The faces of pain: A cluster analysis of individual differences in facial activity patterns of pain

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1 ORIGINAL ARTICLE The fces of pin: A cluster nlysis of individul differences in fcil ctivity ptterns of pin M. Kunz 1,2, S. Lutenbcher 1 1 Physiologicl Psychology, Otto-Friedrich University Bmberg, Germny 2 Biologicl Psychology, Ludwig Mximilin University, Munich, Germny Correspondence Mirim Kunz E-mil: mirim.kunz@uni-bmberg.de Funding sources These studies were supported by reserch grnt from the Deutsche Forschungsgemeinschft (Ku2294/4; study 1) nd FNK grnt of the University of Bmberg (study 2). Conflicts of interest None declred. Accepted for publiction 5 October 2013 doi: /j x Abstrct Bckground: There is generl greement tht fcil ctivity during pin conveys pin-specific informtion but is nevertheless chrcterized by substntil inter-individul differences. With the present study we im to investigte whether these differences represent idiosyncrtic vritions or whether they cn be clustered into distinct fcil ctivity ptterns. Methods: Fcil ctions during het pin were ssessed in two smples of pin-free individuls (n = 128; n = 112) nd were lter nlysed using the Fcil Action Coding System. Hierrchicl cluster nlyses were used to look for combintions of single fcil ctions in episodes of pin. The stbility/replicbility of fcil ctivity ptterns ws determined cross smples s well s cross different bsic socil situtions. Results: Cluster nlyses reveled four distinct ctivity ptterns during pin, which stbly occurred cross smples nd situtions: (I) nrrowed eyes with furrowed brows nd wrinkled nose; (II) opened mouth with nrrowed eyes; (III) rised eyebrows; nd (IV) furrowed brows with nrrowed eyes. In ddition, considerble number of prticipnts were fcilly completely unresponsive during pin induction (stoic cluster). These ctivity ptterns seem to be rection stereotypies in the mjority of individuls (in nerly two-thirds), wheres minority displyed vrying clusters cross situtions. Conclusion: These findings suggest tht there is no uniform set of fcil ctions but insted there re t lest four different fcil ctivity ptterns occurring during pin tht re composed of different configurtions of fcil ctions. Rising wreness bout these different fces of pin might hold the potentil of improving the detection nd, thereby, the communiction of pin. 1. Introduction There is generl greement tht fcil ctivity during pin is not unspecific grimcing but conveys pin-specific informtion (Willims, 2002; Hdjistvropoulos et l., 2011). Evidence for this cn be minly tken from two sources. First, only smll subset of fcil ctivities hs been repetedly observed during pin, nmely, tightening of the muscles surrounding the eyes, furrowed brows, rising the upper lip/nose wrinkling nd eye closure (Prkchin, 1992; Prkchin nd Solomon, 2008). Second, when ctors re tught to disply this subset of pin prototypicl fcil expressions, observers cn recognize pin mong other emotions bove chnce level (Simon et l., 2008). However, despite the evidence tht these key fcil ctivities relibly occur during pin, this does not imply only one uniform fcil expression of pin tht cn be observed t ll time nd in ech individul (Crig et l., 2011). Insted, the frequencies of occurrence of these key ctivities during pin usully only rnge from 10% to 60% (e.g., Kunz et l., 2008), nd thus the likelihood tht ll four key fcil ctivities occur simultneously is very low. Rther, individuls 2013 Europen Pin Federtion - EFIC Eur J Pin (2013) 1

2 Clustering fcil expressions of pin M. Kunz, S. Lutenbcher Wht s lredy known bout this topic? Fcil pin expressions re not unspecific grimcing but convey pin-specific informtion. There re limited number of key fcil ctions tht re displyed relibly in the context of pin. Nevertheless, there re lso gret vritions in fcil expressions of pin, with individuls often displying only prts of these key fcil ctions in different combintions. Wht does this study dd? We could show tht there re t lest four different fcil ctivity ptterns occurring during pin tht re composed of different configurtions of fcil ctions (different fces of pin). These ptterns occur cross smples nd cross situtions. Rising wreness bout the different fces of pin might hold the potentil to improve pin ssessment. often disply only prts of this subset, sometimes even blending it with limited rnge of other fcil ctivities (e.g., smiling; Hle nd Hdjistvropoulos, 1997; Kunz et l., 2009). These inter-individul vritions suggest tht there might be vrious fces of pin, nd this vribility might interfere with the ccurte communiction of pin. Indeed, severl studies hve demonstrted shortcomings of observers to ccurtely recognize nd estimte pin through fcil ctivity (e.g., Kppesser et l., 2006; Eritz nd Hdjistvropoulos, 2011) nd compred with lmost ll six bsic emotions, the recognition ccurcy for pin seems to be mong the lowest (Kppesser nd Willims, 2002; Simon et l., 2008). This is especilly lrming given tht the ccurte decoding of fcil pin ctivity informs the interction nd trnsction between ptient nd cring person, nd cn hereby determine the success of pin mngement (Hdjistvropoulos et l., 2011). Thus, better knowledge nd wreness of different fces of pin is urgently needed since it holds the potentil to improve the ccurte communiction of pin. Although we re not the first ones cknowledging inter-individul