Overshadowing by fixed- and variable-duration stimuli

Abstract

Two experiments investigated the effect of the temporal distribution form of a stimulus on its ability to produce an overshadowing effect. The overshadowing stimuli were either of the same duration on every trial, or of a variable duration drawn from an exponential distribution with the same mean duration as that of the fixed stimulus. Both experiments provided evidence that a variable-duration stimulus was less effective than a fixed-duration cue at overshadowing conditioning to a target conditioned stimulus (CS); moreover, this effect was independent of whether the overshadowed CS was fixed or variable. The findings presented here are consistent with the idea that the strength of the association between CS and unconditioned stimulus (US) is, in part, determined by the temporal distribution form of the CS. These results are discussed in terms of time-accumulation and trial-based theories of conditioning and timing.

Keywords:

• Rats
• Timing
• Overshadowing
• Associative learning
• Stimulus distribution form

The authors would like to thank Eric Tam for helpful discussion and Biomedical Sciences Unit staff for excellent technical support.

This work was funded by the Biotechnology and Biological Sciences Research Council (BBSRC).

Notes

¹According to Rescorla and Wagner (1972), this failure of learning is mediated via a reduction in US processing, according to Pearce and Hall (1980), it is via a reduction in CS processing.

²These models are more accurately described as episode based rather than trial based, but share with trial-based models the assumption that learning occurs incrementally on discrete learning episodes.

³They also demonstrated differences in the acquisition of the CR, in the sense defined by time-accumulation accounts.