Responses to rapid-rate transcranial magnetic stimulation of the human motor cortex

Abstract

We applied trains of focal, rapid-rate transcranial magnetic stimulation (rTMS) to the motor cortex of 14 healthy volunteers with recording of the EMG from the contralateral abductor pollicis brevis, extensor carpi radialis, biceps brachii and deltoid muscles. Modulation of the amplitude of motor evoked potentials (MEPs) produced in the target muscle during rTMS showed a pattern of inhibitory and excitatory effects which depended on the rTMS frequency and intensity. With the magnetic coil situated over the optimal scalp position for activating the abductor pollicis brevis, rTMS led to spread of excitation, as evident from the induction of progressively larger MEPs in the other muscles. The number of pulses inducing this spread of excitation decreased with increasing rTMS frequency and intensity. Latency of the MEPs produced in the other muscles during the spread of excitation was significantly longer than that produced by single-pulse TMS applied to the optimal scalp positions for their activation. The difference in MEP latency could be explained by a delay in intracortical conduction along myelinated cortico-cortical pathways. Following rTMS, a 3-4 min period of increased excitability was demonstrated by an increase in the amplitude of MEPs produced in the target muscles by single-pulse TMS. Nevertheless, repeated rTMS trains applied 1 min apart led to similar modulation of the responses and to spread of excitation after approximately the same number of pulses. This suggests that the spread might be due to the breakdown of inhibitory connections or the recruitment of excitatory pathways, whereas the post-stimulation facilitation may be due to a transient increase in the efficacy of excitatory synapses.

Diagnosis, pathophysiology and management of chronic migraine: a proposal of the Belgian Headache Society

“…1 Hz) have an inhibitory effect [97], whereas high frequencies (C10 Hz) are excitatory [98]. In healthy volunteers and migraine patients, rTMS is able to durably modify excitability of the visual cortex and hence to reverse the abnormalities of evoked potentials found in many migraineurs [99,100].…”

Young patients with focal seizures may have the primary motor area for the hand in the postcentral gyrus

“…The spatial resolution of TMS is estimated to be approximately 1 cm within the primary motor hand area (Brasil-Neto et al, 1992;PascualLeone et al, 1994;Werhahn et al, 1994). It has been speculated that electrical stimulation methods directly stimulate the Betz cells in the motor cortex, whereas TMS methods have the Betz cells indirectly stimulated via interneuronal networks (Pascual-Leone et al, 1994). Previous TMS studies of healthy adult volunteers demonstrated that the TMS-induced current flowing across the central sulcus not in an 'anterior-to-posterior' but 'posterior-to-anterior' direction optimally activated the motor cortex (Brasil-Neto et al, 1992;Werhahn et al, 1994); this well-replicated observation seems to be consistent with the observation in the present study that stimulation of the postcentral gyrus preferably elicited a contralateral hand motor response in a substantial number of young patients with focal epilepsy.…”

Brain Stimulation Techniques in the Treatment of Obsessive-Compulsive Disorder: Current and Future Directions

“…TMS protocols are generally divided into two types of procedures according to the frequency used: low-frequency TMS (<1 Hz) and highfrequency TMS ( < 20 Hz). Depending on stimulation parameters (frequency, rate, and duration), repetitive stimuli to specific cortical regions can either decrease or enhance the excitability of the affected brain structures (ie, cerebral cortex [15][16][17] ) and modify regional cerebral blood flow. 18,19 …”