Association of stimuli at long intervals in conditioned odor aversion

Rats learn to avoid a tasteless odorized solution if they experience visceral malaise after consuming it. This phenomenon is referred as Conditioned Odor Aversion (COA). It is widely accepted that an odor can only be associated with illness if the inter-stimulus interval (ISI) is shorter than 15 min. However, this conclusion is based on long-term memory tests usually made 48 h after conditioning, thus precluding the possibility to discriminate between a specific failure to make the odor–malaise association rather than the failure to consolidate the short-term association into long-term memory. In the present study, we compared the short-term and long-term memories for COA in rats trained with long ISIs.Independent groups of male rats were conditioned using 5, 15, 30, 60 or 90 min ISIs and tested either 4 or 48 h after conditioning. We found a reliable odor aversion at 5, 15, 30 and 60 min, but not at 90 min ISIs, when tested 4 h after conditioning. In contrast, odor aversion was only found at 5 and 15 min ISIs in the groups tested 48 h after training. Our results show that COA can be acquired when malaise follows the odor CS by at least 60 min. This finding indicates that the lack of aversion at long ISIs is not due to an association failure, but rather to a limitation in consolidating short-term memory into long-term memory of COA.

Research highlights
► We analyzed conditioned odor aversion using long inter-stimulus intervals.
► Short-term, but not long-term memory is established with intervals up to 60 min.
► The lack of aversion using long intervals is not due to a stimuli association failure.
► At long intervals consolidating short-term into long-term memory is incomplete.