Cortico-spinal inhibition reflects time but not event preparation: neural mechanisms of preparation dissociated by transcranial magnetic stimulation

Changes in cortico-spinal excitability related to time and event preparatio n were investigated by transcranial magnetic stimulation (TMS) of the motor cortex during the foreperiod of a movement-precuing task. Subjects perform ed a four alternative choice reaction time (RT) task involving a button-pre ss with the index or middle finger (FT) of the left or right hand. Advance information about the to-be-signaled response was provided by a precue, whi ch preceded the response signal by a 1 s foreperiod. The precue either indi cated the hand (right or left) or FI (inder or middle) with which the respo nse would be executed or was uninformative. TMS was delivered to the left o r right cortical hand area at one of five possible times during the foreper iod: -1000, -500, -333, -166 or 0 ms prior to the response signal. Surface EMG activity from a prime mover involved in flexion of the response FIs (Fl exor digitorum superficialis) was used to measure the magnitude of the moto r evoked potential (MEP) elicited by TMS. Cortico-spinal excitability - as assessed by the magnitude of the MEP evoked in the target muscle contralate ral to the stimulated hemisphere - progressively decreased during the forep eriod. The identity of the precued responses, however, had no effect on MEP magnitude. These results suggest that preparation to respond at a particul ar time inhibited excitability of the cortico-spinal tract, while advance p reparation to perform specific responses affected more central structures o nly. (C) 1999 Elsevier Science B.V. All rights reserved. PsycINFO classific ation: 2330; 2560; 2530.