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POTASSIUM IONTOPHORESIS AS AN EXPERIMENTAL

PAIN STIMULUS: ITS PSYCHOPHYSICAL

CHARACTERISTICS AND ITS UTILITY FOR

INVESTIGATING THE SPINAL MODULATION OF PAIN

A Thesis presented in partial fulfilment of the requirements for the

degree of Doctor of Philosophy in Psychology

at Massey University

Steven Albert Humphries

1995

ACKNOWLEDGEMENTS

I would like to thank my fIrst superviser Professor Nigel Long for his helpful assistance,

practical advice and patience; Malcolm Johnson for his help and useful ideas; Dr John Podd

for believing in good experimental design; and Dr. Alan Winton for his constant support, and

for being enthusiastic about psychology.

I would also like to thank the people who gladly volunteered for the experimental studies.

A special thanks to Andrea who proof-read the draft. Thank you for helping me at the

important fmal stage.

And, finally, to my wife Sharon and family who provided love and understanding throughout

the research. Your support has helped me bring this thesis to fruition.

11

ABSTRACT

The present study investigated the psychophysical characteristics of potassium iontophoresis

and its suitability as an experimental pain stimulus. Experiment One investigated the optimal

duration of the pain stimulus for reliable reporting across repeated trials, and whether the

relationship between stimulus and subject response was linear, logarithmic or a power

function. Experiment Two determined the optimal inter-stimulus interval (lSI) for reliable pain

reporting and evaluated stimulus history effects, both in tenns of session effects and the effects

of immediately preceding stimuli. Experiment Three compared potassium iontophoresis with a

sodium iontophoresis control.

Linear functions described the stimulus-pain relationship best. No significant differences in the

goodness-of-fit coefficients of determination, correlations, or coefficients of variation, were

found for the stimulus durations of 1 , 2 and 4 seconds. Significant stimulus history effects

were found across a session, with adaptation and enhancement of responding for low and

moderate intensity stimuli respectively. The effects of the immediately preceding stimuli were

suppression or enhancement of pain response depending on the lSI, the preceding stimulus

intensity and the present stimulus intensity. Potassium iontophoresis was a significantly more

effective pain stimulus than sodium iontophoresis.

It was concluded that potassium iontophoresis is a convenient and reliable experimental pain

stimulus. It can be presented rapidly and repeatedly with minimal loss in consistency of

subject pain report. Potassium iontophoresis provides a technique for investigating the neural

modulation of pain in the relative absence of inflammation processes and tissue damage.

. The properties of potassium iontophoresis determined in Experiments One, Two and Three

indicated that it could be an ideal nociceptive stimulus for a quantitative analysis of some of the

spinal modulation mechanisms predicted by the gate control theory of pain. Clinical and

experimental support for the gate control theory of pain was overviewed.

111

According to the gate control theory of pain a peripheral stimulus that activated both small and

large-diameter afferent fibres would be perceived as painful, though there would be some

reduction in the intensity of the pain due to the inhibitory action of the large fibre activity.

The present study investigated a prediction of gate control theory that there would be a transient

increase in pain above that of the background level - a pulse of pain - as the pain stimulus was

being ramped off due to the large fibre activity at the spinal level falling away more quickly due

to the different peripheral conduction velocities of large and small fibres. A further prediction

was that the more distant the peripheral stimulus was from the spine the greater the pain pulse

would be for any given ramp-off rate. Supraspinal pain modulatory mechanisms were

overviewed but excluded as possibly obscuring the predicted pain pulse generated through the

ramping off of the peripheral nociceptive stimulus.

Fourteen subjects had the experimental pain stimulus of iontophoretically applied potassium

ions (K+) applied to an upper and a lower site on the dominant arm. Each stimulus trial

consisted of four seconds of constant pain followed by the stimulus being ramped off. In a

threshold detection task a double random staircase method was used to adjust the riunp-off rate.

Subjects were asked to indicate if they could detect a brief pulse of additional pain during this

ramp-off phase. Subjects were clearly able to detect a pulse of pain at both sites. The average

rate of stimulus ramp-off in order to detect a pain pulse was statistically greater for the upper­

arm site ( 14.3 J.1.g K+/s), than for the lower-arm site (9.4 J.1.g K+/s). The average ramp-off time

required �o generate a detectable pain pulse was 192 ms and 261 ms for the upper and lower

arm sites respectively.

