Blocking of a CS-US Association by a US-US Association

Abstract

<p>In a typical blocking experiment an association between a conditioned stimulus (CS) and an unconditioned stimulus (US) is first established by CS-US pairings. Concurrent presentation of the previously conditioned CS with a second CS is then shown to prevent or attenuate conditioning to the second CS. In the present blocking experiments a US-US association was first established by preexposure with repeated fixed interval US presentations. A single CS was then paired with a US that followed a prior US by the same fixed US-US interval that was used in preexposure. The interval between the US prior to the CS and the US paired with the CS was called the critical US-US interval. In Experiment 1, a 10.5-sec fixed interval between USs was used to show that this procedure can block acquisition to the CS. In Experiment 2, blocking was also shown with a 100.5-sec fixed interval between USs. Experiments 1 and 2 provided evidence in support of the hypothesis that blocking by a US-US association occurs when the prior US predicts the time of arrival of the US with which the CS is paired (time of arrival hypothesis). In Experiment 3a, manipulating the amount of preexposure at a 10.5-sec US-US interval showed rapid blocking of a CS-US association by a US-US association. Experiment 3b showed that the results from Experiment 3a were not consistent with one alternative account. This alternative account suggested that blocking in Experiment 3a occurred because subjects did learn the CS-US association, but competing responses elicited by learning a short interval US-US association prevented the expression of this learning. Experiment 4 showed that blocking was attenuated with added USs in preexposure at longer US-US intervals than the critical US-US interval: Experiment 5 showed that the results from Experiment 4 were not due to a change in the temporal distribution of USs from preexposure to training. The relationship of US-US blocking to current theories of learning and to other conditioning phenomena was discussed.</p>