The Journal of Pain, Vol 18, No 1 (January), 2017: pp 79-95Available online at www.jpain.org and www.sciencedirect.com

Extinction of Fear Generalization: A Comparison

Between Fibromyalgia Patients and Healthy Control

Participants

Ann Meulders,*,y Michel Meulders,z,x Iris Stouten,* Jozef De Bie,{,k

and Johan W. S. Vlaeyen*,y,***Research Group Health Psychology, yCenter for Excellence on Generalization Research in Health andPsychopathology, zCenter for Information Management, Modeling and Simulation, xResearch Group on QuantitativePsychology and Individual Differences, KU Leuven University, Leuven, Belgium.{Multidisciplinary Pain Center, Ziekenhuis Oost-Limburg, Genk, Belgium.kCentre for Translational Psychological Research (TRACE), Ziekenhuis Oost-Limburg and KU Leuven University, Leuven,Belgium.**Department of Clinical Psychological Science, Maastricht University, Maastricht, The Netherlands.

Received2016.A.M. is a(FWO VlsupporteResearchFlandersEFIC-Gr€uis currenMethusaID: METH

Abstract: Fear learning deficiencies might contribute to the development and maintenance of

chronic pain disability. Fear is often not restricted to movements (conditioned stimulus [CS1]) origi-

nally associated with pain (unconditioned stimulus), but expands to similar movements (generaliza-

tion stimuli [GSs]). This spreading of fear becomes dysfunctional when overgeneralization to safe

stimuli occurs. More importantly, persistence of pain-related fear to GSs despite corrective feedback

might even be more debilitating and maintain long-term chronic pain disability. Yet, research on this

topic is lacking. Using a voluntary joystick movement paradigm, we examined (extinction of) pain-

related fear generalization in fibromyalgia patients (FM) and healthy control participants (HC). Dur-

ing acquisition, one movement (CS1) predicted pain; another did not (CS�). We tested (extinction

of) fear generalization to 5 GSs varying in similarity with the CS1 and CS�. Results revealed flatter

pain expectancy generalization gradients in FM than in HC due to elevated responses to GSs more

similar to the CS�; the fear generalization gradients did not differ. Although pain-related fear and

expectancy to the GSs decreased during extinction, responses to the GSs remained higher for FM

than HC, suggesting that extinction of generalization is impaired in chronic pain patients. Persistence

of excessive protective responses may contribute to maintaining long-term chronic pain disability.

Perspective: Pain-related fear and expectancy to movements—varying in similarity with the original

painful and nonpainful movement—decrease during extinction in HC and FM. Yet, conditioned re-

sponses remain elevated in patients despite corrective feedback, indicating impaired extinction of

generalization. Persistent excessive protective responses may contribute to preserving pain disability.

ª 2016 by the American Pain Society

Key words: Pain-related fear, fear conditioning, fear generalization, voluntary movement paradigm,

fibromyalgia, learning, fear extinction.

Accumulating evidence indicates that pain-relatedfear plays a fundamental role in the transitionfrom acute to chronic disabling pain.59 Fear can

May 3, 2016; Revised September 2, 2016; Accepted October 3,

postdoctoral researcher of the Research Foundation Flandersaanderen), Belgium (grant ID: 12E3714N). This study was alsod by the Odysseus Grant ‘‘The Psychology of Pain and DisabilityProgram’’ to J.W.S.V. and funded by the Research Foundation(FWO-Vlaanderen), Belgium (grant ID: G090208N), and annenthal Research Grant (E-G-G ID: 169518451) to A.M. J.W.S.V.tly supported by the ‘‘Asthenes’’ long-term structural funding-lem grant provided by the Flemish Government, Belgium (grant/15/011).

be defined as an (phasic) emotional response to theanticipation or imminence of threat, harm, or in the spe-cific case of pain-related fear, pain. It is commonly

The authors have no conflicts of interest to declare.Supplementary data accompanying this article are available online atwww.jpain.org and www.sciencedirect.com.Address reprint requests to Ann Meulders, PhD, Department of Psychol-ogy, University of Leuven, Tiensestraat 102, box 3726, Leuven 3000,Belgium. E-mail: ann.meulders@ppw.kuleuven.be

1526-5900/$36.00

ª 2016 by the American Pain Society

http://dx.doi.org/10.1016/j.jpain.2016.10.004

79

80 The Journal of Pain Extinction of Generalization in Fibromyalgia

accepted that fear can at least be expressed in 3 responsesystems23: 1) verbal responding (cognition and affect), 2)psychophysiological responding (changes in autonomicresponding and reflex modulation), and 3) behavioralchanges (such as avoidance or removal from the sourceof threat). It has been shown repeatedly thatpain-related fear can be acquired via associativelearning.30-37,52 An initially neutral movement(conditioned stimulus [CS1]) that is associated withpain (unconditioned stimulus [pain-US]) starts to signaldanger and evokes protective behavior (conditioned re-sponses [CRs]), whereas the control stimulus never pairedwith the pain-US (CS�) does not trigger such responses.Fear learning is an adaptive mechanism, because theability to identify cues signaling threat and safety en-ables protective action against potential bodily harm.In the clinic, however, spreading of fear and avoidanceis observed beyond movements/activities that were asso-ciated with pain during the original pain episode. Forexample, when someone experiences a shooting backpain while lifting a box, it is possible (s)he will not onlyexpect to feel pain and be fearful to lift this specificbox, but also when lifting other objects (eg, a baby, ashopping bag). One possible mechanism accounting forthis spreading of fear is stimulus generalization.14,19,21

Stimulus generalization is also adaptive because itenables individuals to extrapolate the predictive valueof one stimulus to novel, similar stimuli withoutactually having to experience them. From anassociative learning perspective this implies that CRsmay extend to a range of novel stimuli resembling theoriginal CS1, with more similar generalization stimuli(GSs) evoking stronger CRs. Yet, together with reducingthe risk of missing positive threat alarms, which maycontribute to avoiding harm in a swiftly changingenvironment, generalization bears an increased risk torespond to false threat alarms. In particular, when fearspreads in an unbridled way, stimulus generalizationbecomes maladaptive and may lead to dysfunctionalprotective behaviors and culminate in severe paindisability.In a previous study, we showed that fear indeed

spreads selectively toward novel movements resemblingthe original painful movement in healthy pain-free con-trol participants (HC), but fear generalized in a nondif-ferential way in fibromyalgia patients (FM).30 Inanother study using a scenario contingency learningtask with the verbal labels ‘pain’ and ‘no pain’ as out-comes and pictures of handpostures as cues,we reportedthat chronic hand pain patients overgeneralized pain-outcome expectancy to novel cues that were moresimilar to the original ‘safe’ cue compared with HC.29

We argued that excessive generalization might beinvolved in the etiology of chronic pain disability byspreading of undesired protective behaviors. Moreover,persistence of pain-related fear and expectancy to tech-nically safe, unreinforced GSs despite corrective feed-back might even be more debilitating and maintainchronic pain disability in the long run, however, to ourknowledge this has never been tested. Therefore, thisstudy aimed to test pain-related fear generalization

and its extinction in FM and HC using a voluntary joystickmovement conditioning task. We hypothesized that FMwould show: 1) flatter generalization gradients thanHC due to higher responses to the GSs that moreresemble the original CS�, 2) impaired extinction of un-reinforced GSs, whereas generalized pain-related fearwill subside quickly in HC.

Methods

ParticipantsThis study used a convenience sample of 60 partici-

pants including 2 age-matched diagnostic groups: 30FM (29 female, mean 6 SD age = 41 6 11; range, 22–59 years), and 30 HC (mean 6 SD age = 41 6 12; range,21–60 years). In line with Meulders et al,30 we chosenot to use absolute age-matched groups, but used 5-year ranges to match the HC to the FM group. Themost important inclusion criterion for the FM groupwas to be diagnosedwith fibromyalgia and experiencingsome interference in their daily life because of this condi-tion. All patients satisfied the American College of Rheu-matology new diagnostic criteria for fibromyalgia58 onthe basis of the combinedWidespread Pain Index (range,0–19) and Symptom Severity Score (range, 0–12; Table 1).The inclusion criterion for the HC group was to not havefibromyalgia. Exclusion criteria for both groups were:any other chronic pain conditions, diagnosed dyslexiaor analphabetism, pregnancy, current or history of car-diovascular disease, chronic or acute respiratory disease(eg, asthma, bronchitis), neurological diseases (eg, epi-lepsy), uncorrected hearing problems, having pain atthe dominant hand,wrist, or arm that hindersmovementof a joystick painlessly, cardiac pacemaker or the pres-ence of any other electronic medical devices, and thepresence of any other severe medical conditions. Anadditional exclusion criterion only for the HC groupwas: any current or past psychiatric disorder includingclinical depression and panic/anxiety disorder. Becauseof its high comorbidity with depression, other mood dis-orders, and anxiety,22,48,55 this additional criterion wasomitted in the FM group. Participants were recruitedvia social media and from pain clinics in the Limburgregion (Belgium). The study protocol was approved bythe Social and Societal Ethics Committee of the KULeuven (registration number: S-56226), the MedicalEthical Committee of Ziekenhuis Oost-Limburg, and theMedical Ethical Committee of the University Hospital ofKU Leuven (ML10116). All participants signed theinformed consent form, which explicitly stated thatthey were allowed to decline participation at any timeduring the experiment. To compensate for their timeand effort, FM received the book ‘Mastering Your Pain’[de Pijn de Baas] by Frits Winter,57 HC received a box ofBelgian chocolates; both remunerations had an approx-imate value of V15. As expected, FM had lower educa-tional level, were more likely to be unemployed, andwere taking more medication than the HC. Moredetailed demographic and clinical characteristics can befound in Table 1.

