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Inhibitory interactions between pairs of subthreshold conditioning stimuli in the human motor cortex
S Bestmann1, H R Siebner, N Modugno
1Sobell 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
Summary
Two subthreshold conditioning stimuli can induce intracortical inhibition (ICI) without the rapid periodicity of I-wave interactions. This suggests a model where synaptic inputs converge on a common inhibitory interneuron.
Area of Science:
- Neuroscience
- Motor Control
- Electrophysiology
Background:
- Short interval paired-pulse paradigms are used to study intracortical inhibition (ICI) and facilitation (ICF).
- Understanding the interaction of subthreshold conditioning stimuli is crucial for interpreting TMS data.
Purpose of the Study:
- To examine short interval paired-pulse paradigms for ICI and ICF.
- To determine if paired subthreshold conditioning stimuli interact and exhibit rapid periodicity.
Main Methods:
- Transcranial magnetic stimulation (TMS) applied to left M1 to evoke motor-evoked potentials (MEPs) in the first dorsal interosseous (FDI) muscle.
- Subthreshold conditioning stimuli (CS) at intensities below motor threshold were used.
- Interstimulus intervals (ISIs) between 1-15 ms were tested for single and paired CS before a test shock (TS).
Main Results:
- Paired subthreshold CS elicited clear inhibition at specific intervals, unlike single CS.
- Maximum inhibition occurred when CS(1) preceded CS(2) by 1-2.5 ms, with the effect diminishing up to 13 ms.
- A small, non-significant extra-facilitation was observed with paired CS at 6-15 ms ISIs when facilitation was present.
Conclusions:
- Two subthreshold conditioning stimuli can facilitate inhibition.
- This induced inhibition does not exhibit the rapid periodicity characteristic of I-wave interactions.
- The findings support a model involving convergent synaptic inputs onto a common inhibitory interneuron.
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