vritions, our im ws to increse our understnding of fcil expressions by systemtic serch for regulr nd stble ptterns within the mny vritions in fcil expressions of pin. For tht purpose, we used hierrchicl cluster nlyses to investigte whether vritions in fcil ctivity during pin cn be clustered into distinct fcil ctivity ptterns or simply represent idiosyncrtic vritions. If we indeed find stble cluster solutions, this would represent promising pproch to chrcterize types of inter-individul differences in fcil ctivities during pin. We intend to derive the bestfitting cluster solution in one smple nd verify its ppropriteness in second smple s test on the replicbility of clusters cross smples. Furthermore, to test whether potentil fcil ctivity clusters represent stble rection stereotypies, we lso included different socil situtions to test the situtionl stbility of cluster membership. In sum, the present pproch should llow for reveling the existence of fcil ctivity ptterns of pin, testing their stbility of occurrence within nd between individuls, nd identifying potentil individul determinnts of cluster membership (e.g., sex nd ctstrophizing). 2. Mterils nd methods 2.1 Prticipnts Smple 1 consisted of 128 subjects [64 men, 64 women; M = yers; stndrd devition (SD) = 13.59; 64 heterosexul couples being in reltionship >6 months] who were recruited vi dvertisements in the locl newspper (Bmberg). Couples were selected to test prticipnts in different socil situtions (lone, in the presence of the prtner, in the presence of the experimenter) nd, thus, to investigte whether fcil expression clusters re replicble cross different socil situtions. Furthermore, this llows for ssessing how stble one individul shows the sme type of fcil encoding pttern cross situtions (stbility of cluster membership). If individuls were fcilly completely nonexpressive in response to the noxious stimultion, we excluded them from the cluster nlyses becuse of insufficient dt for this type of nlysis [given tht lck of expression ( 0 vlues ) cnnot be clustered into specific type of fcil expression, besides being lck of expression]. However, even though these individuls will not be directly entered into the cluster nlyses, we will tret this group s n dditionl cluster in the Results section ( Stoic cluster ) to stress the fct tht individuls might be experiencing pin lthough they do not show ny pin-relted fcil ctivity nd tht stoic fce cn lso be n expression of pin. Smple 2 consisted of 112 students (54 men, 58 women; M = yers; SD = 4.7) who were recruited vi dvertisements posted in the university buildings of the University of Bmberg. Agin, we excluded those prticipnts who were fcilly not expressive to our pin induction procedure from the cluster nlysis but included them in the Results section s stoic cluster. Exclusion criteri for both smples were current experience of cute or chronic pin, psychologicl nd physicl illnesses. Prticipnts tking psychotropic drugs or nlgesics were excluded from prticiption s well. These criteri led 2 Eur J Pin (2013) 2013 Europen Pin Federtion - EFIC

3 M. Kunz, S. Lutenbcher Clustering fcil expressions of pin to the exclusion of 8% of the prticipnts who hd shown interest in prticipting. All prticipnts provided informed consent nd received either course credit or monetry compenstion. The study protocols were pproved by the Ethics Committee of the University of Bmberg. 2.2 Procedure We investigted fcil ctivities elicited during pinful het stimultion in two different smples. We used two different smples in order to be ble to compre fcil encoding clusters between smples nd, thus, test the cluster replicbility cross smples. The pin stimultion protocol ws the sme in both smples. The min procedurl difference between smples ws tht smple 1 underwent three blocks of therml stimultion (with 10 pinful stimuli ech), wheres smple 2 only underwent one block (of 10 pinful stimuli). The three blocks in smple 1 were necessry to compre cluster solutions cross different socil situtions within one smple. Accordingly, we used within-subject design nd tested prticipnts once while they were lone in the room, once with the prtner present nd once with the experimenter present. Since we hd found stble cluster solutions cross situtions in smple 1 (see Results section of the pper), we decided to test smple 2 in only one situtionl context (while being with the experimenter). The order of situtions in smple 1 ws rndomized cross subjects Pin stimultion Pin stimultion protocols were exctly the sme in both smples. Following previous protocol tht hs been shown to successfully elicit fcil expressions of pin (Kunz et l., 2011, 2012), pin ws induced using Peltier-bsed, computerized therml stimultor (Medoc TSA-2001; Medoc Ltd, Rmt Yishi, Isrel) with 3 3cm 2 contct probe ttched to the outer prt of the left lower leg (midpoint between nkle nd knee). To ensure tht temperture intensities were perceived s pinful but not too pinful in ll subjects (in order to prevent floor s well s ceiling effects), temperture intensities were tilored to the individul pin threshold. Thus, het pin thresholds were determined first, using the method of djustment. Subjects were sked to djust temperture strting from 38 C, using heting nd cooling buttons, until they obtined level tht ws brely pinful. A constnt press of the buttons produced heting or cooling rte of 0.5 C/s. Following fmiliriztion tril, there were four