Conditioning is the change in the response to a stimulus either because of the relation of that stimulus to other stimuli (Pavlovian conditioning), or because of the relation between the response and other stimuli (instrumental conditioning). These relations are formulated in terms of differences in conditional probability known as contingencies. Pavlovian contingencies refer to the difference in the conditional probability of one stimulus (the outcome, or O) given the presence vs. the absence of another stimulus (the conditioned stimulus, or CS). A conditioned response (CR) may be strengthened by a positive Pavlovian contingency (excitatory conditioning) or it may be weakened by a negative Pavlovian contingency (inhibitory conditioning). CRs are anticipatory or modified responses to the O, so their topography depends on the nature of the O (appetitive vs. aversive); the proximity between and congruency of O and CS; prior experience with the CS, O, and their contingency; the magnitude of their contingency; and the characteristics of other stimuli in the environment. Instrumental contingencies refer to the relation between one stimulus (the discriminative stimulus, or SD), a response (or operant, R), and the outcome of that response (O). The nature of the O and of its contingency with the R determines whether the O strengthens or weakens the R: Os that introduce an appetitive stimulus (positive reinforcement) or remove an aversive stimulus (negative reinforcement) strengthen the R. Positive reinforcement is typically arranged on a subset of one or more Rs following a set of rules known as a schedule of reinforcement. The probability that an R is reinforced may depend on the number of Rs (ratio schedules) or the amount of time (interval schedules) since the last reinforcer. The topography and strength of instrumental Rs depend on variables that are analogous to those that affect Pavlovian CRs: the amount and nature of prior experience with the O; the proximity, congruency, and contingency of R and O; and characteristics of other stimuli in past and present environments. Contemporary quantitative models of Pavlovian and instrumental conditioning recognize the importance of contextual stimuli that compete for cognitive and behavioral resources, constraining and shaping the expression of target responses. These models have guided the bulk of recent empirical research and conceptual developments, leading to a progressively unified view of learning and motivation processes. Along the way, Pavlovian and instrumental research have demonstrated their utility in addressing a broad range of consequential societal problems.
The use of operant instrumental conditioning with positive reinforcement for collection of saliva from marmosets (Callithrix jacchus) for research purposes