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To Calibrate or not to Calibrate? A Methodological Dilemma in Experimental Pain Research

  • Waclaw M. Adamczyk
    Correspondence
    Address reprint requests to Dr. Waclaw M. Adamczyk, Laboratory of Pain Research, Institute of Physiotherapy and Health Sciences, The Jerzy Kukuczka Academy of Physical Education, ul. Mikolowska 72A, 40-065 Katowice, Poland.
    Affiliations
    Laboratory of Pain Research, Institute of Physiotherapy and Health Sciences, The Jerzy Kukuczka Academy of Physical Education, Katowice, Poland

    Institute of Health Sciences, Department of Physiotherapy, Pain & Exercise Research Luebeck (P.E.R.L.), University of Lübeck, Lübeck, Germany
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  • Tibor M. Szikszay
    Affiliations
    Institute of Health Sciences, Department of Physiotherapy, Pain & Exercise Research Luebeck (P.E.R.L.), University of Lübeck, Lübeck, Germany
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  • Hadas Nahman-Averbuch
    Affiliations
    Washington University Pain Center, Department of Anesthesiology, Washington University School of Medicine, St. Louis, Missouri
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  • Jacek Skalski
    Affiliations
    Laboratory of Pain Research, Institute of Physiotherapy and Health Sciences, The Jerzy Kukuczka Academy of Physical Education, Katowice, Poland
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  • Jakub Nastaj
    Affiliations
    Laboratory of Pain Research, Institute of Physiotherapy and Health Sciences, The Jerzy Kukuczka Academy of Physical Education, Katowice, Poland
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  • Philip Gouverneur
    Affiliations
    Institute of Medical Informatics, University of Lübeck, Lübeck, Germany
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  • Kerstin Luedtke
    Affiliations
    Institute of Health Sciences, Department of Physiotherapy, Pain & Exercise Research Luebeck (P.E.R.L.), University of Lübeck, Lübeck, Germany

    Center of Brain, Behavior and Metabolism (CBBM), University of Lübeck, Lübeck, Germany
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Open AccessPublished:July 30, 2022DOI:https://doi.org/10.1016/j.jpain.2022.07.007

      Abstract

      To calibrate or not to calibrate? This question is raised by almost everyone designing an experimental pain study with supra-threshold stimulation. The dilemma is whether to individualize stimulus intensity to the pain threshold / supra-threshold pain level of each participant or whether to provide the noxious stimulus at a fixed intensity so that everyone receives the identical input. Each approach has unique pros and cons which need to be considered to i) accurately design an experiment, ii) enhance statistical inference in the given data and, iii) reduce bias and the influence of confounding factors in the individual study e.g., body composition, differences in energy absorption and previous experience. Individualization requires calibration, a procedure already irritating the nociceptive system but allowing to match the pain level across individuals. It leads to a higher variability of the stimulus intensity, thereby influencing the encoding of “noxiousness” by the central nervous system. Results might be less influenced by statistical phenomena such as ceiling/floor effects and the approach does not seem to rise ethical concerns. On the other hand, applying a fixed (standardized) intensity reduces the problem of intensity encoding leading to a large between-subjects variability in pain responses. Fixed stimulation intensities do not require pre-exposure. It can be proposed that one method is not preferable over another, however the choice depends on the study aim and the desired level of external validity. This paper discusses considerations for choosing the optimal approach for experimental pain studies and provides recommendations for different study designs.

      Perspective

      To calibrate pain or not? This dilemma is related to almost every experimental pain research. The decision is a trade-off between statistical power and greater control of stimulus encoding. The article decomposes both approaches and presents the pros and cons of either approach supported by data and simulation experiment.

      Key words

      In basic pain science, the application of noxious stimuli is fundamental to evoke pain experimentally. This allows to study pain mechanisms and support translational research, filling the gap between studies on animal models and clinical populations.
      • Mouraux A
      • Bannister K
      • Becker S
      • Finn DP
      • Pickering G
      • Pogatzki-Zahn E
      • Graven-Nielsen T
      Challenges and opportunities in translational pain research – An opinion paper of the working group on translational pain research of the European pain federation (EFIC).
      ,
      • Pedersen JL
      • Kehlet H
      Hyperalgesia in a human model of acute inflammatory pain: A methodological study.
      ,
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      • Naidu MUR
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      • Ramesh-Kumar-Rao T
      Human experimental pain models: A review of standardized methods in drug development.
      A common dilemma is whether to apply stimuli of fixed or individually calibrated intensity. In some fields, both approaches are equally common,
      • Archibald J
      • MacMillan EL
      • Enzler A
      • Jutzeler CR
      • Schweinhardt P
      • Kramer JLK
      Excitatory and inhibitory responses in the brain to experimental pain: A systematic review of MR spectroscopy studies.
      ,
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      The magnitude of offset analgesia as a measure of endogenous pain modulation in healthy participants and patients with chronic pain: A systematic review and meta-analysis.
      however, the rationale for the choice of one or the other is often tacitly reported in the methods section of experimental pain studies.
      • Sprenger C
      • Stenmans P
      • Tinnermann A
      • Büchel C
      Evidence for a spinal involvement in temporal pain contrast enhancement.
      ,
      • Świder K
      • Ba̧bel P
      The effect of the sex of a model on nocebo hyperalgesia induced by social observational learning.
      ,
      • Zhang S
      • Li T
      • Kobinata H
      • Ikeda E
      • Ota T
      • Kurata J
      Attenuation of offset analgesia is associated with suppression of descending pain modulatory and reward systems in patients with chronic pain.
      Mostly, authors do not give reasons for choosing the particular method (likely due to the routine practice in their lab), and if they do, it is justified by only providing the purpose of the study. For example, Fust et al.
      • Fust J
      • Lalouni M
      • Vadenmark-Lundqvist V
      • Wärnberg E
      • Jensen KB
      Offset analgesia and onset hyperalgesia with different stimulus ranges.
      explain the choice, noting that in their study they calibrated the intensity because they were primarily interested in “pain intensity not stimulus intensity per se”. In the study of Pedersen et al.
      • Pedersen JL
      • Kehlet H
      Hyperalgesia in a human model of acute inflammatory pain: A methodological study.
      authors were interested in reproducing hyperalgesia in a psychophysical model of human inflammatory pain; therefore, they used a fixed stimulus intensity (47°C for 7 min.) capable to induce mild burns. Coghill et al.
      • Coghill RC
      • McHaffie JG
      • Yen Y-F
      Neural correlates of interindividual differences in the subjective experience of pain.
      and Khan et al.
      • Khan HS
      • Stroman PW
      Inter-individual differences in pain processing investigated by functional magnetic resonance imaging of the brainstem and spinal cord.
      applied the same noxious stimulus (49°C) to every subject to identify neural correlates of individual differences in pain perception. Levy et al.
      • Levy D
      • Abdian L
      • Dekel-Steinkeller M
      • Defrin R
      Experimental evidence for weaker endogenous inhibition of trigeminal pain than extra-trigeminal pain in healthy individuals.
      on the other hand chose calibrated stimuli because the primary aim of the study was to control sensitivity of different body regions. Sometimes authors rationalize the choice referring to known inter-individual response variability to noxious stimuli,
      • Kupers R
      • Danielsen ER
      • Kehlet H
      • Christensen R
      • Thomsen C
      Painful tonic heat stimulation induces GABA accumulation in the prefrontal cortex in man.
      individual differences in sensitivity to pain
      • Rischer KM
      • González-Roldán AM
      • Montoya P
      • Gigl S
      • Anton F
      • van der Meulen M
      Distraction from pain: The role of selective attention and pain catastrophizing.
      or the results from a pilot study.
      • Meeuse JJ
      • Löwik MSP
      • Löwik SAM
      • Aarden E
      • van Roon AM
      • Gans ROB
      • van Wijhe M
      • Lefrandt JD
      • Reyners AKL
      Heart rate variability parameters do not correlate with pain intensity in healthy volunteers.
      Some authors advocate a fixed approach pointing out that some people might have problems providing specific ratings which is essential for the calibrated approach.
      • van den Bosch GE
      • van Hemmen J
      • White T
      • Tibboel D
      • Peters JWB
      • van der Geest JN
      Standard and individually determined thermal pain stimuli induce similar brain activations.
      Such discrepancies in published literature call for a systematic summary.

