Abstract
Infant rats (16, 21, and 26 days old) received 3 days of classical conditioning training to Tone/Shock pairings using heart rate as the index of conditioning. The youngest group (16 days old) showed no evidence of conditioning at any stage of acquisition. In the 21- and 26-day-olds, response to the conditioned stimulus (CS) was a deceleration in heart rate during the first day of training. By day 3, the conditioned response (CR) shifted to cardioacceleration. The data were interpreted as offering strong support for a two-stage model of information processing in the infant rat, comprised of a conditioned orienting response in the first stage and a conditioned defensive response in the second.
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