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. SIGNIFICANCE STATEMENT The reaction time, i.e., how quickly we can initiate a movement in response to a stimulus, is important for daily activities such as driving and is also a critical tool in neuroscience, used to probe a multitude of cognitive functions. However, there remains a surprising lack of basic understanding about exactly what determines reaction times, even for simple movements such as reaching to a target. We show that the reaction time for a reaching movement does not reflect the moment that the movement becomes ready to execute. Instead, the reaction time is determined by a separate initiation process. These findings suggest a distinct neural basis for preparation and initiation of movement and provide an explanation for the sluggishness of typical reaction times.