      The Dilemma

      Only a few studies targeted the described problem empirically (see Appendix 1). For instance, in the study conducted by Grouper et al.
      • Grouper H
      • Eisenberg E
      • Pud D
      The relationship between sensitivity to pain and conditioned pain modulation in healthy people.
      , participants were tested using a conditioned pain modulation (CPM) paradigm, which represents the pain-inhibits-pain effect (conditioning stimulus inhibits a testing stimulus). After identifying two distinct subgroups (having high sensitivity to pain – HSP and low sensitivity to pain – LSP) volunteers’ pain self-reports to noxious heat stimuli (so-called test stimuli) were collected before and during concomitant irritation of the opposite hand using cold stimulation (so-called conditioning stimuli). Each group was tested with the two CPM paradigms: with fixed intensity of the test stimulus (47°C) and with an individually calibrated temperature (pain of 60/100). Although the intensity of the conditioning stimulus was always kept fixed (regardless of the group always 12°C), an inhibitory effect was shown using both paradigms. The authors found that there were no differences in CPM magnitude between the paradigms within each group, but they clearly found differences in the variance of perceived pain at baseline. Namely, the standard deviation for pain at baseline was e.g., 25.5 vs. 8.1 in the LSP group and 18.1 vs. 7.5 in the HSP group. Moreover, the CPM assessment with fixed temperature elicited a lower inhibitory effect in HSP participants difference not significant. Since the fixed stimulus elicited only mild pain scores at baseline in the LSP group, the inhibitory effect of the CPM paradigm could be potentially limited by the mild pain ratings at baseline, resulting in skewed data and a possible floor effect for the LPS group.
      • Grouper H
      • Eisenberg E
      • Pud D
      The relationship between sensitivity to pain and conditioned pain modulation in healthy people.
      Thus, the results of the CPM effect in participants with low pain sensitivity might not reflect their full inhibitory capabilities and using a more intense stimulus might have revealed a greater inhibitory effect.
      The dilemma has been also addressed in two fMRI studies in which both approaches were contrasted. For example, results from a study by van den Bosch et al.
      • van den Bosch GE
      • van Hemmen J
      • White T
      • Tibboel D
      • Peters JWB
      • van der Geest JN
      Standard and individually determined thermal pain stimuli induce similar brain activations.
      pointed out that both intensities (calibrated vs. fixed) may activate similar brain regions but also that calibrated intensity was linked to activity detected in a cluster of the white matter within the corpus callosum. As the authors discussed, this can be the result of movement artifacts having higher pain levels in the calibrated compared to the fixed condition (Appendix 1). On the other hand, successful pain ‘match’ between these two conditions has been performed in a later study by Quiton et al.
      • Quiton RL
      • Keaser ML
      • Zhuo J
      • Gullapalli RP
      • Greenspan JD
      Intersession reliability of fMRI activation for heat pain and motor tasks.
      Authors have found that the type of stimulus, i.e., calibrated (pain = 50/100) vs. fixed affected the reliability of the BOLD signal in some of the regions of interests (ROI). For example, the insula, which is a core structure involved in pain perception, showed reliable bilateral activity in response to noxious heat of 48°C compared to calibrated intensity.
      • Starr CJ
      • Sawaki L
      • Wittenberg GF
      • Burdette JH
      • Oshiro Y
      • Quevedo AS
      • Coghill RC
      Roles of the insular cortex in the modulation of pain: Insights from brain lesions.
      Another structure which is part of the “neurological pain signature”, the SII,
      • Quiton RL
      • Keaser ML
      • Zhuo J
      • Gullapalli RP
      • Greenspan JD
      Intersession reliability of fMRI activation for heat pain and motor tasks.
      • Wager TD
      • Atlas LY
      • Lindquist MA
      • Roy M
      • Woo CW
      • Kross E
      An fMRI-Based neurologic signature of physical pain.
      was in contrast, reliably activated with a calibrated intensity. The authors also investigated the spatial patterns of the BOLD signal associated with both stimulus types and found only partial overlap. As described as a coefficient between 0 and 1, the activity in the insula weakly overlapped in its posterior part (ipsilateral), but strong spatial agreement was found in the anterior insula contralaterally.
      • Quiton RL
      • Keaser ML
      • Zhuo J
      • Gullapalli RP
      • Greenspan JD
      Intersession reliability of fMRI activation for heat pain and motor tasks.
      Another study which is (indirectly) relevant for the discussed dilemma is the one by Nir et al.
      • Nir RR
      • Yarnitsky D
      • Honigman L
      • Granot M
      Cognitive manipulation targeted at decreasing the conditioning pain perception reduces the efficacy of conditioned pain modulation.
      Here, a conditioned pain modulation paradigm was used in which thermal pain was applied to one hand before and during exposure of the contralateral hand into a hot water (conditioning stimulus). The temperature was fixed (45.5°C) but the perception of this conditioning stimulus was, however, manipulated in the experiment using a nocebo manipulation. Thus, in one group, participants perceived the conditioning stimulus as ‘more intense’ because of a nocebo cream application that was supposed to increase the level of pain. Results showed that not the actual thermal noxious intensity, but the perceived pain intensity caused by the conditioning stimulus influenced the CPM effect. This might suggest that the use of a calibrated paradigm is accurate to maintain the same pain level and reduce the impact of the conditioning stimulus variability on the CPM response.
      Furthermore, fixed intensity has the potential to elucidate neural mechanisms of individual differences in pain perception. It has been shown that neural responses to fixed stimulus are equal at the thalamic but not cortical level
      • Coghill RC
      • Eisenach J
      Individual differences in pain sensitivity: Implications for treatment decisions.
      which is also supported by a study with spinal cord imaging, showing no correlation between the activity at the spinal cord and the reported pain.
      • Sprenger C
      • Finsterbusch J
      • Büchel C
      Spinal cord-midbrain functional connectivity is related to perceived pain intensity: A combined spino-cortical fMRI study.
      Taken together, these are just a few examples illustrating the problem that the decision whether the stimulus intensity is fixed or not can affect the measured outcome, no matter whether this is physiological
      • Loggia ML
      • Juneau M
      • Bushnell MC
      Autonomic responses to heat pain: Heart rate, skin conductance, and their relation to verbal ratings and stimulus intensity.
      or behavioral (e.g., QST outcomes
      • Nahman-Averbuch H
      • Shefi T
      • Schneider VJ
      • Li D
      • Ding L
      • King CD
      • Coghill RC
      Quantitative sensory testing in patients with migraine: A systematic review and meta-analysis.
      ).

