Fig. 2. Conditioned taste aversion following a one-trial conditioning procedure in snails using sucrose as the conditioned stimulus (CS) and KCl as the unconditioned stimulus (US). (A) The ratio of good and poor performers is assessed by their response to the CS following the one-trial conditioning procedure. We defined good performers as those snails that significantly reduced their response to the CS following the one-trial conditioning procedure. This meant that good performers reduced the number of bites min^–1 in response to the CS from a level of approximately 17 in the pre-test to between 0 and 1 in the post-test session. In poor performers, however, there was no significant difference in the response to the CS in the post-test session compared to the pre-test session. Thus, in each session snails responded to the CS with approximately 17 bites min^–1. We found that 21 of 50 snails could be classified as good performers. That is, about 40% of snails acquired conditioned taste aversion (CTA). (B) The persistence of memory following the one-trial conditioning procedure. The numbers of snails at the pre-test were as follows: 50 for naive snails (open triangles), 50 for backward-conditioning snails (open squares), 21 for good performers (solid circles) and 29 for poor performers (open circles). All values are means ± s.e.m. The x-axis is expressed in a logarithmic scale. The difference between the feeding response of the good performers and that of the poor performers and control snails was maintained for at least 7 days at *P<0.01 (one-way ANOVA followed by the post-hoc Scheffé's test). The numbers of good performers and poor performers became 15 and 23, respectively, at 7 day because some snails withdrew their body into the shell or had died.