trils nd the verge of these trils ws used to constitute the threshold estimte. Following the ssessment of pin thresholds, phsic het stimuli [5 s (plteu); rte of chnge: 4 C/s; bseline temperture: 38 C; inter-stimulus intervls of s] were pplied to the lower leg. Two different stimulus intensities were pplied in rndomized order, nmely, pinful (+3 C bove the pin threshold) s well s non-pinful ( 1 C below the pin threshold) intensities. The non-pinful intensities were interspersed in order to reduce the chnce of hbitution or sensitiztion effects. We refrined from entering fcil responses occurring during non-pinful stimultion into the cluster nlysis becuse hlf of the prticipnts did not disply ny fcil response during the non-pinful stimultion. In ech experimentl block, prticipnts received 10 pinful nd 10 non-pinful stimuli in rndom order. Given tht smple 1 underwent three experimentl blocks (nmely, while being lone, while being with the experimenter nd while being with the prtner), we chnged the site of stimultion between experimentl blocks (moving the thermode 3 5 cm upwrds or downwrds, respectively) to lso reduce the chnce of locl sensitiztion effects Fcil ctions during pin The fce of the prticipnts in smple 1 nd 2 ws videotped throughout the pin induction procedures. The cmer ws positioned in front of the subject (>1 m distnce) nd provided full-fce view (see Fig. 1). A light-emitting diode visible to the cmer, but not to the subject, ws lighted concurrently with the therml stimuli to mrk the onset of stimultion. To ensure tht the fce would lwys be upright nd in frontl view during stimultion, subjects were sked to look t the computer screen or t picture in front of them throughout the therml het stimultion. Subjects were lso instructed not to tlk during therml stimultion. Fcil expressions were coded from the video recordings using the Fcil Action Coding System (FACS; Ekmn nd Friesen, 1987), which is bsed on ntomicl nlysis of fcil movements nd distinguishes 44 different ction units (AUs) produced by single muscles or combintions of muscles. FACS coders identified the frequency nd the intensity (5-point scle) of different AUs. Given the enormous mount of video dt nd the time-consuming FACS coding procedure, five FACS coders were needed to nlyse the video mteril. Among them ws certified FACS coder (the uthor M.K.; qulified by pssing n exmintion given by the developers of the system), who dditionlly FACS coded 5% of the video mteril from ech coder to clculte interrter relibility [using the Ekmn Friesen formul (Ekmn nd Friesen, 1987); inter-rter relibility ly between 0.84 nd 0.93]. A softwre designed for the nlysis of observtionl dt (the Observer Video-Pro; Noldus Informtion Technology, Wgeningen, the Netherlnds) ws used to segment the videos nd to enter the FACS codes into time-relted dtbse. Given tht ech pinful stimulus lsted for 5 s, we selected these time segments of 5 s (beginning just fter stimulus hd reched the trget temperture) for scoring the fcil response occurring during pinful stimultion. In totl, 10 pinful segments (5 s ech) per experimentl block were nlysed in ech subject. As hs been performed in most preceding studies on fcil responses to pin (Prkchin, 1992; Kunz et l., 2004, 2007, 2008), we combined those AUs tht represent similr fcil movements (AU 1_2, AU 6_7, AU 9_10 nd AU 25_26_27) Europen Pin Federtion - EFIC Eur J Pin (2013) 3

4 Clustering fcil expressions of pin M. Kunz, S. Lutenbcher Figure 1 Exmples of fcil ctions occurring during pin tht belong to one of the four fcil ctivity clusters s well s to the stoic cluster. Exmples re given seprtely for smple 1 (for ll three socil situtions) nd for smple 2. To select AUs to be entered into the cluster nlyses, we clculted how often ech AU occurred during pinful stimultion in smple 1 (seprtely for ech of the three situtions) nd in smple 2. Only those AUs tht occurred in t lest 5% of the pinful segments recorded re listed in Tble 1 nd were considered for further nlyses. We decided to only include in the cluster nlyses those AUs tht occurred in more thn 5% in both smples nd in ll three situtions (these were eight Aus; see Tble 1). However, before entering the selected eight AUs into the cluster nlyses, AU vlues (product term: frequency intensity) were z trnsformed within ech prticipnt in order to eliminte differences in the degree of fcil expressiveness between prticipnts. This ws necessry becuse otherwise highly expressive individuls would contribute disproportiontely to the formtion of fcil expression clusters. However, we were not interested in clusters merely reflecting different degrees of expressiveness (referring simply to the vigour nd number of fcil responses), but insted, in the extrction of distinct fcil ctivity ptterns, which my be constnt cross group of individuls regrdless of whether being fintly or more strongly expressed. Individul differences in the degree of expressiveness were thus eliminted using z trnsformtion within ech prticipnt Self-report rtings After the ppliction of ech therml pin stimulus, subjects lwys rted the intensity on visul nlogue scle (VAS). The scle ws lbelled with the verbl nchor fintly pinful in the centre so tht ll non-pinful senstions were rted below 50 nd ll pinful ones t 50 or bove (subjects were instructed tht the lower end ment no felt temperture nd the upper end would indicte extremely strong pin) Pin ctstrophizing In contrst to one of our previous studies (Kunz et l., 2008), others repetedly hve found significnt ssocitions between 4 Eur J Pin (2013) 2013 Europen Pin Federtion - EFIC

