DEPENDENCE OF THE TRANSCRANIALLY INDUCED SILENT PERIOD ON THE INSTRUCTION SET AND THE INDIVIDUAL REACTION-TIME

Objectives and methods: We looked for influences of the experimental c ondition on the silent period (SP) from transcranial motor cortex stim ulation and analyzed how the instruction given to the subject, as well as the individual reaction time, might affect the duration of the SP in the biceps brachii muscle. Results: The duration of the SP was foun d to critically depend on the subject's voluntary reaction of the targ et muscle immediately after the stimulus. With low stimulus intensity and low background force, the duration of the silent period was signif icantly longer in 10 of 15 subjects (P = 0.002) when they were instruc ted to relax quickly after the stimulus rather than to maintain the th e force at a constant level. A significant shortening of the SP (P = 0 .02) was observed when the subjects were instructed to perform a rapid contraction of the target muscle in reaction to the cortical stimulus . With low stimulus intensity and high background force, the same infl uence of the instruction set was found in 6 of 13 subjects. When the s ubjects were left without precise instruction, the SP duration was unp redictable. In 10 subjects, the SP corresponded to that obtained with the instruction to maintain the force at a constant level. However, in 3 subjects it was prolonged to the value observed in the 'relax' inst ruction. With greater stimulus intensities, the effect of the instruct ion set on the SP duration was generally smaller. A significant prolon gation was nevertheless found at low background forces with rapid rela xation (P<0.001), and a significant shortening was found at high backg round forces with rapid contraction (P<0.001) after the stimulus. The SP duration observed with 20% of maximal voluntary contraction (MVC) s ignificantly correlated with the individual reaction time. No such cor relation was found for the SP obtained with 80% MVC. The SP was slight ly longer at 20% MVC, as compared to 80% MVC within each instruction g roup. This effect was significant (P<0.05) at low stimulus intensities . Conclusions: Therefore, when assessing the SP duration for diagnosti c purposes, not only the stimulus intensity but also the background fo rce and the voluntary reaction must be standardized. Furthermore, grea t stimulus intensities and high background forces should be used to mi nimise the effects of instruction set and individual reaction time. (C ) 1998 Elsevier Science Ireland Ltd. All rights reserved.