2 3 Handedness , Dexterity , and Motor Cortical Representations 4 5 6 Jessica

33 34 35 36 2 Abstract 37 Motor system organization varies with handedness. However, previous work has 38 focused almost exclusively on direction of handedness (right or left) as opposed to 39 degree of handedness (strength). In the present study, we determined whether 40 measures of interhemispheric interactions and degree of handedness are related to 41 contralateral and ipsilateral motor cortical representations. Participants completed a 42 battery of handedness assessments including both handedness preference measures 43 and behavioral measures of intermanual differences in dexterity, a computerized 44 version of the Poffenberger Paradigm (PP) to estimate interhemispheric transfer time 45 (IHTT), and they underwent transcranial magnetic stimulation (TMS) mapping of both 46 motor cortices while we recorded muscle activity from the first dorsal interosseous 47 muscle bilaterally. A greater number of ipsilateral motor evoked potentials (iMEPs) were 48 elicited in less lateralized individuals, with the number of iMEPs correlated with IHTT. 49 There were no relationships between handedness or lateralization of dexterity and 50 symmetry of contralateral motor representations, though this symmetry was related to 51 IHTT. Finally, IHTT was positively correlated with multiple measures of laterality and 52 handedness. These findings demonstrate that degree of laterality of dexterity is related 53 to the propensity for exhibiting iMEPs and the speed of interhemispheric interactions. 54 However, it is not clear whether iMEPs are directly mediated via ipsilateral corticospinal 55 projections or are transcallosally transmitted.