Table 1. Demographic and Clinical Characteristics for the FM Group (n = 30) and the HC Group(n = 30) Separately

CHARACTERISTIC

FM GROUP HC GROUP

T DFMEAN SD MEAN SD

Selected pain intensity level, mA 18.73 8.53 22.90 12.18 1.54 58

Selected self-reported pain intensity (range 1–10) 7.87 .35 7.98 .18 1.40 58

Age, y 40.80 11.28 40.63 11.98 �.06 58

WPI (range 0–19) 9.43 3.89

SS (range 0–12) 8.13 1.66

Highest education level, %

Primary school 10 0

Vocational secondary education 23 0

Technical secondary education 17 10

General secondary education 7 13

Professional bachelor’s degree 10 10

Academic bachelor’s degree 20 47

Master’s degree 3 17

Other 10 3

Type of medication, %

Antidepressants 40 0

Anxiolytics 20 0

Analgesics (opioids) 33 0

Analgesics (nonopioids) 43 0

Other 47 7

Abbreviations: SS, Symptom Severity Score; WPI, Widespread Pain Index.

NOTE. Total N = 60. ForWPI, the higher the score, themore pain complaints on different sites of the body during the past week; for SS, 0 = no symptoms, 12 = verymuch

pain symptoms. Other medication includes: muscle relaxants, hormones, antihypertension, antiarrhythmic agents, gastric ulcer medication, dopamine, synthetic thy-

roid hormone, psoriasis medication, magnesium supplements, and probiotics.

Meulders et al The Journal of Pain 81

Stimulus Material and ApparatusThe experiment was run on a Windows 7 computer

(Dell OptiPlex 7010; Dell Inc., Round Rock, Texas) with4096MB RAM and an Intel Core i5-3570 CPU processor(Intel Corp., Santa Clara, California) at 3.40 GHz andan AMD Radeon HD 7570 graphics card (Advanced Mi-cro Devices, Sunnyvale, California) with 2542MB ofvideo RAM. Experimental stimuli were presented on a19-inch computer screen and were controlled withthe free experimental software package Affect 4.0.47

The data were stored using a National Instrumentsdata acquisition card (National Instruments Corp, Aus-tin, TX). The conditioned stimuli (CSs) and the GSswere 7 equally spaced movement quadrants, whichare part of a semicircle (Fig 1). These proprioceptivestimuli consisted of moving a Paccus Hawk joystick (Pac-cus Interfaces BV, Almere, The Netherlands) with thedominant hand within 1 of the 7 movement quadrants.Movements in quadrant 1 (ie, 90� to the left) and inquadrant 7 (ie, 90� to the right) served as CSs, andmovements in the intermediate (2–6) quadrants, servedas GSs. During acquisition, one movement direction(CS1) was followed by the pain-US (75% of the trials),whereas the other movement direction was never fol-lowed by the pain-US (CS�); which movement quad-rant (1 or 7) served as CS1 or CS� wascounterbalanced across participants. The pain-US wasa painful electrocutaneous stimulus (2-ms duration),generated by a commercial constant current stimulator(DS7A, Digitimer, Welwyn Garden City, United

Kingdom) and administered through surface Sensor-Medics electrodes (8 mm; SensorMedics Corp., YorbaLinda, California) filled with K-Y gel (Johnson & John-son, New Brunswick, New Jersey) that were attachedto the wrist of the dominant hand. Before the experi-ment started, participants went through a calibrationprocedure: they received a series of electrocutaneousstimuli of increasing intensity and were asked to indi-cate how intense each stimulus was on a scale from 1to 10 where ‘1’ means: ‘‘you feel something but this isnot painful, it is merely a sensation’’; ‘2’ means: ‘‘thissensation starts to be painful, but it is still a very mod-erate pain’’ up to ’10,’ which means: ‘‘this is the worstpain you can imagine.’’ A subjective stimulus intensityof ‘8,’ which refers to a stimulus that is ‘‘significantlypainful and demanding some effort to tolerate’’ wastargeted. The mean self-reported stimulus intensitywas 7.87 (SD = .35, range = 7–8) for the FM group,and 7.98 (SD = .18, range = 7–8) for the HC group.The mean physical stimulus intensity (in mA) was18.73 (SD = 8.53, range = 8–48) for the FM group, and22.90 (SD = 12.18, range = 11–60) for the HC group.Conditioned pain-related fear was assessed

through self-reports as well as a psychophysiologicalindex of fear learning, that is, the eye blink startleresponse. The eye blink startle response is a compo-nent of the reflexive cross-species, full-body defen-sive response mobilization, which is triggered bystartle-evoking stimuli (eg, acoustic startle probe)and can be measured by the tension in the muscles

Figure 1. Schematic overview of the experimental task during the generalization phase.

82 The Journal of Pain Extinction of Generalization in Fibromyalgia

underneath the eye. Startle modulation refers to thepotentiation of the startle reflex during fear stateselicited by the anticipation of an aversive stimulus(eg, an electrocutaneous stimulus). In the presentsetup, the startle probe was a 100 dBA burst of whitenoise with instantaneous rise time presented binau-rally for 50 ms through headphones (PhilipsSHP2500; Philips, Amsterdam, The Netherlands). Eyeblink startle responses elicited by startle probes deliv-ered during the CS/GS movements served as an indexof cued pain-related fear. Eye blink startle responseselicited by startle probes during the intertrial interval(ITI) served as an index of contextual pain-relatedfear.

Experimental SettingParticipants were seated in an armchair (.6-m screen

distance) in a sound-attenuated and dimmed experi-mental room, adjacent to the experimenter’s room.Further verbal communication was possible through an

Table 2. Study Design Summary

PRACTICE (2 � 8 TRIALS) HABITUATION (8 TRIALS)ACQ

(3 �2 � [4 � CS1 only] 8 Probes 3 � (4

2 � [4 � CS�] 3 � (4

NOTE. CS = movement quadrant 1 and 7; GS = movement quadrants 2-6; pain-US =

refer to the movement direction that is followed by the pain-US (75% reinforcement) a

to indicate nonreinforcement of the CS1movement (ie, during the practice phase). G

the same experimental procedure.

intercom system; the experimenter observed the partici-pants and their physiological responses online by meansof a closed-circuit TV installation and computer monitors.

ProcedureThe experimentwas conducted during a 2-hour session

and comprised 6 experimental phases: a preparationphase, a practice phase, a habituation phase, an acquisi-tion phase, a transfer of acquisition phase, and a gener-alization phase. The procedure is largely based onMeulders et al.32 In a mixed design (Table 2) participantsin both groups (between-subjects factor: FM vs HC)received a pain-US after moving the joystick to theCS1 direction on 75% of the trials, but never receiveda pain-US when moving the joystick to the CS� direction(within-subjects factor: CS1 vs CS�). Note that themove-ment direction that served as the CS1 and CS�was coun-terbalanced across participants. During acquisition,participants freely chose on each trial in which directiontheywere going tomove the joystick. During the transfer

UISITION

8 TRIALS)TRANSFER OF ACQUISITION

(8 TRIALS)GENERALIZATION

(4 � 7 TRIALS)

� CS1) 4 � CS1 4 � CS1

� CS�) 4 � CS� 4 � CS�4 � GS1-5

(painful electrocutaneous stimulus) 2-ms duration; CS1 and CS�, respectively,

nd the movement that is never followed by the pain-US. The suffix ‘‘only’’ is used

S movements are never reinforced. Both groups (FM and HC) were subjected to

Meulders et al The Journal of Pain 83

of acquisition, however, they could no longer choose theorder of the movements themselves, but the movementdirection was ‘‘signaled.’’ During generalization, thesame signaling procedure was used to test the spreadingof conditioned fear to novel intermediate movement di-rections (GSs).

