Topical review The smile of pain Miriam Kunz a,b, * , Kenneth Prkachin c , Stefan Lautenbacher a a Physiological Psychology, Otto-Friedrich University Bamberg, Markuspl. 3, 96045 Bamberg, Germany b Department of Stomatology and Centre de Recherche de l’Institut Universitaire de Geriatrie de Montreal, Universite de Montreal, Canada c Department of Psychology, University of Northern British Columbia, Canada The facial expression of pain has recently attracted considerable interest in experimental and clinical research based on an increas- ing awareness that it supports the communication of pain as a sec- ond signal system besides the verbal one [5,11]. In line with this, facial activity provides the possibility to develop pain assessment tools in individuals with limited abilities to communicate pain ver- bally (e.g. newborns, individuals with pronounced cognitive impairments and dementia [18,20,21,25]). An early and very important observation as regards facial re- sponses associated with pain, was that there is a subset of key fa- cial muscle movements that are displayed consistently across different pain modalities [27]. This subset of pain-relevant facial responses includes brow lowering, orbit tightening, levator con- traction and eye closure [27,29]. Although there is convergent evi- dence that these facial responses constitute the core of the ‘‘pain face”, there are also other facial muscle movements that have fre- quently been observed in the context of pain. The most unexpected accompaniment of pain is the oblique raising of the lip, a facial muscle movement that is caused by contraction of the zygomatic major muscle. Ordinarily, this facial movement is seen when a per- son is smiling and it is assumed to be associated with happiness or positive affect [8,9]. Therefore, it is surprising that in the majority of the studies on facial responses during pain an increased fre- quency of oblique lip raise has been found compared to that of baseline conditions. This observation has been made regardless of whether pain was induced experimentally in pain-free individ- uals (cold pressor task [4,19,27], thermal heat pain [14,15], pres- sure stimulation [16,17,27], electrical current [17,24,26,27]) or whether clinical pain conditions were investigated (e.g. patients with shoulder or knee pathologies undergoing physiotherapy [3,11,28,29]) 1 . Moreover, the percentage of individuals displaying ‘‘smiles” during painful stimulation is rather high. To illustrate this, we computed the percentage of subjects that showed oblique lip raise at least once during phasic or tonic noxious stimulation in pre- vious studies of ours. As can be seen in Table 1, this percentage var- ied between 22% and 57% depending on pain induction method and number/duration of painful stimulation. The occurrence of ‘‘smiles” during painful procedures was not dependent on sex, since males and females did not differ in the frequency of oblique lip raise during painful stimulation. Moreover, ‘‘smiles” seem to occur regardless of whether subjects were aware of being videotaped or whether they were unaware of being directly observed (participants were video- taped from behind a one-way screen). How can it be explained that approximately one-third of individuals facially respond with a ‘‘smile” while experiencing pain, although smiling seems to be abso- lutely incompatible with the experience of pain? Ekman and Friesen [9] reported that smiles can serve diverse communicatory functions rather than simply expressing happiness – and thus one should distinguish among different types of smiles rather than treating smiles as a single class of behaviour. Commu- nicative functions of smiling range from the intention to conceal negative emotions (where smiles are superimposed over muscular actions associated with negative emotions) to the intention to ap- pear as if enjoyment is felt when it is not. These smiles that seem to mainly serve a communicative function have been referred to as non-Duchenne smiles [10,23]. Duchenne [7] described a type of smile that occurs spontaneously while experiencing enjoyment and which is characterized not only by the action of the zygomatic major muscle (oblique raising of the lip), but also by the action of the obicularis oculi muscle (contraction of the muscles around the eyes). This type of smile (the smile of enjoyment) has been called the Duchenne smile [10]. Non-Duchenne smiles on the other hand lack contractions of the orbicularis oculi and do not seem to ex- press ‘‘true” happiness. Ekman and Friesen [9] described various types of non-Duchenne smiles, such as ‘‘embarrassed smiles”, ‘‘masking smiles” (while experiencing negative emotions), ‘‘false smiles” (appearing as if enjoyment is felt when it is not), ‘‘anticipa- tory smiles” or ‘‘miserable smiles” (representing the willingness to endure unpleasant circumstances). To which type of smile does the ‘‘smile of pain” belongs to and what is its function? In order to answer this question, it is impor- tant to look at the temporal dynamic of ‘‘smiles” during pain. It has been argued that the raising of the oblique lip might not occur as a direct response to pain but instead might be secondary to the occurrence of pain as a post-registration response [28,29]. This hypothesis might become more plausible when looking at the fa- cial responses to phasic (5 s) experimental painful stimuli (pres- sure and heat stimuli) in two exemplary subjects as shown in Fig. 1. The first photo always depicts the time of stimulus onset. 0304-3959/$36.00 Ó 2009 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.pain.2009.04.009 * Corresponding author. Address: Physiological Psychology, Otto-Friedrich Uni- versity Bamberg, Markuspl. 3, 96045 Bamberg, Germany. Tel.: +49 951 8631850; fax: +49 951 8631976. E-mail address: miriam.kunz@uni-bamberg.de (M. Kunz). 1 Data on facial expressions of pain so far have been only available for more acute experimental or clinical pain. Therefore, we do not know whether the ‘‘smile of pain” also occurs during the experience of chronic pain. PAIN Ò xxx (2009) xxx–xxx www.elsevier.com/locate/pain ARTICLE IN PRESS Please cite this article in press as: Kunz M et al. The smile of pain. PAIN Ò (2009), doi:10.1016/j.pain.2009.04.009