These results were consistent with the predictions of gate control theory and our ramp-off

. model. Alternative explanations for the results, including intrinsic differences in nociceptive

responding for different dermatomes and anode break excitation, were considered.

IV

It was concluded that the detection of a pain pulse during the ramping off of a peripheral pain

stimulus potentially provides a quantitative measure of the spinal modulation of pain as

described by the gate control theory of pain. However, further studies would be required to

confirm the causal mechanisms that generate the observed pulse of pain.

v

TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION

1.0 In troduc tion ............................................................................ 1

1.1 The role of experimental pain stimuli in pain research.. . . . . . . . . . . . . . . . . . . . . . . . . . . 2

l .2 Requirements for experimental pain stimuli ............ ... . . ......... ...... ....... 4

1.3 Review of commonly used experimental pain stimuli ... ... .. .. . ..... . .... ....... 6

1.3.1 a) Thermal stimulation ........................... .................... ............. 7

l .3 .2 b) Cold Pressor stimulation...................................................... 7

1 . 3 .3 c) Mechanical pressure stimulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1 .3 .4 d) Ischemic stimulation ......... .. ......... .......... ............... ......... ..... 9

1 .3 .5 e) Electrical stimulation . .......................... . .............. .............. .. 10

l .3 .6 f ) Chemical stimulation.......................................................... 11

CHAPTER 2: DEPENDENT MEASURES

2.0 Dependent variable measures of pain ...... ....... . ............. ................... 1 3

2.0. 1 a) Physiological measures ...................................................... 1 3

2.0.2 b) Behavioural measures ........ .. ............... . . ......... ..... ............... 15

2.0.3 c) Signal detection theory measures ........................................... 15

2.0.4 d) Pain tolerance measures...................................................... 1 6

2. 1 Visual Analogue Scale (V AS) .... .. .. ..... .......................................... 16

2.2 Pain threshold measures ............... ............. .... .... .......... ......... .. .... 20

CHAPTER 3: POTASSIUM IONTOPHORESIS

3 .0 Potassium ions as a pain stimulus .. . . ... ............ .... ........................... 23

3 . 1 . Potassium iontophoresis as an experimental pain stimulus . ............. . ....... 25

3 .2 Prior research with potassium iontophoresis .............. ........................ 27

V I

TABLE OF CONTENTS CONTINUED

CHAPTER 4: OVERVIEW OF THE INVESTIGATION OF POTASSIUM

IONTOPHORESIS

4.0 General aims of the investigation of potassium iontophoresis .................. 29

4.1 Overview of the experimental studies .............................................. 29

CHAPTER 5: EXPERIMENTS ONE TO THREE - GENERAL METHOD

5.0 General method for Experiment One, Two and Three ........................... 31

5.1 S ubjects ................................................................................ 31

5.2 Apparatus ...................................................... ....................... . 31

5.2.1 VAS measure . . ... ...... .. . . . . . . . . . . . . . . .. . .. . . .... . . .. . . ........... . ..... . . .. . . . . 33

5.3 Procedure ....... . . ..... ... .... ...... .. .... . . . . . . .. . . . . . . .. . .. . .. .. . . ...... . . . . ... . . . . . . 34

5.3.1 Experimental design ...... .... ......... . . . . .. ....... ...................... . . . . . . . 37

5 .4 Statistical Analysis ................... . . .. . ....... . . . . . . . . ............................ 38

CHAPTER 6: EXPERIMENT ONE - STIMULUS DURATION

6.0 Introduction: The effects of stimulus duration on subj ect responding ... . . ... . 39

6.1 Method . .. . . . . . . .. . . . . . ... .. . '" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

6.2 Results and Discussion .. . ... . . ..... . . .. . . .. . . .. . . .. . ......... . . . ... . . . . . . . . .. . . .. . . . 41

