Facts and Artifacts in Cardiac Cycle Time Analysis: A Simple Model of Vagal Control

ABSTRACT Cardiac cycle time effects occur when stimuli or responses presented at different times within the cardiac cycle induce differential changes in the same or following interbeat interval (IBI). Two related problems regarding cardiac cycle time effects are discussed. One problem concerns how to separate anticipatory from stimulus‐induced changes in interbeat intervals that occur around the time of presentation of an expected stimulus. The other problem is an anomalous finding in reaction time tasks: prestimulus interbeat intervals are longer when they precede stimuli presented early, rather than late, in the cardiac cycle. These two problems can be understood if some simple assumptions are made about anticipatory and stimulus‐induced vagal excitation. If vagal excitation regularly increases prior to an expected event, the anomalous effect of stimulus delay in the cardiac cycle on preceding interbeat intervals can be explained. The presentation of events and IBI times on an ordinal IBI axis induces an inappropriate time shift. Furthermore, estimates of maximal anticipatory deceleration at stimulus onset in the interbeat interval of the stimulus and of subsequent stimulus‐induced vagal changes can be inferred. The relevance of this analysis to earlier experimental results is discussed.