A Re-Examination of One-Trial Blocking in Conditioned Suppression

Three experiments on the conditioned suppression of licking in rats examined the amount of conditioning produced by a single conditioning trial to a clicker-light compound and the effect of prior conditioning to the light on the level of conditioning to the clicker. In Experiment I, prior conditioning to the light, far from blocking conditioning to the clicker, actually enhanced it, whether the clicker was presented in a simultaneous compound with the light or in a serial compound preceding the light. Experiment II, however, showed that this potentiation effect could be abolished if a trace interval was inserted between the clicker and light in the serial compound arrangement. Moreover, Experiment III demonstrated a significant blocking effect when a trace interval separated the clicker and light on the single compound trial. These results establish that one-trial blocking of conditioned suppression is possible, and suggest that in some earlier studies blocking may have been masked by higher-order conditioning to the target stimulus.

Locus of the Effect of a Surprising Reinforcer in the Attenuation of Blocking

Previous experiments on conditioned suppression in rats have shown that prior conditioning to one element of a compound conditioned stimulus paired with shock may block or prevent conditioning to the other element. Reliable conditioning may, however, occur to the added element (blocking may be attenuated), if a surprising second shock is added shortly after each compound trial. Experiment I confirmed this finding, and further showed that blocking was attenuated only when the second shock occurred 10 s after the compound trial, not when it occurred 100 s later. Experiment II showed that the surprising omission of an expected second shock 10 s after each compound trial would also attenuate blocking, thus implying that the surprising event does not itself act to reinforce conditioning to the added element, but rather permits the unconditioned stimulus (the first shock) to play its normal role as an effective reinforcer. This conclusion was confirmed by Experiment III, which showed that a surprising second shock does not produce any increase in conditioning to the added element on the trial on which it occurs; rather it serves to ensure adequate conditioning to that element on a subsequent compound trial. The implication is that the surprising event acts proactively to prevent subjects learning to ignore an otherwise redundant stimulus.

Gsr Conditioning and Pseudoconditionlng with Prolonged Practice

2 groups of Ss received 16 conditioning or pseudoconditioning trials per day for 5 days. The compound (light-tone) CS was paired with a 3.5-ma electric shock (ISI = .5 sec.) in the conditioning group but the stimuli were not paired in the pseudoconditioning group. On Day 6 both groups received 2 counterbalanced CS-element test trials and then 18 CS-compound extinction trials. During acquisition, the conditioning group produced significantly larger GSRs over-all than the pseudoconditioning group, and the former's responses generally increased over test trials on each day while the latter's responses declined. There was an over-all decrease in GSR magnitude overnight in the conditioning group and an over-all rise in the pseudoconditioning group. The GSR was significantly smaller in Group E on the first element test trial than it was on the first compound trial on Day 5, but it was not significantly different from the final compound trial on Day 5, or from the mean of the first two compound trials on Day 6. There was a sustained, statistically significant difference in mean magnitude between the two groups across the extinction trials with very little evidence of any extinction effects.