CHAPTER 7: EXPERIMENT TWO - INTERSTIMULUS INTERVAL

7.0 Introduction: The effects of interstimulus interval on subject responding.. . . . 5 1

7.1. Method........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

7.2 Results and Discussion .... . . . . . . . . . . . .. . . . . . . . . . . .. . .. . ... . . .. . . ... .......... . . .. . . . 55

Vll

. - '

TABLE OF CONTENTS CONTINUED

CHAPTER 8: EXPERIMENT THREE - SODIUM CONTROL

8.0 Introduction: Potassium iontophoresis compared with a sodium control . . . . . 63

V Ill

8 .1 Method ................................................................................. 64

8.2 Results and Discussion .............................................................. 65

CHAPTER 9: GENERAL DISCUSION:

EXPERIMENTS ONE, TWO AND THREE

9.0 General discussion of Experiment One, Two and Three ........................ 69

9.1 Potassium iontophoresis and the requirements for an experimental

pain stimulus.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

9.2 The psychophysics of potassium iontophoresis . .. . ...... . . . ... .............. .. .. 73

.9.3 The use of potassium iontophoresis to investigate pain mechanisms . . . . . . . . .. 77

CHAPTER 10: OVERVIEW OF THE GATE CONTROL THEORY OF PAIN

1 0.0 Overview of the gate control theory of pain ....................................... 79

10.1 Clinical support for gate control theory..................... . . . . . . . . . . . . . . . . . . . . . .. 82

10.2 Transcutaneous Electrical Nerve Stimulation (TENS) . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

10.3 Dorsal column stimulation .......................... ......... .... ....... .... . ........ 87

lOA Peripheral vibratory stimulation . .... . .... ...... .... .. . ... . . ........... ......... . ... 88

10.5 Time course for spinal nociceptive processes .. . .... . ... . .. . . . .. .... ....... .. . .... 90

CHAPTER 11 : SUPRASPINAL INFLUENCES ON PAIN PROCESSING

1 1 .0 Supraspinal influences on spinal nociceptive processes .. . . . ... . . . . . . .. . ... .... . 93

1 1 .1 Descending supraspinal influences.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

1 1 .2 Ascending supraspinal influences ...... ..... . .......... . ......... ... . ....... .. ... .. 95

1 1 .3 Supraspinal controls and gate control theory . ..................................... 96

TABLE OF CONTENTS CONTINUED

CHAPTER 12: INVESTIGATION OF GATE CONTROL THEORY USING

POTASSIUM IONTOPHORESIS

12.0 Investigation of gate control theory by ramping off a potassium

iontophoretic pain stimulus ........ .................................................. 98

12.1 Mathematical simulation model of ramping off a potassium iontophoretic

pain stimulus ............ .......................................... ..... ............. . 104

12.2 Assumptions underlying the ramping off of potassium iontophoresis . . . . . . . 106

12.2.1 Assumptions involving sustained nociceptive neural processes . . . . . . . . . . . . . . . 108

CHAPTER 13: EXPERIMENT FOUR - INVESTIGATION OF THE GATE

CONTROL THEORY OF PAIN

13.0 Introduction: Investigation of the gate control theory of pain................. III

13.1 Method . . . . . . . . . . . . . . . . . . . . . .................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

13.2 Results and discussion: Time of ramp -off of nociceptive stimulus . . . . . . . . . . . 114

13.3 Results and discussion: Reaction times to nociceptive stimulus . . . . . . . . . . .... . 120

CHAPTER 14: EXPERIMENT FIVE - INVESTIGATION OF GATE

CONTROL THEORY WITH A SODIUM CONTROL

14.0 Introduction: Investigation of gate control theory with a sodium

control... . .. . . .. . ... ... . . ... ... . ... .. . ........... .. . . .. .... ........ .. . ....... . .. . . .. .. 126

1 4.1 Method . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 26

14.2 Results and discussion .. . . ...... . ... . . ..... . ..... . . . . . . . . . . . . . . . . . . . . . . . . . ... . .. ... 1 29

CHAPTER 15: GENERAL DISCUSION: EXPERIMENTS FOUR & FIVE

15 .0 General discussion of Experiment Four and Five . . . . . . . . . . . . . . . . . . . ... . . ..... . . 131

