Modulation of the Direction and Magnitude of Hebbian Plasticity in Human Motor Cortex by Stimulus Intensity and...

Area of Science:

• Neuroscience
• Neurophysiology
• Human Motor Cortex Plasticity

Background:

• Paired associative stimulation (PAS) is a non-invasive brain stimulation technique used to induce Hebbian plasticity.
• The precise mechanisms underlying PAS efficacy and its variability remain unclear.
• Neural depolarization level, influenced by stimulation intensity and circuit interactions, may modulate plasticity.

Purpose of the Study:

• To investigate how transcranial magnetic stimulation (TMS) intensity and short latency afferent inhibition (SAI) affect PAS-induced plasticity.
• To elucidate the role of neural activity levels in mediating PAS effects.
• To understand inter-individual variability in PAS response.

Main Methods:

• PAS was applied at different interstimulus intervals (ISIs) relative to the N20 peak of median nerve somatosensory-evoked potentials (PAS_N20+2, PAS_N20+6).
• PAS_N20+2 was tested at varying TMS intensities.
• Short interval intracortical facilitation and inhibition (SICF, SICI) were assessed in the presence of SAI (SICF_SAI, SICI_SAI).

Main Results:

• Long-term potentiation (LTP)-like effects increased with higher PAS_N20+2 TMS intensity; long-term depression (LTD)-like effects occurred at subthreshold intensity.
• Stronger SAI was associated with weaker PAS LTP-like effects.
• PAS_N20+2 was less effective than PAS_N20+6, and SICF_SAI/SICI_SAI did not influence PAS response.

Conclusions:

• Inter-individual differences in SAI contribute significantly to PAS efficacy variability.
• TMS intensity modulates the magnitude and direction of PAS effects.
• The level of neural activity induced by stimulation is critical for determining the direction and magnitude of PAS-evoked Hebbian plasticity in the human cortex.