title: The StartReact effect in tasks requiring end-point accuracy.
abstract: OBJECTIVE
  Fast and accurate movements are often performed in response to a sensory signal. In reaction time tasks, execution of open loop movements is speeded up when a startling auditory stimulus (SAS) is applied together with the imperative signal (IS). In this study, we examined the effects of a SAS on the performance of a task that demands accuracy.
  
  
  METHODS
  Nine subjects were asked to move a monitored pen to a target point located in a table at a fixed angular distance of 30 degrees from a start point. The target was a spot of three possible diameters: 5, 10, and 20mm. Finger force for pen holding, pen tip pressure against the table and kinematic variables of the forearm movement were measured for three conditions: control, SAS delivered at IS (SAS-IS trials) and SAS delivered during movement execution (SAS-MOV trials).
  
  
  RESULTS
  Two movement phases could be identified in the movement trajectory and force profile. The first phase, ballistic, was significantly shortened in SAS-MOV trials, with earlier and larger peak velocity and peak force with respect to control trials. The second phase, slow approach to target, was longer in SAS-IS trials but not in SAS-MOV trials. Accuracy was maintained throughout all conditions and stimulation modes.
  
  
  CONCLUSIONS
  A SAS speeds up only the first (ballistic) part of the movement in an accuracy task. Slower target approach compensates for the accelerated initial movement. No changes in the last part of the movement are seen when a SAS is delivered after movement onset.
  
  
  CONCLUSIONS
  The StartReact effect is restricted to the onset of a complex movement, when muscles are activated in a ballistic mode, without feedback.
authors.affiliation: National School of Occupational Medicine, Carlos III Institute of Health, Madrid, Spain; Department of Physical Medicine and Rehabilitation, School of Medicine, Complutense University of Madrid, Madrid, Spain. Electronic address: jmcastel@isciii.es.
authors.forename: J M
authors.initials: JM
authors.lastname: Castellote
authors.affiliation: Unidad de EMG y Control Motor, Servei de Neurologia, Hospital Clínico, Universidad de Barcelona, Barcelona, Spain.
authors.forename: J
authors.initials: J
authors.lastname: Valls-Solé
mesh_terms: D000161:Acoustic Stimulation / Q000379:methods*; D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009068:Movement / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*; D011897:Random Allocation; D011930:Reaction Time / Q000502:physiology*; D013216:Reflex, Startle / Q000502:physiology*
keywords: Accuracy; Motor control; Motor task; Startle