In delayed conditioning, the presentation of the CS precedes the onset of the US, and the termination of the CS is delayed until the US is presented (either at the US onset or during the presentation of the US). In Watson's study, the rat (CS) was presented before the loud noise (US) and remained in view of Little Albert until the loud noise was presented.
In many situations, the CS is a discrete event that is presented and terminated before the US is presented. Examples of discrete events include the noise of a gun being fired, the flash of a strobe light, one ring of a telephone, and the sound of an automobile horn. This is the procedure of trace conditioning, which can be very effective if the interstimulus interval is very brief. Pavlov used trace conditioning when he produced the tone once and then one second later presented the food. Even though trace conditioning is often used, it is not as effective as delayed conditioning.
Although Pavlov restricted his own research to dogs, other investigators have studied classical conditioning in many animals as well as people. In many situations, the optimum time interval between the CS and US is about one- half to 2 seconds. However, there is variability among species and the particular response being conditioned. For example, in conditioned taste aversion the successful interstimulus interval can be hours rather than seconds.
In simultaneous conditioning the CS and the US are presented at exactly the same time. Pavlov found that conditioning was very weak when the tone and food were presented together. One explanation is that there is no time for the subject to anticipate the US and thus the CS does not take on the properties of the US.
In the procedure sometimes called backward conditioning, the US is presented and terminated before the presentation of the CS. If Pavlov had presented the food and then, after the dog ate, presented the sound of the bell, the tone alone would not elicit much salivation, since it no longer signals that food is imminent. Backward conditioning is controversial because many psychologists argue that it does not work.
