The easy-to-hard effect in a voluntary exposure to toxin paradigm with rats

• A discrimination procedure with ingested LiCl as the US was employed.
• Pre-training on an easy discrimination facilitated a hard discrimination relative to continuous training with the hard discrimination.
• To the best of our knowledge, this is the first demonstration of an easy-to-hard effect in a free-intake toxin paradigm.

In Experiment 1, one group of rats (Group Easy) received initial discrimination training consisting of alternate presentations of two flavor stimuli easily discriminable (presentations of a compound consisting of 0.15% saccharin and 0.15 M lithium chloride, LiCl, and presentations of the saccharin alone). In a subsequent phase, these rats learned a hard version of the discrimination (in which the concentration of the saccharin solution was increased to 1.2%) faster than another group of rats (Group Hard) that received continuous training with the hard discrimination throughout all of the experiment. Experiment 2 led us to discard a possible interpretation of these results in terms of differences in the rates with which the neophobic reaction to the saccharin was habituated in the two groups. This study constitutes the first demonstration of an easy-hard effect in a free-intake toxin paradigm.