5 M. Kunz, S. Lutenbcher Clustering fcil expressions of pin Tble 1 Fcil ction units (AUs) with criticl frequency of occurrence of more thn 5% in pinful segments. Frequency of occurrence (%) Smple 1 Smple 2 Action unit Description Alone With the prtner With the experimenter With the experimenter AU 1_2 Rised eyebrows AU 4 Furrowed brows AU 6_7 Nrrowed eyes AU 9_10 Wrinkled nose AU 12 Smiling AU 14 Dimpler AU 17 Chin riser AU 18 Lip pucker AU 23 Tightened lips AU 25_26_27 Opened mouth Vlues re given seprtely for the three socil situtions in smple 1 nd differently for smples 1 nd 2. AUs selected for the cluster nlyses. pin ctstrophizing nd fcil pin displys (Sullivn et l., 2006; Vervoort et l., 2011). We were interested to investigte whether pin ctsrophizing might not only be relted to the quntity of fcil responses (or the degree of expressiveness) but lso to the more qulittive spects of fcil responses to pin (wht types of fcil ctions re displyed). We therefore included Germn trnsltion of the Pin Ctstrophizing Scle (PCS; Sullivn et l., 1995; Huber et l., 2010) to nlyse whether the trit ctstrophizing might help to explin cluster formtion in fcil encoding of pin. (We decided to use ctstrophizing s trit rther thn stte vrible becuse we ssume tht fcil encoding ptterns of pin re lso more trit chrcteristics.) The PCS ws developed s mesure of ctstrophic thinking relted to pin. Prticipnts re instructed to reflect on thoughts or feelings during the pst pinful experiences. The scle contins 13 items tht re rted on 5-point scle, with the endpoints not t ll nd ll the time. 2.3 Dt nlyses Cluster nlyses Following previous pproches nd recommendtions (Blshfield nd Aldenderfer, 1988; Hir nd Blck, 2000; Rovnik et l., 2010), two-step clustering procedure ws used. In the first step, hierrchicl cluster nlyses were performed using Wrd s method (Wrd, 1963) nd squred Eucliden definition of distnces to determine the number of cluster groups within ech of the three situtions in smple 1 nd in smple 2 (resulting in four hierrchicl cluster nlyses). The clustering process strts with the sme number of clusters s there re cses nd reduces the number of clusters by step-wise combining those clusters whose combintion results in minimum increse of the totl within-group sum of squres. If point is reched where clusters re combined tht re dissimilr, the withingroup sum of squres noticebly increses (s cn be seen in the gglomertion schedule). The number of clusters prior to this rpid increse of the gglomertion coefficient is considered the nturl grouping scheme (Hir nd Blck, 2000). To best determine the correct number of clusters, we inspected the rescled distnces s displyed in the hierrchicl cluster dendrogrm, exmined the chnge in gglomertion coefficient nd pplied the Mojen stopping rule s quntittive criteri to define significnt jump in the gglomertion coefficient (Mojen, 1977; Millign nd Cooper, 1985). In the second step, the cluster mens (centroids), emerged from ech hierrchicl cluster nlysis, were used s initil seed points in non-hierrchicl, k-mens cluster nlyses s method to verify the initil cluster solutions (Hir nd Blck, 2000). This ws gin crried out seprtely for ech of the three situtions in smple 1 nd for the cluster nlysis in smple Cluster replicbility cross smples/situtions nd cluster membership stbility cross situtions In order to ensure tht the found clusters do not only represent chnce findings but insted represent stble fcil encoding ptterns of pin, we ssessed the replicbility nd stbility of the identified fcil expression clusters both cross different situtions (with the experimenter, lone, with the prtner; smple 1) nd cross different smples (smples 1 nd 2) Cluster replicbility cross smples nd situtions To ssess replicbility of fcil encoding clusters cross smples, we compred cluster solutions of smple 1 with cluster solutions in smple 2 (in smple 1, we selected the sitution when subjects were tested in the presence of the 2013 Europen Pin Federtion - EFIC Eur J Pin (2013) 5

6 Clustering fcil expressions of pin M. Kunz, S. Lutenbcher experimenter, which ws the sitution common to both smples). We lso compred cluster solutions between situtions in smple 1 to test replicbility cross situtions. Replicbitity ws tested using McIntyre nd Blshfield s (1980) nerest centroid cross-vlidtion technique. This crossvlidtion procedure uses the cluster solutions from one smple (e.g., smple 1) to clssify nother smple (e.g., smple 2). This is performed by ssigning, for exmple, ech person in smple 2 to one of the cluster centres of the first smple bsed on miniml distnces to these centres, nd vice vers (cross-vlidtion). Thus, for ech individul in ech smple there re now two clustering solutions vilble (the originl clusters nd the nerest centroid-ssigned clusters). We did the sme for cluster solutions in different socil situtions in smple 1. The repliction ccurcy between cluster solutions ws quntified using the kpp coefficient Stbility of cluster membership between situtions In smple 1, we clculted cross tbultions nd kpp sttistics between cluster solutions in different situtions (lone vs. being with the experimenter, lone vs. being with the prtner, being with the prtner vs. being with the experimenter) to investigte whether belonging to certin fcil expression cluster in one sitution predicts the membership to comprble cluster in nother sitution. This procedure llows determining the stbility of cluster membership of ech prticipnt cross situtions Demogrphic nd pin-processing chrcteristics in different fcil expression clusters As lst step, we investigted whether the identified fcil expression clusters might differ with regrd to demogrphic chrcteristics (ge, sex) or pin sensitivity (pin threshold, VAS rtings, PCS) using χ 2 tests (sex) s well s univrite (ge, pin threshold, VAS rtings, PCS score) nlyses of vrince. Findings were considered to be sttisticlly significnt t α<0.05. Sttisticl Pckge for the Socil Sciences version 20 (IBM SPSS, Chicgo, IL, USA) ws used for ll nlyses. 