15.1 Conclusions and implications for future research . . . . . . . ... . . . . ... ... . . . . . . . . . . 132

IX

TABLE OF CONTENTS CONTINUED

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

APPENDIX A: Publication of Experiments One, Two and Three in the journal

'Perception & Psychophysics' ......................... ...... ............ . ... . 173

APPENDIX B: Subject consent fonn used in Experiment One .......... . . . ........ . . . ... ... 186

APPENDIX C: Subject medical check-list used in Experiment One ......................... 188

APPENDIX D: Questionnaire used in Experiment One............... . . . . . . . . . . . . . . . . . . . . . . . . 190

x

Figure 1:

Figure 2:

Figure 3:

Figure 4:

Figure 5:

Figure 6:

Figure 7:

Figure 8:

Figure 9:

Figure 10:

Figure 11:

LIST OF FIGURES

Depolarization of muscle fibre cell membrane potential as a function

of the log of extracellular K + concentration ... .............. ................ .. ... ... 23

Cross-section of human epidermis showing the iontophoretic transport

of ions across the epidermal barrier .................................................. 26

The placement of the anode bowl containing the potassium chloride gel

on the volar surface of the subject's arm, with the opposing cathode

silver plate on the dorsal surface of the arm ..... .............. . ... .. ... . ........... 32

Visual analogue scale (VAS) - sensory intensity scale............................. 33

Three-key control panel for moving the cursor on the computer

VDU presented VAS ................................................................... 34

VAS pain responses as a function of stimulus intensity. Experiment One. . . . .. 46

The change in ratio scores as a function of lSI, intensity level of the present

stimulus, and intensity level of the immediately preceding stimwus in

Experiment Two .......................................................... . . .. . . . ....... 59

The linear regression lines for each subject for sodium and potassium

iontophoresis in Experiment Three ...................... , . . .. . . .. . . .. . ... . . .. . . . . . . .. 67

Schematic diagram of the gate control theory of pain mechanisms, as

proposed by Melzack and Wall (1965) ..... .................. .................... .... 80

The rate of applied K+ 'seen' at the spinal level while a peripheral

nociceptive K+stimulus is ramped off over 300 ms . .. .. .. . . ... . . . ........ . . ... ... 103

The rate of applied K+ 'seen' at the spinal level while a peripheral

nociceptive K+stimulus is ramped off over 100 ms ............................... 104

Figure 12: Mathematical simulation of T cell activity in response to peripheral inputs ..... 105

Figure 13: Double random staircase threshold detection of the pain pulse for Subject

One and Subject Two ................................................................. 116

Figure 14: Double random staircase threshold detection of the pain pulse averaged

over all subjects ........................... ................................ ............ . 1 17

Xl

LIST OF TABLES

Table 1: Coefficients of determination (r2) for linear, logarithmic and power

functions for the relationship between iontophoretic potassium stimulus .......... 43

Table 2: Mean ratio scores, mean VAS responses and Range of VAS means as

a function of stimulus duration. Experiment One .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Table 3: Coefficient of determination (r2) for linear, logarithmic and power

functions for the relationship between iontophoretic potassium stimulus

and pain report on the pain-intensity VAS . Experiment Two . . . . . . . . . . . . . . . . . . . . . . . 55

Table 4: Ratio scores and VAS scores (in brackets) as a function ofISI, session

block and stimulus level. Experiment Two ...... . . . '" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Table 5: Ratio scores and VAS scores (in brackets) as a function of the present and

immediately preceding stimulus pain levels. Experiment Two ..... .......... . . . ... 58

Table 6: Coefficients of determination (r2) for linear, logarithmic and power

functions for the relationship between iontophoretic potassium stimulus

and pain report on the pain-intensity VAS for both the potassium and

sodium groups. ExperiInent Three .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . 65

Table 7 : Average VAS pain response for sodium and potassium iontophoresis.

Experiment Three . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Table 8: Applied stimulus level and the ramp-off rate for the upper and lower

arm sites to produce threshold detection of a pain pulse. Experiment Four ...... 118

Table 9: Median reaction times to the perceived pain pulse. Experiment Four ......... . . . . 122

Table 10: Ramp rate required to produce a perceptible pai� pulse for K+ and Na+

stimulation. Experiment Five .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . ............ . . . . . . 129

xu