The time course of amplitude specification in brief interceptive actions.

The interception of fast moving objects typically allows the object to be seen for only a short period of time. This limits the time available to prepare the movement. To deal with short preparation intervals, performers are likely to prepare a motor program in advance. Although motor preparation may begin before the target is seen, accuracy requires that certain program parameters are determined from observations of the target. In the experiments reported here we sought to determine the last moment at which information about the distance to move (amplitude) can be incorporated into a program. We employed an empirical protocol that allowed us to examine whether new amplitude information is incorporated discretely or continuously into the program during short intervals prior to movement onset (MO)-the preparation interval. Participants were trained to hit targets at two different distances with movements of a specific duration (180 ms): targets were moving in "Experiment 1" and stationary in "Experiment 2". This method permitted an estimate of MO time. Preparation intervals were manipulated by delivering a stimulus cue for movement amplitude at varying times prior to the estimated MO. Results demonstrated that amplitude information could be effectively incorporated into the program provided the preparation interval was greater than about 200 ms. In addition, the results indicated that amplitude was specified predominantly in a discrete manner, though the number of responses directed towards a central default amplitude suggest that the distance between targets was near to a threshold for continuous specification.