Back in the early 20th century, Pavlov managed to train man’s best friend to dribble at the sound of a ringing bell. Quite an achievement back in the day.
By continuously pairing the sound of a bell (the conditioned stimulus) with food (the unconditioned stimulus), Pavlov trained his dog to expect the sound of the bell to be followed by food, and therefore salivate each time it heard the bell (the conditioned response). This has since become one of the classic experiments used to train animals to match a conditioned stimulus with an unconditioned stimulus to produce a conditioned response.
These days the same principle is used to train lab rats but the bell has been replaced by a light. Food is still often used and this drops down into a small container. The conditioned response is to go to the container and collect the food. However, researchers have noticed something rather strange.
Upon the appearance of the light, most rats will correctly go to the container to collect the food. However, there are some rats who seems to get a little confused along the way. These rats will not go straight to the container, but instead will go up to the light and engage with it. And when I say engage, I mean anything such as touching, licking, biting etc……
But why? Are these rats just confused?
A group of researchers in United States, lead by Terry Robinson, have been looking at the biological basis of impulsiveness. In a recent paper they describe evidence that the neurotransmitter dopamine might be a key player in the development of impulsive behaviours.
Robinson and his colleagues used rats that had been selectively bred for particular behaviour traits. They bred rats that behave as goal trackers (they see the light and go for the container, as you might expect) and rats behave as sign trackers (the ones that go “towards the light”). They found that these rats differ in their neurotransmitter responses to the conditioned stimulus (the light).
The researchers looked at the levels of dopamine released in an area called the Nucleus Accumbens. This area is known to be involved pleasure, fear and the development of addiction in humans.
They found that the sign tracking rats showed alterations in dopamine signalling in the Nucleus Accumbens when compared to goal tracking rats. These alterations suggest that dopamine might be important in the development of a feeling of wanting or need for the conditioned stimulus, a phenomena known as “incentive salience”.
Incentive salience is often used to explain the development or relapse of addictive behaviours as it suggests that an important part of the addiction process involves a sensitising of certain brain areas, including the Nucleus Accumbens. This causes a transfer of the pleasure gained from the addictive substance, for example, the rush from gambling, onto other stimuli that might be present at the same time the person goes gambling (eg. pack of cards).
So, to return to the original question, what if Pavlov’s dog had had an addictive personality? Well, we might imagine that if the dog had had an addictive personality, it would probably have behaved as a sign tracker and very might have developed an unusual attraction for the bell rather than the food. This might have made Pavlov misinterpret his results and believe that he had caused the dog to develop a strong wanting for the light by pairing it with the food. Thankfully though, Pavlov’s dog was probably a goal tracker and this allowed Pavlov to make his all important discovery.
If you’d like to read the paper mentioned above the reference is as follows:
Flagel et al. (2011), A selective role for dopamine in stimulus-reward learning, Nature, 469 (7328) : 53-7. doi: 10.1038/nature09588.