Intracortical inhibition and facilitation in different representations of the human motor cortex

Intracortical inhibition (ICI) and intracortical facilitation (ICF) of the human motor cortex can be studied with paired transcranial magnetic stimula tion (TMS). Plastic changes and some neurological disorders in humans are a ssociated with changes in ICI and ICF. Although well characterized in the h and representation, it is not known if ICI and ICF vary across different bo dy part representations. Therefore we studied ICI and ICF in different moto r representations of the human motor cortex. The target muscles were rectus abdominus (RA), biceps brachii (BB), abductor pollicis brevis (APB), quadr iceps femoris (QF), and abductor hallucis (AH). For each muscle, we measure d the rest and active motor thresholds (MTs), the motor-evoked potential (M EP) stimulus-response curve (MEP recruitment), ICI, and ICF. The effects of different interstimulus intervals (ISIs) were studied with a conditioning stimulus (CS) intensity of 80% active MT. The effects of different CS inten sities were studied at ISI of 2 ms for ICI and ISI of 15 ms for ICF. MT was lowest for APE, followed by BE, AH, and QF, and was highest for RA. Except for BE, MEP recruitment was generally steeper for muscles with lower MT. I CI and ICF were present in all the motor representations tested. The stimul us intensity necessary to elicit ICI was consistently lower than that requi red to elicit ICF, suggesting that they are mediated by separate mechanisms . Despite wide differences in MT and MEP recruitment, the absolute CS inten sities (expressed as percentage of the stimulator's output) required to eli cit ICI and ICF appear unrelated to MT and MEP recruitment in the different muscles tested. These findings suggest that the intracortical mechanisms f or inhibition and facilitation in different motor representations are not r elated to the strength of corticospinal projections.