Classical Conditioning
Classical conditioning has come to mean when two or more events are associated because they occur together. Classical conditioning is commonly associated with Ivan P. Pavlov. While he was investigating digestive processes, he noticed that, given food in dishes, dogs would show salivation reactions to the dish itself, whether or not food was present. Pavlov proceeded to explore the range of circumstances that could produce effects such as these. Eventually a standard experimental procedure emerged. Experiments were conducted with two stimuli, the conditioned stimulus and the unconditioned stimulus.
An unconditioned stimulus (US) is an event that reflexively produces a response called an unconditioned response (UR), even where there has been no previous training. A conditioned stimulus (CS) is originally a neutral cue that, when paired with the US, comes to elicit a conditioned response (CR), which often resembles the original UR.
In terms of preliminary experimental work on classical conditioning in humans, emotional development was an early topic of interest. John B. Watson conducted what may be one of the most famous experiments in psychology. In a report published in 1920, Watson and Rayner presented accounts of a project designed to show the conditioning of a fear reaction in a 9-month-old infant, ALbert B. The argument was made that people express different emotional reactions to the environment because of their different experiences.
Acquisition
Acquisition training in classical conditioning involves repeated pairings of the CS and US. Gradual increases in the magnitude of the CR occur over trials until performance levels off, called the asymptote of acquisition.
The selection of US's and CS's may have a major impact on the acquisition of the CR. US strength affects the amount of conditioning that is possible. CS saliency is important in determining the rate of acquisition of the CR. The time at which the CS and US are presented is also an important factor. In simultaneous conditioning, the CS and US are presented at the same time. The most productive procedure is delayed conditioning, in which the CS is presented before the US, but the two overlap. In trace conditioning, the CS is presented and offsets before the US presentation. Time functions as the CS in temporal conditioning. Backward conditioning is when the US is presented before the CS and is usually difficult to obtain.
Extinction
Extinction involves repeatedly presenting the CS in the absence of the US. When the pairings of the two stimuli are discontinued for several trials, the CS loses the capacity to cause the CR. This happens because the CS is a neutral stimulus, and when training stops, the CS resumes a neutral status. It is possible that a CR, having been extinguished, may suddenly reappear even though there has been no reconditioning with the US. This phenomenon is referred to spontaneous recovery.
Generalization
Generalization states that when a CS is paired with a US and comes to elicit a response, other stimuli that vary along the same dimension as the CS may also evoke that conditioned response. The more the new stimuli are like the original CS, the greater should be the strength of the CR. Through the process of generalization, we associate response with different stimuli without having actually to experience each and every stimulus event.
Discrimination
Discrimination involves the organism's ability to detect differences among stimuli and respond to only one or a few such stimuli. Discrimination training signals the behavioral course appropriate to the circumstance.
Second-Order Conditioning
Once a CR has been firmly established, the CS that elicits it may function in a manner to a US and thereby serve to strengthen responses to novel stimuli. In their early experiments, Pavlov and his associates took their observations of this as evidence of second-order conditioning. The significance of second-order effects is that the range for behavior for which classical conditioning might be responsible is extended.
Classical Conditioning Today
Pavlov's account, the contiguity model, states that conditioning occurs when two events are presented closely in time. Leading the opposition to the contiguity model was Robert Rescorla. Rescorla's findings have led many psychologists to think in terms of a contingency model of classical conditioning. The point is that one circumstance is perceived to be dependent on the other.
Blocking occurs when prior experience with one stimulus prevents later conditioning to a second stimulus. The literature on blocking began with Leon Kamin in 1969. There are many theories as to why blocking occurs. Rescorla and Wagner favor the idea that only a certain amount of conditioning can be sustained by a given US. Later conditioning may be blocked simply because some limit has already been reached.
