Response Initiation, Not Completion, Seems to Alter Cardiac Cycle Length

The effects on cardiac interbeat interval (IBI) length of the initiation and completion of speeded motor reactions were examined at different points in the cardiac cycle (different times after the Rwave of the electrocardiogram). Stimuli were presented either at the R‐wave or 350 ms afterward. Responses were paced by instructions to release a start key prior to three different target times (150, 275, or 500 ms). Initiation time was defined as the interval between stimulus onset and the release of the start key; completion time was defined as the interval between the onset of the stimulus and the depression of one of two response keys. The shift from anticipatory deceleration to acceleration varied systematically as a function of time of response relative to the R‐wave. Responses that occurred early in the cardiac cycle were associated with a shift from deceleration to acceleration in the same IBI; for late occurring responses the shift occurred in the subsequent IBI. Most importantly, when response initiation occurred early in the cycle (<300 ms) and response completion occurred late, deceleration was terminated during the same IBI. Initiation rather than completion seems to control the inhibitory vagal input to the heart that is responsible for anticipatory deceleration.