      Fixed Stimulus Intensity

      In this approach, all participants receive the same parameters of stimulation to activate nociceptors through nerve endings (e.g., using heat) or to directly activate nociceptive fibers (electrical stimulation). Although using a fixed intensity is often used as a standalone methodological decision,
      • van den Bosch GE
      • van Hemmen J
      • White T
      • Tibboel D
      • Peters JWB
      • van der Geest JN
      Standard and individually determined thermal pain stimuli induce similar brain activations.
      reducing the number of stimuli applied, it is sometimes coupled with a familiarization/training session.
      • Coghill RC
      • McHaffie JG
      • Yen Y-F
      Neural correlates of interindividual differences in the subjective experience of pain.
      ,
      • Khan HS
      • Stroman PW
      Inter-individual differences in pain processing investigated by functional magnetic resonance imaging of the brainstem and spinal cord.
      ,
      • Yelle MD
      • Oshiro Y
      • Kraft RA
      • Coghill RC
      Temporal filtering of nociceptive information by dynamic activation of endogenous pain modulatory systems.
      The aim of the training session is to teach subjects how to provide pain ratings, to reduce the feeling of novelty or to learn how to use an experimental equipment i.e., the computerized Visual Analogue Scale.
      • Khan HS
      • Stroman PW
      Inter-individual differences in pain processing investigated by functional magnetic resonance imaging of the brainstem and spinal cord.
      ,
      • Yelle MD
      • Oshiro Y
      • Kraft RA
      • Coghill RC
      Temporal filtering of nociceptive information by dynamic activation of endogenous pain modulatory systems.
      With or without familiarization, fixed intensity leads to pain rating variability, that can arise from combinations of multiple biological, psychological, and social factors.
      • Fillingim RB
      Individual differences in pain: Understanding the mosaic that makes pain personal.
      Of note, everyone has his/her own unique history of painful experiences affecting subsequent painful events, and thus perception.
      • Coghill RC
      • McHaffie JG
      • Yen Y-F
      Neural correlates of interindividual differences in the subjective experience of pain.
      ,
      • Vlaeyen JWS.
      Learning to predict and control harmful events: Chronic pain and conditioning.
      Indeed, the fact that prior exposure shapes the pain experience has been shown using procedures of classical conditioning
      • Bąbel P
      • Bajcar EA
      • Adamczyk W
      • Kicman P
      • Lisińska N
      • Świder K
      • Colloca L
      Classical conditioning without verbal suggestions elicits placebo analgesia and nocebo hyperalgesia.
      ,
      • Colloca L
      • Benedetti F
      How prior experience shapes placebo analgesia.
      operant conditioning
      • Jolliffe CD
      • Nicholas MK
      Verbally reinforcing pain reports: An experimental test of the operant model of chronic pain.
      ,
      • Linton SJ
      • Gotestam KG
      Controlling pain reports through operant conditioning: A laboratory demonstration.
      and observational learning of pain.
      • Colloca L
      • Benedetti F
      Placebo analgesia induced by social observational learning.
      For example, in a typical classical conditioning experiment, volunteers underwent a procedure in which they learned that one colour e.g., blue was linked to low pain and another colour e.g., orange preceded high pain. Bąbel et al.
      • Bąbel P
      • Bajcar EA
      • Adamczyk W
      • Kicman P
      • Lisińska N
      • Świder K
      • Colloca L
      Classical conditioning without verbal suggestions elicits placebo analgesia and nocebo hyperalgesia.
      showed that participants continued to feel more intense pain after exposure to orange compared to blue colour, even though the stimulus intensity was equal after the initial conditioning phase.
      An identical stimulus can be perceived and expressed by a myriad of possibilities. Fillingim
      • Fillingim RB
      Individual differences in pain: Understanding the mosaic that makes pain personal.
      reported for example, that a stimulus of 48°C applied via a thermode can provoke perceived pain intensities varying between 4 and 100 with a mean pain rating at the level of 71.8 on a 0 to 100 (most intense pain imaginable) scale. This behavioral effect is also reflected by a profound variability of brain activations. For instance, Coghill et al. showed that the anterior cingulate cortex (ACC), primary sensory cortex (SI) and prefrontal cortex (PFC) exhibited more robust activity in response to a fixed stimulus in highly sensitive compared to less sensitive individuals.
      • Coghill RC
      • Eisenach J
      Individual differences in pain sensitivity: Implications for treatment decisions.
      ,
      • Coghill RC
      • McHaffie JG
      • Yen Y-F
      Neural correlates of interindividual differences in the subjective experience of pain.
      Interestingly, not only different people perceive the same stimulus differently, but there is also a significant within-subject variability, with the most profound example referring to the same stimulus being rated as painful or not within the same session, depending on the trial.
      • Madden VJ
      • Kamerman PR
      • Catley MJ
      • Bellan V
      • Russek LN
      • Camfferman D
      • Lorimer Moseley G
      Variability in experimental pain studies: Nuisance or opportunity?.
      However, the fixed stimulus intensity approach – although frequently used and straightforward – has some important implications (Table 1) as well as advantages and disadvantages (Appendix 2):
      • i.
        The variability of pain ratings can raise ethical concerns as some individuals might be exposed to stimuli that they cannot or can barely tolerate. This variability seems to be dictated by the type of pain modality used in the particular study: Coefficient of Variation (CoV, a ratio of the standard deviation to the mean value) of 16% in pain response was reported using contact heat,
        • Chao CC
        • Hsieh ST
        • Chiu MJ
        • Tseng MT
        • Chang YC
        Effects of aging on contact heat-evoked potentials: The physiological assessment of thermal perception.
        32% using noxious cold-water stimulation
        • Ruscheweyh R
        • Stumpenhorst F
        • Knecht S
        • Marziniak M
        Comparison of the cold pressor test and contact thermode-delivered cold stimuli for the assessment of cold pain sensitivity.
         and 47% using electrical stimuli.
        • Wiercioch-Kuzianik K
        • Babel P
        Color Hurts. The Effect of Color on Pain Perception.
      • ii.
        The approach is prone to statistical effects such as ceiling and/or floor effects
        • Grouper H
        • Eisenberg E
        • Pud D
        The relationship between sensitivity to pain and conditioned pain modulation in healthy people.
        with some volunteers rating the stimulus using extreme values.
        • Fillingim RB
        Individual differences in pain: Understanding the mosaic that makes pain personal.
        This might be problematic in studies that assess a change in pain levels in response to an intervention or a manipulation, as extremely high or low ratings at baseline might limit the visibility of the intervention effect.
      • iii.
        A fixed approach limits the generalizability of the results within a domain. As the aim of basic science research in humans is to fill the gap between animal and clinical research, fixed intensity mimics populations of patients who have similar/comparable (in its magnitude) nociceptive input(s), e.g., size of the injury. The same stimulus e.g., 48°C can serve as a proxy to infer from populations having the same size of injury/nociception and thus, might not be generalized to patients with less intense or more extreme injury.
      • iv.
        Results from studies with fixed stimulus intensities may be more suitable for assessing the relationship between pain and autonomic (physiological) responses. That approach reduces correlations with different inputs that are a feature of the calibrated approach.
        • Nahman-Averbuch H
        • Coghill RC RC
        Pain-autonomic relationships: Implications for experimental design and the search for an “objective marker” for pain.
