title: Lateralized asymmetry of facial motor evoked potentials.
abstract: BACKGROUND
  There is considerable evidence that emotions are expressed more intensely on the left side of the face. This asymmetry could reflect a right hemisphere advantage in processing emotional material or an asymmetry in corticobulbar motor systems. Transcranial magnetic stimulation (TMS) was used to test for lateralized asymmetry in the cortical control of muscles of facial expression in the lower face.
  
  
  METHODS
  We administered TMS to the motor cortex of 50 subjects during contraction of bilateral orbicularis oris muscles. We analyzed motor evoked potentials (MEPs) with a repeated measures analysis of variance (ANOVA) using hemisphere stimulated and orbicularis oris side recorded as within subject factors.
  
  
  RESULTS
  TMS elicited contralateral MEPs in 42 of 50 subjects. Forty of these 42 subjects showed bilateral MEPs. The ANOVA showed a significant main effect of face side, such that MEPs elicited in left face were larger than in right face (p < 0.0001). The analysis also showed a significant interaction between the hemisphere stimulated and face side, such that the difference between contralateral and ipsilateral MEPs with right brain TMS was greater than with left brain TMS (p < 0.0001).
  
  
  CONCLUSIONS
  The results provide evidence of lateralized asymmetry of corticobulbar projections to muscles of facial expression in the lower face.
authors.affiliation: Department of Neurology and Neuropsychology, McKnight Brain Institute, University of Florida, Gainesville, Florida, USA. triggswj@neurology.ufl.edu
authors.forename: William J
authors.initials: WJ
authors.lastname: Triggs
authors.forename: Georges
authors.initials: G
authors.lastname: Ghacibeh
authors.forename: Utaka
authors.initials: U
authors.lastname: Springer
authors.forename: Dawn
authors.initials: D
authors.lastname: Bowers
mesh_terms: D000293:Adolescent; D000328:Adult; D016477:Artifacts; D004566:Electrodes / Q000592:standards; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005152:Facial Muscles / Q000294:innervation* / Q000502:physiology; D005154:Facial Nerve / Q000502:physiology*; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000033:anatomy & histology / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D009431:Neural Conduction / Q000502:physiology; D011712:Pyramidal Tracts / Q000033:anatomy & histology / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D012535:Scalp / Q000502:physiology; D050781:Transcranial Magnetic Stimulation / Q000295:instrumentation / Q000379:methods