Judgements about onset of rapid voluntary movements in man

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Abstract
Subjects made rapid voluntary muscular contractions. In a variable temporal relationship to each contraction they received a reference stimulus (shock to left ankle) and were required to judge whether actual movement or the reference stimulus occurred first. The onset of muscular contraction was measured from the electromyogram (EMG). From the judgments made about the sequence of movement and the variably related reference stimulus in a series of contractions, the temporal relation between the 2 required for them to appear to coincide, was determined. The onset of EMG activity had to precede the reference stimulus by 40-320 ms (depending on the individual subject and muscle group tested) for the movement and reference stimulus to be judged coincident. In a further series of similar contractions the subjects were required to judge whether the command to move or the reference stimulus occurred first. They were asked to attempt to disregard actual movement and to concentrate instead on the moment they told the body part to move. Most subjects now required the reference stimulus to be given in advance of the relevant muscular contraction (preceding the onset of EMG activity by up to 100 ms) for the command to move and the reference to be judged coincident. Subjects attend to different signals when judging the timing of commands than when judging the timing of actual movements. The subjective timing of commands to move was studied in the absence of movements and local sensation. This was achieved by requiring subjects to attempt movements of the hand during local anesthetic or ischemic/pressure blockade of the nerves to the forearm and hand. The timing of the attempted contractions was measured from the onset of the efferent motor volleys recorded through a microelectrode inserted into the appropriate nerve proximal to the block. The efferent neural activity replaced the EMG in trials in which the subject judged whether the command to move or a reference stimulus occurred first. The reference stimulus had to precede the onset of efferent neural activity for command and the reference stimulus to be considered simultaneous, the timing being similar to that found when EMG was matched with the reference stimulus. The signals identified as the commands for movement do not arise from sensory receptors in the part moved. Subjects made rapid voluntary contractions with each of 2 separate muscle groups (those operating the jaw and the ankle). For each muscle the relative timing of EMG and reference stimulus was determined; first, for there to be apparent coincidence of actual movement and the reference stimulus and second, for there to be subjective coincidence of the motor command and the reference stimulus. These data were then used to predict the relative timing of EMG onset in the 2 muscles for voluntary contractions that would be subjectively simultaneous. When contracting the 2 muscles so that the contractions seemed simultaneous, some subjects behaved as if judging simultaneity by reference to commands and others behaved as if judging by reference to felt movements.

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