3. Results The het intensities used to elicit pin (these were djusted to be +3 C bove the individul s pin threshold) were comprble in both smples [smple 1: 49.3 C (SD 1.2); smple 2: 49.0 C (SD 1.4); p > 0.05 for smple differences]. Moreover, VAS pin intensity rtings (rnging from 0 to 100, with 50 indicting pin threshold) did lso not differ between both smples or between the different situtions in smple 1 {smple 1: lone [81.4 (SD 14.4)], with the prtner [81.0 (SD 13.4)], with the experimenter [80.8 (SD 14.1)]; smple 2: 78.2 [SD 15.3]; ll p-vlues > 0.05 for smple nd sitution differences}. Consequently, we were ble to elicit comprble moderte-to-strong pin experiences in both smples nd cross socil situtions. 3.1 Cluster nlyses In ll three hierrchicl cluster nlyses for smple 1 s well s in the hierrchicl cluster nlysis for smple 2, the dendrogrm, the gglomertion coefficient nd the Mojen stopping rule showed tht four-cluster solution ws lwys the most pproprite solution to cluster fcil ctivity during pin into different encoding ptterns. The corresponding cluster mens of different AUs re shown in Tble 2 for smple 1 nd for smple 2. In second step, we verified these initil cluster solutions by using the centroid vlues of the hierrchicl cluster nlyses s initil seed points in non-hierrchicl, k-mens cluster nlyses nd found the results to be very similr to those of the hierrchicl nlyses ( 95% of the prticipnts obtined smegroup membership). In ddition to these four clusters, we lso included the group of non-expressive individuls in Tble 2 s n dditionl cluster nd lbelled it stoic cluster Fcil expression clusters When visully inspecting cluster solutions found in smple 1 for different situtions s well s for smple 2, it becomes evident tht very similr fcil expression clusters were found cross situtions nd cross smples (see Tble 2 nd Fig. 1) Cluster I As cn be seen in Tble 2, the first cluster scored highest on AU 6_7 (contrction of the muscles surrounding the eyes) with slighter co-ctivtion of AU 4 (contrction of the eyebrows) nd (in smple 1) of AU 9_10 (upper lip rise/nose wrinkle). This cluster ws lbelled s nrrowed eyes with furrowed brows nd wrinkled nose. To better illustrte this fcil ctivity pttern, photos re displyed in Fig. 1 (left column). This fcil ctivity pttern ws displyed by 46% of subjects in smple 2 nd by 37 53% of subjects in smple 1 (depending on the socil sitution) nd proved to be the most frequently occurring fcil ctivity pttern during pin in both smples Cluster II As cn be seen in Tble 2, the second cluster scored highest on AU 25_26_27 (mouth opening) with 6 Eur J Pin (2013) 2013 Europen Pin Federtion - EFIC

7 M. Kunz, S. Lutenbcher Clustering fcil expressions of pin Tble 2 Fcil expression cluster profiles for smple 1 (seprtely for the three socil situtions) nd smple 2 cross the selected ction units (z-scores). Sitution Cluster AU 1_2 Rised eyebrows AU 4 Furrowed brows AU 6_7 Nrrowed eyes AU 9_10 Wrinkled nose AU 14 Dimpler AU 17 Chin riser AU 18 Lip pucker AU 25_26_27 Opened mouth Occurrence frequency of ech cluster n % Smple 1 Alone I II III IV Stoic cluster With the prtner I II III IV Stoic cluster With the experimenter Smple 2 With the experimenter I II III IV Stoic cluster I II III IV Stoic cluster Tble 3 Demogrphic (sex, ge) nd pin sensitivity (pin threshold, VAS rtings, PCS score) chrcteristics for ech cluster. Sex Age Pin threshold VAS pin rtings PCS score Sitution Cluster Men : women p Men (SD) p Men (SD) p Men (SD) p Men (SD) p Smple 1 Alone I 16: (15.0) (1.2) (12.2) (7.8) II 12: (13.3) 46.1 (1.2) 83.8 (11.7) 14.3 (7.1) III 9: (14.4) 46.4 (1.3) 77.4 (13.4) 14.5 (6.8) IV 9: (13.5) 46.3 (1.1) 84.5 (12.7) 16.5 (8.5) Stoic 18: (12.0) 46.5 (1.3) 82.5 (11.4) 12.0 (6.8) With the prtner I 24: (14.9) (1.1) (12.2) (6.9) II 8: (14.6) 45.8 (1.3) 78.5 (11.5) 16.4 (6.9) III 7: (14.2) 46.5 (1.1) 76.0 (18.0) 13.1 (6.5) IV 7: (10.8) 46.0 (1.2) 82.3 (13.4) 13.7 (7.5) Stoic 18: (12.0) 46.5 (1.3) 81.0 (12.0) 12.0 (6.8) With the experimenter I 17: (14.9) (1.1) (11.3) (8.0) II 6: (13.6) 46.1 (1.1) 84.8 (9.5) 15.6 (6.2) III 10: (14.2) 46.2 (1.3) 78.8 (15.8) 15.0 (7.7) IV 13: (12.4) 46.5 (1.4) 79.2 (15.8) 13.8 (7.3) Stoic 18: (12.0) 46.5 (1.3) 79.4 (11.8) 12.0 (6.8) Smple 2 With the experimenter I 22: (5.3) (1.5) (16.9) (7.3) II 7: (6.2) 46.0 (1.3) 80.4 (10.2) 20.5 (5.5) III 6: (3.9) 45.9 (1.9) 75.4 (15.1) 15.9 (7.7) IV 8: (2.1) 46.0 (1.1) 78.7 (15.0) 18.3 (8.8) Stoic 10: (3.3) 45.6 (1.1) 69.9 (8.3) 15.9 (7.7) p-vlues indicte differences between clusters. Vlues re given seprtely for smple 1 (for ll three situtions) nd smple 2. PCS, Pin Ctstrophizing Scle; SD, stndrd devition; VAS, visul nlogue scle Europen Pin Federtion - EFIC Eur J Pin (2013) 7

