The flexibility and capacity of adaptation of organisms greatly depends on their learning capabilities. For this reason, animal psychology has devoted great efforts to the study of learning processes. In particular, in the last century a huge body of empirical data has been collected around the two main experimental paradigms of ‘classical conditioning’ (Pavlov, 1927; Lieberman, 1993) and ‘instrumental conditioning’ (Thorndike, 1911; Skinner, 1938; Balleine et al., 2003; Domjan, 2006).
Classical conditioning (also called ‘Pavlovian conditioning’) refers to an experimental paradigm in which a certain basic behaviour such as salivation or approaching (the ‘unconditioned response’ – UR), which is linked to a biologically salient stimulus such as food ingestion (the ‘unconditioned stimulus’ – US), becomes associated to a neutral stimulus like the sound of a bell (the ‘conditioned stimulus’ – CS), after the neutral stimulus is repeatedly presented before the appearance of the salient stimulus. Such acquired associations are referred to as ‘CS-US’ or ‘CS-UR’ associations (Pavlov, 1927; Lieberman, 1993).
Instrumental conditioning (also called ‘operant conditioning’) refers to an experimental paradigm in which an animal, given a certain stimulus/context such as a lever in a cage (the ‘stimulus’ – S), learns to produce a particular action such as pressing the lever (the ‘response’ – R), which produces a certain outcome such as the opening of the cage (the ‘action outcome’ – O), if this outcome is consistently accompanied by a reward such as the access to food.