A Validation of Pain Assessment Using the Visual Analog Scale and Ratio Production Methods
Brian P. Dyre Intellective Consulting and Services, LLC
Nicholas Roome and Tristen Beaudoin University of Idaho
Prepared By: Intellective Consulting Limited, 667 Indian Hills, Moscow, Idaho 83843 PWGSC Contract Number: W7719-165316/001/TOR Technical Authority: Justin Hollands, 416-635-2073
Disclaimer: The scientific or technical validity of this Contract Report is entirely the responsibility of the Contractor and the contents do not necessarily have the approval or endorsement of the Department of National Defence of Canada.
Contract Report DRDC-RDDC-2017-C055 DRDC Date of Publication: March 2017
© Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2017 © Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale, 2017
PAIN METRICS 1
A Validation of Pain Assessment Using the Visual Analog Scale and Ratio Production Methods
Brian P. Dyre
Intellective Consulting and Services, LLC
Nicholas Roome and Tristen Beaudoin
University of Idaho
Technical Report Prepared for:
Defence Research and Development Canada (DRDC)
Contract No. W7719-165316/001/TOR
Aircrew Strain Metrics
DRDC Toronto Research Centre
1133 Sheppard Ave. W., Toronto, ON M3K 2C9 PAIN METRICS 2
Abstract
This research aimed to support development of a valid and reliable psychophysical pain measure with ratio scaling properties. Such a measure is important for assessing the effectiveness of design and procedural interventions introduced to reduce neck strain/pain in Griffon helicopter aircrew. This study included two experiments. Experiment 1 determined whether productions of pain ratios in a dual-stimulus procedure possess ratio scaling properties by testing whether the mathematical property of commutativity is maintained in ratio production of pain. We found commutativity to be maintained, which implies the underlying representation of pain in a pattern of produced ratios of pain has ratio properties. Experiment 2 aimed to determine whether visual analog scales (VAS) can reliably represent these ratio pain productions. In this experiment, we presented participants an experimentally-induced pain stimulus and a numeric ratio. Participants then adjusted a second pain stimulus to represent the given ratio of pain relative to the first stimulus. We then retested participants with the pain stimuli they produced at different ratios and had them rate the pain intensity using VAS. The results demonstrate that VAS scores are linearly related to the produced ratios of two independent pain stimuli. These results suggest that pain can be represented on a ratio scale and that VAS scores can validly represent this underlying ratio scale. Because percent change in VAS pain responses are a reasonable representation of percent changes in actual pain levels experienced, we conclude that the VAS is a convenient and accurate method for assessing interventions aimed at pain reductions in aircrew. PAIN METRICS 3
A Validation of Pain Assessment Using the Visual Analog Scale
and Ratio Production Methods
This research examined a novel dual-stimulus ratio-production approach to measuring subjective pain to develop a valid and reliable psychophysical pain measure with ratio scaling properties. Such a measure is needed to assess the effectiveness of design and procedural interventions introduced to reduce neck strain/pain in Griffon helicopter aircrew, many of whom are being benched or grounded due to neck pain.
Justification of Approach
Physiological indicators of pain are not useful for evaluating design or procedural interventions aimed at reducing pain. Musculoskeletal injuries can result in chronic pain that endures long after injured tissues have finished healing and the peripheral nociceptive activity has returned to normal. Chronic pain typically involves a neuropathic component: pain sensations are the result of damage to peripheral nerves or to the central nervous system (CNS) itself. Neuropathic pain is considered to be a symptom of neuropathic disease, a disease whose most reliable indicator is the report of a subjective pain experience by a patient in the absence of a nociceptive stimulus or obvious damage to peripheral nociceptors or C and A-delta nerve fibers
(Cervero, 2009). It is a sad irony that while the subjective effect of chronic pain is profound and obvious to the sufferer, the objective effects on central nervous system physiology are exceedingly difficult to measure. Indeed, subjective reports of pain are the primary diagnostic tool for assessing neuropathic pain (Baron, 2009).
For any disease, our abilities to diagnose and evaluate treatment are limited by the reliability and validity of our measures of disease symptoms. Pain presents a particularly challenging disease symptom to measure due to its inherent subjectivity, multi-dimensional nature, and unclear scientific classification (a percept vs. a state; Cervero, 2009). Pain is a PAIN METRICS 4 multidimensional construct comprised of quality (e.g., burning, stabbing, throbbing) and intensity. The focus of this report is the measurement of pain intensity rather than quality.
Measurement of the suprathreshold subjective intensity of pain can be achieved using psychophysical scaling techniques such as magnitude estimation and cross-modality matching.
For magnitude estimation, many clinicians and researchers employ discrete categories where a sufferer indicates pain intensity on a fixed 10 or 20 pt. bounded scale, labeled numerically, and sometimes including propositions (Brennum et al., 1992; Melzack, 1975). Common cross- modality matching tasks ask sufferers to adjust grip strength or adjust the spatial placement of a point on a line to indicate a scale value (a visual analogue scale or VAS). These methods all share a common feature: the two end points of the scales provide bounds that—in principle— serve to enhance the reliability of numerical estimates or produced matches.
However, it is probable that fixed bounds at the ends of the response scales (referred to by psychophysicists as moduli with constant subjective values) sacrifice validity of measurement for reliability; that fixed anchors increase the consistency of responses, but at the cost of potentially misrepresenting the underlying subjective state by imposing a particular number system on respondents (Fernandez et al. 1991). For example, a participant may experience a strong stimulus early in a set of trials and assign it a very high value (or grip as hard as they can), not knowing that an even stronger stimulus is possible. When the stronger stimulus is perceived, a ceiling effect artificially compresses the pain measure associated with that stimulus.
Conversely, when sufferers or participants are presented a limited range of pain intensities, they might expand their responses to better fit the range of the scale (Gracely & Eliav, 2009).