8 Clustering fcil expressions of pin M. Kunz, S. Lutenbcher slighter co-ctivtion of AU 6_7 (contrction of the muscles surrounding the eyes). This cluster ws lbelled s opened mouth with nrrowed eyes. To better illustrte this fcil ctivity pttern, photos re displyed in Fig. 1. This fcil encoding pttern ws displyed by 20% of subjects in smple 2 nd by comprble percentge of 15 29% in smple 1 (depending on the socil sitution) Cluster III As cn be seen in Tble 2, the third cluster scored highest on AU 1_2 (lifting the eyebrows) nd ws only found in one nlysis (smple 1, with the experimenter) ccompnied by AU 14 (dimpler). Given tht in three out of four nlyses, this cluster ws solely composed of AU1_2; this cluster ws simply lbelled s rised eyebrows. To better illustrte this fcil ctivity pttern, photos re displyed in Fig. 1. This fcil encoding pttern ws displyed by 14% of subjects in smple 2 nd by 16 24% of subjects in smple 1 (depending on the socil sitution) Cluster IV As cn be seen in Tble 2, the fourth cluster scored highest on AU 4 (contrction of the eyebrows) with slighter co-ctivtion of AU 6_7 (contrction of the muscles surrounding the eyes). In two out of four nlyses, this cluster lso showed co-ctivtion of AU 14 (dimpler). However, given tht this coctivtion only occurred in two of the nlyses, we refrined from including it in the lbelling. Thus, this cluster ws lbelled s furrowed brows with nrrowed eyes. To better illustrte this fcil ctivity pttern, photos re displyed in Fig. 1. This fcil encoding pttern ws displyed by 20% of subjects in smple 2 nd by comprble percentge of 13 21% in smple 1 (depending on the socil sitution) Stoic cluster As cn be seen in Tble 2, considerble number of prticipnts (23% in smple 1 nd 15% in smple 2) did not show ny fcil responses during pin stimultion. 3.2 Cluster replicbility nd cluster membership stbility Replicbility cross smples Compring the originl cluster ssignments for the prticipnts in smple 1 (while being tested with the experimenter) with their nerest centroid clssifiction using the cluster centres of smple 2 resulted into n impressive overll greement of 84% nd kpp vlue of κ = 0.80 (t = 13.3; p 0.001). Similrly, compring the originl cluster ssignments for the prticipnts in smple 2 with their nerest centroid clssifiction using the cluster centres of smple 1 (while being tested with the experimenter) resulted into n impressive overll greement of 87% nd kpp vlue of κ = 0.82 (t = 13.4; p 0.001). Given tht for the interprettion of the κ-vlue, vlue of 0.8 or higher indictes very good greement (Altmn, 1991); the cross-vlidtion procedures reveled high greements nd high replicbility of fcil ctivity clusters cross the two smples Replicbility cross situtions Given tht prticipnts in smple 1 were tested in three situtions, we could test the cluster replicbility lso cross situtions in the sme smple. As cluster centres, we used the cluster clssifictions from the sitution when prticipnts were tested with the experimenter (to mke it comprble with the replicbility nlyses cross smples). Compring the originl cluster ssignments for the lone situtions with their nerest centroid clssifiction using the cluster centres of the with the experimenter sitution resulted into n impressive overll greement of 88% nd kpp vlue of κ = 0.83 (t = 13.8; p 0.001). Similrly, compring the originl cluster ssignments for the prtner sitution to their nerest centroid clssifiction using the cluster centres of the with the experimenter sitution gin resulted into n impressive overll greement of 83% nd kpp vlue of κ = 0.80 (t = 12.9; p 0.001). Thus, the cross-vlidtion procedures reveled high greements nd high replicbility of fcil ctivity clusters lso cross different socil situtions Cluster membership stbility cross situtions In ddition to compring the replicbility of cluster solutions cross situtions, we lso wnted to ssess whether prticipnts of smple 1 remined in the sme fcil ctivity cluster cross situtions or in other words how stble n individul displys certin fcil ctivity pttern cross situtions (cluster membership stbility). Kpp sttistics reveled significnt cluster membership stbility cross situtions (lone vs. with the prtner: κ = 0.433, t = 6.73, p 0.001; lone vs. with the experimenter: κ = 0.456, t = 7.85, p 0.001; with the experimenter vs. with the prtner: κ = 0.414, t = 6.43, 8 Eur J Pin (2013) 2013 Europen Pin Federtion - EFIC

9 M. Kunz, S. Lutenbcher Clustering fcil expressions of pin p 0.001). However, given tht κ-vlues were round 0.4, the greement cn only be interpreted s moderte. Likewise, when considering the cross tbultions, it becomes evident tht the mjority of prticipnts did indeed show the sme type of fcil ctivity ptterns cross situtions, but nevertheless, 34 38% of the prticipnts lso chnged cluster membership between situtions. These chnges in cluster membership between situtions were, however, unsystemtic. This mens tht no obvious pttern of chnge (e.g., members of cluster I chnge most often to cluster II) could be detected. 3.3 Demogrphic nd pin-processing chrcteristics in the different fcil expression clusters In Tble 3, the demogrphic (ge, sex) nd pinprocessing chrcteristics of ech fcil expression cluster of smple 1 (for ll three situtions) nd smple 2 re presented. Neither for ge nor for sex did we find significnt differences between cluster groups (see Tble 3). Moreover, pin processing (s mesured by pin threshold, VAS pin rtings s well s by the tendency to ctstrophize bout pin) did lso not differ between cluster groups (see Tble 3). 