Basic Terms and Procedure
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 easliy collected by temporarily cementing a small ring over the duct opening. From the ring a lenght of surgical tubing led to an apparatus that permitted the drops of saliva to be accurately counted.
Basic Elements of Classical Conditioning
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:
- 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.
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 facilitats 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.
- 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.
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.
- 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 techically a neutral stimulus.
- The Conditioned Response (CR) -- This is the response conditioned to the CS. In Pavlov's experiment, the CR was salivation.
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 unconditioined response to a shock US delivered through the floor is running, whereas the conditioned response may be freezing (immobility).
Basic Procedure for Demonstrating Classical Conditioning
There are three parts to the basic procedure for demonstrating classical conditioning:
- 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 interstimulus interval or ISI. In salivary conditioning, the CS-US interval the works best is on the order of 5 to 10 seconds.
- Repeat this pairing across a number of trials. The interval of time between trials is called the intertrial interval or ITI. In salivary conditioning it is on the order of 40 seconds to a minute.
- Test for conditioning by presenting the CS alone (omit the US). Any salivation to the CS alone must be a conditioned response.
In practice, it may be possible to combine the training and testing phases. If the CS precedes the US by a number of seconds, you can observe whether the CR occurs in the CS-US interval, before the US appears.
Classical Conditioning Preparations
You should not get the idea from reading about Pavlov's experiments with dogs that all classical conditioning involves salivary responses. In fact, many types of response can be classically conditioned, including emotional responses such as fear. Here I review some common alternatives to the salivary preparation:
- Rabbit nictitating membrane response -- The rabbit has a "second eyelid" beneath the first called the "nictitating membrane." A puff of air to the surface of the eye (cornea) elicits a reflexive blink of the nictitating membrane. A brief stimulus presented just prior to the puff will, after many pairings, begin to elicit a conditioned blink.
- Conditioned eyeblink response -- This is similar to the rabbit preparation, only using human participants.
- Conditioned suppression procedure -- Usually done with lab rats, this preparation measures the suppression of ongoing behavior during presentation of a CS that has been paired with some aversive event such as a foot-shock. Typically the ongoing behavior is lever-pressing that is being maintained at a rather steady level by occasional presentations of a pellet of food. One freuqently used measure of the amount of suppression to the CS in this procedure is the suppression ratio. This is computed by observing the number of responses during the CS and during an equivalently long non-CS period, then computing the suppression ratio as CS/(CS + non-CS). A suppression ratio of 0.50 indicates no suppression (half the total responses emitted during the CS). Any value below 0.50 indicates suppression, with zero indicating total suppression.
- Skin conductance response (SCR), also called the galvanic skin response (GSR) -- Measured in human participants, this is a change in skin conductivity owing to sweat-gland activity (under control of the autonomic nervous system) and is picked up via an electrode surface in contact with the hand or finger. The CS is paired repeatedly with some emotion-provoking event; when conditioning occurs a rapid, temporary change in skin conductance (CR) occurs during the CS.
- Conditioned taste aversions -- The subjects usually are lab rats, which are give water to drink that has been doctored with an unfamiliar flavor. After drinking the water, the rats are given something that makes them feel ill, and are then allowed to recover. A conditioned taste aversion is demonstrated if the animal will drink water of other flavors, but avoids water of the same flavor that was paired with the illness.