Consistent with these observations, Hollands and Dyre (2000) showed that scale boundaries significantly bias judgments of proportions, and that changes in the locations of boundaries will affect the magnitude and direction of those biases. PAIN METRICS 5
A more valid ratio scale of stimulus intensity may be obtained using a free modulus procedure, where participants are given no anchors and are encouraged to assign whatever numeral they feel appropriately represents a particular stimulus (Stevens, 1975). Such scales avoid the bias created by fixed scale bounds. However, Stevens’ (1975) assumption that such numerals represent an internal ratio scale of stimulus intensity has been shown repeatedly to be false: participants use and interpret numerals in a manner that violates the properties of ratio scales (Ellermeier & Faulhammer, 2000; Zimmer, 2005). In effect, numerals are simply one more type of stimulus that generates its own unique internal intensity scale (Narens, 1996; Luce,
2002). Thus, even for free modulus magnitude estimation tasks, the numerals that are reported do not necessarily represent the perceived stimulus intensity. Traditional subjective pain measurement techniques such as single stimulus magnitude estimation are analytically incapable of separating the effect of subjective response scales from the internal experience representation of subjective pain. As a result, such techniques can only produce pain scales with ordinal (rather than ratio) properties, and cannot be used to provide estimates of changes in pain intensity that are precise enough to test measures introduced to address neck strain in aircrew.
Clearly a more valid method of assessing a pain sufferer’s true subjective experience of pain independent of their concepts of numerals or other response scales is needed.
Objectives
This study aimed to provide data to help develop a valid and reliable psychophysical pain measure with ratio scaling properties to assess the effectiveness of design and procedural interventions introduced to reduce neck strain/pain in Griffon helicopter aircrew. One commonly-used measure of pain is the visual analog scale (VAS; Price et. Al. 1983). The VAS presents a horizontal line bounded at the left end with the label “no pain” and at the right end with the label “maximum pain intensity imaginable.” Participants place a mark on the line that is PAIN METRICS 6
presumed to represent the percentage of “maximum pain intensity imaginable.” Price et al.
(1983) tested VAS scores for 2 thermal pain stimuli, a standard and a second stimulus intensity
produced by the participant to represent twice (or two-times) the standard intensity. They found
that a doubling of the stimulus led to a doubling of the VAS scores and concluded the VAS to be
a valid ratio scale.
However, this conclusion was based on the untested assumption that ratio pain productions themselves represent a ratio scale. Narens (1996) provided the theoretical basis for establishing whether this assumption was true by testing scaling responses for the properties of commutativity and multiplicativity. The underlying scale is ratio only if commutativity holds for a set of ratio productions. If multiplicativity also holds, then the underlying scale is not only ratio, but the participant’s understanding of numerals representing ratios is also valid. Ellermeier
and Faulhammer (2001) and Steingrimsson and Luce (2007) have demonstrated that for stimulus
domains of loudness and brightness, respectively, commutativity holds, but not necessarily multiplicativity, implying that the intensity of these sensations is represented as a ratio scale, but that participants use of numerals to represent ratios is not necessarily accurate.
Experiment 1 extends these methods to the sensory domain of pain with the aim of determining whether pain is fundamentally represented on a ratio scale. If commutativity fails to
hold for ratio pain production, this implies all attempts at scaling pain on a ratio scale are in vain:
the underlying representation of pain simply does not maintain ratio properties. Hence,
demonstrating the property of commutativity in pain ratio productions is a necessary first step in
establishing a method of measuring pain on a ratio scale.
Even if commutativity is found to hold for pain a second issue remains: How do the
temperatures produced during ratio production map onto meaningful units of pain? To address
this issue, we conducted a second experiment aimed at determining the relationship between the PAIN METRICS 7
VAS, defined by meaningful end-points (“no pain” and “maximum pain imaginable”) and a
vector of temperatures produced during productions of various ratios. If we find commutativity
holds for ratio productions in Experiment 1 and that the function relating temperatures to VAS
scores in Experiment 2 is multiplicative (e.g., linear or a power function), then percent changes in VAS scores will correspond to percent changes in the intensity of pain. Such a result will validate the VAS as a ratio measure of subjective pain.
Experiment 1: Multiplicativity and Commutativity of Ratio Productions of Pain
Method
Participants. We used notices posted around the University of Idaho campus and the city of Moscow, Idaho, as well electronic advertisements to recruit 7 participants who self-reported no current experience of chronic pain. A copy of the posted advertisement is included in
Appendix A. Participants ranged in age from 21-50 years, with a mean of 27 years. Three (3) of the participants were male and 4 were female.
We screened participants before testing by asking these questions regarding their health to ensure their eligibility for the study:
1) Are you currently pregnant?
2) Do you have hereditary risk or personal history of cardiovascular disorders?
3) Do you suffer from chronic pain?
Testing only proceeded if participants responded “No” to all three questions. Prior to testing, all screened participants read and signed an informed consent form describing the study in detail. A copy of this form is included in Appendix B.
All participants were treated in accordance with accordance with the ethical guidelines published by the American Psychological Association (APA) and the United States National
Institutes of Health (NIH). Our research protocol was approved by the University of Idaho PAIN METRICS 8
Institutional Research Board and the Defence Research and Development Canada (DRDC)
Human Research Ethics Committee.
Stimuli and Apparatus. We produced pain sensations using Nocistim, a computer-
controlled dual-thermode thermal stimulator based on the analogue system described by Morrow
and Casey (1981) and developed and built by Intellective Consulting & Services, LLC (ICS).
Nocistim is controlled by a host Microsoft Windows PC running the software package,
Nociscale, also developed by ICS. Together, these components afford precise presentation of two
independently-controlled thermal stimuli over a range of temperatures from 25 to 50 degrees
Celsius (C). Nocistim can raise the temperature of each thermode by 2 C per second, and
lowering the temperatures by 1 C per second. Ten-bit thermistors control the instantaneous temperatures of each thermode to within 0.5 C of the target temperature at a frequency of 10 Hz.
Smoothing over a 1-second square window, temperatures varied by 0.1 C or less. Nociscale provides the ability to use several basic psychophysical methods including threshold measurement (method of adjustment, method of limits) and suprathreshold scaling, including dual-stimulus single-response methods and the VAS. One important feature of Nociscale is that it can repeat a sequence of temperatures produced during a suprathreshold ratio production task
for later assessment using suprathreshold magnitude estimation.
