title: Independence of Movement Preparation and Movement Initiation.
abstract: Initiating a movement in response to a visual stimulus takes significantly longer than might be expected on the basis of neural transmission delays, but it is unclear why. In a visually guided reaching task, we forced human participants to move at lower-than-normal reaction times to test whether normal reaction times are strictly necessary for accurate movement. We found that participants were, in fact, capable of moving accurately ∼80 ms earlier than their reaction times would suggest. Reaction times thus include a seemingly unnecessary delay that accounts for approximately one-third of their duration. Close examination of participants' behavior in conventional reaction-time conditions revealed that they generated occasional, spontaneous errors in trials in which their reaction time was unusually short. The pattern of these errors could be well accounted for by a simple model in which the timing of movement initiation is independent of the timing of movement preparation. This independence provides an explanation for why reaction times are usually so sluggish: delaying the mean time of movement initiation relative to preparation reduces the risk that a movement will be initiated before it has been appropriately prepared. Our results suggest that preparation and initiation of movement are mechanistically independent and may have a distinct neural basis. The results also demonstrate that, even in strongly stimulus-driven tasks, presentation of a stimulus does not directly trigger a movement. Rather, the stimulus appears to trigger an internal decision whether to make a movement, reflecting a volitional rather than reactive mode of control.
authors.affiliation: Departments of Neurology and adrian.haith@jhu.edu.
authors.forename: Adrian M
authors.initials: AM
authors.lastname: Haith
authors.affiliation: Departments of Neurology and School of Clinical Medicine, University of Cambridge, Cambridge, CB2 0SP, United Kingdom.
authors.forename: Jina
authors.initials: J
authors.lastname: Pakpoor
authors.affiliation: Departments of Neurology and Neuroscience, Johns Hopkins University, Baltimore, Maryland 21287, and.
authors.forename: John W
authors.initials: JW
authors.lastname: Krakauer
mesh_terms: D000293:Adolescent; D000328:Adult; D005260:Female; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D008297:Male; D009042:Motivation / Q000502:physiology*; D009068:Movement / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology*; D013998:Time Perception / Q000502:physiology; D055815:Young Adult
keywords: movement initiation; movement preparation; reaching; reaction time; volitional movement