Rats were used in a conditioned-suppression paradigm to investigate why a conditioned inhibition (CS-) does not extinguish when presented alone. Experiment 1 assessed the role of blocking by excitatory contextual cues and/or an evoked representation of the conditioned excitor (CS+), which had been nonreinforced in compound with the CS-. When the CS+ and context were extinguished prior to presentations of the CS- alone, the CS- showed a retardation effect, evidently reflecting latent inhibition, because no inhibition was detected in controls for which presentations of the CS- alone had been omitted. Experiment 2 showed that the loss of conditioned inhibition (CI) was due to excitatory extinction and not to time since conditioning. Furthermore, when excitation was reconditioned to the extinguished CS+ (Experiment 1), or to a novel CS in the same context (Experiment 2), CI was restored. Two other experiments evaluated whether the maintenance of CI depended upon excitation that was generic in form or associatively tied to the training context. They showed no loss of CI when groups received CS+ extinction in that context, along with concomitant presentations in a different context of the US by itself, for a novel CS, or correlated either positively or negatively with the original CS+. Collectively, the findings argue that CI is a "slave" to excitation, for when excitation is extinguished, CI is deactivated; and yet when excitation is reconditioned to the original or a new CS+ in the same or a different context, CI is restored.

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Rats were used in a conditioned-suppression paradigm to assess the effects of contingency variations on responding to a conditioned inhibitor (CS-) and a conditioned excitor (CS+). In Experiment 1, various unconditioned stimulus (US) frequencies were equated across the presence and absence of a CS- in the context of either background cues (continuous-trial procedure) or an explicit neutral event (discrete-trial procedure). With both procedures, a CS-alone treatment enhanced inhibition, whereas treatments involving 50% or 100% reinforcement for the CS- eliminated inhibition without conditioning excitation to that CS. The latter outcome also occurred in Experiment 2, with discrete-trial training equating considerably reduced US frequencies for the presence and absence of the CS-. In further evidence that inhibition was eliminated without conditioning excitation to the CS-, Experiment 3 showed that a novel CS did not acquire excitation when 25%, 50%, or 100% reinforcement was equated across the presence and absence of that CS in the context of a discrete-trial event. Using the procedures of Experiment 1, Experiment 4 showed that a CS+ was extinguished by a CS-alone treatment but was substantially maintained by treatments involving 50% or 100% uncorrelated reinforcement. These effects for a CS+ and a CS- implicate CS-US contiguity, rather than contingency, as the factor determining the extinction of a CS.

Rats were used in a conditioned-suppression paradigm to determine whether an extinction treatment would enhance a moderately developed conditioned inhibitor (CS-), To dissipate unconditioned suppression to the training stimuli, the subjects were first habituated to the stimuli and then given Pavlovian conditioned-inhibition (Cl) training involving reinforced presentations of a clicker and nonreinforced compound presentations ofthat stimulus and the intended CS -, either a light or a tone. Thereafter, experimental subjects received presentations of their CS-by itself, whereas controls received no further training. Following the occurrence and loss of conditioned suppression to the CS -in the extinction phase, summation and retardation tests showed enhanced CI for the experimental subjects relative to both the controls and their own earlier levels of inhibitory performance. In fact, the enhanced inhibition for the experimental subjects approximated that shown by a comparison group for which the CS-had been strongly developed as an inhibitor. These findings suggest that an excitatory representation is associated with the CSearly in CI training, and that subsequent presentations of the CS -by itself strengthen its inhibitory effect by allowing it to be nonreinforced in the presence of that representation.

The present investigation extends our program of studies on factors affecting escape performance (Bower, Fowler, & Trapold, 1959;Trapold & Fowler, 1960) to the delay-of-reinforcement variable. Specifically, the purpose of this study was to assess the effect of different delays of shock termination on the performance of rats running down a charged alley to a goal box that became uncharged at various delays following 5's entry into the goal. A wide range of delays in shock termination was investigated in order to provide a general picture of the function relating escape performance to the delay-of-reinforcement variable. METHODSubjects.-The 5s were 40 male, albino rats, approximately 180 days old at the start of the experiment. All 5s had served previously as control animals in runway experiments wherein the conditions of food reinforcement imposed upon them were constant over training and uncorrelated with speeds. For a period of 2 weeks immediately prior to the start of the present study, 5s were housed individually in the laboratory and were maintained on an ad lib. diet of Purina lab checkers and water.Apparatus.-A straight alley, 68 in. long, 3 in. wide, and 4 in. high, was constructed of two L shaped strips of galvanized sheet metal, and was covered with a transparent Plexiglas top. One of the sheet metal strips formed one side and half of the floor of the apparatus; the other strip formed the other side and other half of the floor. A 1-in. gap separated the two strips at the floor.The alley comprised three sections, a start 1 This experiment was conducted at Yale University while H. Fowler held a predoctoral fellowship from the

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