Effects of viewing affective pictures on sEMG activity of masticatory and postural muscles
Introduction
Looking at pictures with emotional content and intensity effects different physiological systems [2]. As an example, Bradley et al. [3] demonstrated that the specific thematic content of pleasant (e.g., erotic vs adventure) or aversive picture stimuli (e.g., threat vs victim) can specifically modulate the physiological response. Also, it is known from the theory of emotions that unpleasant stimuli generally prime withdrawal reactions, whereas pleasant stimuli prime approach actions. Many studies analyzed the electromyography (EMG) activity of facial muscles in response to viewing pleasant and unpleasant photographs. For example, Zhou observed that the intensity of facial EMG activity on the left side of the face is stronger than the right side of the face during the process of emotional expression [19]. Rymarczyk demonstrated that subjects react spontaneously and rapidly to happy faces with increased EMG activity of zygomaticus major and decreased activity of the corrugator supercilii showing changes in response to dynamic stimuli greater than those to static ones in both muscles. In contrast, angry faces evoked no alteration of EMG activity in zygomaticus muscles and only small changes in the corrugator muscle EMG, and there were found no difference between the responses to static and dynamic stimuli [15]. However, such results are relative only to mimic muscles, while masticatory or body posture responses have been often neglected, which in turn should also be observed as neuro-physiologically and anatomically linked to them [13], [14]. The aim of this study is therefore to evaluate whether pictures with emotional content may modify the surface electromyography (sEMG) of the stomatognathic and postural systems, providing evidence of functional links among emotional reactions, posture and masticatory functionality.
Section snippets
Materials and methods
Twenty male healthy volunteers without visual or hearing impairments (mean age 25.6 years; range 19–34 years) participated in this study. All the subjects were right handed, as determined by self-report [8]. Exclusion criterion was the presence of any musculoskeletal diseases, potentially related to gnathologic or postural disorders (fibromyalgia, myofascial pain, temporomandibular disorders, low back pain, scoliosis, hip pain, trochanteric bursitis, previous orthodontic or gnathologic
Results
None of the subjects scored out of normality values at the STAI test and the overall rating for picture arousal and valence did not differ from the pre-defined IAPS reference values.
The Wilcoxon Signed Rank test applied to assess intra-operator errors due to the positioning of electrodes, did not reveal statistical differences among sEMG recordings acquired twice, confirming the reproducibility of sEMG data on the studied sample (P < 0.05).
The Wilcoxon Signed Rank test applied to compare the
Discussion
In our study, we observed that emotional stimuli seem to be unable to modify significantly the muscular activity of the stomatognathic and postural systems, recorded by sEMG.
The Wilcoxon Signed test applied to compare the first and the last Baseline Set showed statistical differences, randomly distributed, in both block I and II. In accordance with this evidence, the Baseline Sets are not comparable. Consequently, it is not possible to consider the first and the last set of each block as
Conclusion
Viewing emotion eliciting images seems not to have statistically significant effects on body muscular response of masticatory and postural systems. Further studies are needed to better highlight, understand and validate these early evidences.
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