Preparation of response force and movement direction: Onset effects on thelateralized readiness potential

Two experiments assessed the preparatory effects of advance information abo ut response force and movement direction on the lateralized readiness poten tial (LRP). In a choice reaction time (RT) task, an imperative stimulus req uired an isometric flexion ol an extension of the left or right index finge r. Prior information about response force or about movement direction reduc ed RT and shortened the interval from the onset of the imperative stimulus up to the onset of the LRP. Advance information, however, about direction b ut not about force decreased the interval from LRP onset to the onset of th e overt response. The identical pattern of results was obtained in a second experiment, in which each participant performed both precue conditions. Th e findings of both experiments support the notion that response force is sp ecified before movement direction. These results are consistent with the vi ew accordingly different mechanisms are involved in the specification of mu scle force and movement direction.