Age and sex differences in human motor cortex input–output characteristics

Abstract

ProtocolAn input-output curve for the MEP amplitude evoked in the resting FDI by TMS was constructed. Ten TMS were delivered at each intensity, beginning 10-12 % below resting threshold and increasing incrementally in 3 % steps to either 100 % of stimulator output, or to a stimulus intensity where MEP amplitude had Age and sex differences in human motor cortex input-output characteristics Julia B. Pitcher, Kirstin M. Ogston and Timothy S. Miles Department of Physiology, The University of Adelaide, Adelaide, South Australia 5005, AustraliaStimulus-response curves for motor evoked potentials (MEPs) induced in a hand muscle by transcranial magnetic stimulation (TMS) were constructed for 42 subjects with the aim of identifying differences related to age and sex. There was no effect of age on the resting threshold to TMS, the maximal amplitude of the MEP that could be evoked (MEP max ) or the maximal slope of the stimulus-response curve. However, higher stimulus intensities were required to achieve both MEP max and the maximal slope in the older subjects. The trial-to-trial variability of MEPs was greater in the older subjects, particularly at intensities near threshold. There was a significant interaction between age, threshold and trial-to-trial variability of MEP amplitude. Overall, MEP variability fell markedly as stimulus intensity increased above threshold but less rapidly in older than in younger subjects. Females tended to have larger MEP variability than males, but age and threshold were much stronger modulators than sex. These differences in input-output characteristics are likely to be due either to a decreased number of spinal motoneurones being activated synchronously in older subjects, or to the activation of the same number of motoneurones in a less synchronous manner, leading to phase cancellation in the surface electromyogram. Journal of Physiologyreached a plateau. The MEPs evoked at each stimulus intensity were averaged on-line. The peak-to-peak amplitude and onset latency of these averages were then determined with customwritten software (LabVIEW, National Instruments, Austin, TX, USA). All MEP and M-wave data are presented as the ensemble averages of 10 trials.The amplitudes of the MEP averages were plotted against stimulus intensity in each subject, and the Marquardt-Levenberg algorithm for least squares convergence (Sigmaplot for Windows 5.0, 1986-1999; SPSS Inc., IL, USA) was used to calculate the best fit of the cortical stimulus-response curves of each subject. The calculation was based on the highest coefficient of determination, R 2 , and the resulting 5-parameter sigmoidal curve is described by the following equation:where the five parameters are the difference between the smallest and the largest MEP amplitudes observed, i.e. the range between the two asymptotes (a); the minimum MEP amplitude (y 0 ); the difference between the stimulus intensities at 75 % and 25 % of the maximum MEP amplitude (i.e. the width of the transition) (b); the stimulus intensity required to obtain an MEP that ...

Age and gender effects on submental motor-evoked potentials

“…This supports an earlier study that found no gender effects on limb MEP magnitude (Pitcher et al 2003). Another study measured MEPs from the upper limb and found no gender effects on MEP magnitude and latency in younger subjects, after correcting the latencies for the subject's arm length (Livingston et al 2010).…”

Comparison of transcranial magnetic stimulation measures obtained at rest and under active conditions and their reliability

“…Previous studies regarding the reliability of TMS measures have provided equivocal results, with some studies demonstrating moderate to good reproducibility for normalized map volume and for map area (Malcolm et al, 2006;, and other studies showing poor to moderate reproducibility for the same measures (Kiers et al, 1993;Mortifee et al, 1994). The trialto-trial variability of MEP amplitude is well known (Kiers et al, 1993;Pitcher et al, 2003;Smith et al, 2011) and could explain these equivocal results. Our data showed poor reliability for map area either at rest or under active condition, but for map volume a better reliability was observed under active condition.…”

“…Again, the absence of a significant association between resting and active MEPs for measures obtained at 110% of the MT could reflect the large trial-to-trial variability of MEP amplitude. This variability can be particularly important at lower stimulation intensities (Pitcher et al, 2003). Importantly, the presence of an association for MEP amplitudes obtained at 120% of MT but not for…”

“…Our results suggested a similar tendency for the active MEPs to be greater than the resting MEPs, but this difference did not reached significance (p<0.09, both at 110% and 120% of MT). This lack of statistically significant difference might be attributable to the small sample size (limited statistical power increasing the probability of committing a type-II error) and to the considerable trial-to-trial variability in MEP amplitude (Pitcher et al, 2003;Smith et al, 2011)). …”

Increased intracortical inhibition in elderly adults with anterior–posterior current flow: A TMS study

“…The majority of previous studies that have reported RMT and AMT in young and elderly subjects have shown no age-related change in these measures (Kossev et al, 2002;Wassermann, 2002;Pitcher et al, 2003;Oliviero et al, 2006;Rogasch et al, 2009;Smith et al, 2009;Cirillo et al, 2010;Smith et al, 2011). However, in all cases there is typically a trend towards increased RMT and AMT in elderly participants.…”