There are several important safety features built into Nocistim. First, the firmware
programmed into the digital controllers for each thermode ensures temperatures will always be
equal to or less than 50 C. Nocistim also includes hardware for independent testing and
calibration of thermode temperatures. Finally, an “emergency stop” button is available to
participants during psychophysical testing that, when pressed, physically interrupts the power
circuits heating both thermodes, which then immediately begin to cool from circulating water. PAIN METRICS 9
In addition to controlling the temperature of each Nocistim thermode, Nociscale also
presents experimental instructions and visual stimuli to participants and provides real-time
feedback to the experimenter on current temperature levels and participant responses. Participant
and experimenter feedback/instructions can be located on separate monitors. The primary
response device for the Nocistim is an infinite dial/button that can be used to continuously adjust the temperature of either thermode for production tasks or the value of a number presented to participants by Nociscale for an estimation task. Optionally, dual grip force measures and keyboard entry can be used for either producing thermode temperatures or estimating numeric values. Figure 1 illustrates a typical experimental setup of the Nocistim.
Procedure and Experimental Design. After completing the consent form and demographic survey, the experimenter described the safety procedures for the experiment.
Participants were informed that they would experience pain during the experiment, and that if they felt the pain became too intense, they could press a large red button located in front of them to cut off all power to the thermodes and provide immediate relief. We also notified the participants that the thermodes edges could irritate their skin if dragged and urged participants to not try to remove the thermodes themselves. Next, the experimenter described the dial-button response device, and that turning the dial clockwise would increase the magnitude of their response or pain intensity, while turning the dial counter-clockwise would decrease the magnitude of their response or pain intensity. The experimenter further informed participants that pressing the knob down would enter their response into the data log and end the trial.
After reviewing the safety procedures and response device, the experimenter asked for verbal permission to touch the participants’ arms, as well as mark their arms with a felt-tipped marker. Once the participant provided consent, the experimenter placed a metal template that shared the same dimensions as the bottom of the thermodes onto the test location at the center of PAIN METRICS 10
their upper left volar forearm, close to the inside of their elbow joint. These marks allowed the
experimenter to place the thermode in the same location every trial. This process was repeated
for the same location on the right arm.
The experimenter then placed the thermode onto the participant’s left arm, ensuring enough slack in the armband to avoid any non-thermal discomfort, and measured the participant’s pain threshold using the method of adjustment. Before the threshold measurement began, Nociscale presented these instructions to the participant who read them as the experimenter described them aloud and answered any questions:
In this part of the experiment we measuring your pain threshold - the point at which you start to feel pain.
Adjust the temperature with the knob so that you just barely feel pain and if you turn the knob slightly counterclockwise the pain disappears. When satisfied with your adjustment, press the knob to enter your response.
The odd numbered trials will start with a low temperature and you will need to turn the knob clockwise to increase the temperature until you just barely feel pain.
The even numbered trials will start with a high temperature that should produce pain and you will need to turn the knob counterclockwise to decrease the temperature until you just barely feel pain.
Once near your threshold for pain, we want you to make fine adjustments back and forth until you reach a setting that if you turn the knob just slightly counterclockwise the pain is eliminated.
If you have any questions, please ask the experimenter.
Inform the experimenter when you are ready to begin.
The experimenter also urged the participants to try to reliably distinguish between heat
and pain. Each ascending and descending trial also provided this instruction:
Adjust the temperature with the knob so that you just barely feel pain and if you turn the knob slightly counterclockwise the pain disappears.
PAIN METRICS 11
The ascending trials started temperatures at 35 C and participants turned a dial clockwise to increase temperature until they just barely noticed a pain sensation, whereupon they pressed down on the dial/button to enter their response. Descending trials then started temperatures at 44
C and participants turned the dial counter-clockwise until they just barely noticed a pain sensation, and again pressed the dial/button to enter their response. We computed the left arm pain threshold temperature by averaging the ascending and descending trials.
After measuring the participants’ left arm pain threshold, a temperature was determined that produced pain approximately one just noticeable difference (JND) above threshold (JND1).
To determine the JND1 pain stimulus, the experimenter first adjusted the temperature of the left arm thermode to the participant’s threshold. Participants then adjusted the temperature upward until they could just barely notice a difference in pain. Once the participant identified this temperature, they pressed the button to enter the temperature for JND1. This pain level was used during the ratio production trials as the baseline pain level.
The participant then proceeded to the ratio production task, which presented blocks of 8 experimental stimuli. We provided participants the following instructions:
We want you to adjust the temperature of the pain stimulator on the indicated arm such that the pain intensity you feel on the indicated arm is related to the pain intensity on your other arm by the ratio presented to you on each trial.
Use the knob to adjust the temperature of the stimulator. Turning the knob clockwise will increase temperature (pain) and turning the knob counter-clockwise will decrease the temperature (pain).
Once you are satisfied with your adjustment, press the knob to enter your response. If you have any questions ask the Experimenter.
Please inform the Experimenter when you are ready to begin.
The following instruction then preceded each ratio production trial:
PAIN METRICS 12
3 to 1 [or one of the other ratios]
Once you are satisfied with your adjustment, press the knob to enter your response.
The first trial of each block presented the baseline pain level on the left arm and a 35 C
stimulus to the right arm. Participants then adjusted the temperature presented to the right arm
using the dial until the pain it produced stood in a 1:1 ratio with respect to the pain experienced
in the left arm (a match). Trials 2 and 3 presented ratios of 2:1 and 3:1 (in random order) and
participants adjusted the temperature presented to the right arm such that the pain it produced
stood in the presented ratio to the pain experienced in the left arm. Trials 4 and 5 then presented
the adjusted right arm temperatures produced on trials 2 and 3 in random order and asked
participants to adjust the left arm temperature until the pain experienced matched (1:1 ratio) that
experienced in the right arm. These five trials thus produced three levels of left arm pain: 1x, 2x, and 3x the baseline pain level. Trials 6-8 then presented these three pain levels to the left arm, again in a random order, and instructed participants to adjust the pain in their right arm to be either 6x, 3x, or 2x, respectively, the pain in their left arm. Two minute breaks were given between each trial to allow the affected skin region to cool and reduce peripheral adaptation. We instructed participants to take no longer than 15 seconds to respond to reduce thermal exposure and encouraged participants to take breaks from testing whenever necessary.
