Limitations
pain
Our behavioral approach only provides an indirect measure of the activation of the EOS. This limitation warrants further discussion because the mixed-trial design of our study poses the question as to whether the level of endogenous opioids can vary every minute, forty times in a row, without contaminating the subsequent trial. We believe that our results are physiologically plausible, because in a previous functional magnetic resonance imaging (fMRI) study, also using a mixed-trial design, we showed that 30 seconds of pleasant vs. unpleasant music (corresponding to the length of the musical stimuli used in the present study) are sufficient to elicit activity changes in the hippocampus [Another limitation of our study is that our four experimental conditions (especially music with tapping vs. silence with tapping) might have differed in their task difficulty. Thus, we cannot exclude the possibility that our results are confounded by effects of task difficulty. However, our results indicate that pleasantness and familiarity contributed to the modulation of pain (see above), and therefore the pain-reducing effects of sensorimotor synchronization to music cannot be explained by task difficulty only. Nevertheless, it remains a matter of future studies to elucidate the influence of task difficulty on pain-reducing effects of sensorimotor synchronization to music.