Pavlovian conditioning, or classical conditioning, is a form of learning.
Learning is what happens when some past experience in your life alters your
present behavior. In the case of Pavlovian conditioning, learning involves
the association of some stimulus, such as a sound, a light, a smell, with the
experience of either something good or something bad. When the stimulus
reliably predicts that the good or bad thing is about to happen, then we learn
that the stimulus is a predictor of that good or bad thing. That predictor
then becomes a stimulus that allows us to prepare our bodies to deal with the
good or bad thing that is about to happen.
Pavlovian conditioning was of course discovered by Ivan Pavlov. Like so many
other great discoveries in science, it was completely accidental. Pavlov was
a Russian physiologist who was awarded the Nobel Prize in 1904 for his work in
understanding the digestion of food.
At the time of the discovery of Pavlovian conditioning, Pavlov's lab was
engaged in studying the salivation reflex that occurs when we eat. Dogs had
been prepared with a small vial attached to the side of the jaw. The duct
from a salivary glad was surgically exposed and positioned so that the saliva
that was produced by the gland would go into the small vial. Afterwards the
vial would be removed and the amount of saliva was recorded.
To produce the saliva, Pavlov's dog subjects were given "meat powder". After
a few days of getting meat powder to produce saliva, the dogs did something
that would change the history of psychology: The dogs began salivating BEFORE
they were fed! As soon as they noticed the researchers coming to fetch them
for the experiment, they would begin salivating.
Anyone who has ever loved and fed a dog before, or even paid attention to
their own bodies, knows that this happens. Whenever dogs (or any other
critters) get exposed to some signal that they're about to get fed, they wag
their tails, get excited, and salivate. However, this was a very surprising
thing for all the physiologists in Pavlov's lab. Why did the dog start
salivating when it had no food in it's mouth? And why did this odd behavior
take a few days to develop?
Pavlov hypothesized that the dogs were reacting to the expectation of being
fed. That led to the question of how they learned that expectation. Pavlov
hypothesized that the dogs were reacting to cues in their environment that
signaled the arrival of food. He also hypothesized that any stimulus that the
dogs could perceive could be used as a cue to predict the food.
Thus we have the apocryphal story of Pavlov's dogs and the "bell". However,
Pavlov did not use a bell as a stimulus. Instead, to test his hypothesis he
used a ticking metronome, probably because that sound was very unlikely to
have ever been heard by the dogs. The ticking sound was started, and the
followed by meat powder. Sure enough, after a few of these pairings, just the
sound of the metronome was enough to cause the dogs to salivate.
Pavlov went on to characterize in great detail this new learning phenomenon,
which he referred to as a "conditional reflex". Pavlov's understanding of
conditional reflexes was in the context of stimulus-response theory, where
behaviors are the result of stimuli causing responses to occur. Think of the
reflex action of a doctor tapping your patellar tendon with a rubber hammer
(the stimulus) and causing the leg to flex forward (the response). With his
new theory, Pavlov created an elegant way for stimuli and responses to be
altered based on experience.
In Pavlov's framework, the meat powder is called an "unconditional stimulus"
(US) and salivation is the "unconditional response" (UR) to the meat powder
US. US's elicit UR's and that is usually represented like this:
US -------> UR
The US-UR relationship is prewired into the brain of the person or animal, and
is not learned. Thus, dogs don't have to learn to salivate when they get
food. That's a reflexive action that results from the way their nervous
systems are wired.
What happens in Pavlovian conditioning is that a neutral stimulus (NS) gets
paired with a US. When we say "paired" we mean that the NS occurs first,
followed by the US:
NS ------> US ------> UR
Over time, this neutral stimulus starts to become meaningful to the animal,
because it reliably predicts the coming of the US. This is the key to
Pavlovian conditioning. The NS becomes important because it allows the animal
to EXPECT the US. Thus, over time, if the NS becomes a reliable predictor of
the US, it will become a cue to the animal that it needs to prepare for the US
and the UR that follows the US.
Here's the most important part: When the NS acquires this predictive value,
it goes from being a meaningless neutral stimulus to a meaningful "conditional
stimulus" (CS).
A conditional stimulus is one that an animal has learned is a reliable
predictor of some US. As a result, the animal learns to elicit a response to
the CS, in order to help it prepare for the oncoming US-UR. When the animal
starts eliciting this response to the CS, we call this response the
"conditional response" or CR. When the CS comes to elicit the CR like this:
CS ------> CR
We say that conditioning, or learning has occured.
In the case of Pavlov's dogs, the CR was salivation, which happens to be the
same as the UR. The CR is often, but not always, the same as the UR. The key
to figuring out if the CR will be like the UR is to understand that the CR is
always about helping the person or animal PREPARE for the expected US that is
coming. The CS tells the critter that the US is coming, therefore the critter
prepares for that US by responding with the CR.
A classic model of Pavlovian conditioning used nowadays in research is fear
conditioning in rats. A rat is put into a box called a Skinner box. On one
side of the Skinner box is a small light. The floor of the box is made up of
steel rods spaced about a half inch apart. Electric currents are fed to the
steel bars to deliver foot shocks to the rat. The shocks are very
uncomfortable, but do not cause any damage to the rat.
Now imagine that the rat is placed into this Skinner box. A few minutes
later, the light comes on for five seconds, goes off, followed by a two second
foot shock. This process repeats 10 more times. By the time it ends, the rat
will have developed a fear of the light coming on. That fear will have
developed because the light (CS) has come to signal the coming of the foot
shocks (US) which of course produce fear (UR). The response to the light CS,
however, will be FREEZING, where the rat remains completely motionless until
it feels safe again. This freezing behavior is the CR that we always see when
doing fear conditioning in rats. Thus, instead of squeaking and jumping
around like they do when exposed to the foot shock US, the CR is freezing,
which is a response to prepare them for the fear of the footshock.
The process of presenting the CS, then the US, then the UR so the critter
learns that the CS is a reliable signal of the US is called ACQUISITION. At
the start of acquistion, the animal or human of course does not respond to the
CS, which starts off as neutral. Over the repeated presentations (usually
referred to as trials) during acquisition, the animal acquires the
relationship between the CS and US.
What happens if you stop delivering the US, the food or the foot shock? If
the animal starts experiencing the CS not followed by the US, then over time
the CR response will fade away, because the CS no longer signals the coming of
the US. This process of "unconditioning" the CS-US relationship is called
extinction. Extinction is much more than a simple process of forgetting. It
involves learning yet another new thing about the stimulus that was the CS.
Pavlovian conditioning is an extremely powerful and elegant process we all
have built into our brains. It is the mediator for so many of our most basic
behaviors, and can be used to understand everything from certain psychological
disorders to training methods to child rearing to animal behavior.