Preparation Phase

Upon arrival to the laboratory, participants wereinformed that pain-USs and short loud noises (acousticstartle probes) would be administered during the exper-iment. Participants were also told that they were free todecline participation at any time without any conse-quences. Subsequently, written informed consent wasobtained. Before selecting the intensity of the painfulstimulus following the calibration procedure, electrodesfor the eye blink startle responses and the electrocutane-ous stimulus were attached (see the Stimulus Materialand Apparatus section).

Practice

Fig 1 provides an illustrative overview of the designand the voluntary joystick movement task. Before start-ing the practice phase, participants received detailed in-structions about the experimental task. At the beginningof each trial the cursor representing the joystick neededto be positioned in the middle of the screen, so that thejoystick was standing upright and centered. Whenprompted by a starting signal ‘‘1’’ (fixation cross pre-sented in the middle of the computer screen), partici-pants moved the joystick as quickly and accurately aspossible, in whatever order they freely chose. Theywere requested to move the joystick toward the counterbars, each divided in 4 equal segments, positioned at theend of the 2 equally spaced movement quadrants (1 and7). Successful movements always resulted in coloring 1segment of the corresponding counter bar blue. Thatway, participants could instantly ascertain how manymovements in each direction still were to be performed.During this phase, visual feedback about the position ofthe joystick (ie, visualized by the cursor) and the perfor-mance of the movements was provided on the computerscreen. The movement quadrants were delineated withwhite borders. Whenever the cursor (representing thejoystick position) entered a quadrant, this area turnedgreen. When the cursor wrongly left the movementquadrant, this area turned red. During the practice phaseparticipants completed 16 valid movements, that is, 2blocks of 8 trials (4 left/4 right) in total. No pain-USs orstartle probes were presented during this phase, butthe experimenter provided verbal feedback about theperformance of the joystick movements.

Startle Habituation

Because the first responses to the startle probes areusually relatively large, we included a phase to habituateto the probes, to correct for such possible confounds dur-ing the data collection. This habituation phase consistedof 8 trials, each lasting 12 seconds, with an ITI of 2 sec-onds. During each trial, 1 startle probe (100 dBA burst

of white noise) was delivered between the eighth andthe 12th second after trial onset. Participants wore head-phones, and the lights in the experimental room weredimmed. No pain-USs were delivered during this phase.

Acquisition

This phase was basically the same as the practice phase(Fig 1), but now: 1) no verbal feedback was administeredabout the task performance, 2) pain-USs and startleprobes were presented, 3) the movement quadrantswere not delineated with white borders and they didnot turn green/red when the cursor entered/left the cor-responding movement quadrant; the cursor was nolonger visible for the participant, 4) instructions empha-sized to pay close attention to the starting signal ‘‘1’’ andto respond as fast and accurately as possible upon its pre-sentation.The acquisition phase consisted of 3 blocks of 8 trials.

Each block contained 4 trials to the left and 4 trials tothe right. Although a CS movement was of variablelength depending on the participants’ movement speed,a trial typically included an ITI consisting of a pre-CS in-terval of 3 seconds and a post-CS interval of 8 seconds.The pain-US was presented on 75% of the CS1 trials,but never on CS� trials. In each block of 8 CSmovements,4 of the startle probes were presented during the CSmovements, and 4 during the ITI (between 3000 and6000 ms after the movement was executed). Note thatwe did not inform the participants about the contin-gencies between the joystick movements (CSs) and thepain-US. After each conditioning block, participantsrated the pain-related fear elicited by each of the CSmovements.

Transfer of Acquisition

Transfer of acquisition trials were identical as thoseduring the acquisition phase, with the exception thatparticipants could no longer freely choose in which or-der they performed the CS movements. More specif-ically, 3000 ms after trial onset, a green border waspresented around the counter bars of 1 of the move-ment quadrants to indicate in which direction partici-pants were to move. Before actually performing themovement, participants rated to what extent they ex-pected to receive a pain-US after the to-be-performedmovement, and to what extent they were afraid toperform that movement. After answering these ques-tions, participants waited for the starting signal toappear and started moving into the signaled direction.After successfully performing the signaled CS move-ment, a post-CS ITI of 8 seconds followed. During thetransfer phase, 1 block of 8 trials (4 left/4 right) wasrun. The reinforcement scheme and timing scheme ofthe presentation of the startle probes remained thesame as during the acquisition phase.

Generalization

The procedure of the generalization phase was mainlythe same as the transfer of acquisition phase. The differ-ence was that participants now had to perform 5 novel

84 The Journal of Pain Extinction of Generalization in Fibromyalgia

movements (GSs) into intermediate movement quad-rants (2–6) between the CS1 and the CS�. Movementsto quadrant 1 or 7 still served as CS1 and remained rein-forced at the same rate; the CS� and GSs, however, werenever followed by the pain-US. The generalization phaseconsisted of 28 trials, in which participants performed 4trials of each movement (ie, quadrants 1–7). One blockconsists of 7 movements (1 in eachmovement quadrant);the order of the movements was randomized across par-ticipants. As before, after a 3-second pre-CS/GS ITI, themovement direction was signaled by a green borderaround the counter bar of the corresponding movementquadrant and participants rated their pain-US expec-tancy and pain-related fear. Next, the fixation cross ap-peared and participants moved the joystick to thesignaled direction. After successfully performing themovement, again a post-CS/GS ITI of 8 seconds followed.On each trial a startle probewas delivered during the GS/CS movement; no ITI probes were delivered.

Measures

Manipulation Checks

Affective valence, arousal, and control. After thepractice, acquisition, and generalization phase, partici-pants completed 3 Self-Assessment Manikin (SAM)scales4 measuring affective valence, arousal, and thecontrol they experienced when performing the CSmovements. The SAM scales each consisted of 5 differentpictographs of humanlike figures—manikins. Thesemanikins differ in emotional expressions rangingrespectively from ‘‘happy’’ to ‘‘unhappy,’’ ‘‘very aroused’’to ‘‘calm,’’ and ‘‘no control’’ to ‘‘a lot of control.’’ Partic-ipants selected the manikin that matched best how theyfelt when performing the respective CS movements. Re-sponses were scored from 1 to 5 (happy/very aroused/nocontrol–sad/calm/a lot of control).

Main Outcome Variables

Self-reported fear of movement-related pain. Aftereach block, participants answered the following ques-tion: ‘‘How afraid were you to perform the left/rightmovement?’’ on an 11-point Likert scale ranging from0 to 10 with anchors ‘not fearful at all’ to ‘extremelyfearful.’ During the transfer of acquisition and thegeneralization phases, participants rated before eachmovement how afraid they were to actually perform thesignaled movements (CSs/GSs).Pain-US expectancy during transfer of acquisition and

generalization. During the transfer of acquisition andgeneralization phases, participants rated before eachmovement to what extent they expected the painfulstimulus to occur when performing the signaled move-ments (CSs/GSs) on an 11-point Likert scale (range, 0–10) with labels ‘not at all’ to ‘very much.’Eye blink startlemodulation. Orbicularis oculi electro-

myographic activity (EMG) was recorded with 3 Ag/AgClSensorMedics electrodes (4 mm) filled with electrolytegel. After cleaning the skin with exfoliating peelingcream to reduce interelectrode resistance, electrodes

were placed on the left side of the face according tothe site specifications proposed by Blumenthal et al.2

The raw signal was amplified by a Coulbourn isolatedbioamplifier with bandpass filter (LabLinc v75-04;Coulbourn Instruments, Whitehall, Pennsylvania). Therecording bandwidth of the EMG signal was between90 Hz and 1 kHz (63 dB). The signal was rectified onlineand smoothed by a Coulbourn multifunction integrator(LabLinc v76-23 A) with a time constant of 20 ms. TheEMG signal was digitized at 1000 Hz from 200 ms beforethe onset of the auditory startle probe until 1000 msafter probe onset.Pain-US intensity and unpleasantness. After each

block, participants indicated the unpleasantness andthe intensity of the painful stimulus on an 11-point Lik-ert scale (range, 0–10) with labels ‘‘not unpleasant at all’’and ‘‘very unpleasant,’’ and ‘‘not painful at all’’ and ‘‘verypainful.’’

