Long interval intracortical inhibition is asymmetric in young but not older adults.

Aging is typically accompanied by a decline in manual dexterity and handedness; the dominant hand executes tasks of manual dexterity more quickly and accurately than the non-dominant hand in younger adults, but this advantage typically declines with age. Age-related changes in intracortical inhibitory processes might play a role in the age-related decline in manual dexterity. Long interval intracortical inhibition (LICI) is asymmetric in young adults, with more sensitive and more powerful LICI circuits in the dominant hemisphere than the non-dominant hemisphere. Here, we investigated whether the hemispheric asymmetry in LICI in younger adults persists in healthy older adults. Paired-pulse transcranial magnetic stimulation (TMS) was used to measure LICI in the dominant and non-dominant hemisphere of younger and older adults; LICI stimulus-response curves were obtained by varying conditioning stimulus (CS) intensity at two different inter-stimulus intervals (100 ms: LICI100; 150 ms: LICI150). We have replicated the finding that LICI100 circuits are more sensitive and more powerful in the dominant than non-dominant hemisphere of young adults, and extend this finding to show that the hemispheric asymmetry in LICI100 is lost with age. In the context of behavioral observations showing that dominant hand movements in younger adults are more fluent than non-dominant hand movements in younger adults and dominant hand movements in older adults, we speculate a role of LICI100 in the age-related decline in manual dexterity.

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