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The Role of Contextual Stimuli in the Blocking Paradigm

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The Role of Contextual Stimuli in the Blocking Paradigm Psychobiology 1988, Vol. 16 (1), 59-66 The role of contextual stimuli in the blocking paradigm MELANIE S. WEA VER and WILLIAM C. GORDON University of New Mexico, Albuquerque, New Mexico Two experiments used rats in a conditioned lick-suppression paradigm to investigate the role of contextual stimuli in a traditional blocking paradigm. Experiment 1 demonstrated that the blocking effect could be significantly attenuated by changing contexts between the simple and compound stimulus conditioning phases of the blocking procedure. Experiment 2 demonstrated that the blocking efIect could also be attenuated when both phases occurred in the same context as long as substantial context-alone exposure (i.e., extinction) was given between the two train­ ing phases. Experiment 2 also showed that the attenuating efIects produced by extinguishing the training context could be alleviated if additional footshocks were given in either the training or a novel context, suggesting that context extinction may result in both a weakening of direct context-unconditioned stimulus (US) associations and adegradation ofthe US memory represen­ tation. These findings indicate that contextual associations can play an important role in producing the blocking effect. The potential use of contextual manipulations to distinguish between expla­ nations for blocking deficits shown by animals with hippocampal lesions is discussed. It is weIl known that prior conditioning to one element 1972). Thus, if aUS is already fully predicted by stimuli of a reinforced compound conditioned stimulus (CS) often in the environment, new stimuli that occur contiguously reduces conditioning to the other element in the compound with the US will fail to become conditioned. According (Kamin, 1968, 1969). This phenomenon, known as "block­ to this view, the first phase of the blocking paradigm es­ ing," has had important implications for contemporary tablishes a given CS (CS1) as a reliable signal for US oc­ theories of conditioning. It suggests, for example, that the currence. In the second phase of training, when CS1 is contiguous occurrence of an effective CS and uncondi­ placed in compound with a novel CS (CS2) and the com­ tioned stimulus (US) does not necessarily ensure that the pound is reinforced, no conditioning occurs to CS2 be­ CS will become conditioned. On the contrary, condition­ cause the US is already predictable on the basis of the ing to a given CS often depends on the degree to which occurrence of CS 1. other stimuli in the situation already serve as reliable sig­ A second, related type of explanation is that any CS nals for US occurrence. that is paired with a fully predicted US decreases in More recently, the blocking phenomenon has also be­ salience and subsequently loses the capacity for entering come important in evaluating the function of certain brain into an association with that US (see Mackintosh, 1975; structures, such as the hippocampus. It has been shown, Moore & Stickney, 1980; Pearce & Hall, 1980). Thus, for example, that both rats and rabbits with hippocampal according to this view, the first phase of the blocking lesions often exhibit substantial attenuations of blocking procedure establishes CS1 as a reliable signal for the USo as compared with sham animals (e.g., Rickert, Bennett, In the second phase, the presence of CS 1 assures that the Lane, & French, 1978; Solomon, 1977). Thus, current novel CS2 will be paired with a fully predicted USo As theories of hippocampal function have had to account for a result, CS2 loses salience and, on subsequent trials, be­ why blocking deficits sometimes occur in lesioned organ­ comes less capable of entering into an association with isms (see Schmajuk, 1984). the USo Since the blocking effect has implications for both be­ It is dear that these two views make different predic­ havioral and psychobiological theories, it has become in­ tions conceming the blocking effect in some cases (e.g., creasingly important that we understand the mechanisms Dickinson, Nicholas, & Mackintosh, 1983). However, underlying the effect, as weIl as the conditions that con­ these hypotheses do share one important assumption. Ac­ trol its occurrence. To date, most attempts to explain the cording to both of these positions, the degree to which blocking phenomenon have fallen into two general cate­ blocking is obtained should ultimately depend on how sur­ gories. One type of explanation is that a US loses its ability prising the US occurrence is in the compound condition­ to promote conditioning as that US becomes better pre­ ing phase of the blocking procedure. Both of these views dicted by the stimuli preceding it (Rescorla & Wagner, predict that blocking should occur only when stimuli present in the second phase of training have already be­ come reliable signals for US occurrence. Requests for reprints should be sent to William C. Gordon, Depart­ ment of Psychology, University of New Mexico, Albuquerque, NM Given the presumed importance of US predictability in 87131. producing the blocking effect, it is somewhat surprising 59 Copyright 1988 Psychonomic Society, Inc. 60 WEAVER AND GORDON that experimenters have virtually ignored the role ofback­ EXPERIMENT 1 ground or contextual stimuli in the blocking paradigm. It is weH known, for example, that contextual stimuli are The purpose of the first experiment was to assess capable of entering into associations with a US in a con­ whether or not a contextual shift between the simple and ditioning situation (e.g., Balaz, eapra, Hart!, & Miller, compound conditioning phases would attenuate blocking. 