Establishing intermodal equivalence in preweanling and adult rats.

Interactions between olfactory and brightness conditioning were examined in infant (16-day-old) and adult rats. Conditioning to an odor (methyl salicylate [MS] paired with footshock [FS]) was followed by subthreshold conditioning to an unscented black box (paired with FS). Controls were given unpaired presentations in either the 1st or the 2nd phase of this treatment. No age-related differences in conditioning occurred when either phase was presented alone. Significant aversion to the black box was observed after the experimental treatment only in infants, and brightness conditioning potentiated odor conditioning in infants but not adults. In the infants, odor-extinction procedures weakened the aversion to the black box as well as to MS. Olfactory-to-visual transfer required that the same unconditioned stimulus be paired with both conditioned stimuli. These results may reflect an infantile disposition for unitization.