Postexperimental Questionnaires

After the data collection, all participants completed abattery of questionnaires tomap out possible psycholog-ical differences between the FM group and the HC groupusing a Web survey tool. The scores on these question-naires can be found in Table 3.Pain severity. The Chronic Pain Grade Scale (CPGS)53

assesses pain intensity and interference with normaldaily activities using 7 items (eg, How would you rateyour pain at this moment?). Answers on 6 of the 7 itemsrange from 0 ‘‘no pain’’ to 10 ‘‘pain as bad as it could be.’’The one remaining item requires filling in the number ofdays that pain has kept respondents from their typicaldaily activities in the past 6 months (range, 0–180). Onthe basis of the pain intensity score, the disability points(on the basis of the disability score and the days ofdisability) respondents are classified into 4 grades ofchronic pain: grade I, low disability-low intensity; gradeII, low disability-high intensity; grade III, high disability-moderately limiting; and grade IV, high disability-severely limiting.Pain cognitions. The Pain Cognition List (PCL)49 con-

sists of 39 items divided into 5 subscales (catastrophizing,limitation, optimism, internal control, and trust). Eachitem presents a specific pain cognition statement (eg,‘‘My thoughts are always concentrated on the pain’’) andthe respondent is asked to indicate (dis)agreement on a5-point Likert scale. Items are scored from 1 ‘‘totallydisagree’’ to 5 ‘‘totally agree,’’ and a sum score is ob-tained per subscale (catastrophizing: range, 16–80; limi-tation: range, 7–35; optimism: range, 7–35; internalcontrol: range, 5–25; and trust: range, 4–20).Fear of movement. The Tampa Scale of Kinesiophobia

(TSK)42 comprises 17 items intended to assess fear ofmovement and fear of (re)injury. Respondents are askedto indicate to what extent each of the statements (eg,‘‘My body tells me that there is something seriouslywrongwith it’’) reflects a true description of the assumedassociation between movement and (re)injury on a4-point Likert scale, ranging from 1 ‘‘strongly disagree’’to 4 ‘‘strongly agree’’ (total score range, 17–68).

Table 3. Questionnaire Scores for the FM Group and HC Group Separately

QUESTIONNAIRE

FM GROUP (N = 29) HC GROUP (N = 30)

T DFMEAN SD MEAN SD

CPGS, pain intensity* 65.29 14.18 15.56 11.82 14.65 57

CPGS, pain disability* 56.90 17.36 5.22 8.29 14.66 57

CPGS, number of days disability* 44.90 66.47 1.00 2.85 3.61 57

PCL, catastrophizing* 44.17 13.36 30.23 12.75 4.10 57

PCL, limitation* 27.14 5.12 16.20 5.31 8.05 57

PCL, optimism 23.86 4.31 26.37 6.26 �1.79 57

PCL, internal control 16.1 4.18 18.20 3.77 �2.02 57

PCL, trust 13.41 2.82 14.40 3.32 �1.23 57

TSK, total score* 41.07 7.93 34.33 6.80 3.51 57

FIQ, total score* 55.35 15.15 13.82 7.09 13.56 57

PANAS, positive affect* 30.45 7.87 40.43 4.82 �5.90 57

PANAS, negative affect* 24.14 8.53 14.97 4.06 5.30 57

HADS, anxiety* 17.72 3.24 19.90 1.97 �3.13 57

HADS, depression 15.45 2.23 16.00 1.36 �1.15 57

FPQ, medical pain 18.90 6.76 20.27 5.41 �.86 57

FPQ, minor pain 16.72 4.98 13.67 3.19 2.82 57

FPQ, severe pain 24.14 8.36 26.73 8.91 �1.15 57

FPQ, total score 59.76 16.30 60.67 13.16 �.24 57

PTQ, total score* 27.76 13.79 14.30 11.42 4.09 57

NOTE. Total sample N = 59. One patient failed to fill out the questionnaires. CPGS subscales are calculated for pain intensity (items 1-3), pain disability (items 4-6) and

days of disability (item 7); PCL subscales are calculated for catastrophizing, limitation, optimism, internal control, and trust; PANAS subscales are calculated for positive

affect and negative affect; HADS subscales are calculated for anxiety and depression; FPQ subscales are calculated for medical pain, minor pain, and severe pain. On the

basis of the CPGS scales, 10% (3 of 29) of the FMwere classified as grade I (low disability, low intensity), 28% (8 of 29) as grade II (low disability, high intensity), 14% (4

of 29) as grade III (high disability, moderately limiting), and 48% (14 of 29) as grade IV (high disability, severely limiting).

*P < .05, after Holm-Bonferroni corrections.

Meulders et al The Journal of Pain 85

Pain disability. The Fibromyalgia Impact Question-naire (FIQ)5 assesses the effect of fibromyalgia on therespondent’s daily activities. The FIQ is composed of 10items. The first item contains 11 questions (eg, ‘‘Can youindependently do the dishes’’) related to physical func-tioning—each question is rated on a 4-point Likert typescale. Items 2 and 3 ask the respondent to mark thenumber of days they felt well and the number of daysthey were unable to work (including housework)because of pain symptoms. Items 4 through 10 are hori-zontal linear scales marked in 10 increments on whichthe respondent rates work difficulty, pain, fatigue,morning tiredness, stiffness, anxiety, and depression.After normalization, the total score ranges from 0 to 100,with 0 indicating no impairment at all and 100maximumimpairment.Affect. The trait version of the Positive and Negative

Affect Schedule (PANAS)12,54 consists of 20 itemsdivided into 2 subscales. Participants are asked toindicate to what extent, in their normal daily life, theyexperience the feelings defined by the 20 descriptorsusing a 5-point response scale ranging from ‘‘very lit-tle’’ to ‘‘a lot.’’ Ten items describe positive feelings andassess positive affectivity (range, 10–50) and 10 itemsdescribe negative feelings and assess negative affectivity(range, 10–50).Depression and anxiety. The Hospital Anxiety Depres-

sion Scale (HADS)46,60 consists of 14 items divided into 2subscales (anxiety and depression). Respondents areasked to indicate for each item (eg, ‘‘I still enjoy thethings I used to enjoy’’), which answer reflects best

how they felt during past week. Answers are scoredfrom 0 to 3. The scores for the depression subscale andthe anxiety subscale range from 0 to 21.Fear of pain. The Fear of Pain Questionnaire (FPQ)28,43

measures fear and anxiety associated with pain. The FPQis composed of 30 items divided into 3 subscales (severepain, minor pain, and medical pain). Respondents arerequested to indicate how fearful they would be ifthey were experiencing the pain described in the items(eg, ‘‘Breaking your arm’’). Answers are scored from0 ‘‘not fearful’’ to 5 ‘‘extreme fearful.’’Rumination. To explore the way participants typically

think about negative experiences and problems, weadministered the Perseverative Thinking Questionnaire(PTQ).10 This questionnaire comprises 15 items. Theitem pool included 3 items for each of the characteristicsof repetitive negative thinking: repetitive, intrusive,difficulty to disengage from, unproductive, andcapturing mental capacity. Participants had to rate theextent to which the 15 statements applied to them, on ascale ranging from 0 ‘‘never’’ to 4 ‘‘almost always.’’

Response Definition and Data AnalysisOverview

ResponseDefinition of the StartleModulation

Using PSPHA (Psychophysiological Analysis; PSPHAv21/2/2011),8 a modular script-based program, we calculatedthe peak amplitudes defined as the maximum of theresponse curvewithin 21 to 175ms after the startle probe

86 The Journal of Pain Extinction of Generalization in Fibromyalgia

onset. All startle waveforms were visually inspected off-line, and technical abnormalities and artifacts were elim-inated using the PSPHA software. Each peak amplitudewas scored by subtracting its baseline score (averagedEMG level between 1 and 20 ms after the probe onset).A startle response was rejected if the baseline periodwas contaminated with noise or if a voluntary blinkoccurred during a 1 to 20 ms time window after probeonset, also when no visual peak could be detected (ie,nonresponse), the response was rejected. Participantswho failed to reach elevated peak amplitudes comparedwith baseline on more than 50% of the trials wereconsidered nonresponders and were excluded fromfurther analyses. A total of 5 participants (1 HC and 4FM) were excluded because of the absence of reliablestartle eye blink responses. Hence, the statistical analysisof the psychophysiological measure was run on a totalsample of 55 participants. To make data comparable be-tween individuals, despite the interindividual differ-ences in physiological reactivity, raw scores weretransformed to z scores. Furthermore, T scores (a lineartransformation of the z scores) were used in the illustra-tions, to avoid negative values on the Y-axis and for anoptimized visualization of the startle data. Averageswere calculated for responding during CS/GSmovementsand ITI separately for both groups.

