Stimulus and Response: Scientists concerned with human behaviour and attitudes, namely psychologists, have tried to understand the basic process of learning, starting from simple models and situations. The simplest model is that of stimulus and response. The Russian Nobel Prize winner, Ivan Pavlov in early 1900s canied out some experiments on dogs which were perhaps the best examples of a stimulus producing a certain kind of  response. While studying the physiology of digestion in dogs he wanted to measure the flow of saliva. For this he inserted a tube in the cheek of the dog and placed a bowl of meat in front of  it and the dog began to salivate.

apparatus devised by the Russ~an  scientist to  test learned reflexes. Saliva  canied by a tube to beaker, acttvated a lever connwted to the pen beyond the screen at left. Each dmp of saliva was registered by a mark on the revotvtng drum. The dogs evidently  learned to enjoy their work, hopping up onto the platform without being asked. This, of  coutse, is a natural response of any dog. He begins to salivate when he gets his food. But a strange thing happened. The dog began  to salivate at the sight of the apparatus or the experimenter  even before the food was placed in front of  it. Pavlov could have treated this as an experimental nuisance but being a scientist he started asking questions. Pavlov knew that salivation  at the sight  of food was a natural reflex action.  It happens in every dog since birth, but the other reaction was something new, what we can call a learned reflex. Now he decided to investigate if  the dog could be made to associate  food .with other stimuli. In a typical experiment, a bell was sounded  just before the meat was given to the dog. This was repeated several times. Pavlov noticed that the dog now began  to salivate as soon as the bell was rung even if food was not given. The animal associated the two stimuli, food and bell, therefore, one could be substituted for the other. Table shows the steps in  this training process.