Operant conditioning is a form of associative learning and results from the contingency established between a behavioural response and the presentation of a reinforcing stimulus (Mackintosh, 1974). The response–reinforcer contingency can be defined as the probability of receiving the reinforcement for performing the specific behaviour compared with the probability of receiving the reinforcing stimulus in the absence of the behavioural response. In operant conditioning, the frequency of emitting the behavioural responses can be increased or decreased as a result of applying positive or negative reinforcing stimuli, respectively (Domjan and Burkhard, 1993). For optimal conditioning, it is important that the reinforcing stimulus not be presented when the subject is not performing the particular behaviour. Operant conditioning, therefore, results in a specific association between a behavioural response and an external stimulus. Different schedules of reinforcement are used in studies of operant conditioning and memory of the association. A schedule of continuous reinforcement (CR) involves 100 % contingency between the behaviour and the reinforcement; that is, reinforcement is presented every time the animal performs the behaviour. Partial reinforcement (PR), however, refers to any schedule in which there is less than 100 % contingency so that there are instances when the animal’s behaviour is not reinforced. In some operant learning experiments, PR may interfere with the initial acquisition of the operant response, especially when a negative stimulus is used as the reinforcing stimulus; in others, PR has been found to lead eventually to superior performance (e.g. Weinstock, 1958). If, following conditioning, animals receive extinction trials (in which there is no reinforcement), the acquired association is lost, resulting in a behavioural phenotype that resembles the naïve state (Pavlov, 1927). The most important variable determining the magnitude of the behavioural effects of an extinction procedure is the schedule of reinforcement used in the acquisition phase of learning (Domjan and Burkhard, 1993). A PR-induced behaviour is more resistant to extinction than a CR-induced behaviour. This phenomenon has been termed the partial reinforcement extinction effect (PREE) and was first described in the work of both Skinner and Humphreys in the late 1930s (Skinner, 1938; Humphreys, 1939). How can partial, but not continuous, reinforcement offer resistance to extinction? One suggestion proposed by Amsel (1972) is that a ‘disruptive process’, based on non-reward, emerges in partial reinforcement acquisition. This disruptive process does not occur in ordinary CR training, so there is no ‘counterconditioning’; extinction is therefore rapid after CR training. This has sometimes been referred to as the ‘frustration theory’. Another possibility is that, if the subject does not receive reinforcement after each response during training, it may not immediately ‘notice’ when reinforcement ceases, as in extinction training. The change in reinforcement conditions is more dramatic and noticeable if reinforcement ceases after continuous reinforcement. This particular explanation of the PREE is called the discrimination hypothesis (Domjan and Burkhard, 1993) and is somewhat similar to the Pearce–Hall model in that a PR schedule maintains attention because trial outcomes are always ‘surprising’ (Bouton and Sunsay, 2001). Operant conditioning protocols have been used in both vertebrates (Chen and Wolpaw, 1995; Feng-Chen and Wolpaw, 1171 The Journal of Experimental Biology 205, 1171–1178 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB3885 A continuous schedule of reinforcement (CR) in an operant conditioning procedure results in the acquisition of associative learning and the formation of long-term memory. A 50 % partial reinforcement (PR) schedule does not result in learning. The sequence of PR–CR training has different and significant effects on memory retention and resistance to extinction. A CR/PR schedule results in a longer-lasting memory than a PR/CR schedule. Moreover, the memory produced by the CR/PR schedule is resistant to extinction training. In contrast, extinction occurs following the PR/CR schedule. Key words: operant conditioning, partial reinforcement, extinction, long-term memory, Lymnaea stagnalis. Summary Introduction 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 † Author for correspondence (e-mail: lukowiak@acs.ucalgary.ca) Accepted 1 February 2002