Inhibitory interactions between pairs of subthreshold conditioning stimuli in the human motor cortex.
OBJECTIVE
These experiments examined short interval paired-pulse paradigms for intracortical inhibition (ICI) and facilitation (ICF). We tested whether pairs of subthreshold conditioning stimuli interact, and whether they showed rapid periodicity similar to that observed in subthreshold I-wave interaction.


METHODS
Transcranial magnetic stimulation (TMS) was given over left M1 to evoke a motor-evoked potential (MEP) of approximately 1 mV peak-to-peak amplitude in the contralateral first dorsal interosseous (FDI) muscle. Each test shock (TS) was preceded by single or paired subthreshold conditioning stimuli (CS(1) and CS(2)) at short interstimulus intervals (ISIs 1-15 ms). Intensities of CS were set just below thresholds for intracortical inhibition (ICI) or intracortical facilitation (ICF).


RESULTS
Each CS(single) alone had no effect on the test MEP, but with two CS, clear inhibition was elicited at certain intervals. With a CS(2)-TS interval of 2 ms, maximum suppression occurred if CS(1) was applied 1-2.5 ms before CS(2). This inhibitory effect tapered off gradually as the CS(2)-CS(1) interval was increased up to 13 ms. When facilitation was present with a CS(single)-TS interval of 10 ms, a small but non-significant extra-facilitation occurred at ISIs between CS(2) and CS(1) of 6-15 ms.


CONCLUSIONS
Two subthreshold conditioning stimuli facilitate inhibition that lacks the rapid periodicity typical of I-wave interaction. The data would be compatible with a model in which synaptic inputs converge on a common inhibitory interneurone.
Sobell Department of Motor Neuroscience and Movement Disorders, The Institute of Neurology, National Hospital for Neurology and Neurosurgery, Queen Square, London WC1N 3BG, UK. sbestman@ion.ucl.ac.uk
S
S
Bestmann
H R
HR
Siebner
N
N
Modugno
V E
VE
Amassian
J C
JC
Rothwell
D000328:Adult; D003213:Conditioning, Psychological / Q000502:physiology; D004558:Electric Stimulation; D004576:Electromyography; D005260:Female; D006801:Humans; D008280:Magnetics; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D009433:Neural Inhibition / Q000502:physiology*; D010507:Periodicity; D014836:Volition / Q000502:physiology