Data Analysis Overview

Some preparatory analyses were necessary to test ourmain research questions: First, as a manipulation check,we carried out a series of repeated measures analysesof variance (RM ANOVAs) to confirm that before condi-tioning both CSs did not differ with respect to affectivevalence, arousal, and control experienced when per-forming the movements, but that after acquisition andgeneralization the CS1 became more negative, elicitedmore arousal, and less feelings of being in control thanthe CS�. Because we had clear a priori hypotheses, wefurther analyzed the data using planned comparisons.The effect size indication h2

G is reported for all omnibusANOVA effects,1,40 and Greenhouse-Geisser correctionswere applied when appropriate (uncorrected degreesof freedom and corrected P values are reported togetherwith ε). Second, because successful differential fearacquisition is a prerequisite to test fear generalization,we checked whether participants reported more fear inresponse to the CS1 movement than to the CS� move-ment at the end of the (transfer of) acquisition trainingcompared with the beginning. Therefore, we defined arandom intercept 2-level linear regression model toanalyze the test effects of stimulus type (CS1/CS�) onthe change in average pain-related fear ratings during4 repeated measurements (ie, during the 3 blocks ofacquisition [A1/A2/A3] and 1 block of the transfer ofacquisition phase [T1]), nested within persons for bothgroups (FM and HC; see the Supplementary Text for thedetailed statistical model description). The effectsincluded in thismodelwere estimated simultaneously us-ing the SAS procedure MIXED.44,50 The model explains63% of the observed variance in participants’ pain-related fear ratings (Supplementary Table 1). We chose

to include a subject-specific random intercept in themodel, because 48% of the variance was due to differ-ences among participants: s2

q=ðs2q1s2

εÞ. Third, we tested

the acquisition effects in the startle eye blink measuresusing a RM ANOVA, that is, whether participants hadelevated startle responses to the CS1 movementcompared with the CS� movement during (transfer of)acquisition.After these preparatory analyses, we proceeded to test

our main hypotheses: following Meulders et al,30 we ex-pected that: 1) HC would show an immediate transfer ofdifferential pain-related fear and expectancy learning,whereas FM would show an initial loss of differentiallearning due to increased CS� responding, 2) FM wouldshow flatter generalization gradients than HC and thatthis is caused by higher responses to the GSs that aremore similar to the original CS�. Testing extinction offear generalization we expect that: 3) FM would showresistance to extinction to unreinforced GSs, whereasgeneralized fear will subside quickly in HC. To test thefirst hypothesis, we defined two 2-level linear regressionmodels (1 for each dependent variable) with 2 randomintercepts (1 for each stimulus type) including group(FM and HC), linear trend variable Tj (which equals 0, 1,2, and 3 for trials j = T1, T2, T3, T4), as well as the interac-tion between the linear trend and group. The dependentvariables were ratings of pain-US expectancy and fear ofmovement-related pain (see the SupplementaryMaterialtext for the detailed statistical model description). Theeffects included in this model were estimated simulta-neously using the SAS procedure MIXED.44,50 Bothmodels are able to predict participants’ pain-US expec-tancy and fear of movement-related pain ratings(Supplementary Table 2) very well because they explain84% and 83%, respectively, of the observed variance inratings. Including random intercepts was necessarybecause models without random intercepts explainrespectively only 25% and 18% of the variance inobserved ratings. In other words, a substantial part ofthe observed variance in ratings is due to participant dif-ferences.To test the second and third hypothesis, we defined

two 2-level linear regressionmodels (1 for pain-US expec-tancy and 1 for fear of movement-related pain) with asubject-specific random intercept including group (FMand HC), trial (first and last trial of the generalizationphase), a centered linear trend variable Tk (which equals�3, �2, �1, 0, 1, 2, and 3 for stimulus k = CS1, GS1, GS2,GS3, GS4, GS5, GS6, and CS�) modeling the linearcomponent of the generalization gradient togetherwith a quadratic trend variable (Tk)

2 modeling thequadratic component of the generalization gradient,as well as the interaction terms between group andthe linear (quadratic) trend variable (see theSupplementary Material text for the detailed statisticalmodel description). The inclusion of the quadratic trendis supported by the data because the models including aquadratic trend have a better balance betweencomplexity and goodness of fit (ie, they have lowervalues of the Bayesian information criterion).45 Bayesianinformation criterion values for themodel including only

Meulders et al The Journal of Pain 87

the linear trend were higher than for the modelincluding the linear as well as the quadratic trend, forboth dependent variables, respectively pain-US expec-tancy: 3,914 versus 3,874, and fear of movement-related pain: 3,414 versus 3,396. The effects included inthis model were estimated simultaneously using theSAS procedure MIXED.44,50 Both models are able topredict participants’ pain-US expectancy and fear ofmovement-related pain ratings (SupplementaryTable 3) quite well because they explain 62% and 65%,respectively, of the observed variance in ratings.Including random intercepts was necessary becausemodels without random intercepts explain respectively,only 22% and 15% of the observed variance in ratings.Hence, a substantial part of the observed variance in rat-ings is due to participant differences. For all multilevelmodels, follow-up contrasts were calculated to furthertest our a priori hypotheses. Note that we did notobserve a generalization gradient in the startle eye blinkmeasures for either of the groups; so as not to overloadthe results section these analyses are omitted. Finally,to test our fourth hypothesis, pain-US intensity and un-pleasantness ratings were analyzed using RM ANOVAsto test whether pain would be more intense and moreunpleasant for FM than for HC.We decided to use a mixed analysis strategy combining

themultilevel modeling approach as well as the standardmixed RM ANOVA because unlike standard mixed RMANOVA the multilevel approach: 1) can model aquadratic trend, 2) supports the estimation of a large va-riety of planned contrasts (ie, all contrasts can be ex-pressed as a linear function of the underlying regressionparameters), and 3) supports the inclusion of random co-efficients (eg, random intercepts). Therefore, we used themultilevel approach for the analyses where a more flex-ible approach involving these aspects was needed andmore complexity also led to a better model fit. Conse-quently, multilevel modeling was used to conduct themanipulation check on fear acquisition in the self-reports and for testing hypothesis 1, 2, and 3. In contrast,for themanipulation checks on affective valence, arousal,control experienced, and the fear acquisition in the star-tle measures, as well as to test hypothesis 4, a more stan-dard mixed RM ANOVA was satisfactory.

Results

Pain-US Characteristics andQuestionnairesIn contrast with Meulders et al,30 FM did not select a

lower pain-US intensity during the calibration phasethan the HC group, t58 = 1.54, P = .13. There were alsono differences in how the selected stimulus was subjec-tively rated by the FM group and the HC group duringthe calibration, t58 = 1.40, P = .17 (Table 1). For the ana-lyses of the questionnaires, 1 FM patient was excluded,because she failed to complete any of the question-naires. Independent t-tests were conducted using theHolm-Bonferronimethod to correct formultiple compar-isons. The Holm-Bonferroni18 correction is based on the

ranking of the P values of one family of hypothesesfrom smallest to largest. ‘m’ is the number of P values.If the first P value is $a/m, the procedure is stoppedand no P values are significant. If the first P valueis significant the second P value is compared witha/(m�1), etc. Corrected P values are reported. As ex-pected, the groups had significantly different scores onmost of the psychological trait questionnaires (Table 3).Compared with the HC group, FM reported higher painintensity, t57 = 14.65, P < .0001, greater pain disability,t57 = 14.66, P < .0001, and more days of being disabledduring the past 6 months, t57 = 3.61, P < .01 (CPGS). TheFM group had significantly higher scores on the cata-strophizing, t57 = 4.10, P < .01, and the limitation,t57 = 8.05, P < .0001, subscales, but no significant differ-ences on the optimism, t57 = �1.79, P = .48, and trust,t57 = �1.23, P = 1.00, and internal control, t57 = �2.02,P = .33) subscales of the PCL compared with HC. FM re-ported more fear of movement and (re)injury,t57 = 3.51, P < .01 (TSK), reported more impairmentin their daily life activities due to pain, t57 = 13.56,P < .0001 (FIQ), lower positive affect, t57 = �5.90,P < .0001, and higher negative affect, t57 = 5.30,P < .0001 (PANAS). The FM group significantly differedon the anxiety, t57 = �3.13, P < .05), but not on thedepression subscale of the HADS, t57 = �1.15, P = .76,nor did they differ with respect to general fear associ-ated with pain compared with the HC group,t57 = �.24, P = .81 (FPQ). Furthermore, no differenceswere found with respect to the FPQ subscales, medicalpain, t57 = �.86, P = .79, and severe pain, t57 = �1.15,P = 1.00, but FM tended to have higher scores for the mi-nor pain subscale, t57 = 2.82, P = .05. FM also reportedmore repetitive negative thinking, t57 = 4.09, P < .0001,than the HC group (PTQ).

Manipulation ChecksFor all 3 SAM ratings, we performed a 2 (group: FM/

HC) � 2 (stimulus type: CS1/CS�) � 3 (phase: practice/acquisition/generalization) mixed RM ANOVA.

Affective Valence of the CS Movements

This analysis revealed significant main effects forgroup, F1,58 = 4.80, P < .05, h2

G ¼ :03, stimulus type,F1,58 = 37.19, P < .001, h2

G ¼ :13, and phase, F2,116 = 5.99,P < .01, ε = .94, h2

G ¼ :03. Further, there was a significantStimulus Type � Phase interaction, F2,116 = 26.96,P < .0001, ε = .92, h2

G ¼ :09, suggesting that the differ-ences between the CS1 and the CS� changed over theexperimental phases; this interactionwas notmodulatedby group, F < 1. Planned comparisons further revealedthat ratings for the CS1 and the CS� did not significantlydiffer at the end of the practice phase, F < 1, but afteracquisition, participants felt happier when performingthe CS� movement than when performing theCS1 movement, F1,58 = 46.57, P < .001. This differencewas still significant after generalization, F1,58 = 44.61,P < .001. Interestingly, there was no difference betweenthe HC and the FM after acquisition and generalizationwith regard to the affective valence of the painful

Figure 2. Predicted difference of fear of movement-relatedpain ratings in response to CS1 and CS� movements duringacquisition (A1–3) and transfer of acquisition (T) for both groups(HC and FM).

