Abstract
In this study, we examined hemispheric differences in corticospinal excitability and in transcallosal inhibition in a selected group of young adults (n = 34) grouped into three handedness categories (RH: strongly right-handed, n = 17; LH: strongly left-handed, n = 10; MH: mixed-handed, n = 7) based on laterality quotients (LQ) derived from the Edinburgh Handedness Inventory. Performance measures were also used to derive a laterality index reflecting right-left asymmetries in manual dexterity (Dextli) and in finger tapping speed (Speedli). Corticospinal excitability was assessed in each hemisphere by means of transcranial magnetic stimulation (TMS) using the first dorsal interosseus as the target muscle. TMS measures consisted of resting motor threshold (rMT), motor evoked potential (MEP) recruitment curve (RC) and the contralateral silent period (cSP) with the accompanying MEP facilitation. Hemispheric interactions were assessed by means of the ipsilateral silent period (iSP) to determine the onset latency and the duration of transcallosal inhibition (i.e., LTI and DTI). Analysis of hemispheric variations in measures of corticospinal excitability revealed no major asymmetries in relation to degrees of laterality or handedness, with the exception of a rightward increase in rMTs in the LH group. Similarly, no clear asymmetries were found when looking at hemispheric variations in measures of transcallosal inhibition. However, a large group effect was detected for LTI measures, which were found to be significantly shorter in the MH group than in either the LH or RH group. MH participants also tended to show longer DTI than the other participants. Further inspection of overall variations in LTI and DTI measures as a function of LQs revealed that both variables followed a non-linear relationship, which was best described by a 2nd order polynomial function. Overall, these findings provide converging evidence for a link between mixed-handedness and more efficient interhemispheric communication when compared to either right- or left-handedness.
Highlights
The concept of handedness comes from the observation that most humans exhibit a preference for one hand over the other, with,90% of the population showing a rightward preference [1]
To elicit the ipsilateral silent period (iSP), we used the approach described by Giovannelli et al [24], whereby single transcranial magnetic stimulation (TMS) pulses (120% resting motor threshold (rMT)) were delivered ipsilaterally to the maximally contracting hand, while the opposite hand exerted a light force by gently squeezing a soft ball between the thumb and index fingers (,15% of the maximal activation)
laterality quotients (LQ) accounted for .70% of the variance in dexterity laterality index (Dextli), whereas they accounted for .50% of the variance in speed laterality index (Speedli)
Summary
The concept of handedness comes from the observation that most humans exhibit a preference for one hand over the other, with ,90% of the population showing a rightward preference [1]. In a TMS study, Triggs et al [8] used right-left differences in dexterity tests to quantify the degree of lateralization in groups of right-handers and left-handers This index of handedness provided the best correlation with corresponding measures of right-left asymmetry in MT (i.e., the larger the manual asymmetry, the greater the threshold asymmetry). Given recent evidence pointing to a relationship between degree of laterality and asymmetries at the cortical level, we hypothesized that hemispheric differences would emerge in individuals exhibiting strong preference for one hand (either right or left) and large manual asymmetries in performance when compared to individuals with no clear preference for one hand (i.e., mixed handedness) and lower degrees of manual asymmetries
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