Fig. 4. Yoked control snails do not exhibit LTM. Snails were randomly matched to snails in the operantly conditioned group. These yoked control snails received the same number of stimuli (but in a non-contingent fashion) in sessions 1 and 2. In the third session (MT) 24 h later, snails in the yoked group now received a reinforcing stimulus each time they attempted to open their pneumostome. (A) Yoked controls under standard hypoxic conditions. The yoked group (N=11) had significantly more attempted openings in the memory test (MT) session than did the trained group (N=11) (ANOVA F_(21,5)=50.1381, P<0.01). (B) Yoked controls challenged with 75 µmol l^–1 H₂S hypoxic conditions. Yoked snails (N=11) had significantly more attempted openings in the MT session as compared to the trained group (N=11) (ANOVA F_(21,5)=15.4653, P<0.01).