Originally, we intended each participant to complete a total of 15 blocks of trials.
However, two participants experienced mild blistering at test locations during later experimental sessions, and the experiment needed to be terminated prematurely due to this issue. Hence, each
participant completed only a subset of the 15 blocks, discussed further in the results section.
Following the ratio productions, we asked participants to complete a short debriefing
survey (see Appendix C) that asked questions about the pain they experienced, as well as a check
for reactivity to see if participants became aware of our experimental design through testing. PAIN METRICS 13
Once the participants completed the survey, the experimenter read the debriefing script explaining the experiment’s purpose aloud to them as they read along. The experimenter then asked the participants if they would like to be notified of the results, and paid the participants, ending the session.
Results
Threshold and JND1 temperatures. Table 1 lists the threshold and JND1 temperatures established for the seven participants in Experiment 1 along with descriptive statistics. Except for Participant 2, the JND1 temperatures were roughly 0.1-0.6 degree higher than the threshold temperatures, suggesting differences in temperature of fractions of a degree were discernable to most participants. For Participant 2, the JND1 temperature was 2.66 degrees higher than the threshold temperature. Participant 2 also recorded both the lowest threshold temperature and the highest JND1 temperature of the entire sample, suggesting this participant’s responses were generally less precise than those from the remainder of the sample.
Sensitization or Tolerance? To assess whether participants became sensitive to pain throughout the experiment, we computed mean temperatures set by each participant for each block of eight trials and conducted linear regressions of these mean temperatures as a function of block. Figure 2 shows these data for each participant along with the best-fit linear functions.
Five of the seven participants showed increases in mean temperature across the eight blocks with best-fit slope parameters of 0.178, 0.336, 0.233, 0.172 and 0.64 degrees per block. These participants may have become less sensitive to pain as the experiment proceeded. Linear regressions for the remaining two participants (2 and 5) were essentially flat, suggesting no overall change in sensitivity.
Overall, it appears that some degree of pain tolerance developed over the course of the experiment for most participants. This trend indicates that any tests for multiplicativity and PAIN METRICS 14
commutativity that compare adjusted temperatures for different ratios should be conducted within each block so that the variance across blocks due to increased pain tolerance is removed from the error term of the analysis. This is important because multiplicativity and commutativity predict non-significant differences between different ratios. Any increase in variance in the error term of the analysis would thus increase the probability of finding no differences between conditions. Previous research (Ellermeier & Faulhammer, 2000; Steingrimsson & Luce, 2007)
used the non-parametric Wilcoxon U-test to test for multiplicativity and commutativity. Instead
we compared the temperatures produced for different ratios using paired-samples t-tests so that
the temperatures set for each ratio were compared only to other trials within the same block.
Using a paired-samples t-test to compare ratio within blocks increases the power of the analysis
and makes it more likely that violations of commutativity and multiplicativity will be identified.
Multiplicativity and Commutativity. To test whether multiplicativity and
commutativity held for each participant, we compared the temperatures produced for the last
three trials of each block, where participants were presented the temperatures they produced to
represent 1X, 2X, and 3X the JND1 temperature on their left arm and adjusted the right-arm
thermode to form pain ratios of 6X, 3X, and 2X, respectively, relative to the left arm. We label
these three trials as 6X(L1X), 3X(L2X), and 2X(L3X), respectively, with the “L” denoting that
the base stimuli that are located on the left arm, and the participant is adjusting the right-arm
intensity to form the given ratio relative to the left arm. If commutativity holds for pain
production, then the temperature produced for 3X(L2X) should not differ significantly from the
temperature produced for 2X(L3X), just as 2 x 3 = 3 x 2. If multiplicativity holds for pain
production, then 6X(L1X) should not differ significantly from 3X(L2X) or 2X(L3X) (6 = 3 x 2 =
2 x 3). A word of caution is in order here because the commutativity and multiplicativity
conclusions are based on accepting a null (non-significant) result, which could be due to other PAIN METRICS 15
factors such as a lack of statistical power due to a small sample size. Indeed, if all comparisons
are non-significant the simplest explanation is that the experimental design and analysis simply failed to have sufficient statistical power to reliably detect the differences between conditions.
As mentioned above, we chose to analyze the data within blocks using paired-samples t-tests to maximize statistical power for finding violations of commutativity and multiplicativity.
Table 2 and Figure 3 present the results of the comparisons for multiplicativity and
commutativity. For all participants, the t-tests assessing commutativity produced non-significant
results. The sample of temperatures produced for 2X(L3X) was statistically indistinguishable
from the sample produced for 3X(L2X). However, the t-tests assessing multiplicativity determined that for some participants, temperatures produced for 6X(L1X) did differ significantly (p < .05) from those produced for 2X(L3X) and 3X(L2X). Participants 5, 6, and 7 produced 6X(L1X) mean temperatures that were 0.99 to 4.49 C greater than those produced for
2X(L3X) and 3X(L2X). These statistically reliable differences suggest that while the sample sizes for these comparisons were small, there was still sufficient power for detecting violations of multiplicativity. Overall, these results support the conclusion that there is no evidence that pain production violates commutativity for any participant, but clear evidence for violations of multiplicativity.
Discussion
Taken altogether the results of Experiment 1 indicate that our internal scale of subjective pain preserves the commutative property, a property that is necessary and sufficient for demonstrating an internalized ratio scale (Narens, 1996). Thus, human adjustment of pain intensity validly maintains ratio scale properties. While this conclusion is based on accepting null differences between commutative conditions (2 x 3 = 3 x 2), the violations of multiplicativity found in Experiment 1 (6 x 1 ≠ 3 x 2 or 2 x 3) show that the experimental design PAIN METRICS 16
and analysis did have sufficient power to detect differences between conditions. Hence, we
believe it reasonable to conclude that the non-significant differences between 2X(L3X) and
3X(L2X) is due to commutativity holding, and not a lack of statistical power to detect violations
of commutativity.
