Response amendment latencies during discrete arm movements.

Examinations of the ability to rapidly modify ongoing movements have found that amendment latencies are shorter when a visual stimulus specifies an increase in movement velocity as compared to when a movement reversal is required. Our present study examined the hypothesis that amendment latencies for both continue and reverse instructions are dependent on the muscle activation state when the amendment is produced. Visual stimuli representing continue and reverse instructions wer represented at four different phases of a discrete arm movement, with amendment latencies measured to modified electromyographic (EMG) patterns. Amendment latencies for the continue instruction were shortest when the stimulus was presented early in the movement and increased when the stimulus was presented later in the response. The opposite trend was true for the reversal condition with long latencies occurring when the stimulus was presented early in the movement. Our findings support the notion that amendment latencies are directly related to the active state of the motor system when the modification is required, and the nature of the amendment to be generated.