Figure 3. Meaneye blink startle amplitudes (6 standard errors)during the CS movements and the ITI during acquisition (A1–3)and transfer of acquisition (T). For graphic purposes T-scoreswere used.

88 The Journal of Pain Extinction of Generalization in Fibromyalgia

CS1 movement, F1,58 = 1.08, P = .30, but FM were moreunhappy when performing the safe CS� movement,F1,58 = 7.45, P < .01.

Arousal Elicited by the CS Movements

This analysis showed a significant main effect for stim-ulus type, F1,58 = 24.12, P < .001, h2

G ¼ :05. Further therewas a significant Stimulus Type � Phase interaction,F2,116 = 6.52, P < .01, ε = .78, h2

G ¼ :02, suggesting thatthe differences in arousal elicited by the CS1 and theCS� changed over the experimental phases; this interac-tion was not modulated by group, F < 1. Also thePhase � Group interaction was significant, F2,116 = 5.69,P < .01, ε = .84, h2

G ¼ :03, indicating that there was a dif-ference between the groups regarding the arousal expe-rienced during the different phases. Plannedcomparisons further confirmed that there were no differ-ences in arousal elicited by the CS1 and the CS� at theend of the practice phase in both groups, Fs < 1. Interest-ingly, FM reported beingmore arousedwhen performingboth joystick movements before the conditioning pro-cedure than the HC, F1,58 = 5.34, P < .05. After acquisitionhowever, participants in both groups reported morearousal when performing the CS1 movement thanwhen performing the CS� movement, FM: F1,58 = 5.00,P < .05; HC: F1,58 = 11.25, P < .01. This difference was stillsignificant after generalization in both groups, FM:F1,58 = 11.61, P < .01; HC: F1,58 = 8.99, P < .01.

Feelings of Being in Control When PerformingCS Movements

This analysis yielded significant main effects for stim-ulus type, F1,58 = 23.48, P < .001, h2

G ¼ :05, and phase,F2,116 = 10.21, P < .001, ε = .80, h2

G ¼ :05. Further therewas a significant Stimulus Type � Phase interaction,F2,116 = 10.19, P < .001, ε = .86, h2

G ¼ :05, suggesting thatthe differences in control experienced during the CS1and the CS� changed over the experimental phases;this interaction was not modulated by group, F < 1. ThePhase � Group interaction also reached significance,F2,116 = 5.81, P < .01, ε = .80,h2

G ¼ :03, indicating that therewas a difference between the groups regarding the con-trol experienced during the different phases. Plannedcomparisons further confirmed that there were no differ-ences in control experienced during the CS1 and the CS�at the end of the practice phase in both groups, HC: F < 1;FM: F1,58 = 3.99, P = .051. After acquisition however, par-ticipants in both groups reported feeling more in controlwhen performing the CS� movement than when per-forming the CS1 movement, FM: F1,58 = 16.67, P < .001;HC: F1,58 = 11.07, P < .01. This difference was still signifi-cant after generalization in both groups, FM:F1,58 = 14.86, P < .001; HC: F1,58 = 12.74, P < .001.

Acquisition of Self-Reported Fear ofMovement-Related Pain

Self-reported fear of movement-related pain acquisi-tion was tested using a multilevel regression model (seethe Supplementary Material for the detailed statisticalmodel description). Supplementary Table 1 presents the

results of the multilevel regression model for fear ofmovement-related pain ratings during (transfer of)acquisition, follow-up planned contrasts are visualizedin Fig 2. When reporting planned contrasts,bSðkÞl indicates the estimated slope for stimulus k (CS1,

CS�) in group l (HC, FM) and bYðkÞj ðlÞ represents the pre-

dicted rating for stimulus k (CS1, CS�) at block j (A1,A2, A3, T) in group l (HC, FM).There was a significant effect of time on the acquisi-

tion of fear of movement-related pain, indicatingsuccessful fear acquisition to the CS1, but not to theCS� (Fig 2). This was indicated by a significant differencein slopes for the CS1 and the CS� in both groupsðbSð1Þ

FM � bSð�ÞFM ¼ :93; P\:0001; bS

ð1ÞHC � bS

ð�ÞHC ¼ :55; P\:05Þ.

Differential fear of movement-related pain was alreadyacquired at the first rating moment A1 (that is, after 4

Figure 4. Predicted difference in (A) pain-US expectancy ratings (CS1 minus CS�) for both groups (HC and FM), (B) pain-US expec-tancy ratings (FM minus HC) for both CSs, (C) fear of movement-related pain ratings (CS1 minus CS�) for both groups (HC and FM),and (D) fear of movement-related pain ratings (FM minus HC) for both CSs during the 4 trials of the transfer of acquisition (T1–4).

Meulders et al The Journal of Pain 89

movements of each type) in the HC, that is, they reportedhigher fear in response to the CS1 than the CS�(g1 = 1.26, P < .01), whereas this was not the case forthe FM (g2 = .76, P = .06). At the last rating moment T(after the transfer of acquisition phase) the fear elicitedby the CS1 was significantly greater compared withthe CS� movement in both groups (g3 = 2.90, P < .0001;g4 = 3.54, P < .0001). These results indicate that partici-pants in both groups learned to be afraid of theCS1 movement, but not the CS� movement; however,this differential learning was acquired slower by theFM than the HC.

Acquisition of Fear-Potentiated Eye BlinkStartle

A2 (group: FM/HC)� 3 (stimulus type: CS1/CS�/ITI)� 4(block: A1–A3, T) mixed RM ANOVA was carried out totest acquisition of differential fear learning in the eyeblink startlemeasures (Fig 3). The results showed a signif-icant main effect for block, F3,159 = 3.03, P < .05, ε = .92,h2G ¼ :02, indicating habituation, that is, startle re-

sponses declined gradually over time, but increasedagain during the transfer of acquisition phase probablybecause the change in procedure elicited an orientationresponse. Importantly, there was a significant main

effect of stimulus type, F2,106 = 4.11, P < .05, ε = .94,h2G ¼ :02. The Block� Stimulus Type interaction however,

just failed to reach significance, F6,318 = 2.16, P = .05,ε = .89, h2

G ¼ :02. The main effect of group and all the in-teractions with this variable were not significant.Planned comparisons further confirmed that in bothgroups, the mean startle eye blink amplitudes wereelevated during the CS1 movement compared with theCS�movement, F1,53 = 10.95, P < .01. These data confirmthat participants in both groups successfully acquiredsimilar levels of differential eye blink startle responding.

Testing Our Primary Hypotheses

Hypothesis 1: Differences in Transfer ofAcquisition Between HC and FM for Pain-USExpectancy and Fear of Movement-RelatedPain Ratings

Supplementary Table 2 presents the results of themultilevel regression model for the pain-US expectancyand fear of movement-related pain ratings during the 4trials of the transfer of acquisition phase; follow-upplanned contrasts are depicted in Fig 4. When reportingplanned contrasts, bY

ðkÞj ðlÞ is used to indicate the

Figure 5. Predicted difference in (A) pain-US expectancy ratings (FM minus HC) at the first trial (T1) and last trial (T4) of the gener-alization phase, (B) fear of movement-related pain ratings (FMminus HC) at T1 and the last trial (T4) of the generalization phase, (C)pain-US expectancy ratings (T1 minus T4) for both groups (HC and FM), and (D) fear of movement-related pain ratings (T1 minus T4)for both groups (HC and FM).