4. Discussion The im of the present study ws to use hierrchicl cluster nlyses to look for possible systemtic combintions of single fcil ctivities in episodes of pin (distinct fcil ctivity ptterns of pin). So fr, studies on fcil responses during pin hve minly followed the rtionl of reserch on fcil expressions of emotions (e.g., nger, disgust, joy) by trying to identify uniform set of fcil responses tht might distinguish pin from other types of ffective sttes. Prkchin (1992) nd Prkchin nd Solomon (2008) hve been quite successful in describing such set of prototypicl fcil expressions of pin. However, when considering individul responses during pin, considerble interindividul vritions cn be observed (Crig et l., 2011), nd only very smll number of individuls ctully disply the complete set of fcil ctions of the so-clled pin prototypicl expression. Although we re not the first to cknowledge these vritions (e.g., Crig et l., 2011; Ahol Kohut et l., 2012), we hve tried to contribute to the discussion by systemticlly seprting regulr from idiosyncrtic fcil ctivity ptterns during pin. We were ble to show tht in fct multiple-cluster solution seems best fitting for ctegorizing fcil ctivity during pin. Ech cluster ws composed of different combintions of single fcil responses in pin episodes, nmely, (I) nrrowed eyes with furrowed brows nd wrinkled nose; (II) opened mouth with nrrowed eyes; (III) rised eyebrows; nd (IV) furrowed brows with nrrowed eyes (see lso Fig. 1 for better illustrtion). The fct tht these four fcil ctivity ptterns could be replicted cross situtions, s well s cross two different smples, strongly supports the robustness of our findings. Since the ptterns ppered to chrcterize better the individul thn the sitution, one might consider them with cution (given the only moderte replicbility) s individul response stereotypies. Besides these four fcil ctivity ptterns of pin, considerble number of individuls (pproximtely 20%) did not show ny type of fcil responses during pinful stimultion, forming kind of stoic cluster. Thus, even lck of expression cn be fce of pin. 4.1 Detiled description of the four fcil ctivity ptterns of pin The most frequent fcil ctivity pttern in both smples s well s in ll situtions (lone, with their prtner, with the experimenter) ws nrrowed eyes with furrowed brows nd wrinkled nose, displyed by pproximtely 40 50% of the prticipnts. This fcil ctivity pttern ws not only the most frequent but ws lso impressively replicble, given tht the nerest centroid cross-vlidtion technique (McIntyre nd Blshfield s, 1980) reveled greement rtes of 93 97% between smples nd between situtions. In comprison, the verge greement rtes (verged cross ll four clusters) ly between 80% nd 87%. Likewise, cluster IV ( furrowed brows with nrrowed eyes ; in contrst to cluster I, the furrowed brow here is the dominnt ction) lso showed impressive replicbility with greement rtes of % between smples nd situtions. However, this ctivity pttern ws less prevlent (15 20% in both smples) thn the first one. It is interesting tht these two very stble fcil ctivity ptterns of pin show gret overlp with regrd to the single fcil ctivities involved, given tht nrrowed eyes nd furrowed brows ppered in both clusters. Nevertheless, the phenomenology is quite different. Wheres the orbiculris oculi ctivtion (nrrowed eyes) is the most prominent response in the first cluster, this response is surpssed by the corrugtor ctivtion (furrowed brows) in the second cluster (see Fig. 1). Interestingly, the orbiculris oculi ctivtion (nrrowed eyes) ws lso prt of third cluster (frequency = 15 28%), nmely, opened mouth with nr Europen Pin Federtion - EFIC Eur J Pin (2013) 9

10 Clustering fcil expressions of pin M. Kunz, S. Lutenbcher rowed eyes ; however, here it ws ccompnied by strong movement of the mouth (see Fig. 1). Finding orbiculris oculi ctivtion to be prt of three distinct fcil ctivity ptterns is well in line with previous reports tht found nrrowed eyes to be the most common fcil response to pin (Crig et l., 2011). Moreover, we could recently demonstrte tht the orbiculris oculi ctivtion is most closely ssocited with the sensory dimension of pin (Kunz et l., 2012), nd thus three out of four fcil ctivity ptterns seem to include fcil elements encoding the sensory dimension of pin. There ws only one fcil ctivity pttern tht lcked orbiculris oculi ctivtion, nmely, rised eyebrows. This fcil behviour hs been most closely linked to the experience of surprise in erlier studies (Ekmn nd Friesen, 1987). In the context of pin, rised eyebrows might encode the suddenness/novelty component of the pin experience. Those fcil responses tht seem to be mostly linked to the ffective component of pin, nmely, furrowed brows nd wrinkled nose (Kunz et l., 2012) occurred in two of the four fcil expression ptterns (cluster I nd cluster IV). Accordingly, the four distinct ctivity ptterns might be considered behviourl synonyms for pin, which encode slightly different spects of pin. The behviourl synonyms might be comprble with synonyms in verbl lnguges, which convey the sme but minly not identicl messges, llowing for subtle differences in mening. When looking t the photos in Fig. 1, some of the ctivity clusters indeed seem to convey slightly different menings such s pin blended with surprise in cluster III nd pin blended with disgust in cluster I. In future studies, specific self-report mesures ssessing these other ffective sttes should be included to test whether they might help to explin cluster membership. These potentil slight differences in mening of the fcil ctivity ptterns might led to different interctions nd trnsctions (Hdjistvropoulos et l., 2011) between sender (ptient) nd receiver (cring person) in pin mngement. 