        It has been shown, for example that electrodermal activity is stimulus rather than pain-dependent.
        • Nickel MM
        • May ES
        • Tiemann L
        • Postorino M
        • Ta-Dinh S
        • Ploner M
        Autonomic responses to tonic pain are more closely related to stimulus intensity than to pain intensity.
        This might have an advantage in fMRI studies as intensity must not be regressed during analysis.
      • v.
        Interestingly, this method can have profound effects on the reliability of pain measurement, with measurements in the fixed approach having larger reliability compared to the calibrated approach. This aspect has been demonstrated mathematically, in simulations,
        • Woo CW
        • Wager TD.
        What reliability can and cannot tell us about pain report and pain neuroimaging.
        and real data.
        • Quiton RL
        • Keaser ML
        • Zhuo J
        • Gullapalli RP
        • Greenspan JD
        Intersession reliability of fMRI activation for heat pain and motor tasks.
      Table 1Comparison of two different approaches
      ItemCalibration ProcedureFixed Stimulus Intensity
      ConstructEqual pain intensity
      • Adamczyk WM
      • Manthey L
      • Domeier C
      • Szikszay TM
      • Luedtke K
      Nonlinear increase of pain in distance-based and area-based spatial summation.
      ,
      • Gehling J
      • Mainka T
      • Vollert J
      • Pogatzki-Zahn EM
      • Maier C
      • Enax-Krumova EK
      Short-term test-retest-reliability of conditioned pain modulation using the cold-heat-pain method in healthy subjects and its correlation to parameters of standardized quantitative sensory testing.
      ,
      • Levy D
      • Abdian L
      • Dekel-Steinkeller M
      • Defrin R
      Experimental evidence for weaker endogenous inhibition of trigeminal pain than extra-trigeminal pain in healthy individuals.
      ,
      • Szikszay TM
      • Lévénez JLM
      • von Selle J
      • Adamczyk WM
      • Luedtke K
      Investigation of correlations between pain modulation paradigms.
      Equal stimulus intensity
      • Coghill RC
      • Eisenach J
      Individual differences in pain sensitivity: Implications for treatment decisions.
      ,
      • Świder K
      • Ba̧bel P
      The effect of the sex of a model on nocebo hyperalgesia induced by social observational learning.
      ,
      • Yelle MD
      • Oshiro Y
      • Kraft RA
      • Coghill RC
      Temporal filtering of nociceptive information by dynamic activation of endogenous pain modulatory systems.
      VariabilityLarge in terms of input (nociception)
      • Höffken O
      • Özgül ÖS
      • Enax-Krumova EK
      • Tegenthoff M
      • Maier C
      Evoked potentials after painful cutaneous electrical stimulation depict pain relief during a conditioned pain modulation.
      ,
      • King CD
      • Goodin B
      • Kindler LL
      • Caudle RM
      • Edwards RR
      • Gravenstein N
      • Riley JL
      • Fillingim RB
      Reduction of conditioned pain modulation in humans by naltrexone: An exploratory study of the effects of pain catastrophizing.
      ,
      • Sandström A
      • Ellerbrock I
      • Tour J
      • Kadetoff D
      • Jensen KB
      • Kosek E
      Neural correlates of conditioned pain responses in fibromyalgia subjects indicate preferential formation of new pain associations rather than extinction of irrelevant ones.
      , Less variability in pain perception
      • Grouper H
      • Eisenberg E
      • Pud D
      The relationship between sensitivity to pain and conditioned pain modulation in healthy people.
      Large in terms of pain perception
      • Chao CC
      • Hsieh ST
      • Chiu MJ
      • Tseng MT
      • Chang YC
      Effects of aging on contact heat-evoked potentials: The physiological assessment of thermal perception.
      ,
      • Ruscheweyh R
      • Stumpenhorst F
      • Knecht S
      • Marziniak M
      Comparison of the cold pressor test and contact thermode-delivered cold stimuli for the assessment of cold pain sensitivity.
      ,
      • Wiercioch-Kuzianik K
      • Babel P
      Color Hurts. The Effect of Color on Pain Perception.
      , Less variability in terms of input
      • Grouper H
      • Eisenberg E
      • Pud D
      The relationship between sensitivity to pain and conditioned pain modulation in healthy people.
      Statistical inferenceUnlikely to be affected by ceiling and/or floor effects
      • Granot M
      • Granovsky Y
      • Sprecher E
      • Nir RR
      • Yarnitsky D
      Contact heat-evoked temporal summation: Tonic versus repetitive-phasic stimulation.
      ,
      • Nahman-Averbuch H
      • Coghill RC RC
      Pain-autonomic relationships: Implications for experimental design and the search for an “objective marker” for pain.
      Prone to floor/ceiling effects
      • Anderson RJ
      • Craggs JG
      • Bialosky JE
      • Bishop MD
      • George SZ
      • Staud R
      • Robinson ME
      Temporal summation of second pain: Variability in responses to a fixed protocol.
      ,
      • Grouper H
      • Eisenberg E
      • Pud D
      The relationship between sensitivity to pain and conditioned pain modulation in healthy people.
      ,
      • Nahman-Averbuch H
      • Coghill RC RC
      Pain-autonomic relationships: Implications for experimental design and the search for an “objective marker” for pain.
      Neural activityDifferent intensities may distinguish stimulus encoding at the spinal and thalamus level
      • Sprenger C
      • Finsterbusch J
      • Büchel C
      Spinal cord-midbrain functional connectivity is related to perceived pain intensity: A combined spino-cortical fMRI study.
      Same stimulus may produce equal afferent activation patterns until thalamus level
      • Coghill RC
      • McHaffie JG
      • Yen Y-F
      Neural correlates of interindividual differences in the subjective experience of pain.
      ,
      • Sprenger C
      • Finsterbusch J
      • Büchel C
      Spinal cord-midbrain functional connectivity is related to perceived pain intensity: A combined spino-cortical fMRI study.
      FeasibilityTime-consuming technique
      • Adamczyk WM
      • Manthey L
      • Domeier C
      • Szikszay TM
      • Luedtke K
      Nonlinear increase of pain in distance-based and area-based spatial summation.
      ,
      • Posada-Quintero HF
      • Kong Y
      • Nguyen K
      • Tran C
      • Beardslee L
      • Chen L
      • Guo T
      • Cong X
      • Feng B
      • Chon KH
      Using electrodermal activity to validate multilevel pain stimulation in healthy volunteers evoked by thermal grills.
      Time-saving technique
      • van den Bosch GE
      • van Hemmen J
      • White T
      • Tibboel D
      • Peters JWB
      • van der Geest JN
      Standard and individually determined thermal pain stimuli induce similar brain activations.
      TranslationStudies on humans less comparable to studies on other speciesStudies in humans comparable to studies with animal models
      SensitizationDepending on the calibration protocol, potential to irritate the nociceptive system
      • Stroman PW
      • Ioachim G
      • Powers JM
      • Staud R
      • Pukall C
      Pain processing in the human brainstem and spinal cord before, during, and after the application of noxious heat stimuli.
      Lack of pre-exposure in the experiment setup
      • Świder K
      • Ba̧bel P
      The effect of the sex of a model on nocebo hyperalgesia induced by social observational learning.
      Study designCan be preferred in studies targeted at subjective outcomes (e.g., pain ratings)
      • Nahman-Averbuch H
      • Coghill RC RC
      Pain-autonomic relationships: Implications for experimental design and the search for an “objective marker” for pain.
      Can be preferred in studies targeted at physiological outcomes (e.g., fMRI)
      • Nahman-Averbuch H
      • Coghill RC RC
      Pain-autonomic relationships: Implications for experimental design and the search for an “objective marker” for pain.
      Peripheral factorsCalibration reduce influences resulted from body composition, e.g., fat tissue
      • Price RC
      • Asenjo JF
      • Christou NV
      • Backman SB
      • Schweinhardt P
      The role of excess subcutaneous fat in pain and sensory sensitivity in obesity.
      or epidermal thickness

