Three rats were trained under a discrimination procedure in which responding was reinforced only following the repeated presentation ofthree bursts of white noise (8+). S-consisted of presentations of either two or four bursts of noise. All animals responded significantly more in the presence ofS+ and, in two cases, showed lower response rates to both "2" and "4" stimuli. Responding by the third animal revealed differentiation between S+ and the stimulus "2," but no reliable suppression to stimulus "4." The present instances of discriminative control by the stimulus "3" replicate Fernandes and Church's (1982) demonstration of control by sequential auditory stimuli in the rat. Moreover, because the present procedure involves adjacent S-values both greater as well as less than S+, these results extend our knowledge of the rat's abilities with sequential auditory stimuli: Rats are capable of making intermediate numerical discriminations based upon something other than a simple many-versus-few dichotomy.

In four experiments using the conditioned suppression procedure with rats, we compared the effects of extending conditioned stimuli (CSs) before versus after reinforcement (called B vs. A extensions). In Experiments I and 2, Group 0 (no extension) received 2-min noise CS trials (3 per day in Experiment 1, 1 per day in Experiment 2) that terminated with a 1-s grid shock unconditioned stimulus (US). For Group B, the CS began 12 min before the US; for Group A, the CS began 2 min before the US but persisted for 10 min past US termination. In Experiments 3 and 4, similar trials (3 per day in Experiment 3, 1 per day in Experiment 4) included a 2-min light CS that always terminated with the US; thus the noise CS became a systematically manipulated context cue in which lightshock pairings were embedded. In Experiments 1 and 2 we found asymmetrical effects of CS extensions: B extensions weakened conditioning more than did A extensions. In Experiments 3 and 4 we found symmetrical effects: A and B extensions weakened context conditioning equally. A realtime model of conditioning that combined the Frey-Sears (1978) dynamic attention rule with the Rescorla-Wagner (1972) model anticipated the asymmetrical effects preasymptotically and the symmetrical effects at asymptote. John W. Moore for his support throughout the project.

Twenty-eight male albino rats were given a single 4-sec 1-rnA electric-grid-shock unconditioned stimulus (US). In the same session they received two 12-sec conditioned stimuli (CSs). One CS (explicitly unpaired) terminated 180 sec before the US began; the other (backward paired) began immediately after the US terminated. The CSS used were a 1000-Hz 85-dB tone and an 84-dB click; their roles were counterbalanced. Over the next 2 days, each CS was presented for 2 min while the rats drank from a water bottle. The backward-paired CS was found to suppress licking more than the explicitly unpaired CS. This suppression was accompanied by an increase in defensive behavior (freezing and freeze/nod) and by a decrease in other activity. The suppression did not seem to be due to a maintained or enhanced CS-orienting response reflex, nor could it be attributed to an adventitiously reinforced interfering operant. The results support the presumption made in previous reports that the lick suppression evoked by a backward CS reflected onetrial backward excitatory fear conditioning.The topic of backward conditioning has been controversial since the time of Pavlov. The history of that controversy is so well known that it needs no recitation. In recent times, the controversy has continued unabated. For example, in concluding a recent review article on backward conditioning, Spetch, Wilkie, and Pinel (1981) stated that "the empirical evidence for its existence can no longer be ignored. Thus the time for disputing whether backward conditioning is possible is past; it is time instead for systematic exploration of the variables that affect the magnitude and duration of the effect" (p. 174). By way of contrast, an even more recent review (Hall, 1984) concluded that "when only traditional Pavlovian conditioning studies are examined, the experimental fmdings suggest that UCS-CS trials [the backward conditioning procedure] will result in inhibition" (p. 163).The fact that these two reviews reached different conclusions is due in part to a disagreement about what kinds of studies should be reviewed. Spetch et al. leaned heavily on studies that used the conditioned suppression procedure. In that procedure, a conditioned stimulus (CS), such as a light or tone, previously paired with an aversive unconditioned stimulus (US), such as an electric shock, is presented to an animal working for some reward. The suppression in the rate of that rewarded behavior is taken as an index of the CS's conditioned strength. Hall (1984)

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