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The reward circuit (see diagram above) is also known as the mesolimbic pathway. Various activities release dopamine into the nucleus accumbens. This pleasurable experience is passed on to the hippocampus which “remembers” the experience, and the amygdala which can remember a trigger for the event which was associated with the initial dopamine release (all this is taking place unconsciously; the reasoning ability of the conscious brain takes no part). Addiction results in structural changes along this pathway, shortening the link between the pre-frontal cortex and the dopamine release and hence provides little conscious opportunity for us to take remedial action against what can become very destructive behaviour.
All this the world well knows; yet none knows well
To shun the heaven that leads men to this hell. (Shakespeare, Sonnet 129)
It is difficult to treat because dopamine release is also important for normal body and other brain operations, including memory, eating and moving about. Balance is important – too little is bad, and too much is bad. Parkinson’s disease is a result of insufficient dopamine levels in the brain.
There is an unfortunate “bad habit” (addiction) loop which is active in most people to a greater or lesser degree. The output of the action centre is fed to the basal ganglia in the Limbic brain, where there is subconscious feedback through the thalamus to the reward centre and emotional memory centre. Certain actions reward us subconsciously, resulting in us forming a habit to repeat the actions more frequently. We rely on the subconscious reward centre as babies to get us walking and eating and interacting with others, but later on in life it is easy to form addictions to a wide variety of substances such as drugs, food, alcohol, smoking, etc. and to certain activities such as sex, pornography, gambling, etc. The problem with an addiction is that it starts to control us from the subconscious so that the prefrontal cortex executive processing centre is no longer in control. We have much more visibility and control of our conscious minds than we do of the unconscious limbic areas.
Introverts and Extroverts
The distinction between introverts and extroverts are one of the commonly accepted differences between people – extroverts are party types and outgoing, whilst introverts sit quietly at home and can be quite shy. The brain diagram below shows how the reward centre in the brain works differently for these two groups. For Introverts the incoming stimulus reaches the hypothalamus, which sends signals to the thalamus and then to Broca’s area (4 on the diagram) in the prefrontal cortex and then back to the hippocampus and amygdala using Acetylcholine as the signalling neurotransmitter. Extroverts use a shorter dopamine pathway from the hypothalamus directly to the thalamus and the amygdala which sends signals to the motor cortex (to do something!). This is a significant difference, and this is one reason why it is easier for introverts to meditate than extroverts. Dopamine is a much more powerful control mechanism than acetylcholine.