Further, the significant violations of multiplicativity for three of our seven participants
suggest that not all participants are able to accurately use numerals to represent particular ratios
(Narens, 1996). Ellermeier and Faulhammer (2000) and Steingrimsson and Luce (2007) found a
similar pattern of results for ratio productions of auditory loudness. Commutativity of loudness
productions held for all participants, while multiplicativity was violated for a subset of
participants. Hence, it appears that like loudness, pain is experienced on an internalized ratio
scale, but that research participants do not necessarily express that ratio scale accurately with
numerals representing ratios. This has important implications for the use of numbered pain
scales in clinical assessments of pain intensity. We cannot simply assume (as Stevens, 1957,
did) that participants can accurately produce internalized sensations consistent with numerical
ratios or that numerals assigned to represent sensory magnitudes are valid numbers on a ratio scale. PAIN METRICS 17
Experiment 2: Relationship of VAS Ratings to Ratio Productions of Pain
The results of Experiment 1 indicate that our internal experience of pain preserves ratio scale properties but that we do not necessarily assign numerals to represent these internal sensations on a valid ratio scale. If numerals assigned by participants are suspect, could cross- modality measures like the VAS validly represent the ratio properties of our experience of pain intensity? The aim of Experiment 2 was to explore the relationship between the VAS, a relatively simple and easy measure to obtain, with the internal ratio scale associated with pain productions.
Method
Participants. We used notices posted around the University of Idaho campus and the city of Moscow, Idaho, as well electronic advertisements to recruit 10 participants (6 male, 4 female) who self-reported no current experience of chronic pain. A copy of the posted advertisement is included in Appendix A. Participants ranged in age from 19-62 years, with a mean of 28.9 years.
Screening and treatment of participants in Experiment 2 was identical to Experiment 1.
Stimuli and Apparatus. The stimuli and apparatus for Experiment 2 were identical to those used in Experiment 1 except that the ratios presented to participants were 1:3, 1:2, 1:1, 2:1, and 3:1.
Procedure and Experimental Design. Experiment 2 used the same consent and instructional procedures as Experiment 1. However, the experimental design and procedures for measuring pain responses in Experiment 2 differed from Experiment 1 in several important aspects. First, two locations on each volar forearm were tested. The first test location was identical to Experiment 1 at the center of their upper left volar forearm, close to the inside of their elbow joint. A second test location was determined by adding 1.5 inches of distance
(measured with a ruler) toward the bottom of the left forearm and marked. These marks allowed PAIN METRICS 18
the experimenter to easily place the thermode in the same location every other trial. This process
was repeated for two locations on the right arm.
The experimenter then placed the thermode onto the participant’s left arm in Location 1, ensuring enough slack in the armband to avoid any non-thermal discomfort, and measured the participant’s pain threshold using the adjustment procedure as Experiment 1. This process was then repeated for Location 2, and then for the same two locations on the right volar forearm.
After measuring the participants’ pain thresholds at each of the two stimulus locations on each arm, a temperature was determined for the two right-arm locations that produced pain approximately four JNDs above threshold. This pain level was used during the ratio scaling trials as the level of simulated chronic pain, and was established with the following procedure. The experimenter first adjusted the temperature of the thermode to the participant’s threshold.
Participants then adjusted the temperature upward until they could just barely notice a difference in pain. Once the participant identified this temperature, they pressed the button to enter their response for the first JND. The experimenter then adjusted the starting temperature of the next trial to the response on their previous trial, and the participant again adjusted the temperature upward until they could just barely notice a difference in pain and entered the second JND. The experimenter repeated this process two more times to obtain a temperature four JNDs (JND4) above threshold, which was used to simulate chronic pain in the right arm during the scaling trials. This process occurred at both stimulus locations.
Following threshold and JND4 measurements participants matched acute pain produced by the thermode on their left arm to the simulated chronic pain induced by the two JND4 temperatures presented to their right arm. Participants adjusted the temperature of the thermode on their left arm until it matched the pain experienced on their right arm. Measurements were taken with both thermodes in Location 1 and then again in Location 2 on their respective arms. PAIN METRICS 19
The participant then began the ratio production task. This task presented exposures of the
simulated chronic pain (JND4) temperatures to the right arm along with a 35 C temperature to
the left arm. Participants then adjusted the temperature presented to the left arm using the dial
until the pain it produced stood in the ratio provided on the stimulus display screen with respect
to the simulated chronic experienced in the right arm. Before the ratio production task started,
the participant read the following instructions (the experimenter repeated the instructions
verbally answered any questions):
We want you to adjust the temperature of the pain stimulator on your left arm so that the pain intensity you feel on your left arm is related to the pain intensity on your right arm by the ratio presented to you on each trial.
Use the knob to adjust the temperature of the stimulator. Turning the knob clockwise will increase temperature (pain) and turning the knob counter-clockwise will decrease the temperature (pain).
Once you are satisfied with your adjustment, press the knob to enter your response.
If you have any questions ask the Experimenter.
Please inform the Experimenter when you are ready to begin.
The following instruction then preceded each ratio production trial:
Turn the knob to adjust the temperature of the stimulator on your left arm so that the pain intensity it produces stands in the ratio below relative to the pain you feel on your right arm.
3 to 1 [or one of the other ratios]
Once you are satisfied with your adjustment, press the knob to enter your response.
Each trial presented one of five different ratios: 1/3 to 1, 1/2 to 1, 1 to 1, 2 to 1, and 3 to
1. The participants produced each of the five ratios at each of the two JND4 temperatures at each of the two locations for a total of 20 production trials (5 x 2 x 2). Location varied from trial to trial with an interstimulus interval of approximately 30 seconds; thus, each location was tested PAIN METRICS 20
only once per minute to reduce peripheral adaptation. Nociscale determined a unique random
order of the ratios and JND4 temperatures for each location and each participant. We instructed
participants to take no longer than 15 seconds to respond to reduce thermal exposure and
encouraged participants to take breaks from testing whenever necessary. In addition, participants
were asked every 5 trials to consider the need for a short break.
Following the ratio production task, the participants were given a short break before engaging in a cross-modality matching task using the VAS. We determined stimulus magnitudes and order for the magnitude estimation based on the adjusted temperatures produced during the ratio production task. Thus, participants received the same temperature sets in the same order produced during ratio production during the magnitude estimation task. For each stimulus pair presented on a trial, participants were told to indicate on each of two VASs the pain level
associated with the left and right arms, independently. Before the task, participants read these instructions:
Please click on the VAS for LEFT and RIGHT
If you have any questions ask the Experimenter.
