title: Anticipatory postural adjustments under simple and choice reaction time conditions.
abstract: 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.
authors.affiliation: Department of Kinesiology, Rec. Hall-267L, The Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Harm
authors.initials: H
authors.lastname: Slijper
authors.forename: Mark L
authors.initials: ML
authors.lastname: Latash
authors.forename: J Toby
authors.initials: JT
authors.lastname: Mordkoff
mesh_terms: D000328:Adult; D004576:Electromyography; D005260:Female; D006801:Humans; D008297:Male; D009068:Movement / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D011597:Psychomotor Performance; D011930:Reaction Time / Q000502:physiology; D014836:Volition / Q000502:physiology