      Stoll AM, Piergallini JR, Chianta MA: Thermal Conduction Effects in Human Skin. III. Influence of Epidermal Thickness and Exposure Time. 1979.

      Affected by body composition
      • Price RC
      • Asenjo JF
      • Christou NV
      • Backman SB
      • Schweinhardt P
      The role of excess subcutaneous fat in pain and sensory sensitivity in obesity.
      , e.g., epidermal thickness

      Stoll AM, Piergallini JR, Chianta MA: Thermal Conduction Effects in Human Skin. III. Influence of Epidermal Thickness and Exposure Time. 1979.

      innervation density linked with sensitivity
      • Schley M
      • Bayram A
      • Rukwied R
      • Dusch M
      • Konrad C
      • Benrath J
      • Geber C
      • Birklein F
      • Hägglöf B
      • Sjögren N
      • Gee L
      • Albrecht PJ
      • Rice FL
      • Schmelz M
      Skin innervation at different depths correlates with small fibre function but not with pain in neuropathic pain patients.
      Study designDesirable in follow-up studies: control for pain sensitivity over-time
      • Mun CJ
      • Suk HW
      • Davis MC
      • Karoly P
      • Finan P
      • Tennen H
      • Jensen MP
      Investigating intraindividual pain variability: Methods, applications, issues, and directions.
      Beneficial in assessment of inter-individual pain differences
      • Fillingim RB
      Individual differences in pain: Understanding the mosaic that makes pain personal.
      GeneralizabilityGeneralization only to population of a given pain intensity levelGeneralization only to population having the same nociceptive focal
      EthicalAn adapted and tolerable stimulus is always applied (see IASP note)For some individuals, the applied stimulus can be unbearable
      • Zhang S
      • Li T
      • Kobinata H
      • Ikeda E
      • Ota T
      • Kurata J
      Attenuation of offset analgesia is associated with suppression of descending pain modulatory and reward systems in patients with chronic pain.
      or even harmful
      • Naert ALG
      • Kehlet H
      • Kupers R
      Characterization of a novel model of tonic heat pain stimulation in healthy volunteers.
      ,
      • Pedersen JL
      • Kehlet H
      Hyperalgesia in a human model of acute inflammatory pain: A methodological study.
      Pain measurementIn principle affected by lesser reliability
      • Woo CW
      • Wager TD.
      What reliability can and cannot tell us about pain report and pain neuroimaging.
      In principle more reliable due to greater variance in pain
      • Woo CW
      • Wager TD.
      What reliability can and cannot tell us about pain report and pain neuroimaging.
      Abbreviations: fMRI, Functional Magnetic resonance Imaging; IASP, International Association for Studying Pain.
      Thus, it can be suggested that a fixed intensity stimulation is best used in studies whose primary aim is the investigation of physiological responses such as skin conductance or BOLD signal which underly pain perception. Additionally, it has the benefit to not require previous exposure to the experimental stimulus, thereby minimizing the effect of pre-exposure and saving time (Table 1). Furthermore, fixed intensities allow to carefully control the adaptation to e.g., tonic stimulation that strongly depends on the transmitted energy.
      • Hashmi JA
      • Davis KD
      Effects of temperature on heat pain adaptation and habituation in men and women.
      A series of heat stimuli, 45°C lasting 30s each, lead to complete pain reduction on a trial-by-trial basis. In contrast, repeated application of heat of 47°C leads to stable pain level over time, however, such an observation seems to occur more frequently in male subjects.
      • Hashmi JA
      • Davis KD
      Effects of temperature on heat pain adaptation and habituation in men and women.

