Facilitatory effect of thinking about movement on motor-evoked potentials to transcranial magnetic stimulation of the brain

Task-dependent facilitation of motor evoked potentials during dynamic and steady muscle contractions

Task-dependent differences in the facilitation of motor evoked potentials (MEPs) following cortex stimulation were studied in a proximal (deltoid) and a distal muscle (abductor digiti minimi; ADM) in 23 healthy subjects during both dynamic and steady contractions of the target muscle under isometric and under nonisometric conditions. In the deltoid, MEP amplitudes were significantly greater if stimulation was performed during dynamic contractions than during steady contractions, despite equal background electromyographic levels just prior to the stimulus. The same task-specific extra facilitation of deltoid MEP amplitudes was also found with magnetic stimulation of the brain stem instead of the cortex in 3 subjects. In the ADM, no such task-dependent extra facilitation of MEPs during dynamic contractions was found. It is concluded that in the deltoid, during dynamic contractions, a greater proportion of the spinal motoneurons is close to depolarization threshold (greater "subliminal fringe") whereas the number of firing motoneurons is similar to that during steady contraction. The lack of task-dependent extra facilitation of MEPs in the ADM is explained by the predominant recruitment principle for force gradation in small hand muscles, which is in contrast to the predominant frequency principle used in proximal muscles.

Clinical applications of motor evoked potentials

Magnetic stimulation of brain and spinal roots provides a non-invasive evaluation of nervous propagation as well as of motor cortex excitability in healthy subjects and in patients affected by neurological diseases (i.e. multiple sclerosis, stroke, Parkinson's disease, myelopathies etc.). Motor areas can be reliably mapped and short- and long-term 'plastic' changes of neural connections can be studied and monitored over time. By evaluating excitatory and inhibitory phenomena following transcranial stimuli, the mechanisms of action of different drugs, including antiepileptics, can be studied. Moreover, transcranial stimulation of non-motor brain areas represents a probe for the evaluation of lateralized hemispheric properties connected with higher cortical functions. Recent studies suggest a therapeutic role of repetitive magnetic stimulation in psychiatric disorders.

Corticospinal excitability modulation during mental simulation of wrist movements in human subjects

Motor evoked potentials (MEPs) to magnetic transcranial stimulation (TCS) were simultaneously and bilaterally recorded from flexor carpi radialis (FCR) and extensor communis digitorum (ECD) muscles in seven healthy and trained subjects. Latencies and amplitudes characteristics of MEPs were investigated under the following randomised conditions: muscular and mental relaxation; mental simulation, during absolute muscular relaxation, selective flexion or extension of the right or left wrist muscles; arithmetical calculation with muscular relaxation. Unspecific, diffuse facilitatory effects on MEPs amplitude were induced by mental non motor activity (arithmetical calculation). A further specific and lateralised amplitude potentiation on the agonist muscle acting as 'prime mover' for the mentally simulated movement was consistently found in all the subjects, without significant latency changes. Inhibitory effects on antagonists were evident only in two subjects.

Facilitatory effect of thinking about movement on magnetic motor-evoked potentials

To investigate the facilitatory effect of thinking about movement on motor evoked potential (MEP) amplitude, we recorded MEPs in two test muscles during rest, with the subject thinking about contracting the test muscle but without subsequent contraction, and during 10% maximum voluntary contraction. Stimuli were delivered at 10% above resting motor threshold and at 90-100% stimulator output. H-reflexes, recorded in flexor carpi radialis, were obtained during rest and think conditions. MEP threshold was lower during the think condition (P = 0.004). At both stimulus intensities, median MEP amplitudes and areas were significantly (P < 0.001) larger during the think paradigm compared with rest. This effect was greater at the lower stimulus intensity. There was no significant difference in latency (P = 0.15). In 4/8 subjects, H-reflex amplitudes were mildly facilitated (P < 0.05) during the think condition. We conclude that thinking about movement without detectable EMG activity has a facilitatory effect on magnetic MEPs. The absence of a MEP latency shift between rest and think conditions and absence of a consistent increase in H-reflex amplitude suggests this effect occurs largely at the cortical level. In some subjects, however, an increase in spinal motoneuron excitability may also contribute.

Interhemispheric differences of hand muscle representation in human motor cortex

Focal magnetic transcranial stimulation (TCS) is employed for mapping of the motor cortical output to abductor digiti minimi (ADM) muscle. The aim of this study was to evaluate the interhemispheric asymmetries in normals. Motor maps were obtained through motor evoked potentials (MEPs) recordings from ADM muscle in 20 healthy subjects in right and left hemispheres TCS. Measurement of several indexes such as excitability threshold, MEPs amplitude, MEPs latency, and silent period duration did not show differences between the hemispheres. Moreover, no interhemispheric asymmetries were found when the amplitude ratio values were analyzed. The hand motor cortical area, as represented by the number of responsive sites (3.6 vs. 3.5) and the "hot spot" site localization presented a fairly symmetrical organization. Absolute values displayed a relatively wide intersubject variability, while their interhemispheric differences were extremely restricted. This observation can offer a new tool in diagnosing and following up neurological disorders affecting the central motor system, mainly for those concerning monohemispheric lesions.