Cardiac deceleration and reaction time: an evaluation of two hypotheses.

The purpose of this experiment was to evaluate two hypotheses concerning the basis of the association between performance on a simple reaction time (RT) task and the deceleration of heart rate found as the S responds. The RT task consisted of 96 trials in which the foreperiod was randomly varied between 2, 4, 8, and 16 sec. Two groups of 31 Ss each were used, with the cardiac response blocked pharmacologically in one group, in order to determine if the occurrence of the cardiac response facilitated performance through an afferent feedback mechanism. Two aspects of somatic activity, EMG bursts from chin muscles and eye movements and blinks, were also assessed in order to determine if the cardiac response and the associated behavioral facilitative effects were linked to a common mediating process involving cardiac deceleration and the inhibition of ongoing, task-irrelevant somatic activities. The latter hypothesis was consistently supported. Blocking the cardiac response did not significantly influence performance. However, a within-S analysis revealed a pronounced direct relationship between RT and the magnitude of the inhibition of somatic effects and the magnitude of the cardiac deceleration when the latter was not blocked pharmacologically. These data along with several other lines of evidence are considered to indicate that heart rate deceleration may not be significantly involved in an afferent mechanism but rather can be best understood as a peripheral manifestation of central processes.