4.2 A uniform set of fcil responses versus different fcil ctivity ptterns of pin When compring these four different fcil ctivity ptterns with the so-clled prototypicl fcil expression of pin (Prkchin, 1992; Crig et l., 2011) it becomes pprent tht most of the single fcil ctions within this s pin prototypiclly postulted set of fcil responses cn lso be found in three out of the four cluster solutions. Accordingly, there is considerble mount of greement between our findings nd previous reports. However, there is t lest one divergent conclusion. With the present findings, we cn clerly demonstrte tht these single fcil ctions tht often occur in the context of pin do not form uniform set of fcil expression with some unsystemtic vrints but combine to regulr nd distinct fcil ctivity ptterns of pin. These combintions do never include ll single fcil ctions t once. The necessity to consider more thn one fcil ctivity pttern might explin the wek effects of decoder trinings, mking use of the prototypicl set of fcil ctivities (Solomon et l., 1997). The improvement in decoding performnce fter such trinings ws only smll to moderte. Trining observers to recognize the four different fcil ctivity ptterns of pin (insted of one set of prototypicl single ctions) might led to better results. Interestingly, however, we did not find ny demogrphic or pin-relted psychologicl vrible explining which individul shows which type of fcil ctivity pttern. Neither sex, ge, pin sensitivity (pin threshold, pin rtings) nor pin ctstrophizings could explin cluster membership. Wheres pin sensitivity (e.g., Kunz et l., 2004) nd pin ctstrophizing (Sullivn et l., 2006; Vervoort et l., 2011) hve been found to explin differences in the vigour nd mount of fcil expressions of pin, 1 we hve not yet detected the criticl chrcteristics explining which individul shows which type of fcil ctivity pttern. 5. Limittions Although the observed fcil ctivity ptterns stbly occurred cross situtions s well s cross different smples, our findings re nevertheless limited to experimentlly induced pin. It is possible tht fcil ctivity, for exmple, during clinicl pin might be clustered into different fcil ctivity ptterns thn the ones we found, which hs to be exmined in future studies. In ddition, by nture of cluster nlyses ll ctivity ptterns were grouped, which does not exclude the existences of idiosyncrtic nd typicl responses. However, they were not too prevlent becuse the goodness of fit of the four-cluster solution would hve been lower. Furthermore, we s most of previous studies did not consider the temporl sequence of fcil ctions in n ctivity pttern, which might form further source of informtion in the encoding nd decoding processes. Such types of nlyses re even 1 In the present study, however, ctstrophizing could not explin differences in vigour nd mount of fcil expressions of pin (neither in smple 1 nor in smple 2). 10 Eur J Pin (2013) 2013 Europen Pin Federtion - EFIC

11 M. Kunz, S. Lutenbcher Clustering fcil expressions of pin more chllenging, but should be performed s next step to fully cpture the fcil lnguge of pin. 6. Conclusion With the present study, we were ble to show tht pin is not ccompnied by one invrint nd uniform fcil ctivity pttern but insted is encoded in t lest four different ctivity ptterns (s well s in stoic expression) tht could be verified in two independent smples of subjects. The only moderte stbility of these fcil ctivity ptterns within individuls cross situtions suggests the existence of individul fcil rection stereotypies for pin in the mjority of prticipnts but not in ll. Whether the four fcil ctivity ptterns represent behviourl synonyms for the internl stte pin with slightly differing menings remins to be determined. Rising wreness in observers for these different ctivity ptterns might hold the potentil of improving the detection of pin. This might be especilly importnt in clinicl prctice, prticulrly for the ssessment of pin in non-verbl individuls (e.g., ptients with moderte-to-severe dementi). Acknowledgements We thnk Ann Krmnn, Florin Buer, Nin Kunzelmnn, Jessic Peter nd Viktori Zorn for the support in dt collection nd FACS coding. Author contributions M.K. nd S.L. designed the study together nd re responsible for the integrity of the work s whole. M.K. conducted the study nd did the nlyses. Both uthors discussed the results nd wrote the rticle together. References Ahol Kohut, S., Pilli Riddell, R., Flor, D.B., Oster, H. (2012). A longitudinl nlysis of the development of infnt fcil expressions in response to cute pin: Immedite nd regultory expressions. Pin 153, Altmn, D.G. (1991). Prcticl Sttistics for Medicl Reserch (London: Chpmn nd Hll) p Blshfield, R.K., Aldenderfer, M.S. (1988). The methods nd problems of cluster nlysis. In Hndbook of Multivrite Experimentl Psychology, J.R. Nesselrode, R.B. Cttell, eds. 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