Please inform the Experimenter when you are ready to begin.
Each trial was preceded by this instruction:
CLICK VAS
Once participants clicked the VAS the values were entered into the data log and proceed to the
next trial.
Similar to the previous task, we instructed participants to take no longer than 15 seconds
to respond and inquired every 5 trials if they desired a break from testing. After the second task,
we instructed the participants to complete a short debriefing survey (see Appendix C) that asked
questions about the pain they experienced, as well as a check for reactivity to see if participants PAIN METRICS 21
became aware of our experimental design through testing. Once the participants completed the
survey, the experimenter read the debriefing script explaining the experiment’s purpose aloud to them as they read along. The experimenter then asked the participants if they would like to be notified of the results, paid the participants and concluded the experiment.
Results
Thresholds and Pre-Suprathreshold Scaling JND4 values. Table 3 provides a summary of the thresholds measured across the two locations on the left and right arms, the
means and standard deviations of these thresholds within participants, and the JND4 values
established for the right arm locations. These data were collected prior to the ratio production
and VAS cross-modality matching (scaling) tasks.
Two results are worth comment. First, while thresholds varied across participants at each
location, there was less variability within participants across the four locations, which
underscores the importance of examining pain data on an individual basis rather than aggregating
across participants. Second, on average, JND4 values were only about 1.2-1.3 C higher in
temperature than their respective threshold values at the same location.
Stability of ratio production and VAS over trials. Here, we examine whether the
suprathreshold scaling data are valid and reliable by assessing whether participants shifted their
ratio productions and VAS responses across the 20 trials. Figure 4 shows the temperatures
produced and VAS responses averaged across all participants as a function of trial along with
linear regressions representing the trend in these averages. Note that average temperatures
produced did not significantly trend upward or downward over the course of the 20 experimental
trials. However, both left and right VAS responses declined in magnitude as the trials
progressed. These results suggest a dissociation between ratio productions and VAS responses
with repeated testing with the ratio productions showing no evidence of an increase or decrease PAIN METRICS 22
in pain sensitivity over trials, but the decreasing average VAS responses suggesting that
participants are becoming less sensitive to pain as trials progressed. This dissociation suggests
that the VAS responses are less stable than ratio productions for assessing pain and perhaps more
subject to history and range effects.
Relationship of VAS to Ratio Production. Figures 5 and 6 illustrate the relationship
between ratio productions and VAS responses. Figure 5 shows data from a representative
participant and illustrates the steps used to analyze the data. First, panel (a) shows how produced
(adjusted) temperatures are related to the ratios given the participant during the first phase of the
suprathreshold scaling trials. The regression line represents the best-fit power function showing
a compressive non-linearity between temperatures produced and pain ratios. Panels (b) and (c)
show how the VAS ratings assigned to the same adjusted temperatures. Panels (d) and (e) show
the relationships between the VAS ratings and the ratios corresponding to the temperatures
presented. The linear regression lines represent on panels (b) through (d) illustrate how the VAS
ratings map onto the temperatures and ratios presented to the participants. Note that for the
participant shown in Figure 5, there is a clear relationship between ratio given and left VAS
ratings as one would expect since the left temperatures varied with ratio. However, we also see
an upward trend in right VAS rating with ratio, even though the right temperature stimuli did not
vary. This upward trend of right VAS responses with ratio was found for five of the ten
participants and suggests that these participants were not able to perceive the left and right pain
stimuli independently. One other feature to note is that there is considerable variability in the
VAS ratings for a given ratio. This suggests that the VAS is not a reliable measure of pain on a single-trial basis. Rather, stable VAS responses require multiple trials to be averaged together.
Figure 6 presents mean left VAS ratings scaled to the mean left rating at a ratio of 1:1 as
functions of the ratio presented for all participants. Ideally, if the left VAS validly represented PAIN METRICS 23
ratios on an internalized ratio scale these ratings should fall along the identity function illustrated
by the dashed red lines. However, for most participants the slope of the left VAS ratings was far
less than 1: thus, VAS responses were compressed relative to ratio productions.
Discussion and Conclusions
The two experiments reported here demonstrate that ratio production of pain maintains ratio scaling properties. Experiment 1 demonstrated that commutativity holds for pain judgments, which validates a ratio representation of pain in ratio production temperatures.
Experiment 2 showed that the VAS scores assigned to stimuli corresponding to particular ratios are approximately linearly related to those ratios, but that these responses are highly variable and
subject to a compression of range when compared to ratio productions.
These results have important implications for measuring pain in an operational
environment. We conclude that while the VAS is an inexpensive and easy-to-administer scale of pain, ratio productions are far more reliable and less subject to variability due to repeated testing.
We recommend that medical staff and engineers interested in obtaining a valid assessment of percent-changes in pain that result from various interventions such as changes in flight schedules, helmet design, task procedures, etc. use ratio production methods to assess pain. An alternative approach would be to determine a customized transformation of a VAS score to a ratio scale for a given individual. This could be done by 1) obtaining VAS scores on the temperatures produced by adjustment to numeric ratios (as in Experiment 2) regressing the VAS score on the ratios (as in Figure 6); 3) determining the amount of slope adjustment necessary to bring the VAS scores into alignment with the ratios. Assuming that the required adjustment would remain constant over time, each individual would have a customized slope rotation value so that VAS scores obtained from that individual (e.g., before and after a neck strain reduction countermeasure) could be adjusted to be in alignment with a ratio scale. PAIN METRICS 24
References
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PAIN METRICS 26
Table 1. Threshold and JND1 temperatures for the 7 participants in Experiment 1.
Left Left PID Thresh JND1 1 38.54 38.84 2 37.81 40.47 3 38.42 38.97 4 39.12 39.46 5 39.31 40.04 6 40.10 40.20 7 38.73 39.22 Means 38.86 39.60 Stddev 0.73 0.64
Notes: 1) All temperatures are reported in degrees C. 2) PID stands for participant ID. 3) Left Thresh is the temperature corresponding to the pain threshold for the left arm 4) JND1 is the temperature corresponding to one just noticeable difference (JND) above the pain threshold for the left arm
PAIN METRICS 27
Table 2. Experiment 1 comparisons for commutativity and multiplicativity.
