Anticipatory postural adjustments under simple and choice reaction time conditions.

Two different schemes of the central organization of anticipatory postural adjustments (APAs) have been proposed: one of them assumes that postural and focal components of an action are results of a single control process (single-process hypothesis) while the other one suggests that these two components result from two relatively independent control processes (dual-process hypothesis). To distinguish between the two hypotheses, we investigated changes in the relative timing of the postural and focal components under self-paced, simple reaction time (SRT) and choice reaction conditions (CRT). Standing subjects performed one of four small arm movements resulting in a standard postural perturbation (dropping a load). APAs were quantified as changes in the background muscle activity as well as shifts of the center of pressure. APAs occurred at a larger delay prior to the focal movement under the self-paced condition than under the SRT condition. Under the CRT condition, actual RTs were longer than under SRT, but APAs were more similar to those under self-paced conditions. A negative correlation between the reaction time and APA onset was found. The findings demonstrate that the focal and APA components of an action can be decoupled, thus supporting the dual-process hypothesis. Changes in APAs with action suggest a possibility of modifications of a function that transforms two parallel control signals into a single command to the focal and postural muscles, based on postural requirements and behavioral constraints.