Sex differences in somatomotor representations of others' pain: a permutation-based analysis

Sex differences in empathy for pain have been repeatedly observed. However, it is unclear whether this is due to sex differences in "bottom-up" somatomotor representations of others' pain (self-other resonance) or to "top-down" prefrontal control of such responses. Here, we provide data from 70 subjects suggesting that sex differences in empathy for pain lie primarily in pre-reflective, bottom-up resonance mechanisms. Subjects viewed a right hand pierced by a needle during fMRI. They also filled out a self-report measure of trait empathy, the Interpersonal Reactivity Index. A permutation-based analysis (FSL's Randomise) found that females showed greater signal in a cluster in primary somatomotor cortex that includes the motor hand area. No significant differences were observed in other task-implicated areas. An examination of condition-specific parameter estimates found that this difference was due to reduced signal in this cluster in males. No significant differences in resting connectivity or within-task (generalized psychophysiological interaction analysis or gPPI) dynamic connectivity of this region with prefrontal areas were observed. While female subjects scored higher on affective subscales of the IRI, there were no sex differences in Perspective-Taking, the primary index of cognitive, top-down empathy processes. These findings suggest that localized internal somatomotor representations of others' pain, a functional index of bottom-up resonance processes, are stronger in female subjects.