The adult heart rate (HR) response was studied in 18 male subjects following a series of 10 tones and 10 light flashes. Both stimuli were of moderate intensity (54 db and 8.0 ml) and had rise times of 30 milliseconds. The tone evoked a predominantly acceleratory response and no decrement with repetition. Initially, the light flash evoked a predominantly deceleratory response which changed to one of acceleration as the stimulus was repeated. Analysis of respiration revealed that an increase in rate and amplitude followed both stimuli and that the magnitude of respiratory changes did not diminish with stimulus repetition. Viewed according to Sokolov's criteria for identifying orienting, defensive, and adaptive responses the results support the conclusions that: (1) HR deceleration is a component of the orienting response; and (2) HR acceleration is neither an orienting, defensive, nor adaptive response but is largely secondary to respiratory changes. On the basis of the evidence, a tentative model of the HR response to simple auditory and visual stimuli is offered.

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