Effect of long interval interhemispheric inhibition on intracortical inhibitory and facilitatory circuits

Stimulation of the primary motor cortex (M1) of one hemisphere of the brain inhibits the opposite M1, a process known as interhemispheric inhibition (IHI). An early phase of IHI peaks at about∼10ms after stimulation of the opposite hemisphere and is termed short latency interhemispheric inhibition (SIHI). A later phase peaks at about 40ms and has been termed long latency interhemispheric inhibition (LIHI). The objective of the present study is to test how LIHI interacts with cortical inhibitory and facilitatory circuits, including short interval intracortical inhibition (SICI), intracortical facilitation (ICF) and long interval intracortical inhibition (LICI). We studied 10 healthy volunteers. LIHI from right to left hemisphere was elicited by stimulating the rightM1 at an interstimulus interval (ISI) of 40ms before stimulation of the left M1. Conditioning and test stimuli to elicit SICI, ICF and LICI were given to left M1. The effects of different sizes of test motor-evoked potential (MEP amplitudes; 0.2, 1 and 2mV) were examined for SICI, ICF, LICI and LIHI. Using paired-pulse and triple-pulse protocols, ho