      Individual Stimulus Intensity

      The process of individualization of the pain intensity requires a pre-exposure to the stimulus during a so-called calibration procedure. Calibrations differ depending on the psychophysical testing procedures. Two common approaches are the method of limits and the method of levels (ramp and hold). In the former, the intensity is constantly increased (or decreased) while volunteers respond verbally or actively (e.g., by pressing a button) when a pain threshold or pre-specified pain intensity (e.g., 50/100) is reached,
      • Potvin S
      • Marchand S
      Pain facilitation and pain inhibition during conditioned pain modulation in fibromyalgia and in healthy controls.
      while in the latter, volunteers are exposed to series of stimuli of gradually increased and/or decreased intensity (e.g., temperature); volunteers decide posthoc (after the stimulus) weather the stimulus elicited the target sensation (pain threshold, or e.g., pain of 50/100) or provide their ratings in real-time.
      When compared, the method of levels produced lower thresholds compared to the method of limits.
      • Defrin R
      • Shachal-Shiffer M
      • Hadgadg M
      • Peretz C
      Quantitative somatosensory testing of warm and heat-pain thresholds: The effect of body region and testing method.
      Some calibration protocols rely on a random sequence of stimuli. As a result, a stimulus-response function is plotted, and a given value read out from the function. Not only different calibration protocols exist, also, within-protocol diversities have been reported (see
      • Defrin R
      • Shachal-Shiffer M
      • Hadgadg M
      • Peretz C
      Quantitative somatosensory testing of warm and heat-pain thresholds: The effect of body region and testing method.
      ,
      • Nahman-Averbuch H
      • Dayan L
      • Sprecher E
      • Hochberg U
      • Brill S
      • Yarnitsky D
      • Jacob G
      Pain modulation and autonomic function: The effect of clonidine.
      ,
      • Naugle KM
      • Riley JL
      Self-reported physical activity predicts pain inhibitory and facilitatory function.
      ). In that sense, the method of levels could be performed with different baseline temperatures, increase rates, stimulus durations, steps, and intervals. For instance, heat pain thresholds have been determined with rate of 2, 1 or 0.5°C/s. Some participants might require a longer calibration to identify the pre-specified pain intensity level and will be exposed to a larger number of noxious stimuli compared to other participants. This variability might lead to sensitization and could impact the results of the study.
      The outcome of the procedure is a unique individual intensity of noxious stimuli which is supposed to evoke similar pain levels across participants. The main advantage of this approach is that participants are exposed to stimuli causing -in theory- equal pain intensities, thereby reducing the possibility of floor and/or ceiling effects
      • Grouper H
      • Eisenberg E
      • Pud D
      The relationship between sensitivity to pain and conditioned pain modulation in healthy people.
      ,
      • Nahman-Averbuch H
      • Coghill RC RC
      Pain-autonomic relationships: Implications for experimental design and the search for an “objective marker” for pain.
      and at the same time ethical concerns: according to IASP “In any pain research, stimuli should never exceed a subject's tolerance limit and subjects should be able to escape or terminate a painful stimulus at will”. If pain intensity is the primary outcome this approach is likely to be preferable as it seems to be less prone to random noise, as noise might affect one of the tested calibration intensities but is not likely to impact all of them. In case of occurrence of unexpected sensitization or habituation, the original pain level can be restored using recalibration (see e.g.,
      • Traxler J
      • Madden VJ
      • Moseley GL
      • Vlaeyen JWS
      Modulating pain thresholds through classical conditioning.
      ). Furthermore, this approach bridges the inter-individual differences related to body-composition. For example, it has been shown that the body composition (different regions) and, e.g., BMI influence sensory thresholds (but see also
      • Emerson NM
      • Nahman-Averbuch H
      • Peugh JL
      • Coghill RC
      Pain sensitivity does not differ between obese and healthy weight individuals.
      ), and thus, pain sensitivity.
      • Dimova V
      • Oertel BG
      • Lötsch J
      Using a standardized clinical quantitative sensory testing battery to judge the clinical relevance of sensory differences between adjacent body areas.
      ,
      • Price RC
      • Asenjo JF
      • Christou NV
      • Backman SB
      • Schweinhardt P
      The role of excess subcutaneous fat in pain and sensory sensitivity in obesity.
      ,
      • Sierra-Silvestre E
      • Somerville M
      • Bisset L
      • Coppieters MW
      Altered pain processing in patients with type 1 and 2 diabetes: Systematic review and meta-analysis of pain detection thresholds and pain modulation mechanisms.
      Interestingly, in humans, heat perception thresholds are generally expressed as the temperature of the stimulation device, although thresholds for thermoreceptor activation have often been described at the receptor level.
      • Dubin AE
      • Patapoutian A
      Nociceptors: The sensors of the pain pathway.
      However, the temperature of the stimulated skin region depends on several factors, including the initial temperature of the skin, the diffusion capacity between the skin and the stimulation surface, the ability of the skin to distribute heat throughout the tissue, and the depth at which the thermoreceptors are located.
      • Treede RD
      • Meyer RA
      • Raja SN
      • Campbell JN
      Evidence for two different heat transduction mechanisms in nociceptive primary afferents innervating monkey skin.
      Thus, in experimental heat stimulation, the temperatures at the skin surface and at the level of the thermoreceptors may differ significantly. This can be remedied by mathematical (psychophysical) models, a further possible individualization of the stimulation intensity, here related to the parameters of intensity and duration, depending on the receptor depth or the thickness of the epidermis.
      • Dufour A
      • Després O
      • Pebayle T
      • Lithfous S
      Thermal sensitivity in humans at the depth of thermal receptor endings beneath the skin: Validation of a heat transfer model of the skin using high-temporal resolution stimuli.
      Calibration, or re-calibration can have an advantage over fixed intensities (Appendix 2) in studies with multiple observations (e.g., longitudinal), as significant intra-individual differences exist when pain is assessed over days or weeks
      • Mun CJ
      • Suk HW
      • Davis MC
      • Karoly P
      • Finan P
      • Tennen H
      • Jensen MP
      Investigating intraindividual pain variability: Methods, applications, issues, and directions.
      or even within minutes.
      • Madden VJ
      • Kamerman PR
      • Catley MJ
      • Bellan V
      • Russek LN
      • Camfferman D
      • Lorimer Moseley G
      Variability in experimental pain studies: Nuisance or opportunity?.
      Similarly, the effect of inter-individual factors that modulate pain perception is reduced in such an approach. In contrast, individualization hampers the reliability of the measurement. In response to a recent fMRI study focusing on reliability aspects of pain-related brain activity and pain reports,
      • Letzen JE
      • Boissoneault J
      • Sevel LS
      • Robinson ME
      Test-retest reliability of pain-related functional brain connectivity compared with pain self-report.
      Woo & Wager
      • Woo CW
      • Wager TD.
      What reliability can and cannot tell us about pain report and pain neuroimaging.
