Infant rats are more likely than adolescents to orient differentially to amodal (intensity-based) features of single-element and compound stimuli

An infantile predisposition to process quantity, over quality, of stimulati on has been suggested in theories of cognitive and perceptual development, as well as for understanding ontogenetic differences in learning. In the pr esent study, responsivity to stimulus intensity was assessed in preweanling and periadolescent rats by using magnitude of cardiac orienting as an inde x of perceived stimulus intensity. In experiment 1, cardiac orienting was m easured to low- and high-intensity auditory and visual stimuli in 15-, 17-, and 30-day-old rats. The results demonstrated that younger rats are more p redisposed to respond differentially to single-element stimuli on the basis of stimulus intensity. Experiments 2, 3, and 4 examined cardiac orienting in preweanlings to compound stimuli. In accordance with studies of ontogeny of learning, the results of these experiments indicated that preweanlings process compound stimuli on the basis of net intensity, but only if there h ad been no prior experience with the compound's elements. (C) 2000 John Wil ey & Sons, Inc.