1981; Bouton & King, 1983; Marlin, 1982). Thus, it is To make this determination, all animals were exposed to reasonable to assume that in the initial conditioning phase aseries of tone-shock pairings and later were presented of a blocking experiment, both es 1 and contextual stimuli with pairings of a compound tone-light es with shock. have the potential for becoming associated with the USo For some animals, both simple and compound es trials This means that in the compound conditioning phase of occurred in the same context. For other animals, these such an experiment, one rnight expect that both es 1 and two training phases occurred in different contexts. Our contextual stimuli would contribute to the predictability assumption was that the former animals would show rela­ of the USo In effect, contextual stimuli should, in many tively poor conditioning to the light es added during com­ cases, help to make the US less surprising in the second pound training. In other words, these animals should ex­ phase of the blocking procedure. hibit evidence of blocking . The question of interest in this Aside from the possibility that contextual stimuli rnight, study was whether or not blocking would be dirninished themelves, become predictors for the US occurrence, in the animals that experienced a context change. there is at least one other reason to expect that such stimuli rnight influence the blocking effect. Several theorists have Method suggested that contextual stimuli can serve as retrieval Subjects. The subjects were 30 male albino rats derived from cues for the memory of a es-us association (e.g., Hirsh, the Sprague-Dawley strain, bred and reared at the University of 1974; Konorski, 1967; Medin, 1975; Nadel & Willner, New Mexico. They were housed in individual cages and maintained under a 12: 12light:dark cycle. At the beginning ofthe experiment, 1980; Spear, 1973). According to this view, when an or­ all animals were between 90 and 120 days of age, and their body ganism forms a es-us association in the presence of weights ranged from 240 to 300 g. All behavioral testing was con­ specific contextual stimuli, these stimuli somehow become ducted during the light phase of the light:dark cycle. linked to the memory of that association. Later, if the or­ Apparatus. Two identicallick chambers, adapted from operant ganism is reexposed to the training context and the es chambers and manufactured by Lafayette Instrument Co. (Model is presented, the contextual stimuli aid in retrieval of the 8000), were employed. Each chamber measured 27x21 x26 cm and was composed of two alurninum and two Plexiglas walls with es-us memory and a conditioned response occurs. eon­ a Plexiglas lid. The floor of each chamber consisted of 0.6-cm­ ditioned responding is dirninished if the es occurs out­ diameter stainless steel rods, spaced 1.7 cm apart. The rods were side the training context, since retrieval ofthe es-us as­ connected with electrical wire outside the walls of the chamber. sociation is relatively ineffective. An aperture measuring 2.4 cm in diameter was located 12.0 cm From this view, one would also predict that the occur­ from the floor and centered in one alurninum wall to accommodate rence of blocking should depend on the contextual stimuli a drinking tube. A drinkometer (Lafayette Instruments, Model A121) was placed into a circuit that remained open except when an animal present during compound conditioning. Ifthe same con­ stood on the chamber floor and contacted the drinking tube. Each textual stimuli are present during both phases of the block­ such contact resulted in a pulse to a counter that recorded the num­ ing procedure, the memory of the association established ber of tube contacts. in the simple conditioning phase should be retrieved dur­ Conditioning trials were presented in a large Plexiglas chamber ing the compound conditioning phase. Under these con­ measuring 36x36x69 cm. The floor ofthis chamber was identi­ ditions, blocking should occur, since an organism would cal to that described for the liek chambers. A 1.5-mA scrambled remember that es 1 was already a reliable predictor for grid shock would be delivered to the floor by means of a Grason­ Stadler shock generator (Model EI064GS). Two CSs were em­ the USo A change in context between the two phases of ployed.
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