Five experiments tested the effects of experience with a white compartment not paired with footshock (CS-) on conditioning of an aversion to a black compartment paired with footshock. As previously found with odors as CSs, a single pairing of the CS+ with footshock yielded significant conditioning only if the animal was also exposed to CS-, with greater conditioning when the CS-exposure preceded the CS+ than when the CS+ preceded the CS-. If, however, the CSpreceded CS+ by a 24-h interval, it was ineffective and no CS+ conditioning occurred. For adult rats, the effectiveness of the CS-/CS+ "integration" progressively decreased with increasing length of the interval separating their occurrence, although it was still significant (i.e., some CS+ conditioning occurred) with a 12-h CS-to CS+ delay. For preweanlings (16 days postnatal), no conditioning to CS+ occurred ifthe interval between CS-and CS+ was 1 h or longer, although significant conditioning to CS+ did occur with a CS-to CS+ interval as long as 40 min. It was as if active memory for the CS-at the time of CS+ exposure was necessary for CS+ conditioning, and forgetting of the CS-memory proceeded more rapidly for preweanling than for adult rats. Collectively, these experiments extend results previously indicating that (l) the CS+ contiguous to the US mayor may not be "selected" for conditioning, depending on the rat's exposure to, or memory for, a CS-, and (2)this stimulus selection might differ for immature and mature rats. 143A variety of experiential manipulations have identified circumstances that determine which of several contiguous stimuli are "selected" for association in a learning task (e.g., latent inhibition, blocking, overshadowing; Mackintosh, 1974). Other findings indicate that there are certain ontogenetic and biological constraints that determine which stimuli within an episode are and which are not association. Haroutunian and Campbell (1979) found that animals younger than 10 days postnatal could associate an odor CS+ with illness (induced by an injection of lithium chloride) but not with footshock. The young animals were at least as sensitive to the footshock employed as the older animals that readily formed the association. Garcia and Koelling (1966) reported that adult rats could easily form an association between taste and illness, but not between taste and shock. Other studies have demonstrated that both biological and experiential factors can interact to determine similar stimulus selection (Kelley, 1982). Recently, we have studied another experiential manipulation that determines when a contiguous conditioned stimulus (CS) will be selected for association with the unconditioned stimulus (US). Animals given an odor CS-(cue explicitly unpaired with footshock) were found to express greater aversions to the odor CS+ (cue followed by footshock) than animals given only the CS+ (Bryan, 1979;Kucharski & Spear, 1984). Some background to this work is in order.Doctoral dissertation research conducted in our laboratory had indicated that rats as...

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