TEMPERATURES MULTIPLICATIVITY PRODUCED COMMUTATIVITY 2X(L3X) != 6X(L1X) PID N 2X(L3X) 3X(L2X) 6X(L1X) 2X(L3X) != 3X(L2X) 3X(L2X) != 6X(L1X) N.S. 1 12 42.00 41.72 41.35 N.S. N.S. N.S. 2 5 44.28 44.66 44.61 N.S. N.S. N.S. 3 5 43.42 42.10 44.53 N.S. N.S. N.S. 4 12 42.51 43.46 43.41 N.S. N.S. p < .0005 5 9 43.38 44.41 46.77 N.S. p < .001 p < .0005 6 9 40.01 40.92 41.91 N.S. p = .037 p = .009 7 6 42.27 41.70 46.19 N.S. p = .014
Notes: 1) All temperatures are reported in degrees C. 2) PID stands for participant ID. 3) 2x(L3x) represent trials where participants are presented a 3x stimulus on the left arm and adjust the right arm temperature to feel 2 times as painful. 4) 3x(L2x) represent trials where participants are presented a 2x stimulus on the left arm and adjust the right arm temperature to feel 3 times as painful. 5) 6x(L1x) represent trials where participants are presented a 1x stimulus on the left arm and adjust the right arm temperature to feel 6 times as painful. 6) N.S. = not significant by paired samples t-test, p > .05
PAIN METRICS 28
Table 3. Thresholds and JND4 responses for the 10 participants in Experiment 2.
L1 L2 R1 R2 Mean Stddev R1 R2 PID Thresh Thresh Thresh Thresh Thresh Thresh JND4 JND4 1 38.90 37.30 37.30 37.10 37.65 0.73 40.42 40.65 2 38.80 35.14 37.99 37.59 37.38 1.36 38.31 38.73 3 39.06 38.60 38.44 38.12 38.56 0.34 38.98 39.16 4 39.00 38.47 38.51 38.84 38.71 0.22 40.43 40.85 5 39.66 38.77 37.99 38.05 38.62 0.68 39.33 39.47 6 38.45 38.98 36.86 37.74 38.01 0.80 37.67 37.82 7 36.83 36.03 35.92 35.29 36.02 0.55 36.58 36.68 8 37.79 37.50 36.83 37.79 37.48 0.39 38.92 39.00 9 38.38 38.80 39.24 39.53 38.99 0.44 39.67 40.00 10 40.15 39.34 38.14 38.90 39.13 0.73 39.37 40.00 Mean 38.70 37.89 37.72 37.90 38.05 0.62 38.97 39.24 Stddev 0.93 1.38 0.98 1.16 0.95 0.32 1.20 1.28
Notes: 1) All temperatures are reported in degrees C. 2) L1 and L2 refer to Locations 1 & 2 on the left arm 3) R1 and R2 refer to Locations 1 & 2 on the right arm
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Appendix A
Paid Research Participants Needed The Idaho Visual Performance Laboratory (IVPL) is currently recruiting research participants for a study of the subjective experience of pain intensity in response to thermal (hot) stimuli placed on your skin.
The University of Idaho Institutional Review Board has approved this project.
This study will require you to: 1) Report any current chronic pain experience 2) Report any abnormalities you have in experiencing or perceiving pain 3) Report current conditions that preclude your inclusion in the study, such as being pregnant or hereditary risk or personal history of cardiovascular disorders 4) Repeatedly experience some discomfort or pain, which at a maximum will be equivalent to putting your forearm under hot (120 deg F or 49 deg C) water for a few seconds*. Some experimental trials will require you to produce a prescribed intensity level of pain, while other experimental trials will present pain stimuli and require you to indicate the level of pain using numbers, marking a line, or turning a dial. You will receive compensation for your participation in this experiment at the rate of $30 per hour of participation. Moreover, those participating in a multi-session experiment will receive a 20% bonus for completing all experimental sessions in their entirety. Your participation will help increase knowledge of processes underlying perception of pain, which will inform the design of clinical procedures for assessing pain and the effectiveness of pain interventions. Subsequent to your participation the purpose and methods of the study will be described to you and any questions you have about the study will be answered. It is our sincere hope that you will learn something interesting about how your brain processes pain from participating in this study.
Interested in Participating? Questions? Please Contact: Nicholas Roome at [email protected] or Brian Dyre at [email protected]
*Our pain stimuli are digitally-controlled to provide just enough heat (temperatures ranging from 35-49 degrees C) to stimulate the peripheral nerves that trigger pain signals without causing any actual tissue damage, either temporary or permanent. AIRCREW PAIN ASSESSMENT 38
Appendix B
CONSENT FORM
Idaho Visual Performance Laboratory Department of Psychology and Communication Studies College of Liberal Arts and Social Sciences University of Idaho
The University of Idaho Institutional Review Board has approved this project.
This experiment examines the subjective experience of pain and will require that you repeatedly experience some discomfort or pain.
You will receive compensation for your participation in this experiment at the rate of $30 per hour of participation. Moreover, if you are participating in a multi-session experiment you will receive a 20% bonus for completing all experimental sessions in their entirety. You may end the experiment at any time and will be compensated for the time spent in the experiment rounded up to the nearest ½ hour increment (e.g., terminating the experiment between 1-30 minutes = $15 compensation, 31-60 minutes = $30 compensation, etc.).
We will first ask you questions about your own experience of pain, including whether you have a current clinical diagnosis of pain, or if you aware of any other abnormalities in the way you sense pain. We will also ask if you are currently pregnant or have a hereditary risk or personal history of cardiovascular disorder. If you believe these questions invade your privacy, you are not required to answer them and may instead withdraw from the experiment and receive full compensation for your time spent in the experiment up to this point (most likely this will be $15 since reading this consent form is the first step of the experiment).
