Fig. 1. Conditioning procedures for conditioned taste aversion (CTA) in snails. (A) The one-trial conditioning procedure. (B) The 10-trial conditioning procedure. The conditioned stimulus (CS) was 5 ml of either 10 mmol l^–1 sucrose solution or 0.3% carrot juice, and the unconditioned stimulus (US) in all cases was 5 ml of 10 mmol l^–1 KCl solution. The CS and US were added to the Petri dish using a pipette for 15 s with a 15 s inter-stimulus interval. The inter-trial interval was 10 min in the 10-trial conditioning procedure. Before and after the conditioning procedure, the CS was applied to the lips and washed off with distilled water. Then, the feeding response was determined for 1 min as a pre-test or a post-test. A backward-conditioning (US–CS) control procedure and a naive (presented only with distilled water) control procedure were also employed. (C) Inhibitory leaning. After a 10-trial backward-conditioning procedure, a normal one-trial CTA conditioning procedure was used. As a control, the naive procedure was employed instead of the backward-conditioning procedure. We also used two different CSs, CS1 (10 mmol l^–1 sucrose) and CS2 (0.3% carrot juice), and one US (10 mmol l^–1 KCl). All the conditioning procedures were performed with a blind protocol.