Cue competition as a retrieval deficit

Four experiments using rats as subjects investigated the claim of Williams (1996) that cue competition results from an associative acquisition deficit, rather than a performance deficit. In Experiment 1, extinction of an overshadowing stimulus following overshadowing treatment increased responding to the overshadowed stimulus, thereby replicating prior observations with new parameters. In Experiment 2, an overshadowed stimulus failed to support second-order conditioning unless the overshadowing stimulus received prior extinction treatment. Experiment 3 replicated the recovery fromovershadowing effect seen inExperiment 1 using a sensory preconditioning procedure.Most important, in Experiment 4 an overshadowed stimulus failed to block conditioned responding to a novel CS, but blocking by the overshadowed cue was observed following posttraining extinction of the overshadowing stimulus. These results, as well as those of Williams, are discussed in terms of traditional and more recent acquisition-focused models as well as an extension of the comparator hypothesis (Denniston, Savastano, & Miller, 2001). 2002 Elsevier Science (USA). All rights reserved. Learning and Motivation 34 (2003) 1–31 www.elsevier.com/locate/l&m This research was supported by National Institute of Mental Health Grant 33881. The authors would like to thank James Esposito for his assistance in collecting the data, and Francisco Arcediano, Daniel Burger, Raymond Chang, Martha Escobar, Gregory Greer, and Steven Stout for their comments on an earlier version of the manuscript. * Corresponding author. Fax: 1-828-262-2974. E-mail address: [email protected] (J.C. Denniston). 0023-9690/02/$ see front matter 2002 Elsevier Science (USA). All rights reserved. PII: S0023-9690 (02 )00505-2 Cue competition as a retrieval deficit Students of associative learning have long been interested in the conditions under which conditioned stimuli (CSs) trained together will compete with one another for behavioral control. One example of cue competition is the so-called ‘‘blocking’’ effect (Kamin, 1968). In a typical blocking experiment, a CS (A) is paired with an unconditioned stimulus (US) in Phase 1 (i.e., A ! US); then in Phase 2, CS A and another stimulus (CS X) are paired with the US (i.e., AX ! US). Blocking is evidenced by reduced behavioral control by CS X relative to subjects that received Phase 1 training with an irrelevant stimulus (i.e., B ! US). Most contemporary theories of learning explain blocking and other forms of cue competition as a deficit in acquiring an X–US association. For example, Rescorla and Wagner (1972) explained blocking as a result of CS A accruing much of the associative value supportable by the US during Phase 1. This strong A–US association leaves less potential associative strength to the US available for assignment during Phase 2 than if both A and X were novel at the time of Phase 2 training. The comparator hypothesis, a rule for the expression of associations, provides an alternative account of the blocking phenomenon (e.g., Miller & Matzel, 1988; Miller & Schachtman, 1985). The comparator hypothesis assumes that associations are formed based upon a simple associative acquisition mechanism that is entirely dependent upon spatio-temporal contiguity (e.g., Bush & Mostellar, 1955) and the salience of the associates. It treats the weak conditioned responding observed in blocking as a performance deficit originating at the time of testing rather than an acquisition deficit occurring at the time of associative acquisition (i.e., Phase 2). According to the comparator hypothesis, at least three associations are formed in the course of blocking training (see Fig. 1). The first association is between the target CS (X) and the US (Link 1). The second association is between the target CS (X) and the blocking CS (A), which is the so-called comparator stimulus for CS X (Link 2). The third association is between the blocking CS (A) and the US (Link 3). At test, conditioned responding to CS X is assumed to reflect a comparison of the US representations directly and indirectly activated by the target CS (X). The strength of the directly activated US representation reflects the absolute strength of the target CS–US association (Link 1), whereas the strength of the indirectly activated US representation is determined by the product of the strengths of Links 2 and 3. Through the comparator process, excitatory behavioral control by the target CS is presumed to increase (and inhibitory behavioral control to decrease) as the strength of the directly activated US representation increases relative to the strength of the indirectly activated US representation. Conversely, excitatory behavioral control is presumed to decrease (and inhibitory behavioral control to increase) as the strength of the directly activated US representation decreases relative to the strength of the indirectly activated US representation. Applied to blocking, the failure to observe responding to the blocked stimulus X results from strong X–A and A–US associations preventing expression of the acquired X–US association at the time of testing, rather than a failure to acquire the X–US association at the time of training. That is, in a conventional blocking group 2 J.C. Denniston et al. / Learning and Motivation 34 (2003) 1–31