      proved with simulations that the reliability coefficient (ICC) is lower if the sample is characterized by a homogenous level of pain, typical for calibration studies. Finally, individualization solves the ethical dilemma and fully controls the amount of perceived pain, avoiding exposure to unbearable pain. Achieving an exact pain level, however, allows to generalize findings only to the targeted sample of patients who experience this exact pain level, e.g., pain around 50 out of 100. This rises several consequences that must be considered while planning the calibration procedure (Table 1):
      • i.
        This procedure results in a relative variability of stimulus intensity and may depend on the modality being used. As a result, the CoV differs significantly between the modality applied in respect to the determined intensity. For instance, CoV for the temperature used is about 5%,
        • Oudejans LCJ
        • Smit JM
        • van Velzen M
        • Dahan A
        • Niesters M
        The influence of offset analgesia on the onset and offset of pain in patients with fibromyalgia.
        in noxious cold it is 21%,
        • King CD
        • Goodin B
        • Kindler LL
        • Caudle RM
        • Edwards RR
        • Gravenstein N
        • Riley JL
        • Fillingim RB
        Reduction of conditioned pain modulation in humans by naltrexone: An exploratory study of the effects of pain catastrophizing.
        ischemic pressure 22%
        • Sandström A
        • Ellerbrock I
        • Tour J
        • Kadetoff D
        • Jensen KB
        • Kosek E
        Neural correlates of conditioned pain responses in fibromyalgia subjects indicate preferential formation of new pain associations rather than extinction of irrelevant ones.
        and 74% when electrocutaneous stimuli are used.
        • Höffken O
        • Özgül ÖS
        • Enax-Krumova EK
        • Tegenthoff M
        • Maier C
        Evoked potentials after painful cutaneous electrical stimulation depict pain relief during a conditioned pain modulation.
        Resulting in different stimulus intensities, this fact can significantly affect physiological data, for instance, BOLD signal or electro-dermal activity (EDA).
      • ii.
        Studies with this procedure cannot be directly compared to animal research in which individualization, if applied, is dictated by the variance of the physiological response, for instance, the latency of the withdrawal.
        • Tansley SN
        • Macintyre LC
        • Diamond L
        • Sotocinal SG
        • George N
        • Meluban L
        • Austin JS
        • Coderre TJ
        • Martin LJ
        • Mogil JS
        Conditioned pain modulation in rodents can feature hyperalgesia or hypoalgesia depending on test stimulus intensity.
        It is not possible to individualize self-reported pain in non-humans’ species because the way they potentially can do this is markedly different.
        • Crook RJ
        Behavioral and neurophysiological evidence suggests affective pain experience in octopus.
      • iii.
        Calibration, depending on the protocol employed, irritates the nociceptive system which is encompassed by the subsequent assessment.
      • iv.
        Calibration only partly reduces variance in pain ratings. Even though one calibrates the pain, it still varies among individuals and some investigators use a range of acceptable pain ratings such as 5-6 or 40-60.
        • Höffken O
        • Özgül ÖS
        • Enax-Krumova EK
        • Tegenthoff M
        • Maier C
        Evoked potentials after painful cutaneous electrical stimulation depict pain relief during a conditioned pain modulation.
        ,
        • Posada-Quintero HF
        • Kong Y
        • Nguyen K
        • Tran C
        • Beardslee L
        • Chen L
        • Guo T
        • Cong X
        • Feng B
        • Chon KH
        Using electrodermal activity to validate multilevel pain stimulation in healthy volunteers evoked by thermal grills.
        Thus, variability relates to not only peripheral input, but also pain ratings per se. This can be confirmed by our own data (see Fig 1) and all other reports with individual approaches.
        • Adamczyk WM
        • Manthey L
        • Domeier C
        • Szikszay TM
        • Luedtke K
        Nonlinear increase of pain in distance-based and area-based spatial summation.
        ,
        • Grouper H
        • Eisenberg E
        • Pud D
        The relationship between sensitivity to pain and conditioned pain modulation in healthy people.
        ,
        • Szikszay TM
        • Adamczyk WM
        • Luedtke K
        The magnitude of offset analgesia as a measure of endogenous pain modulation in healthy participants and patients with chronic pain: A systematic review and meta-analysis.
        Furthermore, calibration is not 100% precise. Determining the pain level using a calibration is not a prerequisite for that level in the later parts of the experiment (see examples of unsuccessful calibrations here).
        • Grouper H
        • Eisenberg E
        • Pud D
        The relationship between sensitivity to pain and conditioned pain modulation in healthy people.
        • Szikszay TM
        • Adamczyk WM
        • Carvalho GF
        • May A
        • Luedtke K
        Offset analgesia: Somatotopic endogenous pain modulation in migraine.
        • Szikszay TM
        • Adamczyk WM
        • Wojtyna E
        • Luedtke K
        Pain inhibition is not affected by exercise-induced pain.
        Figure 1
        Figure 1Individual differences in offset analgesia (OA) obtained in a within-subject experiment (unpublished data, Adamczyk et al., 2022). In an OA paradigm, noxious heat was applied for 9 seconds (a so-called T1 interval), then increased by 1 degree for another 9s (T2 interval) and then dropped to the original noxious temperature of T1 for 18s (here the T3 interval). On the left, curves from the calibrated paradigm: The T1 temperature was calibrated to elicit pain of 50/100 (temperature from 45 to 48). Note that there is a visibly greater area under the curves during T2 interval indicating that the calibration reduced the variance in the data. On the right, every volunteer received a T1 temperature of 46°C. All curves have been smoothed by applying a moving average. For methodology of obtaining these data please refer to .
      • v.
        Different intensities used in different individuals can hamper the results’ interpretation. This is important as together with the increase in the intensity of stimulation, different fibers are recruited.
        • Mouraux A
        • Iannetti GD
        • Plaghki L
        Low intensity intra-epidermal electrical stimulation can activate Aδ-nociceptors selectively.
        Activation of e.g. A-delta fibers characterizes different sensory quality and latency: when sensation can be described as “pricking”
        • Beissner F
        • Brandau A
        • Henke C
        • Felden L
        • Baumgärtner U
        • Treede RD
        • Oertel BG
        • Lötsch J
        Quick discrimination of Adelta and C fiber mediated pain based on three verbal descriptors.
        or “stabbing”
        • Price DD
        • Dubner R
        Mechanisms of first and second pain in the peripheral and central nervous systems.
        this is an indication for A-delta fiber activity. In turn, C fibers-mediated pain could be distinguishing by a quality of “dull”
        • Beissner F
        • Brandau A
        • Henke C
        • Felden L
        • Baumgärtner U
        • Treede RD
        • Oertel BG
        • Lötsch J
        Quick discrimination of Adelta and C fiber mediated pain based on three verbal descriptors.
        or “throbbing”, “cramping”, “aching”.
        • Price DD
        • Dubner R
        Mechanisms of first and second pain in the peripheral and central nervous systems.
        In general, the increase in the intensity of stimulation (mA) leads to overlapping activity of fibers with large and small dimeters: smaller fibers (e.g A-delta) have higher activation thresholds.

