en Behavioral Neuroscience Behavioral Neuroscience Original Original <p style="text-align: justify;"><strong>Introduction:</strong> Facial expression to pain is an important pain indicator; however, facial movements look unresponsive when perceiving mild pain. The present study investigates whether pain magnitude modulates the relationship between subjective pain rating and an observer&rsquo;s evaluation of facial expression.&nbsp;<br> <strong>Methods:</strong> Twelve healthy volunteers were recruited to obtain 108 samples for pain rating with Visual Analogue Scale (VAS). Subjects underwent three different mechanical painful stimuli (monofilament forces of 100 g, 300 g, and 600 g) over three sessions and their facial expressions were videotaped throughout all sessions. Three observers independently evaluated facial expression of the subjects with a four-point categorical scale (no pain, mild pain, moderate pain, and severe pain). The correlations between subjective pain ratings and the evaluation of facial expression were analyzed in dichotomous group which was low pain ratings (VAS<30), or high pain rating (VAS&ge;30).<br> <strong>Results:</strong> Subjective pain ratings was significantly correlated with the evaluation of facial expression in high pain ratings, however no correlation was found between them in mild pain ratings. In mild pain ratings, most of the subjects (78%) were rated as no pain by observers, despite the fact that subjects reported pain.<br> <strong>Conclusion:</strong> The results suggest that the evaluation of facial expression of pain was difficult for the observer to detect pain severity when the subjects feel mild pain.</p> Pain assessment, Pain perception, Face 43 50 http://bcn.iums.ac.ir/browse.php?a_code=A-10-1044-1&slc_lang=en&sid=1 Kazuhiro Hayashi hayashi.kzhr@gmail.com 13700319475328460014089 13700319475328460014089 Yes Multidisciplinary Pain Center, Aichi Medical University, Nagakute, Japan. Tatsunori Ikemoto tatsunon31@gmail.com 13700319475328460014090 13700319475328460014090 No Multidisciplinary Pain Center, Aichi Medical University, Nagakute, Japan. Takefumi Ueno uenotk@gmail.com 13700319475328460014091 13700319475328460014091 No National Hospital Organization, Hizen Psychiatric Center, Kyushu, Japan. Young-Chang Park Arai arainon@aichi-med-u.ac.jp 13700319475328460014092 13700319475328460014092 No Multidisciplinary Pain Center, Aichi Medical University, Nagakute, Japan. Kazuhiro Shimo shimoe7@gmail.com 13700319475328460014093 13700319475328460014093 No Multidisciplinary Pain Center, Aichi Medical University, Nagakute, Japan. Makoto Nishihara nisihara@aichi-med-u.ac.jp 13700319475328460014094 13700319475328460014094 No Multidisciplinary Pain Center, Aichi Medical University, Nagakute, Japan. Shigeyuki Suzuki suzuki@met.nagoya-u.ac.jp 13700319475328460014095 13700319475328460014095 No Program in Physical and Occupational Therapy, Graduate School of Medicine, Nagoya University, Japan. Takahiro Ushida ushidat@aichi-med-u.ac.jp 13700319475328460014096 13700319475328460014096 No Multidisciplinary Pain Center, Aichi Medical University, Nagakute, Japan.