Standardization of facial electromyographic responses

In behavioral studies, facial electromyographic (EMG) responses to external stimuli or internal events are usually quantified relative to the resting state, presumed to represent a neutral baseline condition. In the large majority of recent studies, EMG responses were expressed as a difference score in terms of microvolts with the resting state. We argue that since EMG activity is measured on a ratio scale rather than on an interval scale, percentage scores should be used instead of difference scores. Reanalyzing results from an earlier study on the relationships between facial EMG responses and affective empathic responses to emotional video clips, we found that the two different types of EMG response quantification were differently related to affective empathy. Relationships between EMG responses and affective empathy were more consistent or stronger for percentage scores than for difference scores. In another study, facial EMG mimicry responses to pictures of emotional facial expressions were stronger for percentage scores than for difference scores. The adequacy of percentage scores relative to difference scores as indices of psychological variables may be simply checked by comparing both types of scores.