The Cover-Stimulus Effect: Role of Similarity in Durations of The CS and Cover Cues

Abstract

Pigeons were exposed to a random relation between a conditioned stimulus (CS) and an unconditioned stimulus (US) under the signaled random procedure. All US presentations that were unpaired with the CS were preceded by a second cue, a cover stimulus. Consistent with prior studies, the signaled random procedure supported responding during the CS; however, magnitude of this cover-stimulus effect varied with the duration of the CS relative to the cover-stimulus duration. Higher levels of responding were maintained when the CS and cover stimulus were equal versus unequal in duration. Experiment 1 provides a systematic replication of a study by Williams (1994), who used rat subjects. Higher levels of CS-directed responding were maintained when the cover stimulus was equal in duration to the CS versus when it was shorter. Experiment 2 provides the first demonstration of the effects of cover-stimulus durations that are both longer and shorter than the CS duration, compared to a procedure in which the durations are equivalent. In support of a stimulus-duration-equality effect, level of responding was higher for all equal stimulus-duration groups in comparison to unequal stimulus-duration comparison groups. These effects were accommodated by a common-elements generalization approach to the Rescorla-Wagner model.

Pavlovian contingencies and temporal information.

“…Jenkins et al (1981, Experiment 11) found no difference in autoshaping to a keylight between unsignaled ITI reinforcers and ITI reinforcers signaled by an auditory cue. In fact, just making the duration of the ITI cue different than the duration of the target cue reduces the facilitative effect of signaling the added reinforcers (Jakubow, Brown, & Hemmes, 2004; Williams, 1994). Future studies will have to decide the contribution of generalization from the ITI cue to the target cue in the two-cue procedure.…”

Sometimes-competing retrieval (SOCR): A formalization of the comparator hypothesis.

“…A challenge to the SOCR account of the cover-stimulus effect is the suggestion that the relative durations of the target cue and cover stimulus determine the magnitude of the effect. Jakubow, Brown, and Hemmes (2004) reported that the effect was maximal when the duration of the cover stimulus was equal to that of the target cue. When it was substantially shorter or longer, degraded contingency was observed.…”