CLASSICAL CONDITIONING.

Classical or Pavlovian Conditioning was first systematically studied by the Russian physiologist Ivan Pavlov, beginning around the turn of the 20th century. Pavlov initially was interested in determining what role the nervous system plays in digestion, and won the Nobel Prize in Medicine or Physiology for this work in 1904. In the course of this research, Pavlov decided to systematically explore a phenomenon that had been noticed in passing by others -- that dogs who were becoming accustomed to being fed during certain laboratory tests began to salivate when merely exposed to stimuli that immediately preceded the offering of the food. To Pavlov, this was exciting stuff, as it suggested that new reflexes could be formed in the dog's nervous system as a result of its experiences. Believing that these nervous-system changes were taking place in the dog's cerebral cortex, Pavlov saw that a thorough study of the conditions under which such reflexes are formed, modified, inhibited, and so on might provide a magnificent new window on the operation of the cerebral cortex, the highest level of the brain. He was to spend the rest of his long career investigating this phenomenon of acquired reflexes.

Pavlov's experiments with dogs made use of a minor surgical procedure in which the end of one of the dog's six salivary ducts was loosened from its attachment to the inner surface of the cheek. A small hole was made through the cheek and the end of the duct, together with the surrounding mucus membrane, was pulled through the hole, sutured to the exterior surface, and allowed to heal. Now when the dog salivated, the output of this duct was conducted to the outside surface of the dog's cheek, where it could be easily collected by temporarily cementing a small ring over the duct opening. From the ring a length of surgical tubing led to an apparatus that permitted the drops of saliva to be accurately counted.

Basic Elements of Classical Conditioning

• The Unconditioned Stimulus (US) -- This is a stimulus that reflexively elicits a response. In Pavlov's experiments the US typically was meat powder.

The Unconditioned Response (UR) -- This is the response to the unconditioned stimulus that the experimenter measures. In Pavlov's experiments the UR was salivation.
The Neutral Stimulus -- This is a stimulus that initially does not elicit the response to be conditioned, in this example, salivation. It is important to note, however, that the initial presentations of the neutral stimulus do produce other responses that Pavlov collectively referred to as the investigatory reflex, or the "what is it?" reflex. The dog would turn toward the source of the stimulus, sniff at it, and so on, as if to determine what it was, and whether it needed to be paid any further attention. The modern term for this behavior is the orienting response.

• The Conditioned Stimulus (CS) -- This is the neutral stimulus after it has acquired the ability, through conditioning, to elicit a response. For simplicity, it is common to refer to this stimulus as "the CS" even while it is still technically a neutral stimulus.

The Conditioned Response (CR) -- This is the response conditioned to the CS. In Pavlov's experiment, the CR was salivation.
Basic Procedure for Demonstrating Classical Conditioning

1. Pair the CS and US together. Usually, this means presenting the CS first and then presenting the US, with the CS continuing to be present until the US occurs. The interval between the start of the CS and the start of the US called the CS-US interval, or alternatively, the inter stimulus interval or ISI. In salivary conditioning, the CS-US interval the works best is on the order of 5 to 10 seconds.

1. Repeat this pairing across a number of trials. The interval of time between trials is called the inter trial interval or ITI. In salivary conditioning it is on the order of 40 seconds to a minute.

1. Test for conditioning by presenting the CS alone (omit the US). Any salivation to the CS alone must be a conditioned response.

To demonstrate classical conditioning, you need at least two types of stimulus and an appropriate procedure for pairing the stimuli. Here are some terms describing the basic elements:

• Pavlov distinguished two kinds of unconditioned salivary reflex: the alimentary reflex to food, and the defensive reflex to weak acids and other tissue irritants. Alimentary salivation tends to be thick with mucus, giving it a lubricating quality that facilitate the mastication and swallowing of food. Defensive salivation tends to be thin and copius, helping to dilute the irritant and wash it out of the mouth.
• Pavlov used a variety of stimuli as neutral stimuli in his research, including an electric light, buzzer, spinning disk, and the ticking of a metronome. In this example we'll use a bell.

• Note that salivation was the response to both the US and the CS. Please keep in mind that the two responses can be (and often are) different. For example, the unconditioned response to a shock US delivered through the floor is running, whereas the conditioned response may be freezing (immobility)