by Thor labs (model number PK4JQP2), had a maximum

displacement of 50µm; less than a tenth of a millimeter. While the motion the piezo produced was

high in force and perceptible to the most sensitive areas of the fingertips, the displacement of the

22

actuator was not observable to the naked eye and difficult to detect in less sensitive areas of the

skin. It was thought that this very small displacement could have contributed to the lack of any

observable experimental effect. Mechanical vibration applied at the surface of the skin attenuates

quickly in the skin. Surface displacement measurements of mechanical waves on the fingertip have

revealed a 90% reduction in peak to peak amplitudes of the signal 2cm away from the input

location (Kurita et al., 2011). Due to such small displacement and rapid attenuation in the skin, it

is possible that the applied signal was not propagating as deep or as far as required to induce any

stochastic resonance phenomenon in the somatosensory system. As such, when designing the

second protocol the piezo actuator was replaced with a C-3 tactor which promised a much greater

displacement, approximately 0.3-0.5mm, in the intended frequency range.

A second shortcoming of the first protocol outlined above involved the monofilament test used for

quantifying tactile sensory levels. Due to the binary nature of the test and the sublet affects

stochastic resonance exerts, it was thought that minor changes in sensory perception thresholds

could have remained undetected. Subjects had to indicate either yes or no to the perception of

discrete tests and the weight of the monofilament used was constant across all subjects. The exact

weight of the monofilament selected was based on subjective trial and error experiences to find a

weight that appeared to be on the borderline of being detectable by most individuals. However,

this assumed that plantar perception levels were similar across subjects and it quickly became

apparent that this was not the case. Thus, some subjects easily perceived the monofilament tests at

certain plantar locations and therefore could not have shown improvements even if stochastic

resonance was present. Conversely, for some subjects the monofilament size selected was far

23

below the sensory perception levels of certain plantar locations and any subtle effect provided by

stochastic resonance may have also gone undetected.

To circumvent these limitations, a new means of quantifying tactile perception levels that offered

a higher degree of fidelity was required. A C-3 tactor was implemented along with the Cornsweet

staircase method, described previously, to provide a more dynamic mechanism of measuring

vibration perception thresholds. Rather than one constant weight monofilament that subjects either

perceived or didn’t perceive, the amplitude of the vibratory signal produced by the tactor was

modulated on a continuum. By allowing each subject to arrive at individual perception thresholds,

the binary nature of the monofilament test was removed and variable baseline tactile sensitivities

were controlled for. The Cornsweet staircase method also provided a more reliable means of

ensuring that subjects accurately honed in on the true perception threshold. Rather than having

only four monofilament detection tests every time, the staircase protocol would continue to

administer detection tests until certain criteria were met before counting detection tests towards

the final reported detection threshold.

Alongside the reasons outlined, another advantage to using C-3 tactors as opposed to the piezo

actuator involved the physical matting of the vibratory element to the skin. The housing holding

the piezo actuator was large and bulky as can be seen in figure 4. As a result, the housing would

frequently shift out of position and require resituating back to the original location. This would

presumably introduce a small, yet present, uncontrolled variability into the data because the

actuator position could have potentially changed marginally. Thus, the C-3 tactor offered another

24

advantage: due to the tactor’s small size and flat shape it could be positioned easily against the

skin and secured with a wrap of medical tape (figure 11).

For these reasons, the piezo actuator and the Semmes Weinstein monofilament test were both

replaced with Engineering Acoustics C-3 tactors in the second protocol. Another change between

the first and second protocols was a reduction in the number of locations where noise was

administered and plantar sensory levels evaluated. Due to considerations in the length of time

required to conduct a Cornsweet staircase, and in an attempt to demonstrate the presence of

stochastic resonance in some capacity, only the most promising ankle and plantar locations were

selected for use in the second protocol. Because the medial plantar nerve innervates the 3rd plantar

metatarsal and runs through the calcaneal tunnel (Figure 2) those two locations were initially

chosen as the sites of noise administration and plantar sensory evaluation respectively.

However, the initial results from the first four data collections of the second protocol indicated that

no stochastic resonance effects were being observed. The second protocol was then further

modified to an even simpler form where the noise and stimulus signal were administered directly

adjacent from one another at the 3rd metatarsal. This was done out of necessity to demonstrate even

the simplest version of stochastic resonance phenomena where the noise and stimulus are directly

superimposed. Cutaneous tactile perception to a mechanical stimulus have been demonstrated to

be enhanced with mechanical noise directly overlaid onto the stimulus signal (J. Collins et al.,

1997). We were unable to replicate such a result using the C-3 tactors side by side; but of note was

that the only statistically significant data collected on any subject was from this condition.

25

The inability to demonstrate stochastic resonance phenomena in the second protocol likely resulted

from one major unforeseen shortcoming of the C-3 tactors. While larger in displacement than the

piezo actuators, the tactors produced significantly less force. Consequently, the amount of

displacement produced was directly dependent on how tightly the tactors were bound to the skin

with medical tape. If pressed too tightly against the skin the movement of the diaphragm of the

tactor would be impeded and little vibration would be transmitted into the skin. Because of the

force of attachment and displacement relationship of the tactors, there was very large variability

between how much vibration was being introduced into the skin at any given moment. Even a

slight repositioning of a subject’s foot or ankle could put tension on the medical wrap and attenuate

the vibratory signal. Thus, large uncontrolled variability in the magnitude of vibration introduced

into the foot would easily reduce the reliability of the Cornsweet staircase in determining vibration

perception thresholds.

With these considerations in mind the failure to demonstrate enhancement of plantar tactile

perception is disappointing but not entirely for naught. After two experimental iterations, a deeper

understanding of the practical challenges associated with psychophysical testing and a grasp of the

limitations of using vibrotactile devices to impart vibration into the skin have been gained. If a

future protocol could be developed that utilized a vibrotactile element that possessed both high

displacement and high force, perhaps the experimental challenges faced over the duration of this

project could be surmounted.

26

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