Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)




Professor H.M. Jenkins


The first experiment of the present thesis replicated the observation made by many investigators that inter-trial interval (ITI) shocks interfere with conditioning. Experiment I extended this observation, however, by demonstrating that all ITI shocks are not equally disruptive of conditioning--the closer an ITI shock occurs prior to a trial, the greater the interference with conditioning.

Two broad classes of theory can explain this temporal location effect. The first class, competition theory, states that a pre-trial shock diminished the effectiveness of a subsequent CS-US pairing because it, directly or indirectly, functions to signal the same shock that is later signalled by the CS. Furthermore, since competition theory postulates that a US can support only a fixed amount of signal value, the more signal value that is acquired by the competing stimuli, the less is available for the CS to acquire. The second class, interference theory, states that the failure of CS-US pairings to produce strong conditioning when preceded by pre-trial shocks is not due to the acquisition of conditioned strength by other stimuli, but rather that less total conditioning occurs.

One kind of competition theory is the Rescorla-Wagner model of classical conditioning. This model suggests that the temporal location effect is due to the conditioning of background cues. Experiment 3 tested this hypothesis by comparing the performance of two groups of rats. Each group received a pre-trial shock prior to each CS-US pairing--but in one group this shock was itself signalled by a salient CS. According to the model, this signalling should reduce the conditioning of background stimuli and hence should increase conditioning. Although the results suggested a small effect in the predicted direction, the important fact was that the temporal location effect was not eliminated, or even substantially reduced, when the pre-CS shock was itself signalled. Conditioning to the background cues does not appear to be the mechanism by which the pre-CS shock reduced the amount of suppression controlled by the CS.

Experiment 4 tested the predictions of two other forms of competition theory--the overshadowing and the Information Hypothesis. The results of Experiment 4 suggested that the temporal location effect does not occur because pre-trial shocks overshadow the CS or because they make the CS "uninformative" or redundant.

In conclusion, the results of the present thesis suggest that pre-trial shocks do not diminish the effectiveness of CS-US pairings because of some form of stimulus competition. Rather, pre-trial shocks interfere with conditioning because they reduce the total amount of suppression controlled by all of the cues in the situation--not just the amount controlled by the CS.

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