90 The Journal of Pain Extinction of Generalization in Fibromyalgia

predicted rating for stimulus k (CS1, CS�) at trial j (T1,T2, T3, T4) in group l (HC, FM). At T1 (the first trial ofthe transfer of acquisition phase; Fig 4A) HC still ex-pected the pain-US to occur more after the CS1 than af-ter the CS� (g5 = 3.37, P < .0001). In contrast with ourexpectations, FM also showed differential pain-USexpectancies for the CS1 and the CS� at T1 (g6 = 1.61,P < .05), but the transferred CS1/CS� difference wasnot significantly smaller in the FM than the HC (g6

–g5 = �1.76, P = .07). At T4, both groups showed stabledifferential pain-US expectancies for the CS1 and theCS� (HC: g7 = 4.47, P < .0001; FM: g8 = 2.88, P < .0001).Interestingly, the pain-US expectancies in response

to the CS1 did not differ between both groups(g9, g10, g11, g12) at any of the transfer of acquisitiontrials, but FM expected the pain-US to occur morewhen performing the CS� than the HC, T1: g13 = 2.06,P < .01; T2: g14 = 1.88, P < .01; T3: g15 = 1.70, P < .01;T4: g16 = 1.53, P < .05, which is indicative of fragilesafety learning (Fig 4B).Similarly, for the fear of movement-related pain rat-

ings, at T1 (the first trial of the transfer of acquisitionphase; Fig 4C) HC reported to be more afraid toperform the CS1 movement than the CS� movement

(g17 = 1.51, P < .01). In line with our expectations, FMdid not transfer the acquired CS1/CS� differentialfear learning and thus were equally afraid to performthe CS1 and the CS� movement at T1 (g18 = .88,P = .09). As expected, this lack of differentiation wasdue to elevated fear responses for the CS� in the FMcompared with the HC (T1: g19 = 1.71, P < .001; T2:g20 = 1.53, P < .001; T3: g21 = 1.35, P < .01; T4:g22 = 1.18, P < .05), whereas no such differences wereobserved for the CS1 (g23, g24, g25, g26). At T4, bothgroups reliably reported more pain-related fear tothe CS1 than to the CS� (HC: g27 = 2.49, P < .0001;FM: g28 = 2.76, P < .0001; Fig 4D).

Hypothesis 2: Differences inGeneralization Between HC and FMSupplementary Table 3 presents the results of themulti-

level regression model for the pain-US expectancy andfear of movement-related pain ratings during the firstand the last trial of the generalization phase, follow-upplanned contrasts are shown in Fig 5. Note that when re-porting contrasts bY

ðkÞj ðlÞ represents the predicted rating

for stimulus k (CS1, GS1, GS2, GS3, GS4, GS5, CS�) at trial

Meulders et al The Journal of Pain 91

j (T1,T4) in group l (HC, FM). The generalization effect isexpected to be the largest at T1, and to extinguish inthe following trials, therefore the difference in general-ization between the HC and FM is assessed at T1 and dif-ferences in extinction of generalization are assessed at T4(and differences from T1 to T4). As predicted, there was asignificant difference in the slope of the linear trend atthe first trial of the generalization phase between bothgroups, bð1ÞLINxG ¼ :25, P < .05, indicating that FM showedflatter pain-US expectancy generalization gradientscompared with the HC. Planned comparisons (Fig 5A)further confirmed that this difference in steepness ofthe slopes is explained by differences at the CS� side ofthe generalization gradient (ie, responses to novel move-ments that are more similar to the safe movements), butthat no differences occurred at the CS1 side of the gener-alization gradient (ie, responses to novel movements thatare more similar to the painful movements). More partic-ularly, FM reported significantly higher pain-US expec-tancies for the CS� (g29 = 2.23, P < .01), and GS5 (thegeneralization movement that was most similar to theCS�), g30 = 1.46, P < .05, than the HC; the pain-US expec-tancy ratings for the other GSs and the originalCS1 however did not differ between both groups (g31,g32, g33, g34, g35).A similar data pattern was observed in the fear of

movement-related pain ratings, however, the statisticalanalyses did not fully corroborate our findings in thepain-US expectancy ratings. The linear trend variabledid not interact with group, bð1ÞLINxG ¼ �:02, P = .78, sug-gesting that the steepness of the slopes for the HC andFM did not significantly differ for the fear ofmovement-related pain ratings. The quadratic trendtended to be different in both groups, but this differencewas not statistically significant, bð1ÞQUADxG ¼ :08, P = .08.Planned contrasts (Fig 5B) further showed that FMwere more afraid of the original CS� (g36 = 1.27,P < .05), than HC, but they also reported more fear ofmovement-related pain in response to the original CS1(g37 = 1.40, P < .05); these elevated fear responses seemedto spread on both sides of the generalization gradient,but failed to reach significance (GS1: g38 = 1.00, P = .07,and GS5: g39 = .91, P = .09). The fear of movement-related pain ratings for the other GSs did not differ be-tween both groups (g40, g41, g42).Taken together, these results provide partial evidence

for our hypothesis: pain-US expectancy generalizationgradients are flatter in FM than HC due to elevatedpain-US expectancies for the technically safe movementswhereas the shape of the fear generalization gradient isnot significantly different between FM and HC, but fearresponses seem to be elevated on both sides of the con-tinuum in FM.

Hypothesis 3: Differences in Extinction ofGeneralization Between HC and FMPredicted ratings of pain-US expectancy and fear of

movement-related fear are depicted in Fig 5. At T4, therewas a significantly different slope in the pain-US expec-tancy ratings for the FM versus HC, bð4ÞLINxG ¼ :22, P < .05.

Planned contrasts (Fig 5C) further showed that pain-USexpectancies in response to all GSs decreased signifi-cantly from T1 to T4 for the HC (g43 = 1.47, g44 = 2.41,g45 = 2.77, g46 = .35, g47 = 1.71; all Ps < .0001), but notfor the CS1 (g48 = �.06, P = .91), that remained rein-forced during the generalization phase, and not forthe CS� (g49 = .30, P = .56; ie, floor effect). The pain-USexpectancy ratings for the FM also declined significantlyfrom T1 to T4 for GS2 (g50 = 1.19), GS3 (g51 = 1.42), GS4(g52 = 1.36), and GS5 (g53 = 1.03; all Ps < .01) but not forthe CS1 (g54 = �.10, P = .84), the CS� (g55 = .42,P = .42), and the GS1 (g56 = .68, P = .05). Interestingly,the decline in pain-US expectancies for GS2(g56 � g43 = �1.22), GS3 (g50 � g44 = �1.35), and GS4(g51 � g45 = �1.17; all Ps < .05), was significantly smallerin the FM group than the HC, suggesting that therewas resistance to extinction of pain-US expectancies tothe novel, unreinforced generalization movements. AtT4, the pain-US expectancies for all GSs (GS1: g57 = 1.26,P < .05; GS2: g58 = 1.64, P < .01; GS3: g59 = 1.91,P < .001; GS4: g60 = 2.08, P < .001; GS5: g61 = 2.15,P < .0001), and the CS� (g62 = 2.11, P < .01), were indeedstill significantly higher for the FM than the HC, whichfurther supports the resistance to extinction of general-ization hypothesis (Fig 5A).A similar analysis was run on the fear of movement-

related pain ratings, however this analysis could onlypartly confirm our findings in the pain-US expectancy rat-ings. At T4, there was no significantly different slope infear of movement-related pain ratings for the FM versusHC,bð4ÞLINxG ¼ �:05, P = .51. Planned contrasts (Fig 5D)further showed that fear in response to all GSs decreasedsignificantly from T1 to T4 for the HC, (GS1: g63 = .66,P < .05; GS2: g64 = .98, P < .001; GS3: g65 = 1.09,P < .0001; GS4: g66 = 1.00, P < .001; GS5: g67 = .70,P < .01), but not for the CS1 (g68 = .15, P = .69), that re-mained reinforced during the generalization phase, andnot for the CS� (g69 = .22, P = .57; ie, floor effect). Thefear of movement-related pain ratings for the FM alsodeclined significantly from T1 to T4 for GS2 (g70 = .73,P < .01), GS3 (g71 = .86, P < .01), GS4 (g72 = .81, P < .01),and GS5 (g73 = .59, P < .05), but not for the CS1(g74 = �.06, P = .87), the CS� (g75 = .18, P = .63), and theGS1 (g76 = .42, P = .10). The decline in fear ofmovement-related pain was not significantly smaller inthe FM group than the HC (Fig 5D). At T4, however thefear of most generalization movements was still signifi-cantly higher for the FM than the HC (GS1: g77 = 1.24,P < .01; GS2: g78 = .99, P < .05; GS3: g79 = .87, P = .06;GS4: g80 = .89, P = .05; GS5: g81 = 1.03, P < .05), which atleast provides partial support for the resistance to extinc-tion of generalization hypothesis (Fig 5C).

Hypothesis 4: Differences in Pain-USIntensity and Unpleasantness BetweenHC and FMWe examined the differences in self-reported intensity

and unpleasantness of the pain-US by performing 2mixed RM ANOVAs including group (FM/HC) and block(A1–A3, T, generalization). These analyses yielded

92 The Journal of Pain Extinction of Generalization in Fibromyalgia

significant main effect of group (unpleasantness:F1,58 = 4.91, P < .05, h2

G ¼ :10; intensity: F1,58 = 8.26,P < .05, h2

G ¼ :06). The main effect and the interactionwith block failed to reach significance in both analyses.These results confirm that FM rated the selected pain-US as more painful and more unpleasant throughoutthe experiment than the HC.