Assuming you consent to answering the above questions, you will then be presented a series of thermal (hot) nociceptive (pain) stimuli to the skin surface of your bare forearm. The maximum pain you will experience will be similar to putting your forearm under hot (120 degrees F or 49 degrees C) tap water for a few seconds. Researchers and pain clinicians have used similar stimuli—carefully calibrated and digitally controlled—to provide just enough heat (temperatures ranging from 35-49 degrees C) to stimulate the peripheral pain receptors that trigger pain signals without causing any actual tissue damage, either temporary or permanent.
Over several trials, you will rate the intensity of the pain experienced using either numerals, marking a line, or by adjusting a dial to represent the intensity of pain. You will also be required to use a dial to adjust the temperature of the stimulus to match ratios (e.g., “half as much,” “three times as much”) of a second pain stimulus.
The data you provide will be kept anonymous. There will be absolutely no link between your identity and your particular set of data.
No study is without some risk, but we believe the benefits outweigh the risks in this study. AIRCREW PAIN ASSESSMENT 39
Our experimental protocol is specifically designed to ensure that pain or discomfort will not persist beyond the acute application of the thermal stimulator. Temperatures will be hot, but not hot enough to cause any actual tissue damage, temporary or permanent. Further, at all times during the experiment you will have control over a safety switch that can instantly turn off the stimulus whenever you wish.
There is some potential for chronic pain sufferers to suffer additional pain sensitization any time they are subjected to pain and this experiment is no exception. However, at the beginning of testing we will carefully assess your sensitivity and tolerance to pain and will adjust the range of pain stimuli applied throughout the remainder of the experiment so as to not exceed your tolerance. Further, breaks will be taken in between pain stimuli to minimize any adaptation or sensitization effects.
We believe these mitigations, along with your knowledge that you can withdraw from the experiment at any time for any reason and still be compensated for your time spent, result in minimal risk.
Your participation will help increase knowledge of processes underlying perception of pain. This knowledge will inform the design of clinical procedures for assessing pain and the effectiveness of pain interventions. Subsequent to your participation the purpose and methods of the study will be described to you and any questions you have about the study will be answered. It is our sincere hope that you will learn something interesting about how your brain works from this debriefing.
[The blanks in the next paragraph represent variables that will be filled in for each experiment.]
Your participation will require __ sessions of approximately ___ minutes each. You may withdraw from this study for any reason and at any time without penalty. If you do wish to withdraw, simply inform the experimenter; you will receive full compensation for your time spent in the experiment up to that point rounded up to the nearest ½-hour increment. However, please be aware that your data will have the greatest scientific value if you complete the experiment in its entirety.
Internal Revenue Service (IRS) and University rules require us to collect your name, address, and Social Security number in order to pay you if the total payment is greater than $50. That information will be kept secure and confidential within the University’s records, to the extent allowed by law. If you are paid a total of $600 or more as a research subject in a calendar year, the University is required to report the payment to the IRS as miscellaneous income. The University will send you an IRS Form 1099 in January documenting the payment total. You can use the form with your income tax return, if appropriate.
If you have further questions or issues please contact: Dr. Brian P. Dyre Dept. of Psychology and Comm. Studies AIRCREW PAIN ASSESSMENT 40
University of Idaho (208) 885-6927 [email protected]
or
University of Idaho Institutional Review Board [email protected] 208-883-6162
If you feel you require pain counseling after participating in this study, please contact
Dr. Mark Yama Dept. of Psychology and Comm. Studies University of Idaho
(208) 885-7376 [email protected]
I have reviewed this consent form and understand and agree to its contents.
Participant Name ______
Signature ______
Date ______
Experimenter Name______Signature______Date ______
Thank you for your participation! AIRCREW PAIN ASSESSMENT 41
This demographics form will be kept separate from the consent form
Participant Number: ______
Age: ______
Gender: ______
Please circle the appropriate responses to the following questions
Are you currently pregnant? YES or NO
Do you have any hereditary risk of cardiovascular disease? YES or NO
Do you have any personal history of cardiovascular disease? YES or NO
Do you experience regular (or chronic) pain? YES or NO
AIRCREW PAIN ASSESSMENT 42
Appendix D
Debriefing
Your answers to the following questions will help us to better understand the data you have provided. 1. Please circle 3 of the following adjectives that best describe the quality of the sensations of pain you experienced in this experiment Throbbing
Shooting
Stabbing
Sharp
Cramping
Gnawing
Hot-Burning
Aching
Heavy
Tender
Splitting
Tiring-Exhausting
Sickening
Fearful
Punishing-Cruel
2. Did you feel that the quality of pain you experienced changed throughout the experimental session? If so, please elaborate:
3. We expect that you experienced different intensities of pain throughout the experimental session, however, did you also feel that the overall intensity of pain you experienced increased or decreased during the experimental session? That is, do you think you became more or less sensitive to pain during the course of the experiment?
AIRCREW PAIN ASSESSMENT 43
4. Do you believe that the pain levels you experienced when making magnitude estimates of pain intensity during the second half of the experiment matched the pain levels you produced by adjusting the dial in the first half of the experiment? Did you realize this during the course of making your pain magnitude estimates or later?
5. Would you describe the overall pain intensities of your second experimental session to be less than, equal to, or greater than the overall pain intensities of your first experimental session?
6. Please provide a range of unpleasantness for the group of pain stimuli you experienced on a scale of 1-10, with 1 equal to very mild unpleasantness and 10 being extremely unpleasant.
7. Pain can often be associated with negative emotional experiences such as anxiety and distress. Please rate on a scale of 1-10, with 1 being mild and 10 being extreme, the range of negative emotion you experienced for the group of pain stimuli in this experiment. AIRCREW PAIN ASSESSMENT 44
Description of Study
This study examines the consistency of pain estimates across different tasks, with the aim of understanding how people make subjective judgments of the intensity of pain. Because pain is a purely subjective phenomenon, it is important that we understand how people use numbers to make judgments of pain intensity. Such understanding is critical to formulating reliable measures of pain in clinical settings.
In particular, we were interested in establishing whether the numbers people assign to perceived pain form a scale with ratio properties, that is, can people reliably assess whether a particular painful stimulus is ½ or double another stimulus in intensity?
Your data will help to develop more reliable and valid measures of pain for clinical applications. This project is sponsored by the Canadian Military in an effort to develop better assessment for Canadian Air Force Personnel experiencing chronic pain.