Dissociation of autonomic and behavioral components of conditioned fear during development in the rat

Classical conditioning of heart rate (HR) was examined in unrestrained preweanling and weanling (16-, 19-, 21-, 25-, and 28-day-old) rats, with tone and light as the conditioned stimuli (CS) and electric shock as the unconditioned stimulus. The conditioned cardiac response was a sustained deceleration in HR which did not emerge until Day 21 for the tone CS and until Day 28 for the light CS. In contrast, when suppression of a behavioral response (running in a straight alley for dry suckling as reward) was used as the index of conditioning, the suppressive effects of the CS were evident around Day 16 for the tone and around Day 19 for the light. These findings indicate that during ontogenesis (a) the behavioral and autonomic responses to the same conditioned stimulus do not develop at the same pace, (b) the emergence of conditioned responses to tone and light stimuli parallel the sequential order in which the relevant sensory modalities achieve maturity (first audition, next vision), and (c) there is no clear-cut interdependence between development of the HR orienting response and conditionability of HR because unconditioned cardiac deceleration to both auditory and visual stimuli first appears about Day 16 in the developing rat, well before conditioned HR responses can be established to either stimulus.