DiscussionTo our knowledge, this is the first study that investi-

gated the differences in generalization gradients ofpain-related fear and expectancy between FM and age-and gender-matched HC, and subsequently comparedthe rate of extinction of generalization between bothgroups.We hypothesized that FMwould show: 1) poorertransfer of safety learning to a novel context, 2) flattergeneralization gradients compared with HC due tohigher responses to the GSs that are more similar to theoriginal CS�, 3) impaired extinction of unreinforcedGSs, whereas generalized pain-related fear and expec-tancy will subside quickly in HC, and 4) higher levels ofpain unpleasantness and intensity than HC.The results can be summarized as follows: First, we suc-

cessfully established acquisition of fear of movement-related pain in both groups. This effect was evident byelevated startle amplitudes, and higher pain-relatedfear and expectancy ratings for the CS1 than for theCS�. Participants also felt unhappier, less in control, andmore aroused while performing the CS1 movementcompared with the CS� movement. Interestingly,whereas HC acquired these CS-US contingencies afteronly 1 acquisition block, it took longer for FM to pick upon these relationships. Second, as predicted, FM showedpoorer transfer of safety learning to a novel contextthan HC. During the transfer of acquisition phase, weswitched from a voluntary to a signaledmovement setup,implying that participants needed to transfer the ac-quired CS-US contingencies to a novel context. HC didtransfer these contingencies impeccably and showed dif-ferential fear responses to the CS1/� from the first trialonward, but FM did not. These results seem to suggestthat when adaptive differential fear learning is acquired,it is fragile and sensitive to context switches in FM.Furthermore, fear responses during the CS1did not differbetween groups, but CS� responses were elevated in FMcompared with HC, indicating disruptive safety learning.This pattern was not completely mirrored in the pain-USexpectancy measures; no loss of transfer of differentiallearning was observed. Nevertheless, again pain-US ex-pectancies in response to the CS� were elevated for FMcompared with HC, whereas no such differences wereobserved for the CS1. These findings also provide evi-dence for the fragility of safety learning in FM. Thevulnerability of safety learning can be understood interms of the associative learning theory. That is, safetylearning can be seen as inhibitory learning to the CS�.In contrast to excitatory learning to the CS1, which gen-eralizes easily to new contexts, safety learning to the CS�is a form of inhibitory learning like extinction learning,which is a more fragile learning process that is under

contextual control. Nowadays, extinction is commonlyviewed as acquiring a new CS–no US association that in-hibits the behavioral expression of the first learned asso-ciation rather than the forgetting/overwriting of theoriginal CS–US association.3 As a consequence, which ofboth coexisting associations controls behavior iscontext-dependent. The effect observed in our study isvery similar to renewal (ie, a return of fear after successfulextinction due to a context switch) because the acquiredsafety learning disappears when a context change occurs(ie, signaled vs voluntary movement setup). These find-ings corroborate previous findings of Meulders et al,30

and suggest that FM, who seem to be characterized byfragile safety learning, might have difficulties transfer-ring CS–no US contingencies to other contexts. Third,with respect to fear generalization, we replicated andextended our previous findings using a design in whichthe GSs either had a feature in common with theCS1 or CS� but no generalization gradients could becalculated.30 More specifically, we showed that pain ex-pectancy generalization gradients are flatter in FM thanin HC due to elevated pain expectancies for the novel,technically safe movements, whereas the shape of thefear generalization gradient did not differ betweenboth groups. These findings are also in linewith our studyon pain expectancy judgements in chronic hand pain pa-tients.29 Fourth, with respect to extinction of generaliza-tion, we found at least partial evidence for ourhypothesis: We showed that although the pain expec-tancy for all generalization movements declined for HCand for all but the GS1 in FM, this decline was still signif-icantly smaller in FM than in HC. Moreover after 4 unrein-forced trials, pain expectancies for all generalizationmovements remained elevated for FM compared withHC. A similar pattern was observed in the pain-relatedfear ratings, fear in response to all generalization move-ments declined for the HC and for all but the GS1 in theFM, this decline was however was not significantlydifferent in FM than in HC. Nevertheless, after 4 unrein-forced trials, pain-related fear of most of the generaliza-tionmovements remained elevated for the FM comparedwith the HC. These results suggest a deficiency in theextinction of fear generalization in FM, which maycontribute to the maintenance of chronic disability in pa-tients. Closely related, Flor and colleagues previouslyshowed that, relative to HC, chronic back pain patientsshowed similar rates of acquisition, but slower extinctionof verbal as well as cortical pain responses.13 The currentfindings are also in line with previous research on fearextinction in anxiety disorder patients. For example,Wessa and Flor56 reported that post-traumatic stress dis-order patients have a deficit in extinction of traumaticresponse. Michael et al38 conducted a study with panicdisorder patients and reported that panic disorder pa-tients showed impaired extinction learning. The currentfindings corroborate previous research in anxiety disor-ders that reported differences in fear extinction betweenHC17,39 and, essentially extend these findings by showingimpaired extinction of generalization. Previous researchalso showed that safety learning is particularlyvulnerable in individuals with high trait anxiety and

Meulders et al The Journal of Pain 93

relatively low levels of positive affect.31 In the presentstudy, FM scored relatively high on trait anxiety, andlowonpositive affect, and showed fragile safety learning,which further corroborates previous findings. Fifth, FMtended to select a lower intensity, however, the physicalintensity did not significantly differ from HC. FM ratedthe selected pain-US as more intense and more unpleas-ant than HC, which might be due to increased pain sensi-tivity, corroborating previous observations.41

There are some limitations that should be addressed aswell. First, 5 participants were excluded from the startleanalyses. Because of reduced statistical power, general in-teractions might have failed to reach statistical signifi-cance. Second, only 1 male participated in our study, sothe results cannot necessarily be generalized to a malepopulation. However, women are affected with fibromy-algia approximately 3 times more often than men.27

Therefore, it can be argued that our sample compositionis justified. Third, no conclusions can be drawn about thecausal relationship between impaired safety learning andovergeneralization of pain-related fear in FM, becausewe did not use a longitudinal design, which is neededto draw such conclusions. Future research might use lon-gitudinal designs to investigate the causal relationship offear learning deficits in the origin and maintenance of fi-bromyalgia. Fourth, the groups also differedwith respecttomedicationuse6,11,16 and comorbiditywith anxiety anddepression20,25,26 thus we cannot exclude the possibilitythat this might have contributed to the observeddifferences in fear learning/expression. Indeed,impaired safety learning,26 fear overgeneralization,24,25

and resistance to extinction38,56 has been reported inanxiety disorder patients as well. Moreover, anxiolyticsmight affect the expression of context conditioning,15

opioids have been shown to impair fear learning,11 andantidepressants may enhance cued fear conditioning.7,16

However, because of the possible opposite effects of thedifferent drugs used in our patient group, it is ratherunlikely that the medication use explains all of theobserved variance between FM and HC.Fear generalization research explains how stimuli that

were never associated with pain may trigger fear. From aclinical perspective, an extensive analysis of crucial stim-uli and their conditioning history should be fed back intoexposure treatment—the clinical analogue of Pavlovianextinction and golden standard for fear reduction. Expo-sure treatment often involves GSs because the original

CSs are unavailable/inaccessible. Previous research hasshown that extinction of the original CS spreads toGSs,51 but not necessarily the other way around. Theobservation that FM show slower extinction of general-ized fear may thus be especially problematic for expo-sure treatment responsivity. A plausible way toovercome this deficit is to use a broad array of GSs (byanalogy of exposure in different contexts to promotethe generalization of extinction).9

ConclusionsThis study showed poorer transfer of safety learning to

a novel context in FM than HC. Further, results providedpartial evidence for our overgeneralization hypothesis:pain expectancy generalization gradients were flatter inFM than HC because of elevated pain expectancies forthe technically safe movements whereas the shape ofthe fear generalization gradient was not different forFM and HC. Fearful responding declined to the general-ization movements in both groups, but extinction ofgeneralization of pain-related fear and expectancy wasimpaired in FM compared with HC. We contend thatthis failure of extinction of generalization might be acontributing factor in the exacerbation andmaintenanceof fibromyalgia syndrome pathology and disability.

AcknowledgmentsPart of these datawere presented in the symposiumon

‘‘Protective responses in pain: the role of threat, associa-tive learning, and perception’’ at the 27th AnnualConvention of the Association for Psychological Science,New York, New York, May 2015, and in the ‘‘EFIC Sympo-sium: New Findings in Clinical Pain Research: Results ofthe EFIC-GR€UNENTHAL Grants 2012’’ at the Ninth EFICCongress, Pain in Europe IX, Vienna, Austria,September 2015.The authors thank the Centre for Translational Psycho-

logical Research (TRACE) for their assistance in therecruitment of the FM.

Supplementary DataSupplementary data related to this article can be

found online at http://dx.doi.org/10.1016/j.jpain.2016.10.004.

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