On the variability of motor-evoked potentials: experimental results and mathematical model

The purpose of this study was to determine the form of the relation between the mean amplitude and variance of motor-evoked potentials (MEP). To this end, single-pulse transcranial magnetic stimulation (TMS) was applied over the motor cortex of seventeen neurologically normal adult human subjects. The coil was positioned at a locus on the scalp that elicited an MEP in the first dorsal interosseous (FDI) at the lowest stimulus intensity. The subjects were instructed to maintain tonic activity in the FDI of 5 or 10% of the maximum voluntary contraction (MVC). The relation between MEP variance and amplitude was found to have an inverted parabolic shape, with maximal variance occurring near the half-maximal MEP amplitude. The coefficient of variation [Formula: see text] of MEPs decreased approximately as a rectangular hyperbolic function of MEP amplitude (i.e. ~ 1/MEP). A probabilistic model is proposed to explain the inverted parabolic relation between MEP variance and MEP amplitude, as well as the sigmoid shape of the MEP input-output relation (i.e. stimulus-response curve). The model is based on a description of α-motoneurons as binary threshold units, with unit thresholds distributed according to a positively skewed probability density function. The units are driven by noisy synaptic input currents having a Gaussian distribution. The model predicts an inverse parabolic relation between MEP variance and amplitude and a sigmoid input-output relation, as experimentally observed. Furthermore, increasing model motoneuron excitability by increasing the background synaptic drive increases MEP variability independently of MEP size, a surprising prediction. The model also explains the approximately rectangular hyperbolic relation between [Formula: see text] and MEP amplitude. The implications of these results for the interpretation of neurophysiological experiments and the statistical analysis of MEPs are discussed.

Changes in recruitment of motor cortex excitation and inhibition in patients with drug-induced tardive syndromes

OBJECTIVES

It has recently been suggested that drug-induced tardive syndromes (TS) might be due to maladaptive plasticity, which increases motor excitability in cerebral cortex and basal ganglia. In order to test this hypothesis, we performed the first measurements of cortical excitability in TS.

METHODS

Motor cortex excitability was examined using transcranial magnetic stimulation (TMS) in 22 TS patients and compared with that in 20 age and sex-matched healthy individuals. Resting and active motor threshold (RMT, AMT) and input-output curves (I/O curves) assessed corticospinal excitability. The duration of the contralateral silent period (cSP) at a range of stimulation intensities and ipsilateral silent period (iSP) were used as measures of inhibition.

RESULTS

There were no significant differences in RMT and AMT between patients and controls, although the input-output curves were significantly steeper in patients. The cSP (at different stimulus intensities) and iSP were both longer in the patients compared to the control group. However, most of this difference could be accounted for by increased recruitment of motor evoked potentials (MEPs) in patients.

CONCLUSION

TS is characterized by hyperexcitability of corticospinal output that might contribute to the lack of selectivity in muscle recruitment and contribute to excess involuntary movement. The findings are opposite to those in naturally-occurring hyperkinesia such as Sydenham's and Huntington's chorea, suggesting a fundamental difference in the pathophysiology.

Effect of coil orientation on strength-duration time constant and I-wave activation with controllable pulse parameter transcranial magnetic stimulation

OBJECTIVE

To compare the strength-duration (S-D) time constants of motor cortex structures activated by current pulses oriented posterior-anterior (PA) or anterior-posterior (AP) across the central sulcus.

METHODS

Motor threshold and input-output curve, along with motor evoked potential (MEP) latencies, of first dorsal interosseus were determined at pulse widths of 30, 60, and 120 μs using a controllable pulse parameter (cTMS) device, with the coil oriented PA or AP. These were used to estimate the S-D time constant and we compared with data for responses evoked by cTMS of the ulnar nerve at the elbow.

RESULTS

The S-D time constant with PA was shorter than for AP stimulation (230.9 ± 97.2 vs. 294.2 ± 90.9 μs; p<0.001). These values were similar to those calculated after stimulation of ulnar nerve (197 ± 47 μs). MEP latencies to AP, but not PA stimulation were affected by pulse width, showing longer latencies following short duration stimuli.

CONCLUSION

PA and AP stimuli appear to activate the axons of neurons with different time constants. Short duration AP pulses are more selective than longer pulses in recruiting longer latency corticospinal output.

SIGNIFICANCE

More selective stimulation of neural elements may be achieved by manipulating pulse width and orientation.

Time course of corticospinal excitability changes following a novel motor training task

Motor learning is known to take place over several days, and there are a number of studies investigating the time course of improvements in motor performance, yet only a limited number that have investigated the time course of neurophysiological changes that accompany motor learning. The aim of this study was to investigate the time course of changes to corticospinal excitability, following novel motor training in the dominant hand, during two sessions of motor training and testing. This study used the slope of transcranial magnetic stimulation (TMS) input-output (I/O) curves elicited at stimulator intensities between 90 and 150% of resting motor threshold for the first dorsal interosseous (FDI) muscle in order to measure corticospinal excitability. The I/O curves for 12 right-handed males (M age: 21.9+-0.5 years, [Laterality Index]=83.42 SD=4.9) were elicited before and after the performance of novel motor tracing task performed with the right hand on two different testing days. Participants had significant improvements in motor performance during both the initial (mean error improvement=31%, SD=7%, F(1, 11)=22.439 with p=0.001) and follow up session (mean error improvement=19%, SD=6%, F(1, 11)=17.85 with p=0.001). The slope of the TMS I/O curve decreased significantly over the four training blocks, F(1,11)=8.149, p=0.016, however pre-planned contrasts within the repeated measures ANOVA indicated that the decrease was only significant relative to baseline following the first day of training F(1,11)=10.476, p=0.008. This study found that corticospinal excitability measured using I/O curves decreases in response to performance of a novel motor training task, and the majority of this excitability change occurs on the first training day.