Characteristics of transcranial magnetic stimulation (TMS) paradigms and related measures
| Tests (TMS paradigms) | Main measures | Definition | Physiological significance | Repeatability/reliability/validity | Safety | Setting (clinical, research) |
| Single-pulse TMS | Noninvasive and painless neurophysiological technique to evaluate the excitability of motor cortical area and the cortical spinal pathways conductivity through the administration of magnetic stimuli over the scalp. | Carries little risk beyond occasional local discomfort at the site of stimulation or a transient headache in susceptible subjects. No change in blood pressure, heart rate, EEG, serum prolactin level, serum cortisol level, or in a variety of memory, cognitive, learning, sensory and motor tests [314]. | ||||
| Motor evoked potential (MEP) | Muscular response obtained after a single TMS pulse applied over the contralateral primary motor cortex at appropriate stimulation intensity. | Integrity of the corticospinal tract and excitability of the corticospinal system. | Moderate to good reliability for MEP amplitude of FDI muscle at rest and under active condition; MEP amplitude is more reliable at 120% intensity of stimulation than those obtained at 100% [315]. | Research | ||
| MEP latency | Time interval between the application of the TMS pulse on the motor cortex area and MEP onset from the contralateral target muscle; it reflects the conductivity of both the central and peripheral nervous systems, as well as neuromuscular junctions and muscles. | Research | ||||
| MEP amplitude | Amplitude of MEP response measured peak-to peak. It reflects the excitatory state of output cells in the motor cortex, nerve roots and the conduction along the peripheral motor pathway to the muscles. | Research | ||||
| Resting motor threshold (RMT) | Lowest TMS intensity able to evoke MEPs in the resting target muscle when single-pulse stimuli are applied to the motor cortex. | Reflects the excitability of a central core of neurons, which arises from the membrane excitability and a balance between inhibitory and excitatory input from local circuits. | Good reliability in FDI for short- and long-term interval [315], also in ADM [316] and APB, EDC, FCR [317]. | Research | ||
| Active motor threshold (AMT) | Lowest TMS intensity required to obtain a MEP response during a weak muscle contraction. | Good to excellent short- and long-term reliability in FDI [315]. | Research | |||
| Cortical silent period (CSP) | Period of suppression of EMG activity following a twitch suprathreshold TMS stimulus of a target muscle during a sustained voluntary contraction of this muscle. | Cortico (spinal) inhibitory mechanisms, possibly GABAb mediated (but not only). | Moderate to good reliability in ADM [315] and FDI [317]. | Research | ||
| Central motor conduction time (CMCT) | Latency difference between the MEPs induced by TMS and by peripheral (motor root) stimulation. | Reflects the integrity of the cortical-spinal tract, from the upper to the lower motor neurons. | Research | |||
| Paired-pulse TMS | TMS paradigm to study intracortical inhibitory and excitatory phenomena by means of a conditioning subthreshold stimulus preceding a suprathreshold test stimulus applied at different interstimulus interval. | Research | ||||
| Intracortical facilitation (ICF) | Paired-pulse TMS measure obtained with long interstimulus interval where the conditioning stimulus is followed by an enhanced response with respect to the test stimulus; it is modulated by multiple neurotransmission pathways. | Expresses the activity of glutamatergic excitatory circuits | Poor reliability in ADM [315]. | Research | ||
| Short latency intracortical inhibition (SICI) | Paired-pulse TMS measure obtained with short interstimulus interval where the conditioning stimulus is followed by an inhibition with respect to the test stimulus; it is attributed to an activation of inhibitory neuronal system transmission. | Reflect the activity of GABAergic inhibitory circuits | Good short-term and long-term reliability under resting, not for active conditions [315]. | Research | ||
| Repetitive TMS (rTMS) | rTMS | Train of TMS pulses of the same intensity applied at a given frequency to a given brain area, that can transiently influence the function of stimulated and connected brain areas, mainly dependent on stimulation frequency. | Even in normal subjects, prolonged, high intensity, rTMS at 10–25 Hz rates can produce partial seizures with or without secondary generalisation [146]. Short inter-train intervals can cause transient degradation in short term verbal memory immediately following rTMS [318]. | Research | ||
| Low-frequency rTMS | Trains of variable duration at ≤1 Hz stimulation frequency. | Depression of the excitability of the stimulated regions, possibly via LTD. | Research | |||
| High-frequency rTMS | Trains of variable duration at ≥1 Hz stimulation frequency. | Increase of the excitability of the stimulated regions, possibly via LTP. | Research | |||
| Theta burst stimulation (TBS) | A form of complex rTMS trains combining different frequencies (i.e. 50 Hz pulse-trains repeated at a rate of 5 Hz) with after-effects on cortical-spinal and cortical-cortical excitability that may reflect changes in synaptic plasticity. | Inhibition when higher than 1 Hz. | Research |
EEG: electroencephalography; LTD: long-term depression; LTP: long-term potentiation; ADM: abductor digiti minimimuscle; FDI: first dorsal interosseous; APB: abductor pollicis brevis; EDC: extensor digitorum communis; FCR: flexor carpi radialis muscles.