        Danner S, Wenger C, Rattay F: Electrical stimulation of myelinated axons: An interactive tutorial supported by computer simulation. 2011.

        ,
        • Handwerker HO
        • Kobal G.
        Psychophysiology of experimentally induced pain.
        ,
        • Inui K
        • Kakigi R
        Pain perception in humans: Use of intraepidermal electrical stimulation.
        This variability of sensory input provokes a perception of different quality.
        • Sunil-Kumar-Reddy K
        • Naidu MUR
        • Usha-Rani P
        • Ramesh-Kumar-Rao T
        Human experimental pain models: A review of standardized methods in drug development.
        Although this phenomenon has been investigated in depth in electrical stimuli, it does not mean that such a problem is irrelevant to other modalities, including heat. This problem seems to be more relevant for the calibration approach in which untilized intensity differs substantially.

      Contrasts in the Two Approaches

      Analytically, the two approaches differ significantly. As mentioned above, variance in the intensity used is only one side of the coin. One could hypothesize that the variance in pain ratings is lower when calibrating the intensity to e.g., an intensity of 50/100. Indeed, the differences can be visually observed in both approaches in Fig 1. Two approaches were compared in terms of pain ratings collected via an offset analgesia (OA) paradigm (Appendix 3). It is assumed that this paradigm reflects the efficiency in descending pain inhibition and relies on the application of stimuli consisting of three temperatures (T1, T2, T3). The intensity of the noxious stimulus during the T1 interval is equal to the intensity in the T3 interval, while the intensity at T2 is slightly higher. Collecting pain data continuously allows to track pain dynamically over time. In a typical OA effect, one can observe a significant drop in pain during the application of the T3 temperature (a so-called disproportional pain reduction). The OA effect can be observed either in the individualized (Fig 1, left) or fixed approach (Fig 1, right). The individualized temperature - even though calibrated for pain 50 - lead to a significant variance in pain ratings, however less than in the fixed approach. The consequence of the fixed approach is a relatively large number of volunteers needed to detect a difference in the effect under investigation. This is caused by the smaller effect size to be detected i.e., Cohen's d.
      • Kim H-Y
      Statistical notes for clinical researchers: Sample size calculation 1. comparison of two independent sample means.
      Thus, sample size underestimation is more likely when designing studies using a fixed approach but calculating the sample size based on data from the other approach.
      Fig 2 presents simulations of p values derived from 1000 paired student t tests comparing pain in the T1 vs the T3 interval (Fig 1). Scatterplot indicates that the likelihood for rejecting the null hypothesis is about 2 times higher if data are collected in an individualized fashion. Thus, one could consider this approach while conducting studies with small sample sizes (e.g., N=10), typical for invasive experiments (Fig 2, left). Problem decreases if simulated experiments are based on greater degrees of freedom e.g., N of 20 (Fig 2, right), or even disappears completely (N = 30, not shown).
      Figure 2
      Figure 2A scatterplot showing p-values distribution obtained from 1000 simulated student t tests. Each simulated test compared pain from T1 to T3 interval in offset analgesia (OA) paradigm obtained via calibrated (red) and fixed intensity (blue). In general, the magnitude of OA is huge (see pain that drops over time on Figure 1). Note that the likelihood for not rejecting a null hypothesis is 3.72 times higher if using a fixed approach (left, simulations for small samples, n=10), the likelihood was much reduced with simulations of larger samples (right, n=20). In this example, the problem vanishes with sufficiently large sample e.g., with n of 40 power in either approach was >0.99. The assumptions for simulations were that i) OA is a true effect as experimental studies clearly replicated that observation (see
      • Szikszay TM
      • Adamczyk WM
      • Luedtke K
      The magnitude of offset analgesia as a measure of endogenous pain modulation in healthy participants and patients with chronic pain: A systematic review and meta-analysis.
      for review), ii) the probability-density-function can be Gaussian described by two parameters: mean(s) and SD(s) for calibrated and fixed T1 and T3, respectively.

      Addressing the Dilemma

      One possibility to tackle the dilemma is a scenario which implies resource challenges. With a large sample size, it would be possible to divide subjects in subgroups by stimulus intensity as well as by pain intensity. A scenario that does not always seem feasible. Alternatively, it seems logical to implement a variety of stimulus ranges applied in a fixed manner to obtain both types of data: calibrated and fixed. In fact, many studies followed such a design.
      • Adamczyk WM
      • Manthey L
      • Domeier C
      • Szikszay TM
      • Luedtke K
      Nonlinear increase of pain in distance-based and area-based spatial summation.
      ,
      • Polianskis R
      • Graven-Nielsen T
      • Arendt-Nielsen L
      Modality-specific facilitation and adaptation to painful tonic stimulation in humans.
      ,
      • Weissman-Fogel I
      • Dror A
      • Defrin R
      Temporal and spatial aspects of experimental tonic pain: Understanding pain adaptation and intensification.
      The reverse is also possible. For example, in a study by Weissman-Fogel et al.
      • Weissman-Fogel I
      • Dror A
      • Defrin R
      Temporal and spatial aspects of experimental tonic pain: Understanding pain adaptation and intensification.
      , participants received tonic noxious heat stimulation calibrated to induce pain of 2, 4 and 6 out of 10. In the end, authors were able to dissect the effect of individualization from the fixed intensity stimulus. Namely, data from subgroups of individual participants that received their comparable stimulus intensities (temperature) were pooled and the absolute effect of temperatures on pain perception was analyzed. As such, a reversed scenario is possible, too. Applying a variety of intensities and subgrouping subjects into different perceptual categories.

      Concluding Remarks

      In summary, both approaches have their own pros and cons (Appendix 2), which should be considered when deciding on the study design, the aim, and the primary outcome. Whether there is a different variance depending on the pain modality should be systematically investigated in further studies. It is advisable to consider multiple factors (Table 1) when deciding if to individualize or fix the intensity. Apart from the solution based on an application of graded intensities, authors should clearly weight pros and cons in the context of their research question(s). For instance, if the main outcome of interest is objective, the fixed approach can be the method of choice. If the sample size is small, due to e.g., invasiveness of the procedure then calibration might be more suitable. Finally, we do agree that novel pain models and stimulation techniques must be developed that reduce the variability in the pain responses while keeping the intensity constant and since no gold standard exist in this methodological step, it is advisable to conduct reproducibility studies with the unused approach (calibration in the secondary report, if fixed was used in the original study). Furthermore, the awareness of the sample size problem and the generalization of results from a given study must be highlighted and future experimental studies are needed to determine the superiority of one method over another.

      Acknowledgments

      None.

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