Cognitive Modulation of Time‐Dependent Primary Bradycardia

In one experiment, 60 male college students counted silently the number of rarely presented tones embedded in a sequence of “standard” tones. The tones differed by 50 Hz. Both rare and standard tones produced a primary bradycardia which exhibited time-dependency: Stimuli which happened to fall relatively early in the cardiac cycle prolonged the duration of that very cycle more than late-occurring stimuli. The latter slowed the subsequent cardiac cycle. These effects are identical qualitatively with those found in acute animal experiments upon direct electrical stimulation of the vagus, and they verified and extended results from previous experiments with human subjects in response to other sensorimotor events. The effects of expectancy, evaluated by serial positional effects on the magnitude and slope of time-dependent cardiac slowing, were different for standard and for rare tones. Comparisons with P300, the late positive component of the average evoked potential, suggest that while P300 is most strongly associated with poststimulus decisional processes, primary bradycardia and its time-dependent aspects reflect both early stimulus registration and later decision making. The results suggest limbic and neocortical modulation of primary bradycardia. In a second experiment with 20 male college students, in which tones were presented at experimentally controlled times within the cardiac cycle, the basic effects obtained with post hoc separation of tones into relative cycle time of occurrence were verified.