Age and hemispheric differences in transcallosal inhibition between motor cortices: an ispsilateral silent period study.

BACKGROUND: In this study, we investigated age and hemispheric differences in transcallosal inhibition (TCI) in the context of active contraction using the ipsilateral silent period (iSP). We also examined whether age-related changes in TCI would be related to corresponding changes in manual performance with age. Participants consisted of right-handed individuals from two age groups (young adults, n=13; seniors, n=17). The iSP was measured for each hemisphere using suprathreshold TMS pulses delivered over the primary motor cortex ipsilateral to the maximally contracting hand while the homologue muscles of the opposite hand were lightly contracting (~15% of the maximum). Manual performance was assessed bilaterally for both grip strength and fine dexterity. RESULTS: Our results yielded two main findings. First, TCI measures derived from iSP were strongly influenced by age, whereas differences between hemispheres were only minor. Second, correlation analyses revealed that age-related variations in TCI measures were related to changes in manual performance, so that left-to-right TCI correlated with right hand performance and vice-versa for the opposite hand/hemisphere. CONCLUSION: Overall, these results concur with other recent reports indicating that mutual inhibition between motor cortices tends to decline with age. In this respect, our observations are in line with the notion that the balance of normally predominantly inhibitory interactions between motor cortices is shifted toward excitatory processes with age.

Using the 6-min Walk Test to Monitor Peak Oxygen Uptake Response to Cardiac Rehabilitation in Patients With Heart Failure.

PURPOSE: We examined the agreement between peak oxygen uptake (V˙o2peak), estimated using prediction equations from the 6-min Walk Test (6MWT), and V˙o2peak measured using a cardiopulmonary exercise test (CPX) to estimate change in V˙o2peak in patients with heart failure (HF) enrolled in cardiac rehabilitation (CR). METHODS: This was secondary analysis of 54 (including 9 women) patients with HF who completed a clinical CR program. Four previously published equations using 6MWT distance were used to estimate V˙o2peak and were compared with a CPX at baseline, follow-up, and change using the standard and modified Bland-Altman method. Analyses were repeated for quartiles of cardiorespiratory fitness (CRF) based on measured V˙o2peak from the CPX. RESULTS: Bland-Altman plots revealed proportional bias between all prediction equations and the measured V˙o2peak. The difference between methods varied by the level of CRF, with overestimation of prediction equations at greater levels of CRF and underestimation at lower levels of CRF. This poor agreement remained when comparisons were made between the estimated and measured V˙o2peak values at quartiles of CRF, indicating prediction equations have limited ability to predict V˙o2peak at any level of CRF. CONCLUSION: Estimated V˙o2peak using 6MWT distance demonstrated poor agreement with measured V˙o2peak from a CPX. While distance ambulated on the 6MWT remains an important measure of physical performance in patients with HF, prediction equations using 6MWT distance are not appropriate to monitor changes in V˙o2peak following CR in patients with HF.

Evidence of alterations in transcallosal motor inhibition as a possible long-term consequence of concussions in sports: A transcranial magnetic stimulation study.

OBJECTIVE: Growing evidence suggests that long-term structural and physiological alterations are present in the brain of previously concussed athletes. In this study, we sought to further explore the long-term consequences of concussions with transcranial magnetic stimulation (TMS) by examining excitability changes both within and between hemispheres. METHODS: Participants (32 young adults with and without a history of concussions (HxC)) first underwent testing to assess cognitive and motor performance using standardized tests. Then, the following TMS measures were derived bilaterally: (1) resting motor threshold and motor evoked potentials (MEP), (2) afferent-induced modulation, (3) contralateral silent period (cSP) and MEP facilitation, and, (4) ipsilateral silent period (iSP). RESULTS: Multivariate analyses of performance data revealed no major group differences. For TMS data, no "hemisphere" effects were detected for all measures. Group differences were detected only for iSP derived measures owing to alterations in the onset latency and duration of transcallosal inhibition in the HxC group. CONCLUSIONS: While no major asymmetries were found between hemispheres, participants in the HxC group showed evidence of impaired transcallosal inhibition. SIGNIFICANCE: Results provide one of the first piece of evidence pointing to alterations in transcallosal inhibition as a potential neurophysiological marker of long-term consequences of concussions in sports.

Predicting Modulation in Corticomotor Excitability and in Transcallosal Inhibition in Response to Anodal Transcranial Direct Current Stimulation.

INTRODUCTION: Responses to neuromodulatory protocols based either on transcranial direct current stimulation (tDCS) or transcranial magnetic stimulation (TMS) are known to be highly variable between individuals. In this study, we examined whether variability of responses to anodal tDCS (a-tDCS) could be predicted from individual differences in the ability to recruit early or late indirect waves (I-waves), as reflected in latency differences of motor evoked potentials (MEPs) evoked by TMS of different coil orientation. METHODS: Participants (n = 20) first underwent TMS to measure latency of MEPs elicited at different coil orientations (i.e., PA, posterior-anterior; AP, anterior-posterior; LM, latero-medial). Then, participants underwent a-tDCS (20 min @ 2 mA) targeting the primary motor cortex of the contralateral preferred hand (right, n = 18). Individual responses to a-tDCS were determined by monitoring changes in MEP amplitude at rest and in the duration of the contralateral silent period (cSP) and ipsilateral silent period (iSP) during contraction; the latter providing an index of the latency and duration of transcallosal inhibition (LTI and DTI). RESULTS: Consistent with previous reports, individual responses to a-tDCS were highly variable when expressed in terms of changes in MEP amplitude or in cSP duration with ~50% of the participants showing either little or no modulation. In contrast, individual variations in measures of transcallosal inhibition were less variable, allowing detection of significant after-effects. The reduced LTI and prolonged DTI observed post-tDCS were indicative of an enhanced excitability of the transcallosal pathway in the stimulated hemisphere. In terms of predictions, AP-LM latency differences proved to be good predictors of responses to a-tDCS when considering MEP modulation. CONCLUSION: The present results corroborate the predictive value of latency differences derived from TMS to determine who is likely to express "canonical" responses to a-tDCS in terms of MEP modulation. The results also provide novel suggestive evidencethat a-tDCS can modulate the excitability of the transcallosal pathway of the stimulated hemisphere.

Hemispheric differences in corticospinal excitability and in transcallosal inhibition in relation to degree of handedness.

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 2(nd) 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.