Four-pulse transcranial magnetic stimulation using multiple conditioning inputs. Normative MEP responses.

Area of Science:

• Neuroscience
• Motor Control
• Electrophysiology

Background:

• Transcranial magnetic stimulation (TMS) is a key tool for probing motor cortical excitability.
• Current dual-pulse TMS protocols for short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) have limitations in precision.
• Investigating novel TMS patterns may enhance the ability to selectively modulate and study cortical circuits.

Purpose of the Study:

• To compare a novel four-pulse superconditioning-test (SC-T) TMS pattern with traditional dual-pulse TMS.
• To evaluate the efficacy of SC-T in modulating motor cortical excitability, specifically inhibition and facilitation.
• To explore the potential of SC-T for investigating abnormal motor cortical circuitry, including in conditions like ALS.

Main Methods:

• A novel four-pulse SC-T pattern was designed, featuring a three-pulse subthreshold conditioning train followed by a suprathreshold test pulse.
• Varied intervals between conditioning pulses (1, 3, 6 ms) and test pulses (1, 3, 10, 25 ms) were tested.
• Motor-evoked potentials (MEPs) from abductor pollicis brevis (APB) and tibialis anterior (TA) muscles were recorded in 15 healthy adults and one individual with ALS.

Main Results:

• The SC-T pattern demonstrated significant modulation of motor cortical excitability, with specific intervals yielding pronounced inhibition (1 ms test interval) or facilitation (10 ms test interval).
• The most effective SC-T inputs consistently outperformed dual-pulse TMS (SICI and ICF) in magnitude of effect for both inhibition and facilitation.
• While overall effects were more pronounced in APB than TA, normalized effects suggested similar underlying mechanisms across muscles.

Conclusions:

• The novel four-pulse SC-T TMS pattern provides a superior method for selectively modulating and investigating motor cortical inhibition and facilitation compared to dual-pulse TMS.
• This technique holds promise for enhanced diagnostic and research applications, particularly in understanding aberrant neural circuitry in neurological disorders.
• The findings highlight the potential of precisely timed conditioning pulse trains to refine TMS-based neuromodulation and circuit analysis.