Progressively increasing the difficulty of a Pavlovian discrimination in a voluntary exposure to toxin paradigm with rats attenuates the magnitude of the easy-to-hard effect

• The discrimination consisted of presenting saccharin either alone or mixed with LiCl.
• Progressive pre-training increasing the difficulty of discrimination facilitated a harder discrimination.
• Progressive pre-training enhanced learning less than continuous pre-training with the easiest discrimination.

Rats received two stages of Pavlovian discrimination training with two flavor stimuli: a compound consisting of saccharin mixed with 0.15 M lithium chloride (LiCl), and the saccharin alone. The concentration of the saccharin solution (i.e., the common element shared by the stimuli to be discriminated) was relatively high in Stage 2 (1.2%). Groups differed in the pre-training that they received in Stage 1. Group Progressive (PROG) was pretrained in easier versions of the discrimination of Stage 2. The difficulty of these discriminations was gradually increased by progressively increasing the initial concentration of saccharin (0.15%). Group PROG learned the hardest discrimination faster than a control group (HARD) that was trained in this discrimination in both Stages 1 and 2 (Experiment 1). We also observed that the enhancement of learning observed in Group PROG was less than that observed after continuous pre-training with the easiest version of the discrimination (Group CONT; Experiment 2). We discuss the implications of these results in relation to other previous demonstrations of the easy-to-hard effect.