Handedness, Dexterity, and Motor Cortical Representations

Abstract

. Motor system organization varies with handedness. However, previous work has focused almost exclusively on direction of handedness (right or left) as opposed to degree of handedness (strength). In the present study, we determined whether measures of interhemispheric interactions and degree of handedness are related to contra-and ipsilateral motor cortical representations. Participants completed a battery of handedness assessments including both handedness preference measures and behavioral measures of intermanual differences in dexterity, a computerized version of the Poffenberger paradigm (PP) to estimate interhemispheric transfer time (IHTT), and they underwent transcranial magnetic stimulation (TMS) mapping of both motor cortices while we recorded muscle activity from the first dorsal interosseous muscle bilaterally. A greater number of ipsilateral motor evoked potentials (iMEPs) were elicited in less lateralized individuals with the number of iMEPs correlated with IHTT. There were no relationships between handedness or lateralization of dexterity and symmetry of contralateral motor representations, although this symmetry was related to IHTT. Finally, IHTT was positively correlated with multiple measures of laterality and handedness. These findings demonstrate that degree of laterality of dexterity is related to the propensity for exhibiting iMEPs and the speed of interhemispheric interactions. However, it is not clear whether iMEPs are directly mediated via ipsilateral corticospinal projections or are transcallosally transmitted. I N T R O D U C T I O NIt is known that handedness is related to the organization of the sensorimotor system. For example, there is some evidence to indicate that left-and right-handers may differ in corpus callosum morphology (Habib et al. 1991;Westerhausen et al. 2004;Witelson 1985) and organization of motor cortical representations (Amunts et al. 1996;Dassonville et al. 1997;Kim et al. 1993;Volkmann et al. 1998). These neuroanatomical differences have been linked to behavior as well with lefthanders typically having faster and more accurate interhemispheric transmission than right-handers (Cherbuin and Brinkman 2006a,b;Marzi et al. 1991). However, the majority of work investigating relationships between handedness, neuroanatomy, and behavior has evaluated direction of handedness (left vs. right) as opposed to degree of handedness (how strongly handed one is), although there is evidence to support the importance of the latter (Dassonville et al. 1997;Luders et al. 2010;Siebner et al. 2002;Solodkin et al. 2001). Organization of motor cortex and handednessMotor cortex physiology is related to handedness. For example, left-and right-handers show different functional activation patterns in the primary motor cortex (M1) when performing motor tasks. Right-handers show more asymmetrical activation in M1, predominantly in the dominant (left) hemisphere, whereas left-handers show more bilateral activity (Kim et al. 1993;Siebner et al. 2002). Those that are more mixedhanded show the most ...

Hemispheric Differences in Corticospinal Excitability and in Transcallosal Inhibition in Relation to Degree of Handedness

“…In fact, the only asymmetry found was for a rightward elevation in rMTs in the LH group. In this regard our observations appear largely consistent with those of Bernard et al [15], who performed similar comparisons of measures of corticomotor excitability (i.e., rMT, MEP size, motor mapping) and found no major asymmetries as a function of handedness using the same three-way classification as we used. As stated earlier, TMS studies have produced mainly conflicting evidence to support the existence of a strong link between hand dominance and laterality differences in corticospinal excitability [see 6 for a review].…”

“…The overall picture that emerges from the TMS literature on handedness is one of difficulty establishing a strong link between the observed behavioural asymmetry in hand use and its neurophysiological correlates at the corticomotor level. As pointed out by Bernard et al [15], beyond variations in experimental protocols, one major reason for the conflicting evidence is the fact that most authors have considered handedness only in terms of direction (i.e., either right or left preference) without consideration for the degree of lateralization (i.e., how strong is your preference). Interestingly, the relatively few studies that have considered the degree of handedness have produced much more consistent findings.…”

Handedness but not dominance influences variability in endurance time for sustained, submaximal contractions

“…In addition, there was no difference in the strength of short-term motor unit synchronization between the dominant and nondominant hands, but motor unit synchronization was less prominent in the dominant hand of right-handed subjects, presumably reflecting a more restricted distribution of direct projections from motor cortical neurons. Motor cortex physiology does vary with handedness, as indicated by differences in functional activation patterns in the primary motor cortex, such as asymmetries in communication between the hemispheres (Bernard et al 2011;Klöppel et al 2007;McCombe et al 2008). Our findings suggest that the neural adjustments underlying the constant net muscle torque during the sustained, submaximal contractions were more variable for left-handed subjects across tasks that required either force or position control than for right-handed subjects.…”

Short-term effects of unilateral lesion of the primary motor cortex (M1) on ipsilesional hand dexterity in adult macaque monkeys

“…In other studies, however, no such ipsilateral MEP activity was observed in hand muscles (Carr et al 1994; Netz et al 1997; Bawa et al 2004). Ipsilateral MEPs are related to the degree of handedness (Bernard et al 2011). When TMS was used to generate a transient inactivation of the motor cortex while the subject was performing hand motor tasks with the ipsilateral hand, deficits were observed in a sequential finger task (Chen et al 1997), and in grip-lift and in step-tracking tasks (Davare et al 2007).…”