Intramodal blocking between olfactory stimuli in rats

The means by which olfactory systems enable the consistent recognition of biologically meaningful odors that may vary in composition over time is a central problem in olfaction. Experiments in honeybees have suggested a solution to one aspect of this problem by demonstrating that the learning of components in an odor mixture suppresses subsequent learning about additional components that are later added to that mixture. We here show, in rats, that intramodal olfactory blocking is also exhibited in vertebrates, and furthermore that the specific characteristics of this blocking are similar to those observed in intermodal blocking studies. Using an olfactory digging task, we submitted two groups of rats to the following olfactory training protocol: one group was first trained (five trials) on an odor (A), followed by five training trials on a binary mixture (A+B), while the second group was similarly trained on a different odor (C) followed by the binary mixture (A+B). After training, both groups' unrewarded responses to the mixture (A+B), odor (B), and an unconditioned control odor (X) were measured. For four out of five odor sets used in this experiment, a significant difference in digging time was observed between the responses of the two groups to odor B (P<.01), but not between their responses to the binary mixture (A+B) or to the control odor (X). These results demonstrate that pretraining rats with an odorant blocks subsequent learning about another odorant when the latter is presented simultaneously with the pretrained odorant, indicating that the components of a mixture are at least partially perceived as independent elements of this mixture.