The need to respond and cardiac cycle effects: an alternative hypothesis

Lacey and Lacey have proposed that stimulation by sensorimotor events produces a discrete burst of vagal activity which alters the length of the cardiac cycle in which such an event falls, and the length of the subsequent cycle, according to the point in the cycle in which the stimulus occurs. To test this hypothesis an experiment was conducted with 24 subjects in which they received three types of trials. These were signalled reaction time tests with a 5-s foreperiod, signalled reaction time tests during which subjects were instructed to sit still and stop breathing, and 5-s periods of immobility. Lacey and Lacey's results were only partially replicated in that no cardiac effect for the beat concurrent with the imperative stimulus was observed, but an effect similar to the one which they describe was found for the beat subsequent to the imperative stimulus. Cycle effects not predicted by the Laceys' hypothesis were found during the beat in which the cycle occurred and the subsequent beat. An alternative explanation of these phenomena is proposed which fits both their data and those of this study. Rather than being a consequence of a burst of vagal activity produced by the stimulus as they suggest, such cycle effects may reflect an underlying monotonic change in the heart rate.