"Learned safety" as a mechanism in long-delay taste-aversion learning in rats.

Rats learn taste aversions with unusually long CS-US delays. This has previously been explained as slow decay of a CS trace or as relative lack of interference. We propose, however, that the CS-US delay gradient is a learning curve: During the delay, a rat gradually learns that a taste is safe. A solution which a rat drinks only once becomes safe and resistant to learned aversions for at least 3 wk., suggesting a learned safety mechanism. If a rat drinks a solution twice (within the effective CS-US interval) before a single poisoning, it learns less aversion than if it received only the second presentation. The learned-safety theory explains this result; a trace-decay or interference model cannot.