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The effects of continuous versus partial reinforcement schedules on associative learning, memory and extinction in Lymnaea stagnalis

Susan Sangha^*, Chloe McComb^*, Andi Scheibenstock, Christine Johannes and Ken Lukowiak

Department of Physiology and Biophysics, University of Calgary, Faculty of Medicine, 3330 Hospital Drive NW, Calgary, Alberta, Canada T2N 4N1
^* Contributed equally to this study

View larger version (12K): [in a new window]   Fig. 1. A partial reinforcement (PR) schedule does not result in learning. Three separate groups of snails were used. (A) A control group of snails (N=15) received three continuous reinforcement 30 min training sessions each separated by a 1 h interval. Learning occurred (ANOVA, F14,2=32.07, P<0.001; session 3 significantly less than session 1, P<0.01), and a long-term memory was formed [the memory test session, 24 h after session 3, was not significantly different from session 3 (P>0.05) but was significantly different (P<0.01) from session 1]. Two other cohorts, each of 20 snails, received three 30 min PR sessions each separated by a 1 h interval. One cohort was administered a tactile stimulus on odd-numbered (B) pokes, while the other was administered a tactile stimulus on even-numbered (C) pokes. In neither group was there learning (i.e. there was no significant difference between session 1 and sessions 2 and 3). Values are means + S.E.M.

View larger version (14K): [in a new window]   Fig. 2. A continuous reinforcement (CR) schedule results in learning and long-term memory that persists for 2 but not 3 days. (A) A group of 20 naïve snails received two 45 min CR training sessions with a 1 h interval between the sessions. Learning occurred (ANOVA, F19,1=51.99, P<0.001; session 2 was significantly less than session 1, P<0.01), and a long-term memory had been formed when tested 2 days later [the memory test session was not significantly different from session 2 (P>0.05) but was significantly different (P<0.01) from session 1]. (B) As in A, except that long-term memory was tested 3 days later and N=19. Learning occurred (ANOVA, F18,1=47.32, P<0.001; session 2 significantly less than session 1, P<0.01), but no long-term memory was formed [the memory test session was significantly different from session 2 (P<0.01) and was not significantly different (P>0.05) from session 1]. Values are means + S.E.M.

View larger version (18K): [in a new window]   Fig. 3. The persistence of memory is affected by the sequence of continuous and partial reinforcement schedules. (A) Two continuous reinforcement (CR) 45 min training sessions on day 1 followed by two partial reinforcement (PR) training sessions on day 2 produces a long-term memory that persists for at least 3 days. The memory test session was significantly different from session 1 (P<0.01) but not significantly different from the last training session. (B) As in A except that snails first received the PR schedule and then the CR schedule. No memory was formed. The memory test session was not significantly different from session 1 but was significantly different from the last CR session on day 2 (P<0.05). Values are means ± S.E.M. (N=15).

View larger version (19K): [in a new window]   Fig. 4. Snails receiving a CR/PR training schedule are more resistant to extinction than snails receiving a PR/CR schedule. (A). Two continuous reinforcement (CR) 45 min training sessions on day 1 followed by two partial reinforcement (PR) training sessions on day 2 produce a long-term memory that is resistant to extinction. On the day following the PR schedule of training (day 3), the snails received two extinction training sessions. When given the test for extinction on the following day (day 4), extinction was not observed (i.e. long-term memory was present). That is, the number of pneumostome openings in the extinction test session was not significantly different from that in the last PR session (P>0.05). (B) As in A except that this naïve group of snails received the PR schedule first (day 1) followed by the CR schedule (day 2). Extinction training on day 3 was sufficient to produce extinction. That is, the number of pneumostome openings in the extinction test session on day 4 was significantly different from that in the the last CR (day 2) training session (P<0.01). Values are means ± S.E.M. (N=20).