Perceptual sharpening in the developing rat.

The development of perceptual sharpening was studied in four experiments by using the heart rate orienting response (marked cardiac deceleration to novel stimuli) as the dependent measure. Preweanling rats either 16-17 or 19-20 days of age were habituated to a 1600-Hz tone over 15 trials. On Trial 16, the stimulus frequency was changed to one of seven tones distributed on either side of the habituated stimulus. Analysis of the heart rate orienting response showed that the younger rats (16-17 days old) failed to respond to any of the new frequencies, which indicates generalization of habituation to the auditory stimuli within the frequency range studied. In marked contrast, rat pups 19-20 days of age displayed a cardiac orienting response proportional to the difference between the habituated stimulus and the test stimulus. Additional experiments demonstrated that these age-related differences were not due to differences in sensitivity to the test stimuli, effector fatigue, or inability to discriminate between the test stimuli. These results indicate that perceptual sharpening develops relatively rapidly in rats and that perceptual sharpening, stimulus detection, and stimulus discrimination follow separate ontogenetic time courses.